In composite construction, a steel beam and a concrete slab act together to resist bending. The slab, in effect, serves as a cover plate and allows use of a lighter steel section. The AISC ASD and LRFD specifications for structural steel buildings treat two cases of composite members: (1) totally encased members that depend on the natural bond between the steel and concrete, and (2) steel members with shear connectors, mechanical anchorages between a concrete slab and the steel.
Composite Beams with Shear Connectors
The most common application of composite construction is a simple or continuous beam with shear connectors. For such applications, the design may be based on an effective concrete- steel T-beam, where the width of the concrete slab on each side of the beam centerline may not be taken more than 1. One-eighth of the beam span, center-to-center of supports 2. One-half the distance of the centerline of the adjacent beam 3. The distance from beam centerline to the edge of the slab
superposition of elastic stresses on the transformed section, with effects of shoring taken into account, where Fy is the specified yield stress of tension flange (ksi), tw is the web thickness (in), and hc is twice the distance (in) from the neutral axis of the steel beam alone to the inside face of the compression flange (less the fillet or corner radius for rolled beams) or to the nearest line of mechanical fasteners at the compression flange.
In the negative-moment region of a composite beam, the design flexural strength should be determined for the steel section alone unless provision is made to utilize composite action.
When the steel beam is an adequately braced, compact shape, shear connectors connect the slab to the steel in the negative-moment region, and the slab reinforcement parallel to the steel beam, within the effective width of the slab, is adequately developed, the negativemoment capacity may be taken as bMn , with b 0.85. Reinforcement parallel to the beam may be included in computations of properties of the composite section.
When a composite beam will be shored during construction until the concrete has developed sufficient strength, composite action may be assumed in design to be available to carry all loads. When shores are not used during construction, the steel section acting alone should be designed to support all loads until the concrete attains 75% of its specified compressive strength.
Composite construction often incorporates steel deck that serves as a form for the concrete deck, is connected to the steel beam, and remains in place after the concrete attains its design strength. The preceding moment-capacity computations may be used for composite systems with metal deck if the deck meets the following criteria:
The steel-deck rib height should not exceed 3 in.
Average width of concrete rib or haunch and slab thickness above the steel deck should be at least 2 in.
Welded stud shear connectors should be 3⁄4 in in diameter or less and extend at least 11⁄2 in above the steel deck.
The slab thickness above the steel deck must be at least 2 in.
The AISC LRFD specification indicates that for ribs perpendicular to the steel beam, concrete below the top of the steel deck should be neglected in determining section properties, whereas for ribs parallel to the steel beam, that concrete may be included (see also Art. 6.26.2). However, it is limited to the minimum clear width near the top of the deck.
ASD for Composite Beams. The AISC ASD specification requires that flexural strength of a composite beam be determined by elastic analysis of the transformed section with an allowable stress of 0.66Fy , where Fy is the minimum specified yield stress (ksi) of the tension flange. This applies whether the beam is temporarily shored or unshored during construction.
The transformed section comprises the steel beam and an equivalent area of steel for the compression area of the concrete slab. The equivalent area is calculated by dividing the concrete area by the modular ratio n, where n = E/Ec , E is the elastic modulus of the steel beam, and Ec is the elastic modulus of the concrete.
If a composite beam will be constructed without shoring, the steel section should be assumed to act alone in carrying loads until the concrete has attained 75% of its specified compressive strength. The maximum allowable stress in the steel beam, in this case, is 0.9Fy.
After that time, the transformed section may be assumed to support all loads. The maximum allowable stress in the concrete is 0.45ƒc , where ƒc is the specified concrete compressive strength.
When shear connectors are used, full composite action is obtained only when sufficient connectors are installed between points of maximum moment and points of zero moment to carry the horizontal shear between those points. When fewer connectors are provided, the increase in bending strength over that of the steel beam alone is directly proportional to the number of steel connectors. Thus, when adequate connectors for full composite action are not provided, the section modulus of the transformed composite section must be reduced accordingly.
For ASD, an effective section modulus may be computed from
The purpose of shear connectors is to ensure composite action between a concrete slab and a steel beam by preventing the slab from slipping relative to or lifting off the flange to which the connectors are welded. Headed-stud or channel shear connectors are generally used. The studs should extend at least four stud diameters above the flange. The welds between the connectors and the steel flange should be designed to resist the shear carried by the connectors.
When the welds are not directly over the beam web, they tend to tear out of a thin flange before their full shear-resisting capacity is attained. Consequently, the AISC ASD and LRFD specifications require that the diameter of studs not set directly over the web be 21⁄2 times the flange thickness or less. The specifications also limit the spacing center-to-center of shear connectors to a maximum of eight times the total slab thickness. The minimum center-to-center spacing of stud connectors should be 6 diameters along the longitudinal axis of the supporting composite beam and 4 diameters transverse to the longitudinal axis of the supporting composite beam, except that within ribs of formed steel decks, oriented perpendicular to the steel beam, the minimum center-to-center spacing should be 4 diameters in any direction.
Shear connectors, except those installed in the ribs of formed steel deck, should have at least 1 in of concrete cover in all directions.
The AISC LRFD specification requires that the total horizontal shear Vh necessary to develop full composite action between the point of maximum positive moment and the point of zero moment be the smaller of Vh (kips) computed from Eqs. (6.80) and (6.81):
or supports. The AISC specifications consequently require that when a concentrated load occurs in a region of positive bending moment, the number of connectors between that load and the nearest point of zero moment should be sufficient to develop the maximum moment at the concentrated load point. The LRFD specification gives no special equation for checking this. The ASD specification gives the following equation for number of shear connectors:
For ASD, the allowable shear q (kips) for a connector is given in Table 6.32 for flat-soffit concrete slabs made with C33 aggregate. Reduction factors apply for studs in formed steel deck. For concrete made with C330 lightweight aggregate, the factors in Table 6.33 should be applied.
Working values Q for shear connectors in Table 6.32 incorporate a safety factor of 2.5 applied to ultimate strength. For use with concrete not conforming to ASTM C33 or C330 and for types of connectors not listed in the table, values should be established by tests.
When shear connectors are not used, composite action may be attained by encasing a steel beam in concrete. To be considered composite construction, concrete and steel must satisfy the following requirements: Steel beams should be encased 2 in or more on their sides and bottom in concrete cast integrally with the slab (Fig. 6.6). The top of the steel beam should be at least 11⁄2 in below the top of the slab and 2 in above its bottom. The encasement should be reinforced throughout its depth and across its bottom to prevent spalling of the concrete.
For such encased beams, composite action may be assumed produced by bond between the steel member and the concrete.
Design of an encased beam depends on whether the steel beam is shored temporarily when the concrete is cast. If the shoring remains in place until the concrete attains 75% of required strength, the composite section can be assumed to carry all loads. Without such shoring, the steel beam carries the dead load unassisted. Only loads applied after the concrete reaches 75% of its required strength can be assumed taken by the composite beam.
Since the beam then is completely braced laterally, the allowable bending stress in the steel flanges for ASD is 0.66Fy , where Fy is the steel yield stress (ksi). Compressive stress in the concrete should not exceed 0.45ƒc , where ƒc is the specified 28-day strength of the concrete (ksi). The concrete should be assumed unable to carry tension. Reinforcement to resist tension, however, may be provided in the concrete for negative moment in continuous beams and cantilevers. This reinforcement should be placed within the effective width.
For LRFD, the design flexural strength bMn should be computed with b 0.90 and Mn determined from superposition of elastic stresses on the transformed composite section, considering the effects of shoring, or from the plastic distribution on the steel section alone (Art. 6.17.2). Also, if shear connectors are provided and concrete meets the requirements of Art.6.26.4, the design flexural strength bMn may be computed based upon the plastic stress distribution on the composite section with b 0.85.
The transformed composite section is obtained by treating the concrete on the compression side of the neutral axis as an equivalent steel area. This is done by dividing that concrete area by n, the ratio of the modulus of elasticity of steel to that of the concrete.
The AISC LRFD specification for structural steel buildings contains provisions for design of concrete-encased compression members. It sets the following requirements for qualification as a composite column: The cross-sectional area of the steel core—shapes, pipe, or tubing—should be at least 4% of the total composite area. The concrete should be reinforced with longitudinal load-carrying bars, continuous at framed levels, and lateral ties and other longitudinal bars to restrain the concrete, all with at least 11⁄2 in of clear concrete cover. The cross-sectional area of transverse and longitudinal reinforcement should be at least 0.007 in2 per in of bar spacing. Spacing of ties should not exceed two-thirds of the smallest dimension of the composite section. Strength of the concrete should be between 3 and 8 ksi for ƒc normal-weight concrete and at least 4 ksi for lightweight concrete. Specified minimum yield