This study investigates the seismic performance of a newly developed self-centering bracing system using a novel experimental technique named as closed-loop dynamic (CLD) testing. The bracing, named piston-based selfcentering (PBSC) apparatus, employs Ni-Ti superelastic shape memory alloy (SMA) bars inside a sleeve-piston assembly for its self-centering mechanism. During cyclic tension-compression loading, the SMA bars are only subjected to tension avoiding buckling and leading to flag-shaped symmetric force-deformation hysteresis. Initially, a braced frame building fitted with PBSC is seismically designed and the preliminary sizing of the brace is determined. For testing, considering the lab capability, the brace is fabricated at a reduced scale. The process of “Closed-loop dynamic testing” starts with the brace test (step 1) under strain-rate loading to characterize the numerical model parameters (step 2), which are then scaled-up as per similitude law and implemented in a finite element software, S-FRAME’s PBSC brace model (step 3). Then the braced frame building is analyzed under an earthquake (step 4) and the axial force-deformation response of the brace under consideration is captured (step 5). In order to further understand and validate the actual response of the brace under earthquake type loading, the axial deformation obtained from S-FRAME is scaled-down (step 6) and used as input parameters for testing the reduced scale brace (step 7). The obtained response (step 8) is further scaled-up and used to match the SFRAME’s PBSC model for validation (step 9). Iterations from step 3 to step 9 will be required until the experimental and numerical results converge. Convergence criteria used for this validation include both the energy dissipation capacity and initial stiffness within 10% accuracy. Reasonable agreement between the numerical and experimental results is achieved in the closed-loop dynamic testing. The PBSC brace shows excellent self-centering capability under various earthquake loadings.

Introduction

A recent study conducted by the Insurance Bureau of Canada estimated the overall loss after a 9.0-magnitude earthquake in British Columbia at almost $75 billion and a $61 billion loss after a 7.1-magnitude earthquake in the Quebec City-Montreal-Ottawa corridor [15], which clearly reflects the vulnerability of Canadian civil infrastructure. To avoid such scenarios in Canada, it is imperative to take immediate measures. Since seismic load, in the form of ground shaking, generates one of the most devastating forces that our infrastructure can experience, designing structures against these large forces are often uneconomic. In various building and infrastructures, different structural elements and systems resist and dissipate earthquake-induced energy by means of deformations. Once permanent deformations take place, a structure becomes difficult to fix. After a major earthquake, these structures may have to be demolished and re-built acquiring huge economic losses. For example, in the Maule (Chile) Earthquake in 2010, the economic losses were estimated to be $30 billion (loss of infrastructure alone was $20.9 billion) which is equivalent to 17% of the GDP of Chile [10]. In the Christchurch (New Zealand) Earthquake in 2011, about $20 billion economic losses (equivalent to 13% of New Zealand’s GDP) were estimated. The destruction was enormous, including demolition of around 70% of downtown buildings, loss of more than 50% of heritage structures, closure of the major business district for over 18 months, and outmigration of thousands of residents [11]. Such seismically damaged infrastructures become a major economic obligation.