The normal stress distribution σ(x) in the concrete slab of a composite beam does not have a constant value but varies where the maximum flexural normal stress occurs at the junction point of the slab and steel girder web, as illustrated in Figure 7.2. This phenomenon is caused by the lag of shear strain at the top of the concrete slab and is referred to as shear lag effect. Effective width of a cross section at a given location depends on the structural layout and loads. For design purposes, it is convenient to define the effective width for the concrete slab. The effective (be) and transformed widths (btr) are illustrated in Figure 7.2. Results of a recent study as shown in NCHRP Report (Chen et al. 2005),
which was later adopted by AASHTO load and resistance factor design (LRFD) specifications (2013), recommend that the full slab width halfway between adjacent girders can be counted as the effective width for the concrete slab. Calculation of the effective widths and their respective section properties will be shown in the example of Section 7.3. For each of the loading stages described earlier in this section, a distinct set of section properties exists and must be used in their respective analyses to properly ascertain design forces and deflections to evaluate strength and serviceability criteria.