Effect of Bottom Flange Bracing on Crane Runway Girders - A Parametric Study on Ultimate Capacity Using Finite Element Analysis

Abstract

Crane runway girders in heavy industry carry large vertical and horizontal loads, placing significant demands on structural stability. While bracing of the top flange to prevent lateral-torsional buckling is well established, guidance on bottom flange bracing is inconsistent: the American Institute of Steel Construction (AISC) has reduced its previous requirement to a recommendation and Eurocode provides no explicit guidance on the matter. This study investigates the effect of bottom flange bracing on crane runway girders through a parametric finite element study in Abaqus, comparing the ultimate load capacity, quantified by the load proportionality factor (LPF), and lateral displacement of the bottom flange between girders braced at both flanges and girders with top flange bracing only. A range of web heights, bottom flange widths, and geometric imperfection amplitudes were considered. The model was verified against analytical hand calculations based on Eurocode, and the results were compared to AISC provisions for web sidesway buckling. The results show no significant capacity gain between the configurations, with a max imum ∆LPF gain of approximately 5-6%. The largest improvements are observed for intermediate web heights, and bottom flange widths, where bottom flange re straint allows for a more even stress distribution over the web. For larger geometric imperfection amplitudes, bracing the bottom flange can reduce capacity due to a self-straightening behavior that restraint prevents. The AISC comparison shows a consistent pattern, where narrower bottom flanges and lower web heights are more susceptible. Based on these findings, no strong case is found for introducing manda tory bottom flange bracing requirements in Eurocode for the geometries and load cases investigated. Rather than mandatory bracing requirements, a more appropri ate approach may be to control the bottom flange behavior through serviceability deflection limits, though further investigation is needed into how the lateral deflec tion of the bottom flange can be analytically determined, as this is dependent on cross sectional geometry.