Effective width of steel–concrete composite beam at ultimate strength state

In a steel–concrete composite beam section, part of the concrete slab acts as the flange of the girder in resisting the longitudinal compression. The well-known shear-lag effect causes a non-uniform stress distribution across the width of the slab and the concept of effective width is usually introduced in the practical design to avoid a direct analytical evaluation of this phenomenon. In the existing studies most researchers have adopted the same definition of effective width which might induce inaccurate bending resistance of composite beam to sagging moments. In this paper, a new definition of effective width is presented for ultimate analysis of composite beam under sagging moments. Through an experimental study and finite element modeling, the distribution of longitudinal strain and stress across the concrete slab are examined and are expressed with some simplified formulae. Based on these simplified formulae and some assumptions commonly used, the effective width of the concrete slab and the depth of the compressive stress block of composite beams with varying parameters under sagging moments are analytically derived at the ultimate strength limit. It is found that the effective width at the ultimate strength is larger than that at the serviceability stage and simplified design formulae are correspondingly suggested for the ultimate strength design.