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Figure 7.57. Composite beam, design example: (a) plan; (b) section.

Composite floor construction: in. of 115 pcf concrete slab over 3 in.

Compressive strength of concrete, f'c = 3.0 ksi

Loading.

1. Dead load of slab with allowance for steel beam = 50 lb/ft2

2. Additional precomposite dead load due to extra concrete required for compensating beam deflection = 5 lb/ft2 (ponding)

3. Additional composite dead loads: partitions = 20 lb/ft2 ceiling plus miscellaneous = 10 lb/ft2

Required. Verification of (W18 x 40) for final design. AISC Specifications (AISCS), 9th Edition.

Solution. The ASD design is based on elastic analysis using the transformed section properties for composite beams. Because the compression flange is continuously braced, the allowable stress in the steel section is 0.66Fy. Lateral-torsional buckling is not a concern for the completed structure, but it must be guarded against during construction.

The allowable stress in the concrete slab is 0.45f C. In building design, typically the neutral axis is close to the top of the section. Therefore, stress in the steel is usually the controlling factor.

The section properties for unshored construction are computed by elastic theory. The bending stress in the steel beam is taken as the sum of: (1) the stress based on the assumption that steel section alone resists all loads applied prior to concrete reaching 75% of its specified strength; and (2) the stress based on the assumption that subsequent loads are resisted by the composite section.

For the example problem we have:

Uniform precomposite load = (50 + 5) x 10 = 550 plf

Uniform postcomposite load = (30 + 50) x 10 = 800 plf

Total = 1350 plf

Maximum moment =-= 270 kip-ft

8 x 1000

Required section modulus =-= 98.18 in.3

Modulus of Elasticity of Concrete Ec. For stress check, AISCS permits the use of normal-weight concrete properties even for lightweight concrete topping. For deflection calculations, however, the use of actual properties is required.

Composite Beam Properties. The effective flange width b, of the composite section, is the smaller of

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