This Preface is not a part of ANSIAISC 34105, Seismic Provisions for Structural Steel Buildings, but is included for informational purposes only. The AISC Specification for Structural Steel Buildings (ANSIAISC 36005) is intended to cover common design criteria. Accordingly, it is not feasible for it to also cover all of the special and unique problems encountered within the full range of structural design practice. This document, the AISC Seismic Provisions for Structural Steel Buildings (ANSIAISC 341 05) with Supplement No. 1 (ANSIAISC 341s105) (hereafter referred to as the Provisions) is a separate consensus standard that addresses one such topic: the design and construction of structural steel and composite structural steelreinforced concrete building systems for high seismic applications. Supplement No. 1 consists of modifications made to Part I, Section 14 of the Provisions after the initial approval had been completed.These Provisions are presented in two parts: Part I is intended for the design and construction of structural steel buildings, and is written in a unified format that addresses both LRFD and ASD; Part II is intended for the design and construction of composite structural steel reinforced concrete buildings, and is written to address LRFD only. In addition, seven mandatory appendices, a list of Symbols, and Glossary are part of this document. Terms that appear in the Glossary are generally italicized where they first appear in a subsection, throughout these Provisions. A nonmandatory Commentary with background information is also provided.
STRUCTURAL STEEL BUILDINGS — PROVISIONS
SCOPE
The Seismic Provisions for Structural Steel Buildings govern the design, fabrication, and erection of structural steel members and connections within seismic load resisting systems (SLRS) and related splices in columns These Provisions apply to buildings and similar structures that incorporate building-like vertical and lateral load-resisting elements They are mandated when the seismic response modification coefficient, R, exceeds 3, regardless of the seismic design category Structures with an R value of 3 or less are exempt from these Provisions unless specifically required by the applicable building code.
These Provisions shall be applied in conjunc tion with the AISC Specification for
Structural Steel Buildings, hereinafter referred to as the Specification Members and connections of the SLRS shall satisfy the requirements of the applicable building code, the Specification, and these Provisions.
In the absence of a local building code, the applicable building provisions will reference SEI/ASCE 7 for loads, load combinations, system limitations, and general design requirements.
Buildings designed with an R factor of 3 or less are typically limited to seismic design categories (SDC) A, B, or C However, certain systems, like cantilever columns with R factors below 3, may be allowed in SDC D and higher For detailed system limitations, refer to the relevant building code.
Part I includes a Glossary that is specifically applicable to this Part, and Appen- dices P, Q, R, S, T, W and X.
REFERENCED SPECIFICATIONS, CODES, AND STANDARDS
The documents referenced in these Provisions shall include those listed in Speci- fication Section A2 with the following additions and modifications:
American Institute of Steel Construction (AISC)
Specification for Structural Steel Buildings, ANSI/AISC 360-05 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, ANSI/AISC 358-05
American Society for Nondestructive Testing (ASNT)
Recommended Practice for the Training and Testing of Nondestructive Testing Personnel, ASNT SNT TC-1a-2001
Standard for the Qualification and Certification of Nondestructive Testing Personnel, ANSI/ASNT CP-189-2001
PART I – REFERENCED SPECIFICATIONS, CODES, AND STANDARDS
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Standard Methods for Determination of the Diffusible Hydrogen Content of Martensitic, Bainitic, and Ferritic Steel Weld Metal Produced by Arc Welding, AWS A4.3-93R
Standard Methods for Mechanical Testing of Welds-U.S Customary, ANSI/
Standard Methods for Mechanical Testing of Welds–Metric Only, ANSI/AWS
Standard for the Qualification of Welding Inspectors, AWS B5.1:2003
Oxygen Cutting Surface Roughness Gauge and Wall Chart for Criteria Describing Oxygen-Cut Surfaces, AWS C4.1
Federal Emergency Management Agency (FEMA)
Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings, FEMA 350, July 2000
GENERAL SEISMIC DESIGN REQUIREMENTS
For seismic design categories (SDC) and seismic use groups, the necessary strength and seismic provisions, along with restrictions on height and irregularity, must adhere to the relevant building code requirements.
The design story drift shall be determined as required in the applicable building code.
LOADS, LOAD COMBINATIONS, AND NOMINAL STRENGTHS
Building codes dictate the necessary loads and load combinations for construction In cases where amplified seismic loads are mandated, the horizontal earthquake load (E) must be multiplied by the overstrength factor (Ωo) specified in the relevant building code.
User Note: When not defined in the applicable building code, Ωo should be taken from SEI/ASCE 7.
The nominal strength of systems, members and connections shall comply with the Specification, except as modified throughout these Provisions.
STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS,
SPECIFICATIONS, SHOP DRAWINGS, AND ERECTION DRAWINGS
PART I – REFERENCED SPECIFICATIONS, CODES, AND STANDARDS
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5.1 Structural Design Drawings and Specifications
Structural design drawings and specifications shall show the work to be per- formed, and include items required by the Specification and the following, as applicable:
(1) Designation of the seismic load resisting system (SLRS)
(2) Designation of the members and connections that are part of the SLRS
(4) Connection material specifications and sizes
(5) Locations of demand critical welds
(6) Lowest anticipated service temperature (LAST) of the steel structure, if the structure is not enclosed and maintained at a temperature of 50 °F (10 °C) or higher
(7) Locations and dimensions of protected zones
(8) Locations where gusset plates are to be detailed to accommodate inelastic rotation
(9) Welding requirements as specified in Appendix W, Section W2.1.
These provisions must align with the Code of Standard Practice outlined in Section A4 of the Specification If there are specific connections and applications not explicitly covered by these provisions, the contract documents should incorporate suitable requirements for those scenarios.
These may include nondestructive testing requirements beyond those in Ap- pendix Q, bolt hole fabrication requirements beyond those permitted by the
Specification, bolting requirements other than those in the Research Council on Structural Connections (RCSC) Specification for Structural Joints Using
ASTM A325 or A490 Bolts, or welding requirements other than those in
Shop drawings shall include items required by the Specification and the follow- ing, as applicable:
(1) Designation of the members and connections that are part of the SLRS
(3) Locations of demand critical shop welds
(4) Locations and dimensions of protected zones
(5) Gusset plates drawn to scale when they are detailed to accommodate inelastic rotation
(6) Welding requirements as specified in Appendix W, Section W2.2.
Sect 5.] PART I – STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS
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When creating shop drawings, it's essential to address any specific connections and applications that may not be covered by the Provisions In such cases, the drawings should incorporate necessary requirements tailored to those applications, which may include additional bolt hole fabrication specifications and bolting criteria that extend beyond the guidelines outlined in the RCSC Specification for Structural Joints using ASTM A325 or A490.
Bolts, and welding requirements other than those in Appendix W See Sec- tion M1 of the Specification for additional provisions on shop drawings
Erection drawings shall include items required by the Specification and the fol- lowing, as applicable:
(1) Designation of the members and connections that are part of the SLRS
(2) Field connection material specifications and sizes
(3) Locations of demand critical field welds
(4) Locations and dimensions of protected zones
(6) Field welding requirements as specified in Appendix W, Section W2.3
When creating erection drawings, it is essential to include any specific requirements for applications not explicitly covered by the Provisions This may involve detailing bolting and welding requirements that differ from the RCSC Specification for Structural Joints Using ASTM A325 or A490 Bolts and Appendix W For further guidance on these additional provisions, refer to Section M1 of the Specification.
MATERIALS
Structural steel utilized in the seismic load resisting system (SLRS) must adhere to the requirements outlined in Specification Section A3.1a, with specific modifications as detailed in these provisions The maximum yield stress for steel members anticipated to undergo inelastic behavior is capped at 50 ksi (345 MPa) for systems in Sections 9, 10, 12, 13, 15, 16, and 17, and at 55 ksi (380 MPa) for those in Sections 11 and 14, unless validated through testing or other rational criteria This yield stress limitation does not apply to columns where the only expected inelastic behavior occurs at the column base.
PART I – STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS [Sect 5.
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The structural steel used in the SLRS desc ribed in Sect ions 9, 10, 11, 12, 13,
14, 15, 16 and 17 shall meet one of the follow ing ASTM Specif ica tions: A36/
A36M, A53/A53M, A500 (Grade B or C), A501, A529/A529M, A572/A572M [Grade 42 (290), 50 (345) or 55 (380)], A588/A588M, A913/A913M [Grade 50
The structural steel used for column base plates shall meet one of the preceding ASTM specifications or ASTM A283/A283M Grade D.
Other steels and non-steel materials in buckling-restrained braced frames are permitted to be used subject to the requirements of Section 16 and Appendix T.
This section focuses solely on the material properties of structural steel as defined in Section 2.1 of the AISC Code of Standard Practice It does not encompass other types of steel, such as cables used for permanent bracing.
6.2 Material Properties for Determination of Required
Strength of Members and Connections
In accordance with these Provisions, the necessary strength of an element, whether a member or a connection, is determined based on the expected yield stress, denoted as R_y F_y Here, F_y represents the specified minimum yield stress for the grade of steel utilized in adjacent members, while R_y indicates the ratio of the expected yield stress to this specified minimum yield stress.
For both LRFD and ASD design methods, the available strength of the element, denoted as φR n for LRFD and R n / Ω for ASD, must meet or exceed the required strength Here, R n represents the nominal strength of the connection, while the expected tensile strength is indicated by R t F u, and the anticipated yield stress is also considered.
R y and F y can be used instead of F u and F y to calculate the nominal strength, R n, for both rupture and yielding limit states within the same member when assessing the required strength.
In certain situations, it is essential to design a member or its connection limit state for forces that reflect the member's expected strength This is particularly relevant for brace fracture limit states, such as block shear rupture and net section fracture in Special Concentrically Braced Frames (SCBF), as well as in the design of beams located outside the link in Eccentrically Braced Frames (EBF) In these instances, using the expected material strength is permissible to determine the available strength of the member However, for connecting elements and other members, the specified material strength should be utilized.
The values of R y and R t for various steels are given in Table I-6-1 Other values of
R y and R t values are permissible when derived from tests on specimens that are similar in size and origin, conducted in accordance with the specified steel grade requirements.
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R y and R t Values for Different Member Types
Hot-rolled structural shapes and bars:
For structural steel in the SLRS, hot rolled shapes with flanges 12 inches thick or thicker must meet a minimum Charpy V-Notch toughness of 20 ft-lb (27 J) at 70 °F (21 °C), tested in the alternate core location as per ASTM A6 Supplementary Requirement S30 Additionally, plates that are 2 inches (50 mm) thick or more must also achieve a minimum Charpy V-Notch toughness of 20 ft-lb (27 J) at 70 °F (21 °C), with testing conducted at any location allowed by ASTM A673.
1 Members built-up from plate
2 Connection plates where inelastic strain under seismic loading is expected
3 As the steel core of buckling-restrained braces
Connection plates are critical components in structural engineering, particularly in scenarios where inelastic behavior is anticipated Notable examples include gusset plates designed to act as hinges, facilitating out-of-plane buckling of braces Additionally, certain bolted flange plates for moment connections and specific end plates used in bolted moment connections exhibit similar characteristics Furthermore, some column base plates are engineered to function as pins, highlighting the diverse applications of connection plates in maintaining structural integrity.
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CONNECTIONS, JOINTS, AND FASTENERS
Connections, joints and fasteners that are part of the seismic load resisting sys- tem (SLRS) shall comply with Specification Chapter J, and with the additional requirements of this Section.
The design of connections in a member of the SLRS must ensure that either the connection or the member itself is governed by a ductile limit state.
When designing connections for members within the Structural Load Resisting System (SLRS), it is crucial to ensure that the strength limit state is governed by ductile limit states, such as tension yielding Designing for nonductile or brittle limit states, like fracture, in either the connection or the member is unacceptable.
All bolts must be high-strength, pretensioned bolts that comply with Specification Section J3.8 for slip-critical faying surfaces, specifically with a Class A surface Installation should occur in standard or short-slotted holes that are perpendicular to the applied load For brace diagonals, oversized holes are acceptable for slip-critical joints, provided they are used in only one ply Alternative hole types are allowed as specified in the Prequalified Connections for Special and Intermediate Moment Frames for Seismic Applications (ANSI/AISC 358), or through connection prequalification per Appendix P, or via qualification testing in accordance with Appendices S or T The shear strength of bolted joints in standard holes should be calculated as bearing-type joints according to Specification Sections J3.7 and J3.10, with the nominal bearing strength at bolt holes not exceeding 2.4dtF u.
End plate moment connections can have their faying surfaces coated with materials that either have not been tested for slip resistance or possess a slip coefficient lower than that of a Class A faying surface.
Bolts and welds shall not be designed to share force in a joint or the same force component in a connection.
A member force, like a brace axial force, must be resisted at the connection using only one type of joint, either bolts or welds Connections that use both bolts and welds to resist different forces, such as a moment connection where welded flanges handle flexure and a bolted web manages shear, do not qualify as sharing the force.
Sect 7.] PART I – CONNECTIONS, JOINTS, AND FASTENERS
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Welding must adhere to Appendix W and follow a welding procedure specification (WPS) as mandated by AWS D1.1, with approval from the engineer of record Additionally, the WPS variables should align with the parameters set by the filler metal manufacturer.
All welds in members and connections of the SLRS must utilize a filler metal capable of achieving a minimum Charpy V-Notch toughness of 20 ft-lb (27 J) at 0 °F (-18 °C), as verified by the relevant AWS A5 classification test method or manufacturer certification This notch toughness requirement is also applicable in other specified instances.
Where welds are designated as demand critical, they shall be made with a filler metal capable of providing a minimum Charpy V-Notch (CVN) toughness of
20 ft-lb (27 J) at 20 °F (29 °C) as determined by the appropriate AWS clas- sification test method or manufacturer certification, and 40 ft-lb (54 J) at 70 °F
When the steel frame is typically enclosed and kept at a minimum temperature of 50 °F (10 °C), the qualification temperature as per Appendix X or other approved methods is set at 21 °C For structures expected to operate at temperatures below 50 °F (10 °C), the qualification temperature for Appendix X must be 20 °F (11 °C) above the lowest anticipated service temperature or adjusted to a lower temperature as necessary.
SMAW electrodes classified under AWS A5.1 as E7018 or E7018-X, as well as those classified under AWS A5.5 as E7018-C3L or E8018-C3, along with GMAW solid electrodes, are not required to undergo production lot testing if their CVN toughness meets or exceeds 20 ft-lb (27 J) at temperatures not exceeding specified limits.
20 °F (29 °C) as determined by AWS classification test methods The man- ufacturer’s certificate of compliance shall be considered sufficient evidence of meeting this requirement.
User Note: Welds designated demand critical are specifically identified in the Provisions in the section applicable to the designated SLRS.
Certain welds may require a demand critical designation, even if they are not explicitly identified as such in the Provisions This designation should be evaluated based on the inelastic strain demand and the potential consequences of failure.
Complete-joint-penetration (CJP) groove welds connecting columns to base plates should be regarded as demand critical, akin to column splice welds, particularly when CJP groove welds employed for column splices in designated Seismic Load Resisting Systems (SLRS) are identified as demand critical.
For special and intermediate moment frames, typical examples of demand critical welds include the following CJP groove welds:
(1) Welds of beam flanges to columns PART I – CONNECTIONS, JOINTS, AND FASTENERS [Sect 7.
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(2) Welds of single plate shear connections to columns
(3) Welds of beam webs to columns
(4) Column splice welds, including column bases
For ordinary moment frames, typical examples include CJP groove welds in items 1, 2, and 3 above.
Eccentrically braced frames (EBF) feature demand critical welds, prominently including CJP groove welds connecting link beams to columns Additionally, welds that attach the web plate to flange plates in built-up EBF link beams, as well as column splice welds utilizing CJP groove welds, should also be classified as demand critical welds.
Where a protected zone is designated by these Provisions or ANSI/AISC 358, it shall comply with the following:
In the designated protected zone, any discontinuities resulting from fabrication or erection activities, including tack welds, erection aids, air-arc gouging, and thermal cutting, must be repaired as specified by the engineer of record.
Welded shear studs and decking attachments must not be installed on beam flanges located within the protected zone However, decking arc spot welds are allowed for securing the decking.
(3) Welded, bolted, screwed or shot-in attachments for perimeter edge angles, exterior facades, partitions, duct work, piping or other construction shall not be placed within the protected zone.
Welded shear studs and other connections are allowed when specified in the Prequalified Connections for Special and Intermediate Moment Frames for Seismic Applications (ANSI/AISC 358) Additionally, they may be permitted based on a connection prequalification outlined in Appendix P or through a qualification testing program as described in Appendix S.
MEMBERS
Members in the seismic load resisting system (SLRS) shall comply with the
Specification and Section 8 For columns that are not part of the SLRS, see Sec- tion 8.4b.
8.2 Classification of Sections for Local Buckling
Members of the SLRS must have flanges that are continuously connected to the web or webs, and the width-thickness ratios of their compression elements must adhere to the maximum limits specified in Table B4.1 of the relevant specifications.
Members of the SLRS must have flanges that are continuously connected to the web or webs, and the width-thickness ratios of their compression elements must adhere to the maximum limits specified in Provisions Table I-8-1, denoted as λ ps.
PART I – CONNECTIONS, JOINTS, AND FASTENERS [Sect 7.
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TABLE I-8-1 Limiting Width-Thickness Ratios for
Limiting Width- Thickness Ratios λ ps
Flexure in flanges of rolled or built-up
Uniform compression in flanges of rolled or built-up I-shaped sections
Uniform compression in flanges of rolled or built-up I-shaped sections [d] b/t 0.38 E/F y
Uniform compression in flanges of channels, outstanding legs of pairs of angles in continuous contact, and braces [c], [g] b/t 0.30 E/F y
Uniform compression in flanges of
Uniform compression in legs of single angles, legs of double angle members with separators, or flanges of tees [g] b/t 0.30 E/F y
Uniform compression in stems of tees [g] d/t 0.30 E/F y
Note: See continued Table I-8-1 for stiffened elements.
When P u /φ c P n (LRFD) > 0.4 or Ω c P a /P n (ASD) > 0.4, as appropriate, without consideration of the amplified seismic load, where φ c = 0.90 (LRFD) Ω c = 1.67 (ASD)
P a = required axial strength of a column using ASD load combinations, kips (N)
P n = nominal axial strength of a column, kips (N)
P u = required axial strength of a column using LRFD load combinations, kips (N) the following requirements shall be met:
To determine the necessary axial compressive and tensile strength without any applied moment, it is essential to use the load combinations specified by the relevant building code, which includes the amplified seismic load.
TABLE I-8-1 (cont.) Limiting Width-Thickness Ratios for
Limiting Width- Thickness Ratios λ ps
Webs in flexural compression in beams in SMF, Section 9, unless noted otherwise h/t w 2.45 E/F y
Webs in flexural compression or combined flexure and axial compression [a], [c], [g], [h], [i], [ j] h/t w for C [ k ]
Round HSS in axial and/or flexural compression [c], [g] D/t 0.044 E /F y
Rectangular HSS in axial and/or flexural compression [c], [g] b/t or h/t w
[a] Required for beams in SMF, Section 9 and SPSW, Section 17.
[b] Required for columns in SMF, Section 9, unless the ratios from Equation 9-3 are greater than 2.0 where it is permitted to use λ p in Specification Table B4.1
[c] Required for braces and columns in SCBF, Section 13 and braces in OCBF, Section 14.
[d] It is permitted to use λ p in Specification Table B4.1 for columns in STMF, Section 12 and columns in EBF,
In accordance with EBF, Section 15, it is necessary to include a link, although the use of λ p from Table B4.1 of the Specification is allowed for flanges of links measuring 1.6M p /V p or shorter, with M p and V p defined in Section 15.
[f] Diagonal web members within the special segment of STMF, Section 12.
[g] Chord members of STMF, Section 12.
[h] Required for beams and columns in BRBF, Section 16.
[i] Required for columns in SPSW, Section 17.
In STMF Section 12, SMF columns with ratios exceeding 2.0 from Equation 9-3, and EBF columns in Section 15, as well as EBF webs with link lengths of 1.6 M p /V p or less, allow for the use of λ p defined as λ p = E when C a is less than or equal to 0.125.
P a = required compressive strength (ASD), kips (N)
P u = required compressive strength (LRFD), kips (N)
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(2) The required axial compressive and tensile strength shall not exceed either of the foll owing:
(a) The maximum load transferred to the column con sidering 1.1R y (LRFD) or (1.1/1.5)R y (ASD), as appropriate, times the nominal strengths of the connecting beam or brace elements of the building.
(b) The limit as deter mined from the resistance of the found ation to over- turning uplift.
The required strength of column splices in the seismic load resisting system (SLRS) shall equal the required strength of the columns, including that deter- mined from Sections 8.3, 9.9, 10.9, 11.9, 13.5 and 16.5b.
Welded column splices must meet specific requirements when subjected to calculated net tensile loads, as determined by the applicable building code, including amplified seismic loads.
(1) The available strength of partial-joint-penetration (PJP) groove welded joints, if used, shall be at least equal to 200 percent of the required strength.
Each flange splice must possess a minimum strength of at least 0.5 R y F y A f for LRFD or (0.5/1.5) R y F y A f for ASD Here, R y F y represents the anticipated yield stress of the column material, while A f denotes the flange area of the smaller connected column.
Beveled transitions are not required when changes in thickness and width of flanges and webs occur in column splices where PJP groove welded joints are used.
Column web splices can be constructed through bolting or welding, or by welding to one column while bolting to another In moment frames that utilize bolted splices, it is essential to incorporate plates or channels on both sides of the column web for added strength and stability.
Column splices using fillet welds or partial-joint-penetration groove welds must be positioned at least 4 feet (1.2 meters) away from beam-to-column connections If the clear height between these connections is less than 8 feet (2.4 meters), the splices should be placed at half of that clear height.
8.4b Columns Not Part of the
Splices of columns that are not a part of the SLRS shall satisfy the following:
Splices must be positioned at least 4 feet (1.2 meters) away from beam-to-column connections In cases where the clear height between these connections is less than 8 feet (2.4 meters), splices should be placed at half of the clear height.
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The shear strength required for column splices must be calculated according to both orthogonal axes of the column, using the formula M pc /H for Load and Resistance Factor Design (LRFD) or M pc /1.5H for Allowable Stress Design (ASD) Here, M pc represents the lesser nominal plastic flexural strength of the column sections in the relevant direction, while H denotes the height of the story.
The required strength of column bases shall be calculated in accordance with Sections 8.5a, 8.5b, and 8.5c The available strength of anchor rods shall be determined in accordance with Specification Section J3
The available strength of concrete elements at the column base, including anchor rod embedment and reinforcing steel, shall be in accordance with ACI 318, Appendix D
When designing anchor embedments with concrete reinforcing steel, it's crucial to recognize the potential anchor failure modes and ensure that reinforcement is adequately developed on both sides of the anticipated failure surface For detailed guidance, refer to ACI 318, Appendix D, Figure RD.4.1 and Section D.4.2.1, along with the accompanying Commentary.
In regions with moderate to high seismic risk, or for structures designated under intermediate to high seismic performance or design categories, the specific requirements outlined in ACI 318, Appendix D, are not mandatory.
The axial strength needed for column bases, along with their connection to the foundation, must equal the total of the vertical strength components of the steel elements linked to the column base.
The shear strength required for column bases and their connections to foundations must equal the total horizontal strength of the connected steel elements.
(1) For diagonal bracing, the horizontal component shall be determined from the required strength of bracing connections for the seismic load resisting system (SLRS).
(2) For columns, the horizontal component shall be at least equal to the lesser of the following:
(a) 2R y F y Z x /H (LRFD) or (2/1.5) R y F y Z x /H (ASD), as appropriate, of the column where
SPECIAL MOMENT FRAMES (SMF)
Special moment frames (SMF) are designed to endure substantial inelastic deformations during the forces generated by a design earthquake Compliance with the specific requirements outlined in this section is essential for SMF.
Beam-to-column connections used in the seismic load resisting system (SLRS) shall satisfy the following three requirements:
PART I – SPECIAL MOMENT FRAMES Sect 9.]
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(1) The connection shall be capable of sustaining an interstory drift angle of at least 0.04 radians.
(2) The measured flexural resistance of the connection, determined at the col- umn face, shall equal at least 0.80M p of the connected beam at an interstory drift angle of 0.04 radians.
(3) The required shear strength of the connection shall be determined using the following quantity for the earthquake load effect E:
R y = ratio of the expected yield stress to the specified minimum yield stress, F y
M p = nominal plastic flexural strength, kip-in (N-mm)
L h = distance between plastic hinge locations, in (mm)
When utilizing E as specified in Equation 9-1 within ASD load combinations that are additive with other transient loads according to SEI/ASCE 7, it is important to note that the 0.75 combination factor for transient loads should not be applied to E.
Connections must be designed to accommodate the necessary interstory drift angle while ensuring the specified flexural resistance and shear strengths Additionally, the design should account for any extra drift caused by connection deformation, ensuring that the overall structure can accommodate these changes Furthermore, a comprehensive analysis of the frame's stability, including second-order effects, is essential for a robust design.
Beam-to-column connections used in the SLRS shall satisfy the requirements of Section 9.2a by one of the following:
(a) Use of SMF connections designed in accordance with ANSI/AISC 358.
(b) Use of a connection prequalified for SMF in accordance with Appendix P.
(c) Provision of qualifying cyclic test results in accordance with Appendix S
Results of at least two cyclic connection tests shall be provided and are permitted to be based on one of the following:
(i) Tests reported in the research literature or documented tests performed for other projects that represent the project conditions, within the limits specified in Appendix S.
Tests tailored for the project are essential, ensuring they accurately reflect the sizes of project members, material strengths, connection configurations, and corresponding connection processes, all within the parameters outlined in Appendix S.
PART I – SPECIAL MOMENT FRAMES [Sect 9.
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Complete-joint-penetration groove welds connecting beam flanges, shear plates, and beam webs to columns are classified as demand critical welds, unless specified otherwise by ANSI/AISC 358, determined through a connection prequalification per Appendix P, or established via qualification testing in accordance with Appendix S, as outlined in Section 7.3b.
User Note: For the designation of demand critical welds, standards such as
ANSI/AISC 358 and tests addressing specific connections and joints should be used in lieu of the more general terms of these Provisions Where these
When a weld is marked as demand critical by certain provisions, but a more specific standard or test does not reflect this designation, the latter should take precedence Additionally, specific standards and tests may classify welds as demand critical even if they are not recognized as such by the provisions.
The areas at both ends of the beam that experience inelastic straining are classified as protected zones, which must adhere to the specifications outlined in Section 7.4 The size of these protected zones is specified in ANSI/AISC 358, can be defined through connection prequalification as per Appendix P, or may be established through qualification testing in accordance with Appendix S.
When designing SMF beams, it's essential to treat the plastic hinging zones at the ends as protected areas These zones must be defined within a prequalification or qualification program for connections, in accordance with Section 9.2b Typically, for unreinforced connections, the protected zone extends from the column face to half the beam depth beyond the plastic hinge point.
9.3 Panel Zone of Beam-to-Column Connections
(beam web parallel to column web)
The thickness of the panel zone must be established based on the method used for proportioning in tested or prequalified connections The minimum required shear strength is calculated by summing the moments at the column faces, which involves projecting the expected moments at the plastic hinge points The design shear strength is represented as φ v R v, while the allowable shear strength is R v /Ω v, with φ v set at 1.0 for LRFD and Ω v at 1.50 for ASD The nominal shear strength, R v, is determined according to the guidelines outlined in Specification Section J10.6, focusing on the limit state of shear yielding.
PART I – SPECIAL MOMENT FRAMES Sect 9.]
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The thickness, t, of column webs and doubler plates must adhere to the requirement t (d z + w z )/90 In this equation, t represents the thickness in inches (or mm), d z denotes the panel zone depth between continuity plates, and w z indicates the panel zone width between column flanges.
Alternatively, when local buckling of the column web and doubler plate is pre- vented by using plug welds joining them, the total panel zone thickness shall satisfy Equation 9-2.
Doubler plates must be welded to column flanges using either a complete joint penetration groove weld or a fillet weld that maximizes the shear strength of the full doubler plate thickness When positioned against the column web, these plates should be welded at the top and bottom edges to effectively transfer a portion of the total force to the doubler plate.
Doubler plates must be symmetrically positioned in pairs away from the column web and welded to continuity plates to effectively transfer the total force to the doubler plate.
9.4 Beam and Column Limi tations
The requirements of Section 8.1 shall be satisfied, in addition to the following.
Beam and column members shall meet the requirements of Section 8.2b, unless otherwise qualified by tests
Abrupt changes in beam flange area are not permitted in plastic hinge regi ons
Drilling flange holes or trimming beam flange width is allowed if testing confirms that the modified design can create stable plastic hinges This configuration must align with a prequalified connection outlined in ANSI/AISC 358, or be established through a connection prequalification per Appendix P, or validated through a qualification testing program in accordance with Appendix S.
Continuity plates must align with the prequalified connection specified in ANSI/AISC 358, or as established through a connection prequalification in Appendix P, or determined via a qualification testing program outlined in Appendix S.
PART I – SPECIAL MOMENT FRAMES [Sect 9.
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The following relationship shall be satisfied at beam-to-column connections: Σ Σ
In structural analysis, the sum of moments at the intersection of beam and column centerlines, denoted as ΣM pc *, is calculated by summing the projections of the nominal flexural strengths of the columns, including any haunches, above and below the joint, adjusted for axial force This can be represented as ΣM pc * = ΣZ c (F yc P uc /A g ) for LRFD or ΣZ c [(F yc /1.5) P ac /A g ] for ASD, depending on the applicable design method When the centerlines of opposing beams do not align, the mid-line between them should be utilized Similarly, ΣM pb *, the sum of moments in the beams at the joint, is determined by the projections of the expected flexural strengths at the plastic hinge locations, which can be expressed as ΣM pb * = Σ(1.1R y F yb Z b + M uv ) for LRFD or Σ[(1.1/1.5)R y F yb Z b + M av ] for ASD It is also permissible to calculate ΣM pb * using prequalified connection designs as outlined in ANSI/AISC 358 or through qualification testing per Appendices P and S, with adjustments made for connections utilizing reduced beam sections.
A g = gross area of column, in 2 (mm 2 )
F yc = specified minimum yield stress of column, ksi (MPa)
M av = the additional moment due to shear amplification from the location of the plastic hinge to the column centerline, based on ASD load combinations, kip-in (N-mm)
M uv = the additional moment due to shear amplification from the location of the plastic hinge to the column centerline, based on LRFD load combinations, kip-in (N-mm)
P ac = required compressive strength using ASD load combinations, kips
P uc = required compressive strength using LRFD load combinations, kips
Z b = plastic section modulus of the beam, in 3 (mm 3 )
Z c = plastic section modulus of the column, in 3 (mm 3 )
Z RBS = minimum plastic section modulus at the reduced beam section, in 3 (mm 3 )
PART I – SPECIAL MOMENT FRAMES Sect 9.]
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Exception: This requ irem ent does not apply if either of the following two condi- tions is satisfied:
(a) Columns with P rc < 0.3P c for all load combinations other than those de- termined using the amplified seismic load that satisfy either of the following:
(i) Columns used in a one-story building or the top story of a multistory building.
INTERMEDIATE MOMENT FRAMES (IMF)
Intermediate moment frames (IMF) are designed to endure limited inelastic deformations in their components and connections during the forces generated by a design earthquake Compliance with the specified requirements in this section is essential for IMF performance.
Beam-to-column connections used in the seismic load resisting system (SLRS) shall satisfy the requirements of Section 9.2a, with the following exceptions:
(1) The required interstory drift angle shall be a minimum of 0.02 radian.
(2) The required strength in shear shall be determined as specified in Section
9.2a, except that a lesser value of V u or V a , as appropriate, is permitted if justified by analysis The required shear strength need not exceed the shear resulting from the application of appropriate load combinations in the applicable building code using the amplified seismic load.
To meet the requirements outlined in Section 10.2a for IMF, conformance demonstration must follow the guidelines specified in Section 9.2b This includes connections that are prequalified for IMF per ANSI/AISC 358, as well as those determined through connection prequalification in Appendix P or through qualification testing as detailed in Appendix S.
PART I – SPECIAL MOMENT FRAMES [Sect 9.
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Complete joint penetration groove welds connecting beam flanges, shear plates, and beam webs to columns are classified as demand critical welds, unless specified otherwise by ANSI/AISC 358, determined through a connection prequalification per Appendix P, or assessed in a qualification testing program in accordance with Appendix S, as outlined in Section 7.3b.
User Note: For the designation of demand critical welds, standards such as
ANSI/AISC 358 and tests addressing specific connections and joints should be used in lieu of the more general terms of these Provisions Where these
When a weld is labeled as demand critical according to provisions, but the specific standard or test does not reflect this designation, the latter should take precedence Additionally, certain standards and tests may classify welds as demand critical, even if they are not recognized as such by these provisions.
The ends of the beam experiencing inelastic straining are classified as protected zones, which must comply with the specifications outlined in Section 7.4 The boundaries of these protected zones will be defined by ANSI/AISC 358, through connection prequalification as per Appendix P, or established via qualification testing as described in Appendix S.
When designing IMF beams, it's crucial to treat the plastic hinging zones at the ends as protected areas These zones should be defined during a prequalification or qualification program for the connection Typically, for unreinforced connections, the protected zone extends from the column face to half the beam depth beyond the plastic hinge point.
10.3 Panel Zone of Beam-to-Column Connections
(beam web parallel to column web)
No additional requirements beyond the Specification.
The requirements of Section 8.1 shall be satisfied, in addition to the following.
Beam and column members shall meet the requirements of Section 8.2a, unless otherwise qualified by tests
Abrupt changes in beam flange area are not permitted in plastic hinge regi ons
Drilling flange holes or trimming beam flange widths is allowed if testing or qualification shows that the new configuration can form stable plastic hinges This configuration must align with a prequalified connection.
PART I – INTERMEDIATE MOMENT FRAMES Sect 10.]
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6.1–38 designated in ANSI/AISC 358, or as otherwise determined in a connection prequalification in accordance with Appendix P, or in a program of qualification testing in accordance with Appendix S
Continuity plates must align with the prequalified connections specified in ANSI/AISC 358, or follow a connection prequalification outlined in Appendix P, or be established through qualification testing as detailed in Appendix S.
No additional requirements beyond the Specification.
10.7 Lateral Bracing at Beam-to-Column Connections
No additional requirements beyond the Specification.
Both flanges must be laterally braced, either directly or indirectly The maximum unbraced length between lateral braces should not exceed 0.17 times the radius of gyration multiplied by the yield strength and divided by the yield stress Additionally, braces must comply with the requirements outlined in Equations A-6-7 and A-6-8 from Appendix 6 of the Specification.
M r = M u = R y Z F y (LRFD) or M r = M a = R y Z F y /1.5 (ASD), as appropriate, of the beam, and C d = 1.0.
Lateral braces must be strategically placed near concentrated loads, changes in cross-section, and locations predicted to develop plastic hinges during inelastic deformations of the IMF When designs are based on assemblies tested per Appendix S, the lateral bracing for beams should align with the test configurations or meet prequalification requirements in Appendix P The necessary strength of the lateral bracing adjacent to plastic hinges is defined as P u = 0.06 M u /h o (LRFD) or P a = 0.06M a /h o (ASD), where h o represents the distance between flange centroids Additionally, the required stiffness must comply with Equation A-6-8 outlined in Appendix 6 of the Specification.
Column splices must adhere to the specifications outlined in Section 8.4a When utilizing groove welds for splicing, it is essential that these are complete-joint-penetration groove welds that conform to the standards set forth in Section 7.3b.
ORDINARY MOMENT FRAMES (OMF)
Ordinary moment frames (OMF) are designed to endure minimal inelastic deformations in their components and connections during seismic events Compliance with the specified requirements is essential for OMF Connections that adhere to Sections 9.2b and 9.5 or Sections 10.2b and 10.5 can be utilized in OMF without the need to satisfy the criteria outlined in Sections 11.2a, 11.2c, and 11.5.
PART I – INTERMEDIATE MOMENT FRAMES [Sect 10.
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The guidelines for OMF were primarily designed for wide flange shapes, but with careful consideration, they can also be applicable to other structural forms, including channels, built-up sections, and hollow structural sections (HSS).
Beam-to-column connections must utilize welds and/or high-strength bolts, allowing for either fully restrained (FR) or partially restrained (PR) moment connections.
11.2a Requirements for FR Moment Connections
FR moment connections within the seismic load resisting system (SLRS) must be designed to achieve a flexural strength of 1.1R y M p according to LRFD standards or (1.1/1.5)R y M p in accordance with ASD standards This design requirement applies to the beam or girder and is capped at the maximum moment that the system can develop, ensuring optimal structural integrity during seismic events.
FR connections shall meet the following requirements
In connections utilizing complete-joint-penetration (CJP) beam flange groove welds, steel backing and tabs must be removed, with the exception of top-flange backing that is continuously fillet welded to the column beneath the CJP groove weld, which does not require removal The process for removing steel backing and tabs should be adhered to as specified.
After the backing is removed, the root pass must be backgouged to reach sound weld metal and subsequently backwelded with a reinforcing fillet This reinforcing fillet should have a minimum leg size of 0.3125 inches (8 mm).
Weld tab removal must occur within 8 inches (3 mm) of the base metal surface, except for continuity plates where a 6 mm (¼ inch) margin is acceptable The edges of the weld tab should achieve a surface roughness of 500 μin (13 μm) or better, without requiring grinding to a flush condition Gouges and notches are prohibited, and any area where they have been removed must have a transitional slope not exceeding 1:5 If grinding results in material being removed more than 2 mm (¼ inch) below the base metal surface, it must be filled with weld metal Additionally, the weld contour at the ends should ensure a smooth transition, avoiding notches and sharp corners.
(2) Where weld access holes are provided, they shall be as shown in Figure 11-1
The weld access hole shall have a surface roughness value not to exceed
The beam web near the end-plate in bolted moment end-plate connections must maintain a smooth surface with a finish of 500 μin (13 μm), free from notches and gouges Any imperfections such as notches and gouges must be repaired as directed by the engineer of record, and weld access holes are strictly prohibited in this area.
The strength requirements for double-sided partial-joint-penetration groove welds and double-sided fillet welds that withstand tensile forces in connections are specified as 1.1R y F y A g (LRFD) or (1.1/1.5)R y F y A g (ASD), depending on the applicable method, based on the connected element or part Single-sided partial-joint-penetration welds have different criteria that must also be considered.
PART I – ORDINARY MOMENT FRAMES Sect 11.]
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6.1–40 groove welds and single-sided fillet welds shall not be used to resist tensile forces in the connections.
(4) For FR moment connections, the required shear strength, V u or V a , as appro- priate, of the connection shall be determined using the following quantity for the earthquake load effect E:
In ASD load combinations that include additive transient loads according to SEI/ASCE 7 guidelines, the 0.75 combination factor for transient loads should not be applied to the E value.
Alternatively, a lesser value of V u or V a is permitted if justified by analysis
The required shear strength need not exceed the shear resulting from the ap- plication of appropriate load combinations in the applicable building code using the amplified seismic load.
Notes: 1 Bevel as required for selected groove weld.
2 Larger of t bf or 2 in (13 mm) (plus 2 t bf , or minus 4 t bf )
3 w t bf to t bf , w in (19 mm) minimum ( ± 4 in.) ( ± 6 mm)
4 a in (10 mm) minimum radius (plus not limited, minus 0)
6 See FEMA-353, “Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications,” for fabrication details including cutting methods and smoothness requirements.
Tolerances shall not accumulate to the extent that the angle of the access hole cut to the flange surface exceeds 25 °
Fig 11–1 Weld access hole detail (from FEMA 350,
“Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings”).
PART I – ORDINARY MOMENT FRAMES [Sect 11.
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11.2b Requirements for PR Moment Connections
PR moment connections are permitted when the following requirements are met:
(1) Such connections shall be designed for the required strength as specified in Section 11.2a above.
(2) The nominal flexural strength of the connection, M n , shall be no less than 50 percent of M p of the connected beam or column, whichever is less.
(3) The stiffness and strength of the PR moment connections shall be consid- ered in the design, including the effect on overall frame stability.
For PR moment connections, the values of V u or V a should be calculated based on the specified load combination, along with the shear generated by the maximum end moment that the connection can withstand.
Complete-joint-penetration groove welds of beam flanges, shear plates, and beam webs to columns shall be demand critical welds as described in Section 7.3b.
11.3 Panel Zone of Beam-to-Column Connections
(beam web parallel to column web)
No additional requirements beyond the Specification.
When constructing FR moment connections using welds between beam flanges or beam-flange connection plates and column flanges, it is essential to include continuity plates as outlined in Section J10 of the Specification Additionally, continuity plates are necessary when the condition t cf < 0.54 b t F F f bf yb / yc is met, or when t cf < b f / 6.
Where continuity plates are required, the thickness of the plates shall be deter- mined as follows:
(a) For one-sided connections, continuity plate thickness shall be at least one half of the thickness of the beam flange.
(b) For two-sided connections the continuity plates shall be at least equal in thickness to the thicker of the beam flanges.
PART I – ORDINARY MOMENT FRAMES Sect 11.]
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Welded joints connecting continuity plates to column flanges must utilize either complete-joint-penetration groove welds, two-sided partial-joint-penetration groove welds with reinforcing fillet welds, or two-sided fillet welds These joints must achieve a strength that meets or exceeds the strength of the contact area between the plate and the column flange Additionally, the required strength for the welded joints of continuity plates to the column web should be determined based on the minimum of specified criteria.
(a) The sum of the available strengths at the connections of the continuity plate to the column flanges.
(b) The available shear strength of the contact area of the plate with the column web.
(c) The weld available strength that develops the available shear strength of the column panel zone.
(d) The actual force transmitted by the stiffener.
11.7 Lateral Bracing at Beam-to-Column Connections
No additional requirements beyond the Specification.
No additional requirements beyond the Specification
Column splices shall comply with the requirements of Section 8.4a.
SPECIAL TRUSS MOMENT FRAMES (STMF)
Special truss moment frames (STMF) are engineered to endure substantial inelastic deformation in designated truss segments during earthquake motions These frames are restricted to a maximum span length of 65 ft (20 m) and an overall depth of 6 ft (1.8 m) Additionally, columns and truss segments outside the specialized areas must be designed to maintain elastic behavior under forces produced by fully yielded and strain-hardened segments Compliance with the specified requirements is essential for STMF.
Each horizontal truss within the seismic load resisting system (SLRS) must include a designated special segment positioned between the quarter points of the truss span This special segment should have a length ranging from 0.1 to 0.5 times the total span of the truss Additionally, the length-to-depth ratio of any panel within this special segment must be maintained between 0.67 and 1.5.
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In a designated special segment, only Vierendeel panels or X-braced panels are allowed, prohibiting any combination of the two or the use of alternative truss diagonal configurations If diagonal members are incorporated within the special segment, they must be arranged in an X pattern and separated by vertical members.
Diagonal members must be interconnected at their crossing points, with the interconnection strength set at 0.25 times the nominal tensile strength of the diagonal member Bolted connections are prohibited for web members within the special segment, and these diagonal web members should consist of flat bars of uniform sections.
Splicing of chord members is prohibited within the special segment and within half the panel length from its ends Additionally, the axial strength required for diagonal web members in the special segment, due to dead and live loads, must not exceed 0.03F y A g (LRFD) or (0.03/1.5)F y A g (ASD), depending on the applicable design method.
The special segment shall be a protected zone meeting the requirements of Section 7.4
12.3 Strength of Special Segment Members
The available shear strength of the special segment is determined by combining the shear strength of the chord members due to flexure with the shear strength derived from the available tensile strength and 0.3 times the available compressive strength of the diagonal members, when applicable.
The top and bottom chord members in the specified segment must consist of identical sections and provide a minimum of 25% of the necessary vertical shear strength The axial strength in these chord members, calculated based on the limit state of tensile yielding, should not surpass 0.45 times φP n (LRFD) or P n / Ω (ASD), with φ set at 0.90 (LRFD) and Ω at 1.67 (ASD).
The end connections of diagonal web members in the special segment must possess a strength that meets or exceeds the expected yield strength in tension, calculated as R y F y A g for LRFD or R y F y A g / 1.5 for ASD, depending on the applicable design method.
12.4 Strength of Non-Special Segment Members
Members and connections of STMF, excluding those in the special segment outlined in Section 12.2, must meet strength requirements based on applicable building code load combinations This involves substituting the earthquake load term E with the lateral loads needed to achieve the expected vertical shear strength of the special segment, represented as V ne (LRFD) or V ne /1.5 (ASD), at mid-length.
PART I – SPECIAL TRUSS MOMENT FRAMES Sect 12.]
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M nc = nominal flexural strength of a chord member of the special segment, kip-in (N-mm)
EI = flexural elastic stiffness of a chord member of the special segment, kip-in 2 (N-mm 2 )
L = span length of the truss, in (mm)
L s = length of the special segment, in (mm)
P nt = nominal tensile strength of a diagonal member of the special segment, kips (N)
P nc = nominal compressive strength of a diagonal member of the special segment, kips (N) α = angle of diagonal members with the horizontal
Chord members and diagonal web members within the special segment shall meet the requirements of Section 8.2b
The top and bottom chords of trusses must be laterally braced at the ends of the special segment and at intervals not exceeding Lp, as specified in Chapter F This bracing is required along the entire length of the truss, ensuring that each lateral brace at the ends and within the special segment meets the necessary strength requirements.
P a = (0.06/1.5) R y P nc (ASD), as appropriate, where P nc is the nominal compressive strength of the special segment chord member
Lateral braces outside of the special segment shall have a required strength of
The required brace stiffness shall meet the provisions of Equation A-6-4 of Appendix 6 of the Specification, where
SPECIAL CONCENTRICALLY BRACED FRAMES (SCBF)
Special concentrically braced frames (SCBF) are designed to endure substantial inelastic deformations during the forces generated by design earthquake motions Compliance with the requirements outlined in this section is essential for SCBF.
PART I – SPECIAL TRUSS MOMENT FRAMES [Sect 12.
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User Note: Section 14 (OCBF) should be used for the design of tension-only bracing.
Bracing members shall have Kl r/ ≤4 E F/ y
Braces with 4 E/F where y < Kl/r ≤ 200 are allowed in frames if the column's available strength meets or exceeds the maximum load transferred to it, based on either R y (LRFD) or (1/1.5)R y (ASD) Additionally, column forces should not surpass those calculated through inelastic analysis or the maximum load effects achievable by the system.
When the effective net area of bracing members is smaller than the gross area, the necessary tensile strength of the brace, determined by the limit state of fracture in the net section, must exceed the smaller value of the two following criteria.
(a) The expected yield strength, in tension, of the bracing member, determined as R y F y A g (LRFD) or R y F y A g /1.5 (ASD), as appropriate.
(b) The maximum load effect, indicated by analysis that can be trans ferred to the brace by the system.
User Note: This provision applies to bracing members where the section is reduced A typical case is a slotted HSS brace at the gusset plate connection.
Bracing must be installed in alternating directions along any line of bracing, ensuring that for forces parallel to the bracing, 30 to 70 percent of the total horizontal force is resisted by tension braces This requirement holds unless the compression strength of each brace exceeds the necessary strength determined by applicable building codes, including amplified seismic loads A line of bracing is defined as a single line or parallel lines with a plan offset of 10 percent or less of the building's dimension perpendicular to the bracing line.
Column and brace members shall meet the requirements of Section 8.2b.
User Note: HSS walls may be stiffened to comply with this requirement.
PART I – SPECIAL CONCENTRICALLY BRACED FRAMES Sect 13.]
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The spacing of stitches must ensure that the slenderness ratio (l/r) of each element between stitches remains at or below 0.4 times the governing slenderness ratio of the built-up member.
The total shear strengths of the stitches must meet or surpass the tensile strength of each component Stitches should be evenly spaced, and a minimum of two stitches is required for built-up members Additionally, bolted stitches must not be positioned within the central quarter of the clear brace length.
In cases where brace buckling around the critical buckling axis does not induce shear in the stitches, it is essential that the spacing of the stitches is designed to ensure that the slenderness ratio (l/r) of the individual elements between the stitches remains within 0.75 times the governing slenderness ratio of the built-up member.
13.3 Required Strength of Bracing Connections
The required tensile strength of bracing connections (including beam- to-column connections if part of the bracing system) shall be the lesser of the following:
(a) The expected yield strength, in tension, of the bracing member, determined as R y F y A g (LRFD) or R y F y A g /1.5 (ASD), as appropriate.
(b) The maximum load effect, indicated by analysis that can be trans ferred to the brace by the system.
The required flexural strength of bracing connections shall be equal to 1.1R y M p
(LRFD) or (1.1/1.5)R y M p (ASD), as appropriate, of the brace about the critical buckling axis.
Exception: Brace connections that meet the requirements of Section 13.3a and can accommodate the inelastic rotations associated with brace post-buckling deformations need not meet this requirement.
Accommodation of inelastic rotation is usually achieved using a single gusset plate, where the brace ends before reaching the line of restraint The specific detailing requirements for this type of connection are outlined in the accompanying commentary.
Bracing connections shall be designed for a required compressive strength based on buckling limit states that is at least equal to 1.1R y P n (LRFD) or (1.1/1.5)R y P n
(ASD), as appropriate, where P n is the nominal compressive strength of the brace.
PART I – SPECIAL CONCENTRICALLY BRACED FRAMES [Sect 13.
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13.4 Special Bracing Conf igur ation Requirements
13.4a V-Type and Inverted-V-Type Bracing
V-type and inverted V-type SCBF shall meet the following requirements:
The strength requirements for beams intersected by braces, along with their connections and supporting members, must be established according to the load combinations specified in the relevant building code, assuming that the braces do not contribute to supporting dead and live loads When evaluating load combinations that factor in earthquake effects, the earthquake impact, denoted as E, on the beam must be calculated accordingly.
(a) The forces in all braces in tension shall be assumed to be equal to
(b) The forces in all adjoining braces in compression shall be assumed to be equal to 0.3P n
Beams must be continuous between columns, with both flanges laterally braced at a maximum spacing defined by Lb = Lpd, according to Equations A-1-7 and A-1-8 in Appendix 1 of the Specification The lateral braces must comply with Equations A-6-7 and A-6-8 in Appendix 6, where M_r equals M_u for LRFD or M_a for ASD, calculated as M_r = R_y Z F_y or M_r = R_y Z F_y / 1.5, respectively, and C_d is set at 1.0.
At least one set of lateral braces is necessary at the intersection of V-type or inverted V-type bracing, unless the beam possesses adequate out-of-plane strength and stiffness to maintain stability between neighboring brace points.
To demonstrate adequate out-of-plane strength and stiffness of a beam, it is essential to apply the bracing force specified in Equation A-6-7 of Appendix 6 of the Specification to each flange, creating a torsional couple This loading must be considered alongside the flexural forces mentioned previously Additionally, the beam's stiffness, along with its restraints, must meet the criteria outlined in Equation A-6-8 to ensure proper performance under torsional loading.
K-type braced frames are not permitted for SCBF
Column splices in Special Concentrically Braced Frames (SCBF) must meet the criteria outlined in Section 8.4, ensuring they develop 50 percent of the lower available flexural strength of the connected members The necessary shear strength is calculated using ΣM pc /H for Load and Resistance Factor Design (LRFD) or ΣM pc /1.5H for Allowable Stress Design (ASD), where ΣM pc represents the total nominal plastic flexural strengths of the columns situated above and below the splice.
The protected zone for bracing members in a Special Concentrically Braced Frame (SCBF) encompasses the central quarter of the brace length, along with an area adjacent to each connection that is equal to the brace depth in the buckling plane This defined protected zone is crucial for ensuring the structural integrity of SCBF systems.
PART I – SPECIAL CONCENTRICALLY BRACED FRAMES Sect 13.]
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6.1–48 elements that connect braces to beams and columns and shall satisfy the require- ments of Section 7.4.
ORDINARY CONCENTRICALLY BRACED FRAMES (OCBF)
Ordinary concentrically braced frames (OCBF) are designed to endure limited inelastic deformations in their components and connections during seismic events Compliance with the specified requirements in this section is essential for OCBF Additionally, OCBF located above the isolation system in seismically isolated structures must adhere to the criteria outlined in Sections 14.4 and 14.5, while exemptions from Sections 14.2 and 14.3 apply.
Recent updates to the Provisions have removed the requirement for OCBF members to be designed for amplified seismic loads, which previously halved the effective R factor This change aligns the design of OCBF with other systems and reflects a corresponding reduction in the R factor as outlined in SEI/ASCE 7-05 Supplement Number 1 Consequently, the strength of OCBF members will now be determined based on the loading combinations specified by the relevant building code, utilizing the reduced R factors without considering amplified seismic loads.
Bracing members shall meet the requirements of Section 8.2b.
Exception: HSS braces that are filled with concrete need not comply with this provision.
Bracing members in K, V, or inverted-V configurations shall have
Bracing members intended solely for tension should not be utilized in K, V, and inverted-V configurations due to their neglect of compressive strength However, these tension-only braces can be effectively employed in other structural configurations without needing to meet this specific requirement Examples of such bracing members include slender angles, plates, or cable bracing, which remain permissible under Section 6.1.
In V-type and inverted V-type OCBF systems, beams must remain continuous at bracing connections that are positioned away from the beam-column junction Additionally, columns in K-type OCBF should adhere to these same continuity requirements at their respective bracing connections.
PART I – SPECIAL CONCENTRICALLY BRACED FRAMES [Sect 13.
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The required strength of structural elements must be calculated according to the load combinations specified in the relevant building code, with the assumption that braces do not contribute to supporting dead and live loads For load combinations that factor in earthquake impacts, the earthquake effect, denoted as E, on the member should be assessed accordingly.
In V-type and inverted V-type OCBFs, the tension forces in braces should be considered equal to RyFyAg, but they must not surpass the maximum force that the system can generate.
(b) The forces in braces in compression shall be assumed to be equal to 0.3P n
(2) Both flanges shall be laterally braced, with a maximum spacing of
L b = L pd , as specified by Equations A-1-7 and A-1-8 of Appendix 1 of the
Specification Lateral braces shall meet the provisions of Equations A-6-7 and A-6-8 of Appendix 6 of the Specification, where M r = M u = R y Z F y
(LRFD) or M r = M a = R y Z F y /1.5 (ASD), as appropriate, of the beam and
For effective structural stability, a minimum of one set of lateral braces must be installed at the intersection point of the bracing system when C d = 1.0 This requirement holds unless the member exhibits adequate out-of-plane strength and stiffness to maintain stability between adjacent brace points.
User Note: See User Note in Section 13.4 for a method of establishing suf- ficient out-of-plane strength and stiffness of the beam.
The required strength of bracing connections shall be determined as follows.
For the limit state of bolt slip, the strength required for bracing connections must be determined based on the load combinations specified by the relevant building code, excluding any amplified seismic loads.
(2) For other limit states, the required strength of bracing connections is the expected yield strength, in tension, of the brace, determined as R y F y A g
(LRFD) or R y F y A g /1.5 (ASD), as appropriate
Exception: The required strength of the brace connection need not exceed either of the following:
(a) The maximum force that can be developed by the system (b) A load effect based upon using the amplified seismic load
14.5 OCBF above Seismic Isolation Systems
Bracing members shall meet the requirements of Section 8.2a and shall have
K-type braced frames are not permitted
PART I – ORDINARY CONCENTRICALLY BRACED FRAMES Sect 14.]
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14.5c V-Type and Inverted-V-Type Bracing
Beams in V-type and inverted V-type bracing shall be continuous between columns.
ECCENTRICALLY BRACED FRAMES (EBF)
Eccentrically braced frames (EBFs) are designed to endure substantial inelastic deformations in the links during design earthquake motions The diagonal braces, columns, and beam segments outside the links must remain primarily elastic under maximum forces generated by fully yielded and strain-hardened links, with specific exceptions allowed For buildings taller than five stories, the upper story of an EBF system may be designed as an OCBF or SCBF, while still qualifying as part of the EBF system for determining system factors in the relevant building code Compliance with the outlined requirements is essential for EBFs.
Links shall meet the requirements of Section 8.2b.
The web of a link shall be single thickness Doubler-plate reinforcement and web penetrations are not permitted.
Except as limited below, the link design shear strength, φ v V n , and the allowable shear strength, V n /Ω v , according to the limit state of shear yielding shall be de- termined as follows:
V n = nominal shear stre ngth of the link, equal to the lesser of V p or 2M p /e, kips (N) φ v = 0.90 (LRFD) Ω v = 1.67 (ASD) where
V p = 0.6 F y A w , kips (N) e = link length, in (mm)
The effect of axial force on the link available shear strength need not be con- sidered if
P u = required axial strength using LRFD load combinations, kips (N)
P a = required axial strength using ASD load combinations, kips (N)
P y = nominal axial yield strength = F y A g , kips (N)
PART I – ORDINARY CONCENTRICALLY BRACED FRAMES [Sect 14.
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P a > (0.15/1.5)P y (ASD), as appropriate, the following additional requirements shall be met:
(1) The available shear strength of the link shall be the lesser of φ v V pa and 2φ v M pa /e (LRFD) or
V pa / Ω v and 2 (M pa /e)/Ω v (ASD), as appropriate, where φ v = 0.90 (LRFD) Ω v = 1.67 (ASD)
P r = P u (LRFD) or P a (ASD), as appropriate
P c = P y (LRFD) or P y /1.5 (ASD), as appropriate
(2) The length of the link shall not exceed:
V r = V u (LRFD) or V a (ASD), as appropriate
V u = required shear strength based on LRFD load combinations, kips
V a = required shear strength based on ASD load combinations, kips
The link rotation angle refers to the inelastic angle formed between the link and the beam outside of the link when the total story drift matches the design story drift, Δ It is crucial to ensure that the link rotation angle does not surpass specified limits.
(a) 0.08 radians for links of length 1.6M p /V p or less.
(b) 0.02 radians for links of length 2.6M p /V p or greater.
(c) The value determined by linear interpolation between the above values for links of length between 1.6M p /V p and 2.6M p /V p
Full-depth web stiffeners must be installed on both sides of the link web at the ends of the diagonal braces These stiffeners should have a total width of at least (b f + 2t w) and a minimum thickness of 0.75t w or 0.375 inches (10 mm).
PART I – ECCENTRICALLY BRACED FRAMES Sect 15.]
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6.1–52 whichever is larger, where b f and t w are the link flange width and link web thickness, respectively.
Links shall be provided with intermediate web stiffeners as fol lows:
Links measuring 1.6M p/V p or shorter must include intermediate web stiffeners positioned at intervals not exceeding (30t w – d/5) for a link rotation angle of 0.08 radians, or (52t w – d/5) for rotation angles of 0.02 radians or less For angles between 0.08 and 0.02 radians, linear interpolation should be applied.
Links longer than 2.6M p/V p and shorter than 5M p/V p must include intermediate web stiffeners positioned at a distance of 1.5 times b f from each end of the link.
(c) Links of length between 1.6M p /V p and 2.6M p /V p shall be provided with intermediate web stiffeners meeting the re quirements of (a) and (b) above.
(d) Intermediate web stiffeners are not required in links of lengths greater than 5M p /V p
(e) Intermediate web stiffeners shall be full depth For links that are less than
25 in (635 mm) in depth, stiffeners are required on only one side of the link web The thickness of one-sided stiffeners shall not be less than t w or a in
(10 mm), whichever is larger, and the width shall be not less than (b f /2) t w
For links that are 25 in (635 mm) in depth or greater, similar intermediate stiffeners are required on both sides of the web
The required strength of fillet welds that connect a link stiffener to the link web is determined by A st F y (LRFD) or A st F y / 1.5 (ASD), where A st represents the area of the stiffener Additionally, the strength of fillet welds joining the stiffener to the link flanges is specified as A st F y / 4 (LRFD) or A st F y / 6 (ASD).
Link-to-column connections must support the maximum rotation angle of the link, as outlined in Section 15.2c Additionally, the connection's strength at the column face should be no less than the nominal shear strength of the link, Vn, as detailed in Section 15.2b, at this maximum rotation angle.
Link-to-column connections shall satisfy the above requirements by one of the following:
(a) Use a connection prequalified for EBF in accordance with Appendix P.
To comply with Appendix S, it is essential to provide qualifying cyclic test results, including at least two cyclic connection tests These results may be derived from one of the specified methods outlined in the appendix.
(i) Tests reported in research literature or documented tests performed for other projects that are representative of project conditions, within the limits specified in Appendix S.
PART I – ECCENTRICALLY BRACED FRAMES [Sect 15.
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Tests tailored for the project are essential, reflecting the sizes of project members, material strengths, connection configurations, and corresponding connection processes, all adhering to the limits outlined in Appendix S.
In cases where reinforcement at the beam-to-column connection prevents yielding of the beam over the reinforced length, the link may extend from the end of the reinforcement to the brace connection If the link length is within the limit of 1.6M p /V p, cyclic testing of the reinforced connection is unnecessary, provided that the strength of the reinforced section and connection meets or exceeds the required strength calculated based on the strain-hardened link.
Section 15.6 Full depth stiffeners as required in Section 15.3 shall be placed at the link-to-reinforcement interface.
Lateral bracing must be installed at both the top and bottom link flanges at the ends of the link The necessary strength for each lateral brace at these ends is calculated using the formula P b = 0.06 M r /h o, where h o represents the distance between the flange centroids in inches (mm).
For design according to Specification Section B3.3 (LRFD)
For design according to Specification Section B3.4 (ASD)
The required brace stiffness shall meet the provisions of Equation A-6-8 of the
Specification, where M r is defined above,C d = 1, and L b is the link length.
15.6 Dia gonal Brace and Beam Outside of Link
The combined axial and flexural strength required for diagonal braces must be calculated according to the load combinations specified by the relevant building code In cases where seismic effects are considered, the load Q1 is used in place of the term E, defined as the axial forces and moments resulting from at least 1.25 times the expected nominal shear strength of the link RyVn, with Vn as outlined in Section 15.2b Additionally, the diagonal brace's available strength must meet the criteria established in Specification Chapter H.
Brace members shall meet the requirements of Section 8.2a.
The combined axial and flexural strength of the beam outside the link must be calculated according to the load combinations specified by the relevant building code In cases where seismic effects are considered, the load Q1 should replace the term E, with Q1 representing forces that are at least 1.1 times the expected nominal shear strength of the link, defined as RyVn, where Vn is the nominal shear strength.
PART I – ECCENTRICALLY BRACED FRAMES Sect 15.]
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6.1–54 is as defined in Section 15.2b The available strength of the beam outside of the link shall be determined by the Specification, multiplied by R y
The diagonal brace and beam segment outside the link are designed to remain primarily elastic under the forces exerted by the fully yielded and strain-hardened link These components typically experience a combination of significant axial forces and bending moments, necessitating their consideration as beam-columns in the design process The available strength for these elements is outlined in Chapter H of the Specification.
At the junction of the diagonal brace and the beam at the link end, the centerlines of the brace and beam must intersect either at the end of the link or within the link itself.
BUCKLING-RESTRAINED BRACED FRAMES (BRBF)
Buckling-restrained braced frames (BRBF) are designed to endure substantial inelastic deformations during design earthquake forces Compliance with the specified requirements in this section is essential for BRBF In cases where the relevant building code lacks design coefficients for BRBF, the guidelines outlined in Appendix R will be applicable.
Bracing members shall be composed of a structural steel core and a system that restrains the steel core from buckling.
The steel core shall be designed to resist the entire axial force in the brace.
The axial strength of the brace design, represented as φP ysc (LRFD), along with the allowable axial strength of the brace, noted as P ysc /Ω (ASD), must be calculated for both tension and compression based on the limit state of yielding.
P ysc = F ysc A sc (16-1) φ = 0.90 (LRFD) Ω = 1.67 (ASD) where
F ysc = specified minimum yield stress of the steel core, or actual yield stress of the steel core as determined from a coupon test, ksi (MPa)
A sc = net area of steel core, in 2 (mm 2 )
Plates used in the steel core that are 2 in (50 mm) thick or greater shall satisfy the minimum notch toughness requirements of Section 6.3.
Splices in the steel core are not permitted.
The buckling-restraining system is comprised of a casing that encases the steel core For stability calculations, it is essential to include beams, columns, and gussets that connect to the core as integral components of this system.
The buckling-restraining system must effectively prevent both local and overall buckling of the steel core under deformations up to 2.0 times the design story drift It is essential that the system remains stable and does not experience buckling within this deformation limit.
User Note: Conformance to this provision is demonstrated by means of test- ing as described in Section 16.2c.
PART I – BUCKLING-RESTRAINED BRACED FRAMES Sect 16.]
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The design of braces must adhere to the qualifying cyclic tests outlined in Appendix T, which includes specific procedures and acceptance criteria To qualify, at least two successful cyclic tests are required: one must involve a brace subassemblage that meets the rotational demands specified in Appendix T, Section T4, while the other can be either a uniaxial or a subassemblage test in accordance with Appendix T, Section T5 Both types of tests may be based on the specified guidelines.
(a) Tests reported in research or documented tests performed for other projects.
(b) Tests that are conducted specifically for the project.
Interpolation or extrapolation of test results for varying member sizes must be supported by a logical analysis This analysis should show that the stress distributions and internal strain magnitudes are consistent with, or less severe than, those of the tested assemblies Additionally, it must take into account the potential negative impacts of variations in material properties.
Test results can be extrapolated for similar combinations of steel core and buckling-restraining system sizes Designs can be qualified through testing, provided that the criteria outlined in Appendix T are satisfied.
Where required by these Provisions, bracing connections and adjoining mem- bers shall be designed to resist forces calculated based on the adjusted brace strength.
The adjusted brace strength in compression shall be βωR y P ysc The adjusted brace strength in tension shall be ωR y P ysc
Exception: The factor R y need not be applied if P ysc is established using yield stress determined from a coupon test.
The compression strength adjustment factor, β, is determined by the ratio of the maximum compression force to the maximum tension force of the test specimen, as outlined in Appendix T, Section T6.3, for deformations up to 2.0 times the design story drift The higher value of β from the two brace qualification tests must be utilized, with a minimum value of 1.0 enforced.
The strain hardening adjustment factor, ω, is determined by the ratio of the maximum tension force obtained from qualification tests in Appendix T, Section T6.3, corresponding to 2.0 times the design story drift, to P ysc of the test specimen The higher value of ω from the two qualification tests must be utilized If the tested steel core material differs from the prototype, ω should be based on coupon testing of the prototype material.
PART I – BUCKLING-RESTRAINED BRACED FRAMES [Sect 16.
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Bracing connections must meet specific strength requirements, with tension and compression connections, including beam-to-column connections within the bracing system, needing to be 1.1 times the adjusted brace strength in compression according to LRFD, or 1.1/1.5 times the adjusted brace strength in compression as per ASD standards.
The design of connections shall include considerations of local and overall buck- ling Bracing consistent with that used in the tests upon which the design is based is required.
To meet the specified provision for gusset plates, designers can either engineer the plate to withstand transverse forces based on testing results, incorporate a stiffener for added resistance, or install a brace connected to the gusset plate or the brace itself In cases where supporting tests did not encompass transverse bracing, it is not necessary to include such bracing Additionally, any connections of bracing to the steel core must be part of the qualification testing process.
16.4 Special Requirements Related to Bracing
V-type and inverted-V-type braced frames shall meet the following requirements:
The strength requirements for beams intersected by braces, along with their connections and supporting members, must be calculated according to the load combinations specified by the relevant building code, assuming that the braces do not support dead or live loads When considering load combinations that account for earthquake effects, the vertical and horizontal earthquake forces on the beam should be derived from the adjusted brace strengths in both tension and compression.
Beams must be continuous between columns, with both flanges laterally braced Lateral braces should comply with the provisions outlined in Equations A-6-7 and A-6-8 of Appendix 6 of the Specification, specifically for the beam's moment requirements (M r) based on either LRFD or ASD methods, where C d is set to 1.0 At a minimum, one set of lateral braces is necessary at the intersection of V-type or inverted V-type bracing, unless the beam possesses adequate out-of-plane strength and stiffness to maintain stability between adjacent brace points.
For a beam to possess adequate out-of-plane strength and stiffness, it must bend in the horizontal plane while meeting the necessary brace strength and stiffness criteria for column nodal bracing as outlined in the Specification The required compressive strength of the brace can be represented as P u.
PART I – BUCKLING-RESTRAINED BRACED FRAMES Sect 16.]
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In brace design and testing, the maximum deformation of braces must be adjusted to account for the vertical deflection of the beam under the specified loading conditions outlined in Section 16.4(1).
K-type braced frames are not permitted for BRBF.
Beams and columns in BRBF shall meet the following requirements.
Beam and column members shall meet the requirements of Section 8.2b
SPECIAL PLATE SHEAR WALLS (SPSW)
Special plate shear walls (SPSW) are designed to endure significant inelastic deformations in their webs during design earthquake motions The horizontal boundary elements (HBEs) and vertical boundary elements (VBEs) adjacent to these webs must remain primarily elastic under the maximum forces generated by fully yielded webs, allowing for plastic hinging at the ends of the HBEs Compliance with the relevant requirements for SPSW is essential, particularly in cases where the applicable building code lacks specific design coefficients for these structures.
PART I – BUCKLING-RESTRAINED BRACED FRAMES [Sect 16.
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The panel design shear strength, φV n (LRFD), and the allowable shear strength,
V n /Ω (ASD), according to the limit state of shear yielding, shall be determined as follows:
V n = 0.42 F y t w L cf sin2α (17-1) φ = 0.90 (LRFD) Ω = 1.67 (ASD) where t w = thickness of the web, in (mm)
L cf = clear distance between VBE flanges, in (mm) α is the angle of web yielding in radians, as measured relative to the vertical, and it is given by: tan 4
I L w c w b c α (17-2) h = distance between HBE centerlines, in (mm)
A b = cross-sectional area of a HBE, in 2 (mm 2 )
A c = cross-sectional area of a VBE, in 2 (mm 2 )
I c = moment of inertia of a VBE taken perpendicular to the direction of the web plate line, in 4 (mm 4 )
L = distance between VBE centerlines, in (mm)
The ratio of panel length to height, L/h, shall be limited to 0.8 < L/h ≤ 2.5.
Openings in webs must be fully enclosed by Horizontal Boundary Elements (HBE) and Vertical Boundary Elements (VBE) that span the entire width and height of the panel, unless alternative solutions are validated through testing and analysis.
17.3 Connections of Webs to Boundary Elements
The strength of web connections to the surrounding HBE and VBE must match the anticipated yield strength in tension of the web, as determined at an angle α according to Equation 17-2.
17.4 Horizontal and Vertical Boundary Elements
The required strength of the Vertical Bracing Element (VBE) must be determined based on the forces associated with the expected yield strength in tension of the web, calculated at an angle α, in accordance with the stipulations of Section 8.3.
PART I – SPECIAL PLATE SHEAR WALLS Sect 17.]
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The strength requirement for HBE must be based on the higher value between the expected yield strength of the web in tension at an angle α and the load combinations specified in the relevant building code, assuming the web does not support any gravity loads.
The beam-column moment ratio provisions in Section 9.6 shall be met for all HBE/VBE intersections without consideration of the effects of the webs.
HBE-to-VBE connections must comply with the standards outlined in Section 11.2 The required shear strength, denoted as V u, for these connections is determined per Section 11.2 guidelines, ensuring it meets or exceeds the shear strength corresponding to moments at each end of 1.1R y M p (for LRFD) or (1.1/1.5)R y M p (for ASD), along with the shear due to the anticipated yield strength in tension of the webs yielding at an angle α.
HBE and VBE members shall meet the requirements of Section 8.2b.
HBE must be laterally braced at all intersections with VBE, with a maximum spacing of 0.086r y E/F y Both flanges of the HBE require direct or indirect bracing, ensuring that the lateral bracing strength is at least 2% of the nominal strength of the HBE flange, F y b f t f The stiffness of all lateral bracing should be calculated according to Equation A-6-8 in Appendix 6 of the Specification, where M r is determined as R y ZF y (LRFD).
M r shall be computed as R y ZF y /1.5 (ASD), as appropriate, and C d = 1.0
VBE splices shall comply with the requirements of Section 8.4.
The VBE panel zone next to the top and base HBE of the SPSW shall comply with the requirements in Section 9.3
17.4g Stiffness of Vertical Boundary Elements
The VBE shall have moments of inertia about an axis taken perpendicular to the plane of the web, I c , not less than 0.00307 t w h 4 /L.
WELD METAL/WELDING PROCEDURE SPECIFICATION
MATERIALS
Structural steel members and connections used in composite seismic load resist- ing systems (SLRS) shall meet the requirements of Specification Section A3
Structural steel used in the composite SLRS described in Sections 8, 9, 12, 14,
16 and 17 shall also meet the requirements in Part I Sections 6 and 7.
Concrete and steel reinforcement used in composite components in composite SLRS shall meet the requirements of ACI 318, Sections 21.2.4 through 21.2.8.
Exception: Concrete and steel reinforcement used in the composite ordinary seismic systems described in Sections 11, 13, and 15 shall meet the requirements of Specification Chapter I and ACI 318, excluding Chapter 21.
PART II – GENERAL SEISMIC DESIGN REQUIREMENTS [Sect 3.
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COMPOSITE MEMBERS
The design of composite members in the SLRS described in Sections 8 through
17 shall meet the requirements of this Section and the material requirements of Section 5.
6.2 Composite Floor and Roof Slabs
The design of composite floor and roof slabs shall meet the requirements of ASCE 3 Composite slab diaphragms shall meet the requirements in this Section.
Details shall be designed so as to transfer loads between the diaphragm and boundary members, collector elements, and elements of the horizontal framing system.
The nominal shear strength of composite diaphragms and concrete-filled steel deck diaphragms is defined as the nominal shear strength of the reinforced concrete located above the top of the steel deck ribs, following ACI 318 guidelines, excluding Chapter 22 Alternatively, this shear strength can be established through in-plane shear tests conducted on concrete-filled diaphragms.
Composite beams shall meet the requirements of Specification Chapter I Com- posite beams that are part of composite-special moment frames (C-SMF) shall also meet the requirements of Section 9.3.
This section pertains to columns that are composed of reinforced concrete encasing shapes, where the structural steel area constitutes a minimum of 1 percent of the total cross section of the composite column, while also adhering to specific additional limitations.
Columns must adhere to the standards set forth in Specification Chapter I, with modifications outlined in Section I2.1 Furthermore, additional criteria for intermediate and special seismic systems, as detailed in Sections 6.4b and 6.4c, are applicable as specified in the descriptions of composite seismic systems found in Sections 8 through 17.
Columns that consist of reinforced-concrete-encased shapes shall meet the re- quirements for reinforced concrete columns of ACI 318 except as modified for
(1) The structural steel section shear connectors in Section 6.4a(2).
(2) The contribution of the reinforced-concrete-encased shape to the strength of the column as provided in ACI 318.
(3) The seismic requirements for reinforced concrete columns as specified in the description of the composite seismic systems in Sections 8 through 17.
PART II – COMPOSITE MEMBERS Sect 6.]
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The following requirements for encased composite columns are applicable to all composite systems, including ordinary seismic systems:
The shear strength of a column must be assessed according to Specification Section I2.1d, while the nominal shear strength of the tie reinforcement is determined by ACI 318 Sections 11.5.6.2 to 11.5.6.9 Specifically, in ACI 318 Sections 11.5.6.5 and 11.5.6.9, the dimension b w is defined as the width of the concrete cross-section minus the width of the structural shape, measured perpendicular to the shear direction.
Composite columns must effectively distribute applied loads between the structural steel section and the reinforced concrete encasement To achieve this, shear connectors must comply with the standards outlined in Specification Section I2.1.
(3) The maximum spacing of transverse ties shall meet the requirements of
Transverse ties must be positioned vertically within half of the tie spacing above the top of the footing or the lowest beam or slab in any given story Additionally, they should be spaced within half of the tie spacing below the lowest beam or slab that frames into the column.
Transverse bars must have a diameter of at least one-fiftieth of the largest side dimension of the composite member, with a minimum size of No 3 bars and a maximum size of No 5 bars Additionally, welded wire fabric with an equivalent area can be used as transverse reinforcement, unless it is specifically prohibited for intermediate and special seismic systems.
Load-carrying reinforcement must adhere to the detailing and splice requirements outlined in ACI 318 Sections 7.8.1 and 12.17 It is essential to include load-carrying reinforcement at each corner of a rectangular cross-section Additionally, the maximum spacing for other longitudinal reinforcement, whether load-carrying or restraining, should not exceed one-half of the smallest side dimension of the composite member.
(5) Splices and end bearing details for encased composite columns in ordinary seismic systems shall meet the requirements of the Specification and ACI
The design must adhere to ACI 318 Sections 21.2.6, 21.2.7, and 21.10, taking into account potential adverse effects from abrupt changes in member stiffness or nominal tensile strength Critical areas for consideration include transitions to reinforced concrete sections lacking embedded structural steel, transitions to bare structural steel sections, and column bases.
PART II – COMPOSITE MEMBERS [Sect 6.
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Encased composite columns in intermediate seismic systems shall meet the fol- lowing requirements in addition to those of Section 6.4a:
(1) The maximum spacing of transverse bars at the top and bottom shall be the least of the following:
(a) one-half the least dimension of the section (b) 8 longitudinal bar diameters
(c) 24 tie bar diameters (d) 12 in (300 mm)
Maintain the specified spacings over a vertical distance equal to the greatest length measured from each joint face, on both sides of any section where flexural yielding is anticipated.
(a) one-sixth the vertical clear height of the column (b) the maximum cross-sectional dimension (c) 18 in (450 mm)
(2) Tie spacing over the remaining column length shall not exceed twice the spacing defined above.
(3) Welded wire fabric is not permitted as transverse reinforcement in interme- diate seismic systems.
Encased composite columns in special seismic systems shall meet the following requirements in addition to those of Sections 6.4a and 6.4b:
(1) The required axial strength for encased composite columns and splice de- tails shall meet the requirements in Part I Section 8.3.
(2) Longitudinal load-carrying reinforcement shall meet the requirements of
(3) Transverse reinforcement shall be hoop reinforcement as defined in ACI
318 Chapter 21 and shall meet the following requirements:
(i) The minimum area of tie reinforcement A sh shall meet the following:
The cross-sectional dimension of the confined core, denoted as h_cc, is measured from the center-to-center of the tie reinforcement in inches (mm) Additionally, the spacing of the transverse reinforcement, represented by s, is measured along the longitudinal axis of the structural member in inches (mm).
PART II – COMPOSITE MEMBERS Sect 6.]
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F y = specified minimum yield stress of the structural steel core, ksi (MPa)
A s = cross-sectional area of the structural core, in 2 (mm 2 )
P n = nominal compressive strength of the composite column calculated in accordance with the Specification, kips (N) f′ c = specified compressive strength of concrete, ksi (MPa)
F yh = specified minimum yield stress of the ties, ksi (MPa)
Equation 6-1 need not be satisfied if the nominal strength of the rein- forced-concrete-encased structural steel section alone is greater than the load effect from a load combination of 1.0D + 0.5L.
(ii) The maximum spacing of transverse reinforcement along the length of the column shall be the lesser of six longitudinal load-carrying bar diameters or 6 in (150 mm).
According to Sections 6.4c(4), 6.4c(5), and 6.4c(6), the maximum spacing for transverse reinforcement must not exceed one-fourth of the smallest member dimension or 4 inches (100 mm) Additionally, for this reinforcement, cross ties, overlapping hoop legs, and other confining reinforcement should be spaced no more than 14 inches (350 mm) apart in the transverse direction.
Encased composite columns in braced frames must include transverse reinforcement over their entire length when nominal compressive loads exceed 0.2 times Pn, as outlined in Section 6.4c(3)(iii) However, this requirement can be waived if the nominal strength of the reinforced-concrete-encased steel section is greater than the load effect from the combination of 1.0D + 0.5L.
Composite columns that support reactions from discontinued stiff members, like walls or braced frames, must include transverse reinforcement as outlined in Section 6.4c(3)(iii) This reinforcement should extend the entire length beneath the discontinuity level if the nominal compressive load surpasses 0.1 times the specified limit.
Transverse reinforcement must extend into the discontinued member sufficiently to ensure full yielding of the reinforced-concrete-encased shape and longitudinal reinforcement However, this requirement can be waived if the nominal strength of the reinforced-concrete-encased structural steel section exceeds the load effect from a load combination of 1.0D + 0.5L.
(6) Encased composite columns used in a C-SMF shall meet the following requirements:
(i) Transverse reinforcement shall meet the requirements in Section 6.4c(3)(iii) at the top and bottom of the column over the region speci- fied in Section 6.4b.
PART II – COMPOSITE MEMBERS [Sect 6.
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(ii) The strong-column/weak-beam design requirements in Section 9.5 shall be satisfied Column bases shall be detailed to sustain inelastic flexural hinging.
(iii) The required shear strength of the column shall meet the requirements of ACI 318 Section 21.4.5.1.
COMPOSITE CONNECTIONS
This Section is applicable to connections in buildings that utilize composite or dual steel and concrete systems wherein seismic load is transferred between structural steel and reinforced concrete components.
Composite connections must demonstrate strength, ductility, and toughness equivalent to that of comparable structural steel or reinforced concrete connections, in accordance with Part I and ACI 318 standards Additionally, the methods used to calculate connection strength must adhere to the specified requirements in this section.
Connections must possess sufficient deformation capacity to withstand the required strength during design story drift Furthermore, connections essential for the building's lateral stability under seismic loads must comply with the specifications outlined in Sections 8 through 17, tailored to the specific system in which the connection operates When evaluating the available strength of connected members, which relies on nominal material strengths and dimensions, it is crucial to consider the effects stemming from the actual increase in the nominal strength of these connected members.
The nominal strength of connections in composite structural systems must be established using rational models that ensure equilibrium of internal forces and adhere to the strength limitations of materials and components based on potential limit states If connection strength is not determined through analysis and testing, the analytical models employed must comply with the stipulations outlined in Sections 7.3(1) to 7.3(5).
Force transfer between structural steel and reinforced concrete can occur through several methods: direct bearing of headed shear studs or suitable alternatives, mechanical means, shear friction aided by reinforcement perpendicular to the shear transfer plane, or a combination of these methods For connection force transfer, any potential bond strength between the structural steel and reinforced concrete is disregarded It is essential that the stiffness and deformation capacity of the different mechanisms are compatible for their contributions to be effectively combined.
The nominal bearing and shear-friction strengths must comply with the standards set forth in ACI 318 Chapters 10 and 11 For composite seismic systems outlined in Sections 9, 12, 14, 16, and 17, these strengths will be reduced by 25 percent unless higher strengths are verified through cyclic testing.
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(2) The available strength of structural steel components in composite connec- tions shall be determined in accordance with Part I and the Specification
Structural steel elements that are encased in confined reinforced concrete are permitted to be considered to be braced against out-of-plane buckling
Face bearing plates consisting of stiffeners between the flanges of steel beams are required when beams are embedded in reinforced concrete col- umns or walls.
The nominal shear strength of reinforced-concrete-encased steel panel zones in beam-to-column connections is determined by summing the nominal strengths of the structural steel and the confined reinforced concrete shear elements, as outlined in Part I Section 9.3 and ACI 318 Section 21.5.
Reinforcement in reinforced concrete connections must effectively counteract all tensile forces, with concrete being confined by transverse reinforcement It is essential that all reinforcement is fully developed in tension or compression beyond the point where it is no longer needed to resist forces Development lengths should comply with ACI 318 Chapter 12, and specific requirements for systems outlined in Sections 9, 12, 14, 16, and 17 must adhere to ACI 318 Section 21.5.4.
(5) Connections shall meet the following additional requirements:
When a slab transmits horizontal diaphragm forces, it is essential that the slab reinforcement is properly designed and anchored to accommodate in-plane tensile forces at all critical locations, including connections to collector beams, columns, braces, and walls.
For connections between structural steel or composite beams and reinforced concrete or encased composite columns, it is essential to include transverse hoop reinforcement in the connection area of the column This reinforcement must comply with the specifications outlined in ACI 318 Section 21.5, with noted modifications.
(a) Structural steel sections framing into the connections are consid- ered to provide confinement over a width equal to that of face bearing plates welded to the beams between the flanges.
Lap splices are allowed for perimeter ties if they are confined by face bearing plates or other methods that prevent spalling of the concrete cover, as outlined in Sections 10, 11, 13, and 15.
To minimize slippage of longitudinal bars in reinforced concrete and composite columns at beam-to-column connections, it is essential to detail the sizes and layout of these bars This is crucial due to the significant force transfer resulting from changes in column moments along the height of the connection.
PART II – COMPOSITE CONNECTIONS Sect 7.]
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COMPOSITE PARTIALLY RESTRAINED (PR) MOMENT FRAMES (C-PRMF)
This section pertains to frames made of structural steel columns and composite beams connected by partially restrained (PR) moment connections, in accordance with Specification Section B3.6b(b) Composite partially restrained moment frames (C-PRMF) must be designed to ensure that yielding occurs in the ductile components of the composite PR beam-to-column moment connections during earthquake loading Limited yielding is allowed at other locations, such as column base connections It is essential to consider connection flexibility and composite beam action when assessing the dynamic characteristics, strength, and drift of C-PRMF.
Structural steel columns shall meet the requirements of Part I Sections 6 and 8 and the Specification
Composite beams must be unencased and fully composite, adhering to the requirements outlined in Specification Chapter I For analysis purposes, the stiffness of these beams should be calculated using the effective moment of inertia of the composite section.
The strength of beam-to-column PR moment connections must account for connection flexibility and second-order moments Composite connections should possess a nominal strength of no less than 50 percent of the nominal plastic flexural strength (M p) of the connected structural steel beam, disregarding composite action Additionally, these connections must comply with Section 7 requirements and demonstrate a total interstory drift angle of 0.04 radians, validated through cyclic testing.
COMPOSITE SPECIAL MOMENT FRAMES (C-SMF)
This Section is applicable to moment frames that consist of either composite or reinforced concrete columns and either structural steel or composite beams
Composite special moment frames (C-SMF) must be designed to accommodate substantial inelastic deformations during a design earthquake, predominantly in the beams, while ensuring that inelastic deformations in the columns and connections are minimized.
PART II – COMPOSITE PARTIALLY RESTRAINED MOMENT FRAMES [Sect 8.
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Composite columns shall meet the requirements for special seismic systems of Sections 6.4 or 6.5, as appropriate Reinforced concrete columns shall meet the requirements of ACI 318 Chapter 21, excluding Section 21.10.
Composite beams that are part of C-SMF shall also meet the following requirements:
(1) The distance from the maximum concrete compression fiber to the plastic neutral axis shall not exceed
Y con = distance from the top of the steel beam to the top of concrete, in (mm) d b = depth of the steel beam, in (mm)
F y = specified minimum yield stress of the steel beam, ksi (MPa)
E = elastic modulus of the steel beam, ksi (MPa)
Beam flanges must comply with the specifications outlined in Part I Section 9.4, unless they are part of reinforced-concrete-encased compression elements that have a minimum reinforced concrete cover of 2 inches (50 mm) In areas where plastic hinges are anticipated during seismic events, hoop reinforcement is required This hoop reinforcement must adhere to the standards set forth in ACI 318 Section 21.3.3.
In C-SMF, the use of structural steel or composite trusses as flexural members to withstand seismic loads is not allowed unless testing and analysis prove that the specific system offers sufficient ductility and energy dissipation capacity.
The required strength of beam-to-column moment connections shall be deter- mined from the shear and flexure associated with the expected flexural strength,
The nominal strength of connections in beams must comply with the requirements outlined in Section 7, ensuring they can sustain a total interstory drift angle of 0.04 radians For connections where beam flanges are interrupted, cyclic tests must demonstrate this interstory drift angle In cases involving reinforced concrete columns with continuous beams, where welded joints are unnecessary, the inelastic rotation capacity must be validated through testing or supporting data.
PART II – COMPOSITE SPECIAL MOMENT FRAMES Sect 9.]
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Reinforced concrete column design must adhere to ACI 318 Section 21.4.2 standards, while the column-to-beam moment ratio for composite columns should comply with Part I Section 9.6, incorporating specified modifications.
(1) The available flexural strength of the composite column shall meet the re- quirements of Specification Chapter I with consideration of the required axial strength, P rc
(2) The force limit for Exception (a) in Part I Section 9.6 shall be P rc < 0.1P c
(3) Composite columns exempted by the minimum flexural strength require- ment in Part I Section 9.6(a) shall have transverse reinforcement that meets the requirements in Section 6.4c(3).
COMPOSITE INTERMEDIATE MOMENT FRAMES (C- IMF)
This Section is applicable to moment frames that consist of either composite or reinforced concrete columns and either structural steel or composite beams
Composite intermediate moment frames (C-IMF) should be designed with the expectation that the primary inelastic deformation during a design earthquake will occur in the beams, while allowing for moderate inelastic deformation in the columns and connections.
Composite columns shall meet the requirements for intermediate seismic sys- tems of Section 6.4 or 6.5 Reinforced concrete columns shall meet the require- ments of ACI 318 Section 21.12
Structural steel and composite beams shall meet the requirements of the
The nominal strength of the connections shall meet the requirements of Section
7 The required strength of beam-to-column connections shall meet one of the following requirements:
(a) The required strength of the connection shall be based on the forces associ- ated with plastic hinging of the beams adjacent to the connection.
(b) Connections shall meet the requirements of Section 7 and shall demonstrate a total interstory drift angle of at least 0.03 radian in cyclic tests.
PART II – COMPOSITE SPECIAL MOMENT FRAMES [Sect 9.
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COMPOSITE ORDINARY MOMENT FRAMES (C-OMF)
This section addresses moment frames made of composite or reinforced concrete columns paired with structural steel or composite beams Composite ordinary moment frames (C-OMF) must be designed with the assumption that only limited inelastic action will take place in the beams, columns, and connections during the design earthquake.
Composite columns shall meet the requirements for ordinary seismic systems in Section 6.4 or 6.5, as appropriate Reinforced concrete columns shall meet the requirements of ACI 318, excluding Chapter 21.
Structural steel and composite beams shall meet the requirements of the
Connections shall be designed for the load combinations in accordance with
Specification Sections B3.3 and B3.4, and the available strength of the connec- tions shall meet the requirements in Section 7 and Section 11.2 of Part I.
COMPOSITE SPECIAL CONCENTRICALLY BRACED FRAMES (C-CBF)
This section pertains to concentrically connected braced frames, allowing for minor eccentricities if considered in the design Structural components must include steel, composite steel, or reinforced concrete for columns, while beams and braces can be made of structural or composite steel Composite special concentrically braced frames (C-CBF) should be designed with the expectation that inelastic behavior during the design earthquake will mainly result from tension yielding and/or brace buckling.
Structural steel columns shall meet the requirements of Part I Sections 6 and 8
Composite columns shall meet the requirements for special seismic systems of Section 6.4 or 6.5 Reinforced concrete columns shall meet the requirements for structural truss elements of ACI 318 Chapter 21.
Structural steel beams must comply with the specifications for special concentrically braced frames (SCBF) outlined in Part I Section 13 Additionally, composite beams are required to adhere to the standards set forth in Specification Chapter I and the SCBF criteria from Part I Section 13.
PART II – COMPOSITE SPECIAL CONCENTRICALLY BRACED FRAMES Sect 12.]
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Structural steel braces shall meet the requirements for SCBF of Part I Section
13 Composite braces shall meet the requirements for composite columns of Section 12.2.
Bracing connections shall meet the requirements of Section 7 and Part I Section 13.
COMPOSITE ORDINARY BRACED FRAMES (C-OBF)
This section pertains to concentrically braced frame systems that include composite or reinforced concrete columns, structural steel or composite beams, and structural steel or composite braces, specifically focusing on composite ordinary braced frames.
(C-OBF) shall be designed assuming that limited inelastic action under the design earthquake will occur in the beams, columns, braces, and/or connections.
Encased composite columns must comply with the ordinary seismic system requirements outlined in Section 6.4, while filled composite columns are subject to the standards specified in Section 6.5 for ordinary seismic systems Additionally, reinforced concrete columns should adhere to the guidelines set forth in ACI 318, excluding Chapter 21.
Structural steel and composite beams shall meet the requirements of the
Structural steel braces shall meet the requirements of the Specification Compos- ite braces shall meet the requirements for composite columns of Sections 6.4a,
Connections shall be designed for the load combinations in accordance with
Specification Sections B3.3 and B3.4, and the available strength of the connec- tions shall meet the requirements in Section 7.
COMPOSITE ECCENTRICALLY BRACED FRAMES (C-EBF)
This section pertains to braced frames where each brace's end intersects a beam with eccentricity from the centerlines of the beam and column or from the beam and an adjacent brace Composite eccentrically braced frames (C-EBF) must be designed to ensure that inelastic deformations during the design earthquake are limited to shear yielding in the links.
PART II – COMPOSITE SPECIAL CONCENTRICALLY BRACED FRAMES [Sect 12.
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Diagonal braces, columns, and beam segments outside the link must be designed to remain primarily elastic under the maximum forces generated by a fully yielded and strain-hardened link Columns can be either composite or reinforced concrete, while braces and links should be made of structural steel The strength of these members must comply with the specifications outlined, with modifications specified in this section Additionally, C-EBF must adhere to the requirements of Part I Section 15, with applicable modifications.
Reinforced concrete columns must comply with the structural truss element standards outlined in ACI 318 Chapter 21 Composite columns are required to adhere to the specifications for special seismic systems as detailed in Sections 6.4 or 6.5 Furthermore, when a link is positioned next to a reinforced concrete column or an encased composite column, it is essential to provide transverse column reinforcement as stipulated in ACI 318 Section 21.4.4, or Section 6.4c(6)(i) for composite columns, both above and below the link connection.
All columns shall meet the requirements of Part I Section 15.8.
Links must consist of unencased structural steel that complies with the requirements for eccentrically braced frame (EBF) links as specified in Part I Section 15 The section of the beam outside the link may be encased in reinforced concrete Additionally, beams that include the link can function compositely with the floor slab through the use of shear connectors along any part of the beam, provided that this composite action is factored into the calculation of the link's nominal strength.
Structural steel braces shall meet the requirements for EBF of Part I Section 15.
In addition to the requirements for EBF of Part I Section 15, connections shall meet the requirements of Section 7.
ORDINARY REINFORCED CONCRETE SHEAR WALLS
WALLS COMPOSITE WITH STRUCTURAL STEEL ELEMENTS (C-ORCW)
This section outlines the requirements for reinforced concrete walls that incorporate structural steel elements, including infill panels within structural steel frames and structural steel coupling beams connecting adjacent reinforced concrete walls These reinforced concrete walls must comply with the standards set forth in ACI 318, with the exception of Chapter 21.
PART II – ORDINARY REINFORCED CONCRETE SHEAR WALLS COMPOSITE Sect 15.]
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Boundary members shall meet the requirements of this Section:
When structural steel sections are used as boundary members in reinforced concrete infill panels, they must comply with specific specifications The axial strength required for these boundary members is calculated by assuming that the shear forces are managed by the reinforced concrete wall, while the boundary members, along with the shear wall, support all gravity and overturning forces.
The reinforced concrete wall shall meet the requirements of ACI 318 ex- cluding Chapter 21.
When reinforced-concrete-encased shapes serve as boundary members in reinforced concrete infill panels, their analysis should utilize a transformed concrete section with elastic material properties The wall must adhere to ACI 318 standards, excluding Chapter 21 If the reinforced-concrete-encased structural steel boundary member qualifies as a composite column per Specification Chapter I, it should be designed according to the ordinary seismic system requirements outlined in Section 6.4a Otherwise, it must be designed as a composite column in compliance with ACI 318 Section 10.16 and Chapter I of the Specification.
Headed shear studs or welded reinforcement anchors are essential for transferring vertical shear forces between structural steel and reinforced concrete When utilizing headed shear studs, they must comply with the standards outlined in Specification Chapter I Similarly, if welded reinforcement anchors are employed, they should adhere to the requirements specified in AWS D1.4.
Structural steel coupling beams that are used between two adjacent reinforced con- crete walls shall meet the requirements of the Specification and this Section:
Coupling beams must have an embedment length into the reinforced concrete wall that effectively develops the maximum potential moment and shear generated by the nominal bending and shear strength of the beam This embedment length begins within the first layer of confining reinforcement in the wall boundary member, ensuring adequate connection strength for load transfer between the coupling beam and the wall, in accordance with Section 7 requirements.
Vertical wall reinforcement must have a nominal axial strength that matches the nominal shear strength of the coupling beam, with two-thirds of the steel positioned over the first half of the embedment length This reinforcement should extend at least one tension development length above and below the coupling beam's flanges Additionally, vertical reinforcement intended for other uses, such as vertical boundary members, can be utilized as part of the required vertical reinforcement.
PART II – ORDINARY REINFORCED CONCRETE SHEAR WALLS COMPOSITE [Sect 15.
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Encased composite sections serving as coupling beams shall meet the require- ments of Section 15.3 as modified in this Section:
Coupling beams must be embedded into the reinforced concrete wall to a length that ensures the full development of the maximum moment and shear capacities of the encased composite steel coupling beam.
(2) The nominal shear capacity of the encased composite steel coupling beam shall be used to meet the requirement in Section 15.3(1).
(3) The stiffness of the encased composite steel coupling beams shall be used for calculating the required strength of the shear wall and coupling beam.
16 SPECIAL REINFORCED CONCRETE SHEAR WALLS
COMPOSITE WITH STRUCTURAL STEEL ELEMENTS (C-SRCW)
Special reinforced concrete shear walls composite with structural steel elements
(C-SRCW) systems shall meet the requirements of Section 15 for C-ORCW and the shear-wall requirement of ACI 318 including Chapter 21, except as modified in this Section
In addition to the requirements of Section 15.2(1), unencased structural steel columns shall meet the requirements of Part I Sections 6 and 8.
In addition to the stipulations outlined in Section 15.2(2), this section mandates that walls featuring reinforced-concrete-encased structural steel boundary members must adhere to the requirements of ACI 318, including its specific provisions detailed in Chapter.
21 Reinforced-concrete-encased structural steel boundary members that qualify as composite columns in Specification Chapter I shall meet the special seismic system requirements of Section 6.4 Otherwise, such members shall be designed as composite compression members to meet the requirements of ACI 318 Sec- tion 10.16 including the special seismic requirements for boundary members in ACI 318 Section 21.7.6 Transverse reinforcement for confinement of the com- posite boundary member shall extend a distance of 2h into the wall, where h is the overall depth of the boundary member in the plane of the wall.
Headed shear studs or welded reinforcing bar anchors must be installed as outlined in Section 15.2(3) When connecting to unencased structural steel sections, it is important to note that the nominal strength of the welded reinforcing bar anchors will be reduced accordingly.
25 percent from their static yield strength.
PART II – SPECIAL REINFORCED CONCRETE SHEAR WALLS COMPOSITE Sect 16.]
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Structural steel coupling beams must comply with the requirements outlined in Part I Sections 15.2 and 15.3, in addition to the specifications of Section 15.3 Unless justified by a rational analysis of expected inelastic deformations during the design earthquake, the coupling rotation should be assumed to be 0.08 radian Additionally, face bearing plates are required on both sides of the coupling beams at the interface with the reinforced concrete wall.
These stiffeners shall meet the detailing requirements of Part I Section 15.3.
Vertical wall reinforcement as specified in Section 15.3(2) shall be confined by transverse reinforcement that meets the requirements for boundary members of ACI 318 Section 21.7.6.
Encased composite sections serving as coupling beams shall meet the require- ments of Section 16.3, except the requirements of Part I Section 15.3 need not be met.
COMPOSITE STEEL PLATE SHEAR WALLS (C-SPW)
This section pertains to structural walls made of steel plates that are encased in reinforced concrete on one or both sides, along with structural steel or composite boundary members.
The shear strength for composite steel plate shear walls (C-SPW) with a stiffened plate is determined by φV ns (LRFD) or V ns / Ω (ASD), depending on the applicable limit state of shear yielding as outlined in Section 17.2(1).
V ns = nominal shear strength of the steel plate, kips (N)
A sp = horizontal area of stiffened steel plate, in 2 (mm 2 )
F y = specified minimum yield stress of the plate, ksi (MPa)
The shear strength of C-SPW with a non-compliant plate, as outlined in Section 17.2(1), should be determined solely by the plate's strength, excluding any contributions from the reinforced concrete Additionally, it must adhere to the specifications detailed in Sections G2 and G3.
To ensure the steel plate is adequately stiffened, it must be encased or attached to reinforced concrete, demonstrating through elastic plate buckling analysis that the composite wall can withstand a nominal shear force of V ns The concrete thickness should be at least 4 inches (100 mm) on each side when applied to both sides of the steel plate, and 8 inches (200 mm) when applied to one side Additionally, headed shear stud connectors or other mechanical connectors must be utilized to enhance stability.
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To ensure structural integrity against local buckling and plate separation, it is essential to incorporate both horizontal and vertical reinforcement within the concrete encasement, adhering to the detailing requirements specified in ACI 318 Section 14.3.
The reinforcement ratio in both directions shall not be less than 0.0025; the maximum spacing between bars shall not exceed 18 in (450 mm)
Seismic forces acting perpendicular to the plane of the wall as specified by the applicable building code shall be considered in the design of the com- posite wall system
The steel plate must be continuously welded or bolted to structural steel framing and boundary members on all edges to ensure it achieves its nominal shear strength Additionally, the design of these welded and bolted connections must comply with the specific requirements outlined in Part I Section 7.
Structural steel and composite boundary members must be engineered to withstand the shear capacity of the plate and any active reinforced concrete sections of the wall during the design story drift Additionally, composite and reinforced concrete boundary members are required to comply with the standards outlined in Section 16.2, while steel boundary members must adhere to the specifications in Part I, Section 17.
Boundary members shall be provided around openings as required by analysis.
STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS,
SPECIFICATIONS, SHOP DRAWINGS, AND ERECTION DRAWINGS
Structural design drawings and specifications, shop drawings, and erection draw- ings for composite steel and steel building construction shall meet the require- ments of Part I Section 5
In reinforced concrete and composite steel building construction, it is essential for contract documents, shop drawings, and erection drawings to clearly specify bar placement, cutoffs, lap and mechanical splices, hooks, and mechanical anchorages Additionally, they must outline tolerances for the placement of ties and transverse reinforcement, as well as provisions for dimensional changes due to temperature fluctuations, creep, and shrinkage The documentation should also detail the location, magnitude, and sequencing of any prestressing or post-tensioning Furthermore, if concrete floor slabs or slabs on grade function as diaphragms, the connection details between the diaphragm and the main lateral-load resisting system must be explicitly identified.
PART II – STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS Sect 18.]
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For the construction of reinforced concrete and composite steel buildings, it is essential to adhere to several key documents, including ACI 315-04, which focuses on the details and detailing of concrete reinforcement, and ACI 315R-94, a manual for engineering and placing drawings for reinforced concrete structures Additionally, the ACI Detailing Manual (ACI SP-66) provides further guidance, alongside necessary modifications outlined in Chapter 21 of ACI 318-02 and ACI 352, which addresses monolithic joints in concrete.
QUALITY ASSURANCE PLAN
A quality assurance plan must be provided when mandated by the applicable building code (ABC) or the engineer of record, specifically for the steel components of the construction, in accordance with the provisions outlined in Part I, Section 18.
User Note: For the reinforced concrete portion, the provisions of ACI 121R-
98 (Quality Management Systems for Concrete Construction), ACI 309.3R-
97 (Guide to Consolidation of Concrete in Congested Areas and Difficult
Placing Conditions), ACI 311.1R-01 (ACI Manual of Concrete Inspection) and ACI 311.4R-00 (Guide for Concrete Inspection) may apply
PART II – STRUCTURAL DESIGN DRAWINGS AND SPECIFICATIONS [Sect 18.
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COMMENTARY on the Seismic Provisions for Structural Steel Buildings
Seismic Provisions for Structural Steel Buildings dated March 9, 2005
(The Commentary is not a part of ANSI/AISC 341-05, Seismic Provisions for Structural
Steel Buildings, or ANSI/AISC 341s1-05, Supplement No 1 to ANSI/AISC 341-05, but is included for informational purposes only.)
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STRUCTURAL STEEL BUILDINGS C1 SCOPE
The 1994 Northridge and 1995 Kobe earthquakes greatly enhanced our understanding of the seismic response of welded steel moment frames In the aftermath of the Northridge earthquake, the SAC Joint Venture conducted an extensive study on the seismic performance of these structures, with funding from the Federal Emergency Management Agency (FEMA) This initiative led to the development of guidelines aimed at structural engineers, building officials, and other stakeholders for evaluating, repairing, modifying, and designing welded steel moment frame structures in seismic zones, with active participation from AISC.
Many recommendations in the Recommended Seismic Design Criteria for New Steel Moment-
Frame Buildings—FEMA 350 (FEMA, 2000a) formed the basis for Supplement No 2 to the 1997 AISC Seismic Provisions for Structural Steel Buildings (AISC, 1997b, 2000b)
Supplement No 2 to the 1997 Provisions was created in conjunction with the revisions to the Building Seismic Safety Council's National Earthquake Hazard Reduction Program This supplement served as the foundation for the steel seismic design provisions included in the 2000 NEHRP Provisions and the 2000 International Building Code 2002 Supplement, published by the International Code Council.
These 2005 AISC Seismic Provisions for Structural Steel Buildings, hereinafter referred to as the Provisions or ANSI/AISC 341, continue incorporating the recommendations of
The Committee has revised the Provisions based on the best available knowledge, while ongoing research, including FEMA 350, continues to inform these updates This revision coincided with a significant overhaul of SEI/ASCE 7, which has since been completed and published in its 2005 edition.
Due to this timing, these Provisions adopt the 2002 edition of SEI/ASCE 7 (ASCE, 2002) but are intended to be compatible and used in conjunction with the 2005 edition of SEI/ASCE 7
This Commentary will thus reference the requirements in the latter (ASCE, 2005)
It is also anticipated that these Provisions will be adopted by the International Building
The 2006 edition of the Code and the 2005 National Fire Protection Association (NFPA) Building Code are anticipated to reference SEI/ASCE 7 for seismic loading, while lacking specific seismic requirements within their provisions.
The latest Provisions integrate both LRFD and ASD design methods into a unified format, aligning with the AISC 2005 Specification for Structural Steel Buildings (ANSI/AISC 360), unlike the previous edition (AISC, 2002) which presented them separately in Parts I and III This streamlined approach enhances clarity and usability, as both design methodologies are now included in Part I, eliminating the need for a separate Part III for seismic considerations.
Provisions for Structural Steel Buildings devoted to ASD has been eliminated in this edition of the Provisions.
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To effectively utilize these Provisions alongside a model code that has not yet adopted them, it is crucial to reference ANSI/AISC 360 (AISC, 2005) as they are companion documents Additionally, for a comprehensive and coordinated approach, users should also incorporate SEI/ASCE 7 (ASCE, 2005).
In previous editions of these Provisions and the predecessor specifications to the new AISC Specification for Structural Steel Buildings, ANSI/AISC 360 (AISC,
In 2005, the scope of ANSI/AISC 360 was initially limited to buildings; however, it has since been expanded to encompass other structures that are designed, fabricated, and erected similarly to buildings, featuring vertical and lateral load-resisting elements akin to those found in buildings This modification ensures consistency with ANSI/AISC 360, and for clarity, we will use the terms steel buildings and structures interchangeably in this commentary.
These provisions were specifically designed for buildings and may not fully apply to nonbuilding structures lacking building-like characteristics Caution should be exercised when considering their application to such structures, as there are inherent differences in response characteristics between buildings and nonbuilding structures.
Structural steel systems in seismic regions are generally expected to dissipate seismic input energy through controlled inelastic deformations of the structure
These provisions enhance ANSI/AISC 360 for specific applications, taking into account the seismic design loads outlined in building codes, which are formulated based on the energy dissipation that occurs during inelastic response.
The Provisions are intended to be mandatory for structures where ANSI/AISC
The R factor, as defined in SEI/ASCE 7 (ASCE, 2005), is specifically referenced as 341, particularly in seismic design categories D and above, where it typically exceeds 3 However, there are cases where a system may be assigned an R factor of less than 3, yet still requires compliance with ANSI/AISC 341.
Table 12.2–1 (ASCE, 2005) outlines specific cases for cantilevered column systems, while Table 15.4–1 addresses intermediate and ordinary moment frames with height increases For structures with response modification factors (R factors) below 3, adherence to these provisions is mandatory This is particularly relevant for structures categorized under seismic design considerations.
A to C the designer is given a choice to either solely use ANSI/AISC 360 and the
R factor given for structural steel buildings not specifically detailed for seismic resistance (typically, a factor of 3) or the designer may choose to assign a higher
R factor to a system detailed for seismic resistance and follow the requirements of these Provisions
1 A joint venture of the Structural Engineers Association of California (SEAOC), Applied Technology (ATC), and California Universities for Research in Earthquake Engineering (CUREe).
Seismic Provisions for Structural Steel Buildings, March 9, 2005, incl Supplement No 1
A MERICAN I NSTITUTE OF S TEEL C ONSTRUCTION , I NC
Seismic Provisions for Structural Steel Buildings, March 9, 2005, incl Supplement No 1
A MERICAN I NSTITUTE OF S TEEL C ONSTRUCTION , I NC
The latest edition of the Provisions expands upon previous versions by not only defining requirements for members and connections within the seismic load resisting system (SLRS) but also incorporating new requirements for columns that are not part of the SLRS, specifically outlined in Section 8.4b.
The specifications, codes, and standards mentioned in Part I are detailed with their respective revision dates in this section or Section A2 of ANSI/AISC 360 As the Provisions serve as a supplement to ANSI/AISC 360, the references found in Section A2 are not reiterated within the Provisions.
When designing earthquake-resistant structures, it is essential to categorize each structure based on its occupancy and intended use to assess potential earthquake hazards The determination of available strength varies significantly across different building codes and specifications The main goal of these provisions is to offer critical information for evaluating the required and available strengths of steel structures This overview highlights how various seismic codes categorize structures and establish the necessary strength and stiffness For factors influencing seismic design categories, such as height limitations, irregularities, and site characteristics, consulting the relevant building code is crucial.
According to SEI/ASCE 7 (ASCE, 2005), structures are classified into four occupancy categories, with Category IV designated for essential facilities Based on these occupancy categories and the site's seismicity, structures are then categorized into seismic use groups For structures in seismic design categories A, B, and C, which are associated with moderate seismic risk, special seismic provisions are optional In contrast, mandatory special seismic provisions apply to structures in seismic design categories D, E, and F, which pertain to regions with high seismic risk.