In composite construction, rolled or built-up steel shapes are combined with reinforcedÂ concrete to form a structural member. Examples of this type of constructionÂ include: (a) concrete-encased steel beams (Fig. 7.37c), (b) concrete decks interactiveÂ with steel beams (Fig. 7.37a and b), (c) concrete encased steel columns, and (d)Â concrete filled steel columns. The most common use of this type of construction isÂ for composite beams, where the steel beam supports and works with the concreteÂ slab to form an economical building element.

Design procedures require that a decision be made regarding the use of shoringÂ for the deck pour. (Procedures for ASD and LRFD differ in this regard.) If shoringÂ is not used, the steel beam must carry all dead loads applied until the concreteÂ hardens, even if full plastic capacity is permitted for the composite section afterward.

The assumed composite cross section is the same for ASD and LRFD procedures.

The effective width of the slab is governed by beam span and beam spacingÂ or edge distance (Fig. 7.37a and b).

Slab compressive stresses are seldom critical for interior beams but should beÂ investigated, especially for edge beams. Thickening the slab key and minimumÂ requirements for strength of concrete can be economical.

Connector Details. In composite construction, shear connectors welded to the topÂ flange of the steel beam are typically used to ensure composite action by transferringÂ shear between the concrete deck and steel beam. Location, spacing, and sizeÂ limitations for shear connectors are the same for ASD and LRFD procedures. Connectors,Â except those installed in ribs of formed steel decks, should have a minimumÂ lateral concrete cover of 1 in. The diameter of a stud connector, unless locatedÂ directly over the beam web, is limited to 2.5 times the thickness of the beam flangeÂ to which it is welded. Minimum center-to-center stud spacing is 6 diameters alongÂ the longitudinal axis, 4 diameters transversely. Studs may be spaced uniformly,Â rather than in proportion to horizontal shear, inasmuch as tests show a redistributionÂ of shear under high loads similar to the stress redistribution in large bolted joints.

Maximum spacing is 8 times the slab thickness.

Formed Steel Decking. Concrete slabs are frequently cast on permanent steelÂ decking with a ribbed, corrugated, cellular, or blended cellular cross section (seeÂ Sec. 8). Two distinct composite-design configurations are inherent: ribs parallel orÂ ribs perpendicular to the supporting beams or girders (Fig. 7.38) The design procedures,Â for both ASD and LRFD, prescribed for composite concrete-slab and steelbeamÂ construction are also applicable for systems utilizing formed steel decking,Â subject to additional requirements of the AISC â€˜â€˜Specification for Structural SteelÂ for Buildingsâ€™â€™ and as illustrated in Fig. 7.38.

Shear and Deflection of Composite Beams. In ASD and LRFD, shear forces areÂ assumed to be resisted by the steel beam. Deflections are calculated based on compositeÂ section properties. It should be noted that, because of creep of the concrete,Â the actual deflections of composite beams under long-term loads, such as dead load,Â will be greater than those computed.

## ASD of Encased Beams

Two design methods are allowed for encased beams. In one method, stresses areÂ computed on the assumption that the steel beam alone supports all the dead loadÂ applied prior to concrete hardening (unless the beam is temporarily shored), andÂ the composite beam supports the remaining dead and live loads. Then, for positiveÂ bending moments, the total stress, ksi, on the steel-beam bottom flange is

## ASD of Beams with Shear Connectors

For composite construction where shear connectors transfer shear between slab andÂ beam, the design is based on behavior at ultimate load. It assumes that all loadsÂ are resisted by the composite section, even if shores are not used during constructionÂ to support the steel beam until the concrete gains strength. For this case, the computedÂ stress in the bottom flange for positive bending moment is

In continuous composite beams, where shear connectors are installed in negativemomentÂ regions, the longitudinal reinforcing steel in the concrete slab may beÂ considered to act compositely with the steel beam in those regions. In such cases,Â the total horizontal shear to be resisted by the shear connectors between an interiorÂ support and each adjacent infection point is

where Asr total area, in2, of longitudinal reinforcing steel within the effectiveÂ width of the concrete slab at the interior supportÂ Fyr specified yield stress of the reinforcing steel, ksi

These formulas represent the horizontal shear at ultimate load divided by 2 toÂ approximate conditions at working load.

Number of Connectors. The minimum number of connectors N1, spaced uniformlyÂ between the point of maximum moment and adjacent points of zero moment,Â is Vh /q, where q is the allowable shear load on a single connector, as given inÂ Table 7.22. Values in this table, however, are applicable only to concrete made withÂ aggregates conforming to ASTM C33. For concrete made with rotary-kiln-producedÂ aggregates conforming to ASTM C330 and with concrete weight of 90 pcf or more,Â the allowable shear load for one connector is obtained by multiplying the valuesÂ in Table 7.22 by the factors in Table 7.23.

The allowable shear loads for connectors incorporate a safety factor of aboutÂ 2.5 applied to ultimate load for the commonly used concrete strengths. Not to beÂ confused with shear values for fasteners given in Art. 7.30, the allowable shearÂ loads for connectors are applicable only with Eqs. (7.67) to (7.69).

The allowable horizontal shear loads given in Tables 7.22 and 7.23 may haveÂ to be adjusted for use with formed steel decking. For decking with ribs parallel toÂ supports (Fig. 7.38a), the allowable loads should be reduced when w/h is less thanÂ 1.5 by multiplying the tabulated values by

## LRFD of Encased Beams

Two methods of design are allowed, the difference being whether or not shoring isÂ used. In both cases, the design strength is bMn, where b 0.90. Mn is calculatedÂ for the elastic stress distribution on the composite section if shoring is used or theÂ plastic stress distribution on the steel section alone if shoring is not used.

## LRFD of Composite Beams

As with ASD, the use of shoring to carry deal loads prior to the time the concreteÂ has hardened determines which design procedures are used. For composite constructionÂ where the steel beams are exposed, the design flexural strength for positiveÂ moment (compression in the concrete) is bMn. It is dependent on the depththicknessÂ ratio hc / tw of the steel beam, where tw is the web thickness and, for websÂ of rolled or formed sections, hc is twice the distance from the neutral axis to theÂ toe of the fillet at the compression flange, and for webs of built-up sections, hc isÂ twice the distance from the neutral axis to the nearest line of fasteners at theÂ compression flange or the inside face of a welded compression flange.