Figure 7.49f. Composite beam with narrow hump metal deck.
Metal decks for composite construction are available in the United States in three depths—1 1 in. (38 mm), 2 in. (51 mm), and 3 in. (76 mm). The earlier types of metal deck did not have embossments, and the interlocking between concrete and metal deck was achieved by welding reinforcement transverse to the beam. Later developments of metal deck introduced embossments to engage the concrete and metal deck and dispensed
with the transverse-welded reinforcement. Typical spans for composite metal deck are generally in the range of 8 to 15 ft (2.4 to 4.6 m).
In floor systems using 1V2-in. (38-mm)-deep decks, provision for electrical and telephone services is made by punching holes through the slab at various locations and passing the under-floor ducts through them. A deeper deck is required if the power distribution system is integrated as part of the structural slab; 2- or 3-in. (51- or 76-mm)-deep metal deck is sufficient. Tests have shown that there is very little loss of composite beam stiffness due to the ribbed configuration of metal deck in the depth range of 1V2 to 3 in. (38 to 76 mm). As long as the ratio of width to depth of the metal deck is at least 1.75, the entire capacity of the shear stud can be developed similar to that for beams with solid slabs. However, with deeper deck, a substantial decrease in shear strength of the stud occurs, which is attributed to a different type of failure mechanism. Instead of the failure of shear stud, the mode of failure is initiated by cracking of the concrete in the rib corners. Eventual failure takes place by separation of concrete from the metal deck. When more than one stud is used in a metal deck flute, a failure cone can develop over the shear stud group, resulting in lesser shear capacity per each stud. The shear stud strength is therefore closely related to the metal deck configuration and factors related to the surface area of the shear cone.
Often special considerations are required in composite design when openings interrupt slab continuity. For example, beams adjacent to elevator and stair openings may have full effective width for part of their length and perhaps half that value adjacent to the openings. Elevator sill details normally require a recess in the slab for door installations, rendering the slab ineffective for part of the beam length. A similar problem occurs in the case of trench header ducts, which require elimination of concrete, as opposed to the standard header duct, which is completely encased in concrete. When the trench is parallel to the composite beam, its effect can easily be incorporated into the design by suitably modifying the effective width of compression flange. The effect of the trench oriented perpendicular to the composite beam could range from negligible to severe depending upon its location. If the trench can be located in the region of minimum bending moment, such as near the supports in a simply supported beam, and if the required number of connectors could be placed between the trench and the point of maximum bending moment, its effect on the composite beam design is minimal. If, on the other hand, the trench must be placed in an area of high bending moment, its effect may be so severe as to require that the beam be designed as a noncomposite beam.
This slab thickness in composite construction is usually governed by fire-rating requirements rather than by the bending capacity of the slab. In certain parts of the United States it may be economical to use the minimum thickness required for strength and to use sprayedon or some other method of fireproofing the deck to obtain the required ratings. Some major projects have used a 2V2-in. (63.5-mm)-thick concrete slab on 3-in. (76.2-mm)-deep metal deck spanning as much as 15 ft (4.57 m).
In continuous composite beams the negative moment regions can be designed such that: 1) the steel beam alone resists the negative moment; or 2) it acts compositely with mild steel reinforcement placed in the slab parallel to the beam. In the latter case, shear connectors must be provided through the negative moment region.
Careful attention should be paid to the deflection characteristics of composite construction because the slender not-yet-composite shape deflects as wet concrete is placed on it. There are three ways to alleviate the deflection problem.
1. Use relatively heavy steel beams to limit the dead-load deflection and place lens-shaped tapering slabs to obtain a nearly flat top. Although a reasonably flat surface results from this construction, the economic restraints of speculative office buildings do not usually permit the luxury of the added cost of additional concrete and heavier steel beams.
2. Camber the steel beam to compensate for the deflection due to weight of steel beam and concrete. Place a constant thickness of slab by finishing the concrete to screeds set from the cambered steel. Continuous lateral bracing as provided by the metal deck is required to prevent the lateral torsion buckling of beam. If steel deck is not used, this system requires a substantial temporary bracing system to stabilize the beam during construction.
3. Camber and shore the steel beam. The beam is fabricated with a camber calculated to compensate for the deflection of the final cured composite section. Shores are placed to hold the steel at its curved position while the concrete is being poured. As in method 2, slab is finished to screeds set from cambered steel. Although methods 1 and 3 are occasionally used, the trend is to use method 2 because it is the least expensive.
126.96.36.199. AISC Allowable Stress Design (ASD)
Including provisions for solid slab, there are three categories of composite beams in the AISC specifications each with a differing effective concrete area.
188.8.131.52.1. Solid Slab. The total slab depth is effective in compression unless the neutral axis is above the top of the steel beam. ( In typical floor systems with relatively thin slabs, the neutral axis of steel beams is invariably below the slab, rendering the total slab depth effective in compression).
1. As illustrated in Fig. 7.51, concrete below the top of steel decking shall be neglected in computations of section properties and in calculating the number of shear studs, but the concrete below the top flange of deck may be included for calculating the effective width.
2. The maximum spacing of shear connectors shall not exceed 32 in. (813 mm) along the beam length.
3. The steel deck shall be anchored to the beam either by welding or by other means at a spacing not exceeding 16 in. (406 mm).
4. A reduction factor as given by the AISC formula I5-1
kK A hr
should be used for reducing the allowable horizontal shear capacity of stud connectors. In the above formula hr is the nominal rib height in inches; Hs is length of stud connector after welding in inches. An upper limit of (hr + 3) is placed on the length of shear connectors used in computations even when longer studs are installed in metal decks. Nr is the number of studs in one rib. A maximum value of 3 can be used in computations although more than three studs may be installed. wr is average width of concrete rib.
1. The major difference between perpendicular and parallel orientation of deck ribs is that when the deck is parallel to the beam, the concrete below the top of the decking can be included in the calculations of section properties and must be included when calculating the number of shear studs, as illustrated in Fig. 7.52.
2. If steel deck ribs occur on supporting beam flanges, it is permissible to cut high-hat to form a concrete haunch.
3. When the nominal rib height is 1in. (38.1 mm) or greater, the minimum average width of deck flute should not be less than 2 in. for the first stud in the transverse row plus four stud diameters for each additional stud. This gives minimum average widths of 2 in. (51 mm) for one stud, 2 in. plus 4 d for two studs, 2 in. plus 8d for three studs, etc., where d is the diameter of the stud. Note that if a metal deck cannot accommodate this width requirement, the deck can be split over the girder to form a haunch.
4. A reduction factor as given by AISC formula I5-2
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