Numerical Investigation of the Seismic Response of Square HSS Braces with Intentional Eccentricity

Abstract

Concentrically Braced Frames (CBFs) comprising Hollow Structural Section (HSS) bracing members possess high stiffness and are susceptible to premature local buckling at the plastic hinge region, leading to low-cycle fatigue induced fracture. Intentionally offsetting the axis of otherwise conventional steel braces with respect to the working points has been proposed to overcome these shortcomings. Braces with Intentional Eccentricity (BIEs), the novel type of brace proposed by researchers in Japan, are subject to bending moment in addition to axial force under seismic action and, as such, inherently possess lower axial stiffness than Conventional Concentric Braces (CCBs). Their pre- and post-yielding stiffness can be adjusted by varying the eccentricity, allowing for better control of the structure’s dynamic response to ground motion excitations. A single experimental study has been performed on BIEs, with results indicating that, in comparison with CCBs, local buckling and fracture occurred in BIEs at significantly higher drift ratios due to the strain demand being more evenly distributed along the brace length. A numerical investigation has been undertaken to verify the generalisation of this behaviour to square HSS BIEs with different global and local slenderness ratios and to shed light on the range of imposed axial deformation these braces are able to sustain safely, as function of the eccentricity and their global and local slenderness. The investigation consists of a parametric study of finite element models of BIEs considering global slenderness ratios (𝐿/𝑟) ranging from 50 to 200, local slenderness ratios (𝑏/𝑡) between 4 and 36 and eccentricity ratios (𝑒/𝐻) from 0 (i.e. that of a CCB) to 2. Based on cost-effectiveness from a constructive point of view, and since the study is planned to be continued by the physical testing of full-scale BIE specimens, the BIE models were designed considering that the eccentricity is introduced by an assembly consisting of welded side plates linking the bracing members to the end connections. It was observed that the introduction of the eccentricity does delay, in terms of axial displacement, or imposed drift ratio, the onset of local buckling and thus, presumably, the fracturing of the brace. The article presents the numerical study and discusses the results and their implications for the design of Frames with Intentionally Eccentric Braces (FIEBs). An equation for predicting the deformation capacity of BIEs is proposed. Recommendations regarding the equivalent damping properties of BIEs for their use in the context of a Displacement-Based Design approach are also provided.