Improved Noncompact Web-Slenderness Limit for Steel I-Girders

Abstract

The noncompact web-slenderness limit in current design specifications is based on an assumed value of the plate buckling coefficient for a web subjected to flexure that is between the values for fixed and simply-supported edge conditions. While the limit provided in current specifications is a good representation in many situations, there is evidence that the limit overestimates the magnitude of the restraint provided by smaller flanges. This paper shows that the ratio of the area of the compression flange to the area of the web in compression is a good indicator of the restraint provided to the bend-buckling of the web plate. The overestimation of the plate buckling coefficient can result in the classification of cross sections having relatively small flanges as noncompact web sections when their behavior is more akin to that of slender web sections. Slender web sections have a maximum possible flexural resistance, referred to as the plateau strength, less than the yield moment Myc, because of load shedding from the web. Furthermore, the St. Venant torsional stiffness contribution to the lateral torsional buckling resistance is neglected for these sections due to web distortional flexibility. This paper evaluates the appropriate noncompact web-slenderness limit via test simulation of a targeted set of short girder specimens in which the calculated resistance is governed by the plateau strength and proposes a modified equation to rectify the problem. The impact of this change on the prediction of a broad range of member limit states is illustrated via comparison to other test simulations and to experimental results.