Steel Girder Stability during Bridge Erection: AASHTO LRFD Check on $L/b$ Ratios

The erection of steel plate girders during the construction process of a steel bridge is a complex operation, which is often left to the contractor and/or the subcontractor to plan and execute. Rules of thumb have been developed through experience to check the lateral torsional buckling of the steel girder during erection using the maximum $L/b$ (unbraced length/compressive flange width) ratio, below which no lateral torsional buckling would occur. Although the $L/b$ ratio check has proven to be useful and convenient on-site, it is necessary to provide a more rational basis for the rules of thumb, and find the maximum $L/b$ ratios by checking the lateral torsional buckling failure of girders under erection according to the latest AASHTO LRFD code. A series of parametric studies were conducted on cantilever and simply supported girders under self-weight as well as self-weight plus wind load, in order to: (1) check the rules of thumb on $L/b$ ratios and (2) determine the effects of girder flange width, flange thickness, web depth, web thickness, and yield strength on the maximum $L/b$ ratio and girder stability during erection. From the results, rules of thumb were modified for girders with common shapes, and it was obvious that (1) self-weight plus wind load controls the girder stability during erection in most cases and (2) flange width and web depth have the most effects on the maximum $L/b$ ratio and girder stability during erection.