ON THE DEFORMATION CAPACITY OF CONCENTRICALLY STEEL BRACES

In this paper, the current provisions for seismic design of concentric steel braces are reviewed. Due to these provisions, the overall deformation capacity predominantly depends on the overall slenderness ratio representing the overall buckling. However, in many cases this may not hold true as the local buckling becomes the predominant cause of rupture. Finite Element (FE) analysis is carried out to investigate the behaviour of steel braces subjected to cyclic loadings. These models proved to be capable of predicting the formation of plastic hinge, and the occurrence of overall and local buckling at the mid-span with reasonable accuracy. Measurement of the strain at the mid-span plastic hinge demonstrates a sharp rise after the occurrence of local buckling. This severe growth of strain results in the rupture failure at the mid-span plastic hinge, as confirmed by the general observations in experiments. As a result, the local buckling is identified and introduced as the main limit state that can be used as an index for evaluation of the overall deformation capacity of braces. The difference between strains at outer and inner layers of the shell elements located at the mid-span is introduced as an index for detecting the occurrence of local buckling. This index could be employed to quantify the overall axial deformation that produces local buckling. INTRODUCTION Concentrically steel braced frames (CBF) are a common type of lateral load resisting systems. Tubular members, pipes and hollow structural shapes (HSS), have been frequently used as CBFs. Seismic performance of tubular braces greatly depends on slenderness ratio, and width-to-thickness ratio. Usage of braces with lower slenderness ratio might postpone overall buckling, and obtain higher compression strength. However, such braces exhibited less ductile fracture behaviour in experiments (Tremblay 2002). Overall compression loading triggers two inevitable geometric changes entitled as overall and local buckling. The overall buckling causes a reduction in load carrying capacity, although the member can still experience further inelastic deformations. On the contrary, there is no reduction in load carrying capacity to be identified as occurrence of local buckling, and nevertheless, the local buckling deteriorates deformation capacity. Thus, the local buckling is not detectable compared to the overall buckling. The occurrence of local buckling depends on the compactness of braces, and more compacted members can experience further inelastic deformations (Elchalakani et al. 2003; Fell 2008). Experimental investigations showed that cyclic loading forms a plastic hinge at the mid-span, and another one at each end of the brace (Haddad 2004). Formation of plastic hinge at the mid-span is accompanied by concentration of stress and strain (Han et al. 2007). Strain concentration sharply rises after the occurrence of local buckling resulted in a rupture failure at the mid-span (Haddad 2004). In 1 PhD student, Sharif University of Technology, Tehran, Iran, sedehi-omid@outlook.com 2 Professor, Sharif University of Technology, Tehran, Iran, hsnmoghaddam@yahoo.com