OPTIMAL DESIGN OF ISOLATED BRIDGES AND ISOLATION SYSTEMS FOR EXISTING BRIDGES

Designing new isolated bridges, the selection of optimal characteristics for strength, stiffness and displacement capacity for both piers and isolators is not at all obvious. When designing an isolation system for an existing bridge, the variable parameters are less, but it should be assumed as a principle that all piers have to be isolated. A study of this subject has been recently presented by the same authors where an optimization procedure was outlined, even if it was more an idea to be developed and verified than a reliable design tool. Since then, the idea has become a proposal for a design method, and the method has been implemented in an efficient software program and applied to a large number of cases which confirmed the soundness of the principles adopted in the design philosophy. The design approach assumes that a displacement profile predicted using a linear equivalent model, will be reproduced by the envelope of the maximum displacement obtained from a series of non-linear analyses, considered to be representative of the real response. A displacement reduction factor is proposed in analogy with the force reduction factor commonly used in force-based approaches, and a response predicted by the linear equivalent model. The response index is then used as the main parameter to be optimized, to design an appropriate isolation system, keeping the appropriate boundary conditions to assure a capacity protection of the foundation-pier systems. The procedure has been applied to several cases, comparing the results with those obtained from non-linear analysis, showing that the design approach seems to be efficient and reliable.