Experimental and Analytical Investigations of Doubly Symmetric Built-Up I-Girders with Large Moment Gradient Factors

Recent computational studies have shown that the current AISC Design Specifications (2016) overpredict the flexural resistance of certain built-up I-girders. The largest reductions were observed in I-girders subjected to reverse curvature bending that have unstiffened webs, a large web slenderness ratio, and a high moment gradient factor resulting in a high shear-to-moment ratio. To further investigate these reductions, this study evaluated six experimental specimens and a suite of complimentary parametric finite element analysis (FEA) simulations. The experimental specimens include three single curvature and three reverse curvature experiments of steel I-girder members, where each loading configuration explored a range of web slenderness ratios. Parametric FEA simulations extended the laboratory specimens through a range of unbraced lengths using nominal specimen dimensions, material properties, geometric imperfections, and residual stress patterns. Results from the study demonstrate strength reductions for built-up steel I-girders meeting the criteria specified above. The largest strength reduction was observed in the specimen with the largest web slenderness ratio subjected to reverse curvature bending (Mtest / MDesign = 0.68), and the smallest strength reduction was observed in the specimen with the smaller web slenderness ratio that was subjected to single curvature bending (Mtest / MDesign = 0.96). The largest strength reductions for all the specimens generally occurred in the inelastic lateral torsional buckling (LTB) region, and the observed strengths converge to the elastic LTB design resistance at larger LTB slenderness values. The results suggest that the overprediction in strength can be attributed to the direct scaling of the LTB resistance by the moment gradient factor and the LTB mode of failure being prematurely induced due to web distortion effects.M.S