Sổ tay kết cấu thép - Section 8

FLOOR AND ROOF SYSTEMS

8.1SECTION 8

FLOOR AND ROOF SYSTEMSDaniel 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 forfloor and roof construction Steel framing can achieve longer spans more efficiently thanother types of construction This minimizes the number of columns and footings therebyincreasing speed of erection Longer spans also provide more flexibility for interior-spaceplanning.

Another advantage of steel construction is its ability to readily accommodate future 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 bysuch measures as addition of framing members connected to existing members and fieldwelding of additional steel plates to strengthen existing members.

struc-FLOOR DECK

The most common types of floor-deck systems currently used with structural steel tion are concrete fill on metal deck, precast-concrete planks, and cast-in-place concrete slabs.

construc-8.1CONCRETE 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 ayield strength of at least 33 ksi Design requirements for metal deck are contained in theAmerican Iron and Steel Institute’s ‘‘Specification for the Design of Cold-Formed SteelStructural Members.’’

The concrete fill is usually specified to have a 28-day compressive strength of at least3000 psi Requirements for concrete design are contained in the American Concrete InstituteStandard ACI 318, ‘‘Building Code Requirements for Reinforced Concrete.’’

Sheet thicknesses of metal deck usually range between 24 and 18 ga, although thicknessesoutside this range are sometimes used The design thicknesses corresponding to typical gagedesignations are shown in Table 8.1.

TABLE 8.1 Equivalent Thicknesses forCold-Formed Steel

Designthickness, in

FIGURE 8.1 Cold-formed steel decking used in composite construction with concrete fill.

Metal deck is commonly available in depths of 11⁄2, 2, and 3 in Generally, it is preferableto use a deeper deck that can span longer distances between supports and thereby reducethe number of beams required For example, a beam spacing of about 15 ft can be achievedwith 3-in deck However, each project must be evaluated on an individual basis to determinethe most efficient combination of deck depth and beam spacing.

For special long-span applications, metal deck is available with depths of 41⁄2, 6, and 71⁄2

in from some manufacturers.

Composite versus Noncomposite Construction. Ordinarily, composite construction withmetal deck and structural-steel framing is used In this case, the deck acts not only as apermanent form for the concrete slab but also, after the concrete hardens, as the positivebending reinforcement for the slab To achieve this composite action, deformations areformed in the deck to provide a mechanical interlock with the concrete (Fig 8.1) Althoughnot serving a primary structural purpose, welded wire fabric is usually placed within theconcrete slab about 1 in below the top surface to minimize cracking due to concrete shrinkageand thermal effects This welded wire fabric also provides, to a limited degree, some amountof crack control in negative-moment regions of the slab over supporting members.

FIGURE 8.2 Cellular steel deck with concrete slab.

Noncomposite metal deck is used as a form for concrete and is considered to be ineffectivein resisting superimposed loadings In cases where the deck is shored, or where the deck isunshored but the long-term reliability of the deck will be questionable, the deck is alsoconsidered 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 precautionsare taken In such cases, the metal deck should be used solely as a form to support theconcrete until it hardens Reinforcement should be placed within the slab to resist all designloadings.

Noncellular versus Cellular Deck. It is sometimes desirable to distribute a building’s trical wiring within the floor deck system, in which case cellular metal deck can be used inlieu of noncellular deck However, in cases where floor depth is not critical, maximum wiringflexibility and capacity can be attained by using a raised access floor above the structuralfloor deck.

elec-Cellular deck is essentially noncellular deck, such as that shown in Fig 8.1, with a flatsheet added to the bottom of the deck to create cells (Fig 8.2) Electrical, power, andtelephone wiring is placed within the cells for distribution over the entire floor area In manycases, a sufficient number of cells is obtained by combining alternate panels of cellular deckand noncellular deck, which is called a blended system (Fig 8.3) When cellular deck isused, the 3-in depth is the minimum preferred because it provides convenient space forwiring The 11⁄2-in depth is rarely used.

For feeding wiring into the cells, a trench header is placed within the concrete above themetal deck, in a direction perpendicular to the cells (Fig 8.4) Special attention should begiven 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, thedirection of the cells should be selected to minimize the total length of trench header re-

FIGURE 8.3 Blended deck, alternating cellular and noncellular panels, in composite construction.

AIR CELLSSPRAY-ON FIREPROOFING(NOT ALWAYS REQUIRED)ELECTRICAL

If a uniform grid of power outlets is desired, such as 5 ft by 5 ft on centers, preset outletscan be positioned above the cells and cast into the concrete fill However, in many cases theoutlet locations will be dictated by subsequent tenant layouts In such cases, the concrete fillcan be cored and afterset outlets can be installed at any desired location.

Shored versus Unshored Construction. To support the weight of newly placed concreteand the construction live loads applied to the metal deck, the deck can either be shored orbe designed to span between supporting members If the deck is shored, a shallower-depth

them is less for (a) cells in the longitudinal direction than for (b) cells in the transverse direction.

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, aswell as concrete cracking over supporting members Another consideration is that use ofshoring can sometimes affect the construction schedule, since the shoring is usually kept inplace until the concrete fill has reached at least 75% of its specified 28-day compressivestrength In addition, when shoring is used, the concrete must resist the stresses resultingfrom the total dead load combined with all superimposed loadings.

When concrete is cast on unshored metal deck, the weight of the concrete causes thedeck to deflect between supports This deflection is usually limited to the lesser of1⁄180thedeck span or3⁄4in If the resulting effect on floor flatness is objectionable, the top surfacecan be finished level, but this will result in additional concrete being placed to compensatefor the deflection The added weight of this additional concrete must be taken into accountin design of the metal deck to ensure adequate strength The concrete fill, however, needonly resist the stresses resulting from superimposed loadings.

Unshored metal-deck construction is the system most commonly used The additionalcost of the deeper or thicker deck is generally much less than the cost of shoring To increasethe efficiency of the unshored deck in supporting the weight of the unhardened concrete andconstruction live loads, from both a strength and deflection standpoint, the deck is normallyextended 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 slabis designed for the total load, including slab self-weight, with the slab treated as a singlespan, unless negative-moment reinforcement is provided over supports in accordance withconventional reinforced-concrete-slab design (disregarding the metal deck as compressivereinforcement).

Lightweight versus Normal-Weight Concrete. Either lightweight or normal-weight crete can serve the structural function of the concrete fill placed on the metal deck Althoughthere is a cost premium associated with lightweight concrete, sometimes the savings in steelframing 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 eliminatethe need for additional slab fire protection (see ‘‘Fire Protection’’ below).

con-The tradeoffs in use of lightweight concrete versus normal-weight concrete plus fire tection should be evaluated on a project-by-project basis.

pro-Fire Protection. Most applications of concrete fill on metal deck in buildings require thatthe floor-deck assembly have a fire rating For noncellular metal deck, the fire rating isusually obtained either by providing sufficient concrete thickness above the metal deck orby applying spray-on fire protection to the underside of the metal deck For cellular metaldeck, which utilizes outlets that penetrate the concrete fill, the fire rating is usually obtainedby the latter method As an alternative, a fire-rated ceiling system can be installed below thecellular or noncellular deck.

When the required fire rating is obtained by concrete-fill thickness alone, lightweightconcrete requires a lesser thickness than normal-weight concrete for the same rating Forexample, a 2-hour rating can be obtained by using either 31⁄4 in of lightweight concrete or41⁄2 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., heavierbeams, girders, and columns) and the foundations.

If spray-on fire protection is used on the underside of the metal deck, the thickness ofconcrete above the deck can be the minimum required to resist the applied floor loads Thisminimum thickness is usually 21⁄2in, and the less expensive normal-weight concrete may beused instead of lightweight concrete Therefore, the two options that are frequently consid-ered for a 2-hour-rated, noncellular floor-deck system are 31⁄4-in lightweight concrete above

FIGURE 8.6 Two-hour fire-rated floor systems, with cold-formed steel deck (a) With lightweightconcrete fill; (b) with normal-weight concrete fill.

the metal deck without spray-on fire protection and 21⁄2-in normal-weight concrete above themetal deck with spray-on fire protection (Fig 8.6) Since the dead load of the floor deck forthe two options is essentially the same, the steel framing and foundations will also be thesame Thus, the comparison reduces to the cost of the more expensive lightweight concreteversus the cost of the normal-weight concrete plus the spray-on fire protection Since thecosts, and contractor preferences, vary with geographical location, the evaluation must bemade on an individual project basis (See also Art 6.32.)

Diaphragm Action of Metal-Deck Systems. Concrete fill on metal deck readily serves asa relatively stiff diaphragm that transfers lateral loads, such as wind and seismic forces, ateach 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 foradditional reinforcement Attachment of the metal deck to the steel framing, as well asattachment between adjacent deck units, must be sufficient to transfer the resulting shearstresses (see ‘‘Attachment of Metal Deck to Framing’’ below).

Additional shear reinforcement may be required in floor decks with large openings, suchas those for stairs or shafts, with trench headers for electrical distribution, or with other sheardiscontinuities Also, floors in multistory buildings in which cumulative lateral loads are

FIGURE 8.7 Precast-concrete plank floor with concrete topping.

transferred from one lateral load-resisting system to another (for example, from perimeterframes to interior shear walls), may be subjected to unusually large shear stresses that requirea diaphragm strength significantly greater than that for a typical floor.

Attachment of Metal Deck to Framing. Metal deck can be attached to the steel framingwith puddle (arc spot) welds, screws, or powder-driven fasteners These attachments providelateral bracing for the steel framing and, when applicable, transfer shear stresses resultingfrom diaphragm action The maximum spacing of attachments to steel framing is generally12 in.

Attachment of adjacent deck units to each other, that is, sidelap connection, can be madewith 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, andspacing 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 Whenmetal deck is welded to steel framing, welding washers should be used if the deck thicknessis less than 22 ga to minimize the possibility of burning through the deck Sidelap weldingis not recommended for deck thicknesses of 22 ga and thinner.

Screws can be either self-drilling or self-tapping Self-drilling screws have drill pointsand threads formed at the screw end This enables direct installation without the need forpredrilling of holes in the steel framing or metal deck Self-tapping screws require that ahole 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) arefrequently used for sidelap connections.

Powder-driven fasteners are installed through the metal deck into the steel framing withpneumatic or powder-actuated equipment Predrilled holes are not required These types offasteners are not used for sidelap connections.

Button punches can be used for sidelap connections of certain types of metal deck thatutilize upstanding seams at the sidelaps However, since uniformity of installation is difficultto control, button punches are not usually considered to contribute significantly to diaphragmstrength.

The diaphragm capacity of various types and arrangements of metal deck and attachments

are given in the Steel Deck Institute Diaphragm Design Manual.

8.2PRECAST-CONCRETE PLANK

This is another type of floor deck that is used with steel-framed construction (Fig 8.7) Theplank is prefabricated in standard widths, usually ranging between 4 and 8 ft, and is normallyprestressed with high-strength steel tendons Shear keys formed at the edges of the plankare subsequently grouted, to allow loads to be distributed between adjacent planks Voidsare usually placed within the thickness of the plank to reduce the deadweight without causing

significant reduction in plank strength The inherent fire resistance of the precast concreteplank obviates the need for supplementary fire protection.

Topped versus Untopped Planks. Precast planks can be structurally designed to sustainrequired 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 elevationbetween adjacent planks at the joints and thus provide a smooth slab top surface When atopping is used, the top surface of the plank may be intentionally roughened to achievecomposite action between topping and plank Thereby, the topping also serves as a structuralcomponent of the floor-deck system.

A cast-in-place concrete topping can also be used for embedment of conduits and outletsthat supply electricity and communication services Voids within the planks can also be usedas part of the distribution system When the topping is designed to act compositely with theplank, 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, theprestressing load balances a substantial portion of the dead load As a result, relatively smalldead-load deflections occur For planks subjected to significant superimposed dead-load con-ditions of a sustained nature, for example, perimeter plank supporting an exterior masonrywall, additional prestressing to compensate for the added dead load, or some other stiffeningmethod, is required to prevent large initial and creep deflections of the plank.

Diaphragm Action of Concrete-Plank Systems. The diaphragm action of a floor deckcomposed of precast-concrete planks can be enhanced by making field-welded connectionsbetween 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 andprovide lateral bracing for supporting steel framing A typical method of attachment is afield-welded connection between the supporting steel and steel embedments in the precastplanks.

8.3CAST-IN-PLACE CONCRETE SLABS

Use of cast-in-place concrete for floor decks in steel-framed construction is a traditionalapproach that was much more prevalent prior to the advent of metal deck and spray-on fireprotection For one of the more common types of cast-in-place concrete floors, the formworkis configured to encase the steel framing, to provide fire protection and lateral bracing forthe steel (see Fig 8.8) If the proper confinement details are provided, this encasement canalso 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, theformwork should be cambered to provide a level deck surface after removal of the formworkshoring Diaphragm action is readily attainable with cast-in-place concrete floor decks (Seealso Art 8.1.)

ROOF DECKS

The systems used for floor decks (Arts 8.1 to 8.3) can also be used for roof decks Whenused as roof decks, these systems are overlaid by roofing materials, to provide a weathertightenclosure Other roof deck systems are described in Arts 8.4 to 8.7.

FIGURE 8.8 Minimum requirements for composite action with concrete-encased steelframing.

8.4METAL ROOF DECK

Steel-framed buildings often utilize a roof deck composed simply of metal deck Whenproperly 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, diaphragmaction can be achieved by proper sizing and attachment of the metal deck A fire rating canbe provided by applying spray-on fire protection to the underside of the roof deck, or byinstalling a fire-rated ceiling system below the deck.

Metal roof deck usually is used for noncomposite construction It is commonly availablein depths of 11⁄2, 2, and 3 in Long-span roof deck is available with depths of 41⁄2, 6, and71⁄2in from some manufacturers Cellular roof deck is sometimes used to provide a smoothsoffit When a lightweight insulating concrete fill is placed over the roof deck, the deckshould 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 seamabove 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 machineprovided by the deck manufacturer Some deck types utilize an additional cap piece over theseam, which is mechanically seamed in the field (see Fig 8.10) Frequently, the seamscontain a factory-applied sealant for added weather protection.

Thicknesses of standing-seam roof decks usually range between 26 and 20 ga Typicalspans range between 3 and 8 ft A roof slope of at least1⁄4in per ft should be provided fordrainage of rainwater.

Standing-seam systems are typically attached to the supporting members with concealedanchor clips (Fig 8.11) that allow unimpeded longitudinal thermal movement of the deckrelative to the supporting structure This eliminates buildup of stresses within the system andpossible leakage at connections However, the effect on the lateral bracing of supportingmembers 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 ‘‘Specificationfor the Design of Cold-Formed Steel Structural Members.’’ (See Art 10.12.4.)

FIGURE 8.9 Standing-seam roof deck (a) With snapped seam; (b) with mechanical seam; (c)

steps in forming a seam.

8.5LIGHTWEIGHT PRECAST-CONCRETE ROOF PANELS

Roof decks of lightweight precast-concrete panels typically span 5 to 10) ft between supports.Panel thicknesses range from 2 to 4 in, and widths are usually 16 to 24 in Depending onthe product, concrete density can vary from 50 to 115 lb per ft3 Certain types of panelshave diaphragm capacities depending upon the edge and support connections used Manypanels can achieve a fire rating when used as part of an approved ceiling assembly.

The panels are typically attached to steel framing with cold-formed-steel clips (see Fig.8.12) The joints between panels are cemented on the upper side, usually with an asphalticmastic compound Insulation and roofing materials are normally placed on top of the panels.Some panels are nailable for application of certain types of roof finishes, such as slate, tile,and copper.

8.6WOOD-FIBER PLANKS

Planks formed of wood fibers bonded with portland cement provide a lightweight roof deckwith insulating and acoustical properties The typical density of this material ranges between30 and 40 lb per ft3 Some plank types have diaphragm capacities When used as part of anapproved ceiling assembly, many planks can achieve a fire rating.

FIGURE 8.10 Standing-seam roof deck with cap installed over the seams (a) Channel cap withflanges 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), which are nominallyspaced 32 to 48 in on centers The joint over the bulb tee is typically grouted with a gypsum-concrete grout and roofing materials are applied to the top surface of the planks.

ANCHOR CLIP

FIGURE 8.11 Typical anchor clip for standing-seam roof deck.

FIGURE 8.12 Typical clips for attachment of precast-concrete panels to steel framing Theclips are driven into place for a wedge fit at diagonal corners of the panels Minimum flangewidth for supporting member is preferably 4 in.

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 theroof deck by varying the thickness of gypsum.

FLOOR FRAMING

With a large variety of structural steel floor-framing systems available, designers frequentlyinvestigate several systems during the preliminary design stage of a project The lightest

FIGURE 8.13 (a) Wood-fiber planks form roof deck (b) Plank is supported by

a steel bulb tee.

framing system, although the most efficient from a structural engineering standpoint, maynot be the best selection from an overall project standpoint, since it may have such disad-vantages as high fabrication costs, large floor-to-floor heights, and difficulties in interfacingwith mechanical ductwork.

Spandrel members are frequently subjected to torsional loadings induced by facade ments and thus require special consideration In addition, design of these members is fre-quently governed by deflection criteria established to avoid damage to, or to permit properfunctioning of, the facade construction.

ele-8.8ROLLED SHAPES

Hot-rolled, wide-flange steel shapes are the most commonly used members for multistorysteel-framed construction These shapes, which are relatively simple to fabricate, are eco-nomical for beams and girders with short to moderate spans In general, wide-flange shapesare readily available in several grades of steel, including ASTM A36 and the higher-strengthASTM A572 and A992 steels.

FIGURE 8.14 (a) Gypsum-concrete roof deck (b) Cast on formboard, thc deck is supported by

a steel bulb tee.

Interfacing with mechanical ductwork is usually accomplished in one of two ways First,the steel framing can be designed to incorporate the shallowest members that provide therequired strength and stiffness, and the mechanical ductwork can be routed beneath the floorframing As an alternative, deeper beams and girders than would otherwise be necessary canbe used, and these members can be fabricated with penetrations, or openings, that allowpassage of ductwork and pipes Openings can be either unreinforced, when located in zonessubjected 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, ican Institute of Steel Construction.)

Amer-Composite versus Noncomposite Construction. Wide-flange beams and girders are quently designed to act compositely with the floor deck This enables the use of lighter orshallower members Composite action is readily achieved through the use of shear connectorswelded to the top flange of the beam or girder (Fig 8.16) When the floor deck is composedof concrete fill on metal deck the shear connectors are field-welded through the metal deckand onto the top flange of the beam or girder, prior to concrete placement.

fre-FIGURE 8.15 Penetrations for ducts and pipes in beam or girder webs (a) Rectangular ing, unreinforced (b) Circular opening reinforced with a steel-pipe segment (c) Rectangularpenetration reinforced with steel bars welded to the web (d ) Reinforced cope at a column.

open-FIGURE 8.16 Beam and girder with shear connectors for composite action with concreteslab.

FIGURE 8.17 Open-web steel joist supports gypsum deck.

Composite strength is usually controlled by shear transfer or by bottom flange tension.In cases where increased future loadings are likely, such as file storage loading in officeareas, additional shear connectors can be provided in the original design at minimal addi-tional cost When the increased loadings must be accommodated, reinforcement plates needonly be welded to the easily accessible bottom flange of the beams and girders, since theadded shear connectors have already been installed.

Noncomposite design is generally found to be more economical for relatively short spans,inasmuch as the added cost of shear connectors tends not to justify the savings in steelframing.

Shored versus Unshored Construction. Composite floor framing can be designed as beingeither shored or unshored during construction In most cases, unshored construction is used.This allows dead-load deflections to occur during the concrete placement, and the floors tobe finished with a level surface In such cases, the additional concrete dead load must betaken into account when designing the beams and girders, and other components of thestructure.

When unshored construction is used for moderate spans with relatively large dead-loaddeflections, the beams and girders can be cambered for the dead-load deflection, therebyresulting in a level floor surface after placement of the concrete When camber is specified,however, careful consideration should be given to the end restraint of the beam (for example,whether the beam frames into girders or into columns), even if simple connections are usedthroughout End restraint reduces deflections, and camber that exceeds the actual dead-loaddeflection can sometimes be troublesome, since it may affect the fire rating (because ofinsufficient concrete-fill thickness over metal deck), the elevation of preset inserts in anelectrified floor system, or installation of interior finishes.

Shored construction will result in lighter or shallower beams and girders than unshoredconstruction, since the flexural members will act compositely with the floor deck in resistingthe weight of the concrete when the shores are removed However, consideration must begiven to the deflections that will occur after shore removal, and whether the resulting floorlevelness will be acceptable.

8.9OPEN-WEB JOISTS

Although more frequently used for moderate- to long-span roof framing, open-web steeljoists (Fig 8.17) are sometimes used for floor framing in multistory buildings Joists as floor