Unseating prevention for bridges using superelastic elements

Unseating of bridge spans due to excessive relative hinge opening is a common problem for bridges subjected to strong ground motion. Various unseating prevention devices have been developed in both the United States and Japan to try to reduce the likelihood of collapse due to unseating. This paper presents the results of the evaluation of unseating prevention devices using Nitinol Shape Memory Alloy materials. Superelastic shape memory alloys have the ability to remain elastic under very large deformations, due to a solid-state martensitic transformation. This unique property leads to enhanced performance of the adaptive superelastic unseating prevention device, compared with conventional devices used in the United States and Japan. To assess the effectiveness of the devices, nonlinear time history analyses are performed on two typical bridges in the United States and Japan. A multi-frame reinforced concrete box girder bridge is analyzed using a suite of representative ground motions. The results show that for both bridge types the adaptive superelastic devices are very effective in limiting the relative hinge displacement and preventing unseating.