AISC LRFD 1.pdf

214Comm. ICHAPTER ICOMPOSITE MEMBERSI1. DESIGN ASSUMPTIONS AND DEFINITIONSForce Determination. Loads applied to an unshored beam before the concrete hashardened are resisted by the steel section alone, and only loads applied after theconcrete has hardened are considered as resisted by the composite section. It is usuallyassumed for design purposes that concrete has hardened when it attains 75 percentof its design strength. In beams properly shored during construction, all loadsmay be assumed as resisted by the composite cross section. Loads applied to a continuouscomposite beam with shear connectors throughout its length, after the slabis cracked in the negative moment region, are resisted in that region by the steel sectionand by properly anchored longitudinal slab reinforcement.For purposes of plastic analysis all loads are considered resisted by the compositecross section, since a fully plastic strength is reached only after considerable yieldingat the locations of plastic hinges.Elastic Analysis. The use of constant stiffness in elastic analyses of continuousbeams is analogous to the practice in reinforced concrete design. The stiffness calculatedusing a weighted average of moments of inertia in the positive momentregion and negative moment regions may take the following form:It = aI pos + bInegwhereI pos = effective moment of inertia for positive moment, in. 4 (mm 4 )I neg = effective moment of inertia for negative moment, in. 4 (mm 4 )The effective moment of inertia shall be based on the cracked transformed sectionconsidering degree of composite actions. For continuous beams subjected to gravityloads only, the value of a may be taken as 0.6 and the value of b may be taken as0.4. For the case of composite beams in moment resisting frames, the value of a andb may be taken as 0.5.Plastic Analysis. For composite beams with shear connectors, plastic analysis maybe used only when the steel section in the positive moment region has a compactweb, i.e., h/ tw≤ 376 . E/Fyf, and when the steel section in the negative momentregion is compact, as required for steel beams alone. No compactness limitationsare placed on encased beams, but plastic analysis is permitted only if the direct contributionof concrete to the strength of sections is neglected; the concrete is reliedupon only to prevent buckling.Plastic Stress Distribution for Positive Moment. Plastic stress distributions areLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION