A major cause of bridge failure during earthquakes is the excessive movement of the bridge superstructure in the longitudinal direction. Excessive movement can result in damage to the bridge diaphragms and supports or unseating and subsequent collapse of the bridge deck. A significant number of bridges damaged during the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake were due to excessive movement at the hinges. To limit the relative hinge displacement, steel restrainers in the form of rods or bundled wires are used at intermediate hinges. However, due to their high plasticity and since they are only engaged in tension, steel restrainers have shown several deficiencies in recent earthquakes. On the other hand, shape memory alloys (SMAs) possess unique mechanical properties such as superelasticity effect which provides SMAs with high recentering capability. Superelastic SMAs are also characterized by a nonlinear stress-strain hysteresis which provides constraints on the forces transmitted to the connected members. Despite of all of the above characteristics many questions still need to be addressed regarding the behavior of the SMAs in bridges such as (1) How would the SMAs behave under cyclic loading, (2) Is the performance of SMAs in bridges sensitive to their hysteretic properties (3) How would the SMA devices behave at various temperatures, and (4) How would the SMA devices compare to other devices.

The outcomes of this research showed an agreement in the results of the simplified SMA models and the more complex SMA models. Considering the sublooping effects in the SMA models affected the maximum structural response by less than 9% in average. The hinge opening is not sensitive to the SMAs hysteretic height, while the slope of the SMAs hysteresis has a slight effect in the range of 5% to 10% on the hinge opening. It was observed that SMA devices are more effective at higher ambient temperatures. The effect of temperature changes was more pronounced at temperatures near the austenite finish temperature and in the case of bridges with large ductility and/or moderate-to-high period ratios. Comparing the SMA devices with other retrofit devices showed that in all cases, SMA devices are more effective compared to the steel restrainers. However, SMAs are more effective compared to metallic dampers in the case of bridges with low-to-moderate period ratios. Finally, SMAs were more effective than the viscoelastic dampers in the case of bridges with large ductility and higher period ratios.

 

Selected Publications:

- Andrawes B and DesRoches R. Unseating prevention for multiple frame bridges using superelastic devices. Smart Materials and Structures 2005; 14 :S60-S67

- Andrawes B and DesRoches R. Effect of hysteretic properties of superelastic shape memory alloys on the seismic performance of structures. Submitted to the Journal of Structural Control and Health monitoring (2005).