Effect of flange and stiffener rigidity on the boundary conditions and shear buckling stress of plate girders

The two essential functions of the web plate in a plate girder are to maintain a relative distance between the top and bottom flanges and to resist shear stresses. In most practical ranges of plate girder bridge spans, the shear stresses are relatively low compared to bending stresses in the flanges induced by flexure. As a result, the web plate is typically much thinner than the flanges. The web panel is therefore prone to buckling at comparatively low shear forces. To enhance the web’s buckling strength, it is often reinforced with transverse stiffeners. The web design then involves a selection of plate thickness and stiffener spacing to provide optimum economy in terms of the material and fabrication costs.

In the design of plate girder web panels, according to AISC, AASHTO and EN 1993-1-5 specifications, the post-buckling strength is added to the elastic buckling strength. To calculate the elastic buckling strength, the boundary conditions of the web panel that is stiffened by transverse intermediate stiffeners must be determined.

In current practice, the elastic shear buckling stress of a web panel with transverse stiffeners is estimated using the conservative assumption that the web panel has simply supported boundary conditions at the junctions with the flange and the stiffener. The study presented herein explores this assumption using a more realistic approach, both numerically, through approximately 5000 numerical analysis runs, and experimentally. The results indicate that more realistic boundary conditions and critical buckling shear stress estimation could be obtained by accounting for the rigidity of the flanges and the stiffeners, in addition to initial web imperfections.