Experimental and analytical study of the inelastic behavior of double angle bracing members under severe cyclic loading.

Abstract

It has recently been recognized that built-up bracing members designed by current practice are quite susceptible to premature failures when subjected to severe earthquake motions, thus, endangering the survival of braced structures during severe ground motions. Bracing members experience large excursions in cyclic tension and post-buckling range. During a cyclic deformation, plastic hinges form in a member at the ends and at mid length. The Ductility and energy dissipation capacity of a bracing member in post-buckling range mainly depend on the performance of these plastic hinges under reversed cyclic rotations. Current design practice does not consider this aspect of the behavior. Seventeen full-scale bracing members made of double angles in A36 steel with welded stitches and end gusset plates were tested under large cyclic deformations representative of severe earthquakes. Three types of section configuration were investigated which included conventional back-to-back, strengthened back-to-back, and boxed section configuration. The following four conclusions are made. First, early fracture due to local buckling is the common mode of failure of conventional back-to-back angles. Second, strengthening of conventional back-to-back angles by welding two inclined plates does not improve member response due to unsymmetric buckling. Third, boxed angles have the most effective section configuration in which, with continuous stitch plate and adequate stitch strength, section distortion can be virtually eliminated. Fourth, end fixity causes shift of the end plastic hinges into the member which significantly enhances the member response. An analytical criterion has been established to modify the overall strength of built-up struts to account for shear flexibility of these members. Furthermore, analytical equations have been derived to calculate the shear strength required between the components of built-up struts to ensure behavior identical to that of an integral section, at the time of first buckling as well as in post-buckling range which is of interest in seismic design. The equations are applicable to any built-up strut with general end conditions.