Cyclic Performance of Composite Bracing Members

In seismic regions, steel tubular bracing members have widely been used in steel structures. Although there are some shortages with such systems, due to their economical advantages, they are being used in many structures. The steel hollow brace members exhibit stable hysteretic behavior up to the point of first local buckling, and subsequently they exhibit significant degradation in strength and ductility. Comparing composite braces with steel braces, the presence of concrete infill has shown to influence the possible mode of failure in the specimens. It is also found that the infill supplies the extra compression resistance of the brace after several load reversals; consequently improves ductility capacity of the system. In this research, using the finite element method, the seismic behavior of the composite bracing system with and/or without the infill is simulated. Analytical models with the capability of displaying the cyclic behavior are utilized for concrete filled tubular bracing members. The analysis has been implemented numerically by employing the general purpose finite element software. Overall, it is shown that the difference in improvement gained by concrete infilling was attributed to pertinent variables such as the width to thickness ratio, shape of the section and concrete compressive strength, yield stress of the steel tube and the axial load level on the stiffness, strength and ductility of concrete-filled steel tube braces. In addition, this paper provides information on key response parameters, including tensile and compressive strength and post-buckling capacity, as well as, ductility and energy dissipation capabilities. Particular attention is given to the influence of member slenderness. The analytical findings are compared with the recommendations of a number of international codes of practice; and areas of agreement or discrepancy are highlighted. Finally, special considerations for enhancing the seismic performance goals are discussed.