Effective flange width of composite girders in negative moment region

In the analysis and design of steel-concrete composite bridges, stresses and displacements are typically computed on the basis of elementary beam theory by using the effective flange width concept. Currently, the AASHTO load and resistance factor design (LRFD) code specifies the same effective flange width design criteria for both positive moment sections and negative moment sections. The effective flange width concept for the positive moment has been well established by many researchers. However, the classical effective flange width definition does not take into account the strain variation through the slab thickness or the stress transfer mechanism from concrete to steel reinforcements after cracking. A more appropriate effective flange width definition for the negative moment section is introduced to account for these factors. This definition was developed on the basis of the effective flange width definition for positive moment sections proposed previously. The proposed definition for the negative moment section is explored by using finite element analyses. The finite element modeling is briefly discussed, and the model is successfully verified by comparison of the results obtained through the model with the experimental results. A parametric study was performed to evaluate the effective flange width for the negative moment section. The force transfer mechanism between concrete and reinforcements is addressed quantitatively. The finite element model-extracted effective flange widths demonstrate that full width can be used as the effective flange width in the design and analyses of the negative moment region in most cases. Numerical results also show that the effective flange width criterion in the current AASHTO LRFD specification is typically conservative, especially for bridges with wide girder spacings. The recommended revised criterion is the use of the full width.