This analytical work focuses on enhancing the ductility capacity and damage mitigation of reinforced concrete bridge
columns during earthquakes by using innovative active confinement technique. The high recovery stress associated with
the shape recovery of shape memory alloys (SMAs) is exploited to apply the confining pressure. A 2-D analytical model
for a single column is developed and analyzed. The model is used to evaluate the seismic behavior of the column
retrofitted with SMA rings and compare it with the behavior of the column retrofitted with the more conventional
approach using carbon fiber reinforced polymer (CFRP) sheets. The stress-strain behavior of the concrete confined with
internal ties only, internal ties and external SMA rings, and internal ties and external CFRP sheets is described based on
two different constitutive models. The column model is subjected to cyclic loading with increasing amplitude and a
ground motion excitation. The analysis shows that the SMA rings provide the column with more damage protection
represented by a reduction in the maximum strain by up to 273% compared to CFRP sheets. In addition, the column
retrofitted with SMA rings shows smaller lateral drifts compared to the column retrofitted with the CFRP sheets when
subjected to the same ground motion excitation. The superior performance of the SMA rings is primarily attributed to the
increase in the compressive strength at early stages of loading associated with applying the active confinement pressure.
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 Af 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 Af 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.
The cyclic behavior of shape memory alloys (SMAs) in their austenitic form is studied to determine the most appropriate method of modeling in terms of both accuracy and ease of implementation. Four different models for SMA behavior are evaluated: (a) a simple nonlinear elastic model, (b) a trigger-line model, (c) a one-dimensional thermomechanical model, and (d) a one-dimensional thermomechanical model which accounts for the behavior of SMAs under cyclic loading. Using a two degree-of-freedom bridge model with SMA restrainers and a single degree-of-freedom building model with SMA cross-braces, the effect of using the different models on the seismic response of the bridge and building is evaluated. Using a suite of nine earthquake ground motions, the displacement response histories with the four different models are compared. The results illustrate that although the models show quite different behaviors for the SMAs, the resulting responses of the bridge and building are insensitive to the type of model used. For most of the ground motion records used, the difference in the maximum displacement for the four models was less than 15%. This study lends support to the use of more simplified models when evaluating the effectiveness of the SMAs for seismic response modification.