Cyclic loading of simple braced frames

Abstract

The purpose of this study is to obtain experimental information about the load-deflection behavior under monotonic and reversed lateral loading of simple one-bay, one-story steel frames with corner-to-corner bracing. Model frames with pinned and with rigidly welded joints are considered. Slowly applied lateral loads that carry the structures well into the inelastic range of deformation are included, but vertical loading is not considered. The primary test variables are the stiffnesses and strengths of the bracing members and of the frames. It is found that diagonal members composed of cables with standard clamps cannot be counted upon to maintain their strength beyond the elastic limit but that mild steel diagonal members with larger effective areas at the ends than in the middle do provide useful strength and ductility in the inelastic range. For a pinned frame with diagonals of the latter type, the idealized elastoplastic "slip" model provides a good approximation to the load-deflection relationship of the structure, so long as strain-hardening effects are neglected and the compressive buckling loads of the diagonals are small compared to the tensile yield loads. For a rigid-jointed frame a more complicated idealized diagram made up of the "slip" model for the diagonals plus a conventional bilinear hysteretic model for the frame is found to provide somewhat less accuracy due to gradual yielding in the frame. It is found that in most practical cases the diagonals in a rigid-jointed structure of this type will yield at a lower deflection than the frame and will limit the deflection at which both components remain active. However, if the joints are welded, then local stresses in the welds may cause earlier fracture in the frame with cyclic loading.