Effect of ambient temperature on the performance of shape memory alloy seismic devices

Shape memory alloys (SMAs) are a class of metallic alloys that exhibit unique characteristics such as shape memory effect and superelasticity effect. SMAs are found in two main phases: the high temperature phase, which is known as austenite (superelastic), and the low temperature phase, which is known as martensite. Although there are few civil engineering applications using SMAs, there have been considerably large numbers of research studies focusing on exploiting SMAs in seismic resistant design and retrofit of buildings and bridges. Most of these studies focus on utilizing the superelasticity phenomenon exhibited by SMAs at high temperatures. The effect of ambient temperature variation on the efficacy of superelastic SMA devices that are used in seismic applications is a major concern. This paper presents an analytical investigation on the effect of ambient temperature variation on the performance of superelastic SMA bridge restrainers during earthquakes. A thermomechanical constitutive model is developed to describe the constitutive behavior of the SMA restrainers at various temperatures. The SMA model is implemented in a 2-DOF bridge model and tested using 20 historical ground motion records. The ambient temperature is varied from a temperature below A_f to a relatively high temperature. The results of the study showed that SMAs are more effective when used in its austenitic phase and thus when the temperature decreases below A_f SMA devices lose a major part of their efficiency. On the other hand, the study also showed that at high temperatures the ductility demand of the bridge frames increases.