Stiffness and strength of shear diaphragms used for stability bracing of slender beams

Abstract

Light gage metal decking is often used in structures as concrete deck formwork, roof cladding or siding. In the steel building and bridge industries, decking acts like a shear diaphragm and provides continuous lateral bracing to the top flange of non-composite beams and girders that they are attached to. The building industry has long relied on the in-plane stiffness and strength of metal decking to brace steel beams during construction. Although the current AASHTO LRFD specifications do not allow bridge deck forms to be relied upon as a bracing source for steel bridge I-girders, recent studies have demonstrated that deck forms can significantly increase the buckling capacity of bridge girders by providing a relatively simple modification to the connection. Shear diaphragm bracing of steel I-beams have been studied in the past. These studies mainly focused on beams with stocky webs. The purpose of the study outlined in this paper is to enhance the understanding of both the stiffness and strength of shear diaphragms used to brace slender steel I-beams. The parameters that are investigated include diaphragm stiffness, sheet thickness, number of side-lap fasteners, flange width, and web slenderness ratio. Beams with web slenderness ratios of 100 to 160 and span/depth ratios of 10, 15, and 20 are considered. A simple finite element analytical (FEA) model is utilized in the study. The results indicate that web slenderness ratio does not have a major effect on fastener forces and the strength behavior of shear diaphragms is dependent on the number of side-lap fasteners. The findings of the study will be used to develop strength and stiffness requirements for shear diaphragms used to brace slender steel beams.