Behavior of double-channel bracing members under large cyclic deformations

This study is concerned with seismic behavior of double channel toe-to-toe built-up bracing members subjected to large cyclic inelastic deformations simulating severe earthquake ground motions. Seventeen full size bracing members made of representative structural channel sections were tested under a prescribed cyclic axial deformation history. Either a steel plate or groove welds were used to stitch two channels together. Fifteen specimens had fixed end connections while the remaining two used single gusset plates at both ends simulating hinged ends. The effective slenderness ratio of the specimens varied between 50 and 86.;In some specimens an unusual phenomenon involving the movement of the center plastic hinge to a location next to a stitch from one end, was noticed for the first time. The responses and the hysteretic behavior of the specimens, and the effects of stitch spacing were studied with special attention paid to buckling strength and energy dissipation capacity of the specimens.;The test results showed that the use of current specifications for the design of this type of built-up section was inadequate for seismic conditions. The stitch spacing in such members was found to be a dominant factor in the performance of this type of bracing members with stitch design in accordance with current practice leading to early failure of the bracing members.;Based on the experimental observations and theoretical analysis, design criteria are suggested to ensure maximum ductility and energy dissipation capacity of built-up double channel bracing members. The suggested design criteria for stitch spacing were used in some specimens and their test results verified the expected performance. A modified method for calculation of the compressive strength of built-up bracing members is presented.;Recommendations are also made in regard to the seismic design of such members, including several design details to improve their seismic performance. Design details found to be more desirable are: end connections producing fixed end conditions, and use of continuous stitch plate with cut-off at quarter points of member length.