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FLOOR AND ROOF SYSTEMS
SECTION 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 structural modifications, such as openings for tenants’ stairs and changes for heavier floor loadings 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 construction 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 8.1 8.2 SECTION EIGHT TABLE 8.1 Equivalent Thicknesses for Cold-Formed Steel Gage designation Design thickness, in 28 26 24 22 20 18 16 0.0149 0.0179 0.0239 0.0299 0.0359 0.0478 0.0598 Metal deck is commonly available in depths of 11⁄2, 2, and 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 41⁄2, 6, and 71⁄2 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 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 FIGURE 8.1 Cold-formed steel decking used in composite construction with concrete fill FLOOR AND ROOF SYSTEMS 8.3 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 structures 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 electrical 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 11⁄2-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 composite 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- FIGURE 8.2 Cellular steel deck with concrete slab 8.4 SECTION EIGHT FIGURE 8.3 Blended deck, alternating cellular and noncellular panels, in composite construction 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 ft by 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 TRENCH HEADERS CONCRETE SLAB AIR CELLS SPRAY-ON FIREPROOFING (NOT ALWAYS REQUIRED) ELECTRICAL CELLS FIGURE 8.4 Cellular steel deck with trench header placed within the concrete slab to feed wiring to cells 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 8.5 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 singlespan 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 concrete 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 sufficient 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 protection 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 31⁄4 in of lightweight concrete or 41⁄2 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 21⁄2 in, and the less expensive normal-weight concrete may be used instead of lightweight concrete Therefore, the two options that are frequently considered for a 2-hour-rated, noncellular floor-deck system are 31⁄4-in lightweight concrete above FLOOR AND ROOF SYSTEMS 8.7 the metal deck without spray-on fire protection and 21⁄2-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 accommodated 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 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 8.8 SECTION EIGHT 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 attachments 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 0.216in and 0.242-in shank diameter, respectively) for attachment of metal deck to steel framing No 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 and 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 FIGURE 8.7 Precast-concrete plank floor with concrete topping 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 conditions 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 Alternatively, 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 11⁄2, 2, and in Long-span roof deck is available with depths of 41⁄2, 6, and 71⁄2 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 insulating 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 and 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.) ... 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... lateral loads are 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 8. 8 SECTION EIGHT transferred.. .8. 2 SECTION EIGHT TABLE 8. 1 Equivalent Thicknesses for Cold-Formed Steel Gage designation Design thickness, in 28 26 24 22 20 18 16 0.0149 0.0179 0.0239 0.0299 0.0359 0.04 78 0.05 98 Metal