Cyclic Testing of Braces Laterally Restrained by Steel Studs to Enhance Performance during Earthquakes

Graduate Students: Oguz C. Celik, PhD, Jeffrey W. Berman

Principal Investigator: Michel Bruneau

Project Completion Date: 08-05-2003

Braces of steel frames are subjected to large axial displacements in tension and compression under severe earthquake excitations. Hysteretic behavior of brace members is complex, especially in the inelastic range. Previous studies have revealed that substantial amounts of cumulative energy can be dissipated in steel braces in the post buckling range when those members are subjected to reversed cyclic displacements. Many studies and codes suggest that stocky braces be used in seismically active regions. In fact, following that thinking, buckling restrained brace technologies have been developed and implemented in the seismic design and retrofit of buildings.

This report investigates the effects of KL/r ratio, bracing configuration, and cross sectional type of braces on the hysteretic characteristics of concentrically braced frames with and without cold formed steel studs (CFSS) infills designed to laterally restrain braces and delay their buckling. Four specimens were designed and cyclically tested. Specimens have either single diagonal tube or solid bar braces with and without CFSS and U brackets, providing out-of-plane and in-plane buckling restraint. Behavioral characteristics of the specimens are quantified with an emphasis on hysteretic energy dissipation.

Experimental results show that, at the same ductility levels, the cumulative energy dissipation of braced frames can be significantly increased when CFSS members are used to laterally restrain the braces against buckling. However, when tubular cross sections are used for braces, local buckling led to a reduced fracture life compared to the case without CFSS members. On the other hand, CFSS members appear to be relatively more effective when solid bar braces having large slenderness (tension- only braces) are used, since the difference between dissipated energies with and without studs is substantial.

This research was supported in part by the Earthquake Engineering Research Centers Program of the National Science Foundation (NSF) under Award Number EEC-9701471 to the Multidisciplinary Center for Earthquake Engineering Research (MCEER).