Structure Steel Design''''s Handbook 2009 part 10 potx

8.1 SECTION 8 FLOOR AND ROOF SYSTEMS Daniel A. Cuoco, P.E. Principal, LZA Technology/Thornton-Tomasetti Engineers, New York, New York Structural-steel framing provides designers with a wide selection of economical systems for floor and roof construction. Steel framing can achieve longer spans more efficiently than other types of construction. This minimizes the number of columns and footings thereby increasing speed of erection. Longer spans also provide more flexibility for interior-space planning. Another advantage of steel construction is its ability to readily accommodate future struc- tural modifications, such as openings for tenants’ stairs and changes for heavier floor load- ings. When reinforcement of existing steel structures is required, it can be accomplished by such measures as addition of framing members connected to existing members and field welding of additional steel plates to strengthen existing members. FLOOR DECK The most common types of floor-deck systems currently used with structural steel construc- tion are concrete fill on metal deck, precast-concrete planks, and cast-in-place concrete slabs. 8.1 CONCRETE FILL ON METAL DECK The most prevalent type of floor deck used with steel frames is concrete fill on metal deck. The metal deck consists of cold-formed profiles made from steel sheet, usually having a yield strength of at least 33 ksi. Design requirements for metal deck are contained in the American Iron and Steel Institute’s ‘‘Specification for the Design of Cold-Formed Steel Structural Members.’’ The concrete fill is usually specified to have a 28-day compressive strength of at least 3000 psi. Requirements for concrete design are contained in the American Concrete Institute Standard ACI 318, ‘‘Building Code Requirements for Reinforced Concrete.’’ Sheet thicknesses of metal deck usually range between 24 and 18 ga, although thicknesses outside this range are sometimes used. The design thicknesses corresponding to typical gage designations are shown in Table 8.1. 8.2 SECTION EIGHT TABLE 8.1 Equivalent Thicknesses for Cold-Formed Steel Gage designation Design thickness, in 28 0.0149 26 0.0179 24 0.0239 22 0.0299 20 0.0359 18 0.0478 16 0.0598 FIGURE 8.1 Cold-formed steel decking used in composite construction with concrete fill. Metal deck is commonly available in depths of 1 1 ⁄ 2 , 2, and 3 in. Generally, it is preferable to use a deeper deck that can span longer distances between supports and thereby reduce the number of beams required. For example, a beam spacing of about 15 ft can be achieved with 3-in deck. However, each project must be evaluated on an individual basis to determine the most efficient combination of deck depth and beam spacing. For special long-span applications, metal deck is available with depths of 4 1 ⁄ 2 , 6, and 7 1 ⁄ 2 in from some manufacturers. Composite versus Noncomposite Construction. Ordinarily, composite construction with metal deck and structural-steel framing is used. In this case, the deck acts not only as a permanent form for the concrete slab but also, after the concrete hardens, as the positive bending reinforcement for the slab. To achieve this composite action, deformations are formed in the deck to provide a mechanical interlock with the concrete (Fig. 8.1). Although not serving a primary structural purpose, welded wire fabric is usually placed within the concrete slab about 1 in below the top surface to minimize cracking due to concrete shrinkage and thermal effects. This welded wire fabric also provides, to a limited degree, some amount of crack control in negative-moment regions of the slab over supporting members. FLOOR AND ROOF SYSTEMS 8.3 FIGURE 8.2 Cellular steel deck with concrete slab. Noncomposite metal deck is used as a form for concrete and is considered to be ineffective in resisting superimposed loadings. In cases where the deck is shored, or where the deck is unshored but the long-term reliability of the deck will be questionable, the deck is also considered to be ineffective in supporting the dead load of the concrete slab. For example, in regions where deicing chemicals are applied to streets, metal deck used in parking struc- tures is susceptible to corrosion and may eventually be ineffective unless special precautions are taken. In such cases, the metal deck should be used solely as a form to support the concrete until it hardens. Reinforcement should be placed within the slab to resist all design loadings. Noncellular versus Cellular Deck. It is sometimes desirable to distribute a building’s elec- trical wiring within the floor deck system, in which case cellular metal deck can be used in lieu of noncellular deck. However, in cases where floor depth is not critical, maximum wiring flexibility and capacity can be attained by using a raised access floor above the structural floor deck. Cellular deck is essentially noncellular deck, such as that shown in Fig. 8.1, with a flat sheet added to the bottom of the deck to create cells (Fig. 8.2). Electrical, power, and telephone wiring is placed within the cells for distribution over the entire floor area. In many cases, a sufficient number of cells is obtained by combining alternate panels of cellular deck and noncellular deck, which is called a blended system (Fig. 8.3). When cellular deck is used, the 3-in depth is the minimum preferred because it provides convenient space for wiring. The 1 1 ⁄ 2 -in depth is rarely used. For feeding wiring into the cells, a trench header is placed within the concrete above the metal deck, in a direction perpendicular to the cells (Fig. 8.4). Special attention should be given to the design of the structural components adjacent to the trench header, since com- posite action for both the floor deck and beams is lost in these areas. Where possible, the direction of the cells should be selected to minimize the total length of trench header re- 8.4 SECTION EIGHT FIGURE 8.3 Blended deck, alternating cellular and noncellular panels, in composite construction. CONCRETE SLAB AIR CELLS SPRAY-ON FIREPROOFING (NOT ALWAYS REQUIRED) ELECTRICAL CELLS TRENCH HEADERS FIGURE 8.4 Cellular steel deck with trench header placed within the concrete slab to feed wiring to cells. quired. Generally, by running the cells in the longitudinal direction of the building, the total length of trench header is significantly less than if the cells were run in the transverse direction (Fig. 8.5). If a uniform grid of power outlets is desired, such as 5 ft by 5 ft on centers, preset outlets can be positioned above the cells and cast into the concrete fill. However, in many cases the outlet locations will be dictated by subsequent tenant layouts. In such cases, the concrete fill can be cored and afterset outlets can be installed at any desired location. Shored versus Unshored Construction. To support the weight of newly placed concrete and the construction live loads applied to the metal deck, the deck can either be shored or be designed to span between supporting members. If the deck is shored, a shallower-depth 8.5 FIGURE 8.5 Floor layout for cellular deck with cells in different directions. Length of trench header serving them is less for (a) cells in the longitudinal direction than for (b) cells in the transverse direction. 8.6 SECTION EIGHT or thinner-gage deck can be used. The economy of shoring, however, should be investigated, inasmuch as the savings in deck cost may be more than offset by the cost of the shoring. Also, slab deflections that will occur after the shoring is removed should be evaluated, as well as concrete cracking over supporting members. Another consideration is that use of shoring can sometimes affect the construction schedule, since the shoring is usually kept in place until the concrete fill has reached at least 75% of its specified 28-day compressive strength. In addition, when shoring is used, the concrete must resist the stresses resulting from the total dead load combined with all superimposed loadings. When concrete is cast on unshored metal deck, the weight of the concrete causes the deck to deflect between supports. This deflection is usually limited to the lesser of 1 ⁄ 180 the deck span or 3 ⁄ 4 in. If the resulting effect on floor flatness is objectionable, the top surface can be finished level, but this will result in additional concrete being placed to compensate for the deflection. The added weight of this additional concrete must be taken into account in design of the metal deck to ensure adequate strength. The concrete fill, however, need only resist the stresses resulting from superimposed loadings. Unshored metal-deck construction is the system most commonly used. The additional cost of the deeper or thicker deck is generally much less than the cost of shoring. To increase the efficiency of the unshored deck in supporting the weight of the unhardened concrete and construction live loads, from both a strength and deflection standpoint, the deck is normally extended continuously over supporting members for two or three spans, in lieu of single- span construction. However, for loadings once the concrete is hardened, the composite slab is designed for the total load, including slab self-weight, with the slab treated as a single span, unless negative-moment reinforcement is provided over supports in accordance with conventional reinforced-concrete-slab design (disregarding the metal deck as compressive reinforcement). Lightweight versus Normal-Weight Concrete. Either lightweight or normal-weight con- crete can serve the structural function of the concrete fill placed on the metal deck. Although there is a cost premium associated with lightweight concrete, sometimes the savings in steel framing and foundation costs can outweigh the premium. Also, lightweight concrete in suf- ficient thickness can provide the necessary fire rating for the floor system and thus eliminate the need for additional slab fire protection (see ‘‘Fire Protection’’ below). The tradeoffs in use of lightweight concrete versus normal-weight concrete plus fire pro- tection should be evaluated on a project-by-project basis. Fire Protection. Most applications of concrete fill on metal deck in buildings require that the floor-deck assembly have a fire rating. For noncellular metal deck, the fire rating is usually obtained either by providing sufficient concrete thickness above the metal deck or by applying spray-on fire protection to the underside of the metal deck. For cellular metal deck, which utilizes outlets that penetrate the concrete fill, the fire rating is usually obtained by the latter method. As an alternative, a fire-rated ceiling system can be installed below the cellular or noncellular deck. When the required fire rating is obtained by concrete-fill thickness alone, lightweight concrete requires a lesser thickness than normal-weight concrete for the same rating. For example, a 2-hour rating can be obtained by using either 3 1 ⁄ 4 in of lightweight concrete or 4 1 ⁄ 2 in of normal-weight concrete above the metal deck. The latter option is rarely used, since the additional thickness of heavier concrete penalizes the steel tonnage (i.e., heavier beams, girders, and columns) and the foundations. If spray-on fire protection is used on the underside of the metal deck, the thickness of concrete above the deck can be the minimum required to resist the applied floor loads. This minimum thickness is usually 2 1 ⁄ 2 in, and the less expensive normal-weight concrete may be used instead of lightweight concrete. Therefore, the two options that are frequently consid- ered for a 2-hour-rated, noncellular floor-deck system are 3 1 ⁄ 4 -in lightweight concrete above FLOOR AND ROOF SYSTEMS 8.7 FIGURE 8.6 Two-hour fire-rated floor systems, with cold-formed steel deck. (a) With lightweight concrete fill; (b) with normal-weight concrete fill. the metal deck without spray-on fire protection and 2 1 ⁄ 2 -in normal-weight concrete above the metal deck with spray-on fire protection (Fig. 8.6). Since the dead load of the floor deck for the two options is essentially the same, the steel framing and foundations will also be the same. Thus, the comparison reduces to the cost of the more expensive lightweight concrete versus the cost of the normal-weight concrete plus the spray-on fire protection. Since the costs, and contractor preferences, vary with geographical location, the evaluation must be made on an individual project basis. (See also Art. 6.32.) Diaphragm Action of Metal-Deck Systems. Concrete fill on metal deck readily serves as a relatively stiff diaphragm that transfers lateral loads, such as wind and seismic forces, at each floor level through in-plane shear to the lateral load-resisting elements of the structure, such as shear walls and braced frames. The resulting shear stresses can usually be accom- modated by the combined strength of the concrete fill and metal deck, without need for additional reinforcement. Attachment of the metal deck to the steel framing, as well as attachment between adjacent deck units, must be sufficient to transfer the resulting shear stresses (see ‘‘Attachment of Metal Deck to Framing’’ below). Additional shear reinforcement may be required in floor decks with large openings, such as those for stairs or shafts, with trench headers for electrical distribution, or with other shear discontinuities. Also, floors in multistory buildings in which cumulative lateral loads are 8.8 SECTION EIGHT FIGURE 8.7 Precast-concrete plank floor with concrete topping. transferred from one lateral load-resisting system to another (for example, from perimeter frames to interior shear walls), may be subjected to unusually large shear stresses that require a diaphragm strength significantly greater than that for a typical floor. Attachment of Metal Deck to Framing. Metal deck can be attached to the steel framing with puddle (arc spot) welds, screws, or powder-driven fasteners. These attachments provide lateral bracing for the steel framing and, when applicable, transfer shear stresses resulting from diaphragm action. The maximum spacing of attachments to steel framing is generally 12 in. Attachment of adjacent deck units to each other, that is, sidelap connection, can be made with welds, screws, or button punches. Generally, the maximum spacing of sidelap attach- ments is 36 in. In addition to diaphragm or other loading requirements, the type, size, and spacing of attachments is sometimes dictated by insurance (Factory Mutual or Underwriters’ Laboratories) requirements. Weld sizes generally range between 1 ⁄ 2 -in and 3 ⁄ 4 -in minimum visible diameter. When metal deck is welded to steel framing, welding washers should be used if the deck thickness is less than 22 ga to minimize the possibility of burning through the deck. Sidelap welding is not recommended for deck thicknesses of 22 ga and thinner. Screws can be either self-drilling or self-tapping. Self-drilling screws have drill points and threads formed at the screw end. This enables direct installation without the need for predrilling of holes in the steel framing or metal deck. Self-tapping screws require that a hole be drilled prior to installation. Typical screw sizes are No. 12 and No. 14 (with 0.216- in and 0.242-in shank diameter, respectively) for attachment of metal deck to steel framing. No. 8 and No. 10 screws (with 0.164-in and 0.190-in shank diameter, respectively) are frequently used for sidelap connections. Powder-driven fasteners are installed through the metal deck into the steel framing with pneumatic or powder-actuated equipment. Predrilled holes are not required. These types of fasteners are not used for sidelap connections. Button punches can be used for sidelap connections of certain types of metal deck that utilize upstanding seams at the sidelaps. However, since uniformity of installation is difficult to control, button punches are not usually considered to contribute significantly to diaphragm strength. The diaphragm capacity of various types and arrangements of metal deck and attachments are given in the Steel Deck Institute Diaphragm Design Manual. 8.2 PRECAST-CONCRETE PLANK This is another type of floor deck that is used with steel-framed construction (Fig. 8.7). The plank is prefabricated in standard widths, usually ranging between 4 and 8 ft, and is normally prestressed with high-strength steel tendons. Shear keys formed at the edges of the plank are subsequently grouted, to allow loads to be distributed between adjacent planks. Voids are usually placed within the thickness of the plank to reduce the deadweight without causing FLOOR AND ROOF SYSTEMS 8.9 significant reduction in plank strength. The inherent fire resistance of the precast concrete plank obviates the need for supplementary fire protection. Topped versus Untopped Planks. Precast planks can be structurally designed to sustain required loadings without need for a cast-in-place concrete topping. However, in many cases, it is advantageous to utilize a topping to eliminate differences in camber and elevation between adjacent planks at the joints and thus provide a smooth slab top surface. When a topping is used, the top surface of the plank may be intentionally roughened to achieve composite action between topping and plank. Thereby, the topping also serves as a structural component of the floor-deck system. A cast-in-place concrete topping can also be used for embedment of conduits and outlets that supply electricity and communication services. Voids within the planks can also be used as part of the distribution system. When the topping is designed to act compositely with the plank, however, careful consideration must be given to the effects of these embedded items. Dead-Load Deflection of Concrete Plank. In design of prestressed-concrete planks, the prestressing load balances a substantial portion of the dead load. As a result, relatively small dead-load deflections occur. For planks subjected to significant superimposed dead-load con- ditions of a sustained nature, for example, perimeter plank supporting an exterior masonry wall, additional prestressing to compensate for the added dead load, or some other stiffening method, is required to prevent large initial and creep deflections of the plank. Diaphragm Action of Concrete-Plank Systems. The diaphragm action of a floor deck composed of precast-concrete planks can be enhanced by making field-welded connections between steel embedments located intermittently along the shear keys of adjacent planks. (See also Art. 8.1.) Attachments of Concrete Plank to Framing. Precast-concrete planks are attached to and provide lateral bracing for supporting steel framing. A typical method of attachment is a field-welded connection between the supporting steel and steel embedments in the precast planks. 8.3 CAST-IN-PLACE CONCRETE SLABS Use of cast-in-place concrete for floor decks in steel-framed construction is a traditional approach that was much more prevalent prior to the advent of metal deck and spray-on fire protection. For one of the more common types of cast-in-place concrete floors, the formwork is configured to encase the steel framing, to provide fire protection and lateral bracing for the steel (see Fig. 8.8). If the proper confinement details are provided, this encasement can also serve to achieve composite action between the steel framing and the floor deck. Dead-load deflections should be calculated and, for long spans with large deflections, the formwork should be cambered to provide a level deck surface after removal of the formwork shoring. Diaphragm action is readily attainable with cast-in-place concrete floor decks. (See also Art. 8.1.) ROOF DECKS The systems used for floor decks (Arts. 8.1 to 8.3) can also be used for roof decks. When used as roof decks, these systems are overlaid by roofing materials, to provide a weathertight enclosure. Other roof deck systems are described in Arts. 8.4 to 8.7. 8.10 SECTION EIGHT FIGURE 8.8 Minimum requirements for composite action with concrete-encased steel framing. 8.4 METAL ROOF DECK Steel-framed buildings often utilize a roof deck composed simply of metal deck. When properly sloped for drainage, the metal deck itself can serve as a watertight enclosure. Al- ternatively, roofing materials can be placed on top of the deck. In either case, diaphragm action can be achieved by proper sizing and attachment of the metal deck. A fire rating can be provided by applying spray-on fire protection to the underside of the roof deck, or by installing a fire-rated ceiling system below the deck. Metal roof deck usually is used for noncomposite construction. It is commonly available in depths of 1 1 ⁄ 2 , 2, and 3 in. Long-span roof deck is available with depths of 4 1 ⁄ 2 ,6,and 7 1 ⁄ 2 in from some manufacturers. Cellular roof deck is sometimes used to provide a smooth soffit. When a lightweight insulating concrete fill is placed over the roof deck, the deck should be galvanized and also vented (perforated) to accelerate the drying time of the in- sulating fill, and prevent entrapment of water vapor. Standing-Seam System. When the metal roof deck is to serve as a weathertight enclosure, connection of deck units with standing seams offers the advantage of placing the deck seam above the drainage surface of the roof, thereby minimizing the potential for water leakage (Fig. 8.9). The seams can simply be snapped together or, to enhance their weathertightness, can be continuously seamed by mechanical means with a field-operated seaming machine provided by the deck manufacturer. Some deck types utilize an additional cap piece over the seam, which is mechanically seamed in the field (see Fig. 8.10). Frequently, the seams contain a factory-applied sealant for added weather protection. Thicknesses of standing-seam roof decks usually range between 26 and 20 ga. Typical spans range between 3 and 8 ft. A roof slope of at least 1 ⁄ 4 in per ft should be provided for drainage of rainwater. Standing-seam systems are typically attached to the supporting members with concealed anchor clips (Fig. 8.11) that allow unimpeded longitudinal thermal movement of the deck relative to the supporting structure. This eliminates buildup of stresses within the system and possible leakage at connections. However, the effect on the lateral bracing of supporting members must be carefully evaluated, which may result in a need for supplementary bracing. An evaluation method is presented in the American Iron and Steel Institute’s ‘‘Specification for the Design of Cold-Formed Steel Structural Members.’’ (See Art. 10.12.4.) [...]... SHAPES Hot-rolled, wide-flange steel shapes are the most commonly used members for multistory steel- framed construction These shapes, which are relatively simple to fabricate, are economical for beams and girders with short to moderate spans In general, wide-flange shapes are readily available in several grades of steel, including ASTM A36 and the higher-strength ASTM A572 and A992 steels FLOOR AND ROOF SYSTEMS... and pipes Openings can be either unreinforced, when located in zones subjected to low stress levels, or reinforced with localized steel plates, pipes, or angles (Fig 8.15) (‘ Steel and Composite Beams with Openings,’’ Steel Design Guide Series no 2, American Institute of Steel Construction.) Composite versus Noncomposite Construction Wide-flange beams and girders are frequently designed to act compositely... with cold-formed steel load-bearing wall studs for low-rise construction Spans are usually short to keep depth of floor system small This depth has a direct bearing on the overall height of structure to which costs of several building components are proportional Space in apartment buildings often is so arranged that beams and columns can be confined, hidden from view, within walls and partitions Since... structural steel hot-rolled shapes, or round or rectangular tubes, or cold-formed steel sections Many space frames are capable of utilizing two or more different member types For some space-frame roof structures, the top chords also act as purlins to directly support the roofing system In these cases, the top chords must be designed for a combination of axial and bending stresses For other roof structures,... reinforcement plates need only be welded to the easily accessible bottom chord of the joists, since the added shear connectors and increased web sizes have already been provided 8 .10 LIGHTWEIGHT STEEL FRAMING Cold-formed steel structural members can provide an extremely lightweight floor framing system These members, usually C or Z shapes, are normally spaced 24 in center to center (c to c) and can... diaphragm capacities When used as part of an approved ceiling assembly, many planks can achieve a fire rating 8.12 SECTION EIGHT (c) FIGURE 8 .10 Standing-seam roof deck with cap installed over the seams (a) Channel cap with flanges folded over lip of seam (b) U-shaped cap clamps over clips on seam (c) Steps in forming a seam with clamped cap The planks are usually supported by steel bulb tees (Fig 8.13),... vibrations for concrete slab (including concrete fill on metal deck) floor systems framed with steel joists or steel beams (T M Murray, ‘‘Acceptability Criterion for Occupant-Induced Floor Vibrations,’’ AISC Engineering Journal, vol 18, no 2 T M Murray, D E Allen, E E Ungar, ‘‘Floor Vibrations Due to Human Activity,’’ AISC Steel Design Guide Series, no 11.) ROOF FRAMING The systems used for floor framing (Arts... Although more frequently used for moderate- to long-span roof framing, open-web steel joists (Fig 8.17) are sometimes used for floor framing in multistory buildings Joists as floor FIGURE 8.17 Open-web steel joist supports gypsum deck 8.18 SECTION EIGHT members subjected to gravity loadings represent an efficient use of material, particularly since net uplift loadings that are sometimes applicable for roof... placement 8.16 SECTION EIGHT FIGURE 8.15 Penetrations for ducts and pipes in beam or girder webs (a) Rectangular opening, unreinforced (b) Circular opening reinforced with a steel- pipe segment (c) Rectangular penetration reinforced with steel bars welded to the web (d ) Reinforced cope at a column FIGURE 8.16 Beam and girder with shear connectors for composite action with concrete slab FLOOR AND ROOF SYSTEMS... roof deck 8.7 GYPSUM-CONCRETE DECKS Poured gypsum concrete is typically used in conjunction with steel bulb tees, formboards, and galvanized reinforcing mesh (Fig 8.14) Drainage slopes can be readily built into the roof deck by varying the thickness of gypsum FLOOR FRAMING With a large variety of structural steel floor-framing systems available, designers frequently investigate several systems during the . reinforced with localized steel plates, pipes, or angles (Fig. 8.15). (‘ Steel and Composite Beams with Openings,’’ Steel Design Guide Series no. 2, Amer- ican Institute of Steel Construction.) Composite. and provide lateral bracing for supporting steel framing. A typical method of attachment is a field-welded connection between the supporting steel and steel embedments in the precast planks. 8.3. described in Arts. 8.4 to 8.7. 8 .10 SECTION EIGHT FIGURE 8.8 Minimum requirements for composite action with concrete-encased steel framing. 8.4 METAL ROOF DECK Steel- framed buildings often utilize