Out-of-Flatness Effect on Flexural Strength of Steel Bridge Girders

This numerical study determines the effects of out-of-flatness on flexural strength at the onset of yielding in continuous I-shaped and tub (box) steel highway bridge girders. This moment at onset of yielding is the strength limit state for flexural design of steel highway bridge girders, according to the AASHTO standard. Finite element analysis is used to obtain values of flexural strength reduction for girders with various magnitudes of out-of-flatness, covering a range of continuous I-shaped and tub (box) cross sections and spans. Straight girders were used since the evaluated behavior is local buckling. Models are built with co-existing out-of-flatness imperfections in both webs and flanges. The imperfection pattern is set to be compatible with the first buckling mode of the built-up cross section to match the case having the theoretical maximum effect on local buckling. ANSYS heat analysis was used to create appropriate residual stress pattern in the models. Models are laterally supported to ensure the local buckling limit state is the governing failure mode. Both Grade 50 steel and Grade 100 steel plate are considered with elastic-perfectly plastic material behavior. Large deflection theory is used to iteratively capture the secondary moments due to out-of-flatness. Maximum strength reduction implicitly allowed in accordance with the most restrictive value of out-of-flatness for continuous two-span unstiffened I-shaped plate girders by the American Welding Society D1.5 Bridge Welding Code is obtained. Strength-based out-of-flatness criteria for the bottom flange of steel tub girders are proposed as functions of bottom flange slenderness.