SUPERB

Abstract

Nowadays, it is generally accepted that, for a modern transportation system to be reliable, the design process must ensure an acceptable earthquake risk for the bridge infrastructures. In the case of existing structures, unacceptable seismic safety conditions must be clearly identified and promptly corrected. Past earthquakes have demonstrated that the damage induced in bridges can assume a multitude of different forms, depending, among others, on factors like the ground motion itself, conditions depending on the building site, the adopted bridge structural solution and its specific detailing provisions.

Unseating of the bridge superstructure at in-span hinges, or at simple supports, is one of the most severe forms of seismic damage, leading to eventual collapse. This type of failure is either due to shaking or to differential support movement associated with ground motion. The problem of unseating is generally associated with inadequate seat lengths or restraint and it is worsened by skewed, curved, or complex bridge configurations.

In order to reduce the seismic response of bridge structures, they can be provided with special restraining devices called seismic links. According to EC8, these connection devices may be responsible for the partial or full transmission of the design seismic action, provided that dynamic shock effects are mitigated and taken into account in the design. They are designed to ensure the structural integrity of the bridge and avoid unseating under extreme seismic displacements, while allowing the non-seismic displacements of the bridge to develop without transmitting significant loads.

While the new design strategies contemplated in EC8 aim to mitigate potential unseating problems in new bridges, there are still many existing bridges susceptible to span unseating, due either to the lack of adequate seismic detailing, like the shorter seats usually associated with old constructions, either to potential stronger shaking than the one considered in the original design. These structures require seismic retrofitting and several solutions are currently used, namely steel restrainer cables, metallic dampers and seat extenders. In order to overcome some of the limitations presented by these devices, and taking advantage of recent advances in
Material Science, the use of new materials has been proposed in the literature. Among them, the shape memory alloys (SMAs), a classof metallic alloys exhibiting two important properties: the shape-memory effect, which allows the material to recover its original geometry during heating, even after severe deformation, and the superelasticity, which enables the material to withstand large cyclic deformations, without residual strains, while dissipating energy.

The main objective of this project is to study a seismic retrofitting solution for existing bridges in Portugal, using seismic links built up of superelastic NiTi SMA restrainer cables to reduce the deck’s response and therefore the risk of span unseating during earthquakes. Several activities are planned to be executed during the project. First, taking advantage of the information stored in the EP's archives, a database will be created and populated with relevant data regarding the dynamic characteristics of old and non-ductile bridges existing in Portugal. This database will support the definition of several typical old bridge structural models that, after being equipped
with superelastic NiTi SMA restrainers, will be analysed numerically under seismic excitation to asses the effectiveness of this type of retrofitting. Finally, taking advantage of the exquisite facilities available at LNEC, a large scale experimental test programme will be implemented in order to confirm the feasibility of the application of SMAs on bridge structures retrofitting. To establish the supporting conditions necessary to guarantee the accomplishment of all project goals, the research team is composed by specialists with large expertise in seismic analysis of structures (IST, LNEC, FCT), bridges maintenance, retrofitting and rehabilitation (EP), numerical modelling of complex SMA constitutive models (FCT). Professor Reginald DesRoches from the Georgia Institute of Technology, well known researcher in Earthquake Engineering, design and analysis of bridge structures and structural applications of smart materials and Professor António Reis from IST, one of the most conceited bridges specialists in Portugal, will contribute with their knowledge by acting as external consultants.

The expertise gained during the project will help in training young post-graduate researchers and ensure the longevity of the achievements of this research, and also constitute a pool of knowledge for all involved partners and the bridge community at large.