Multiobjective sizing optimization of seismic-isolated reinforced concrete structures

It is known that seismic isolation is able to protect structures from damage by reducing the earthquake effects on the superstructure rather than increasing the structural resistance. Base-isolated buildings are becoming more numerous all around the world, especially in areas subject to a high seismic hazard or where high safety levels are required. The cost of the isolation devices for ordinary buildings hinders a widespread adoption of the new technology. However, a well-designed base isolation system can largely reduce seismic loadings transferred to the superstructure and it not only enables to immediately reduce the superstructure building cost, but also to reduce the maintenance costs incurred after every earthquake during the building lifetime. To better understand these factors, this paper presents an efficient numerical optimization technique for comparing the responses of a base-isolated and a traditional fixed-base reinforced concrete ordinary building under the same type of solicitations and seismic spectra, as appropriate for each case. We start from a multiobjective optimization. The superstructure and the isolation system are generally designed separately in a building. In this work, we consider elastomeric isolators and we optimize at the same time the structural elements of the building (superstructure column and beam sections and reinforcements) and the isolator parameters (rubber type, maximum allowed displacement and elastomer size). We consider three objectives: minimization of the superstructure material cost, minimization of the top-floor acceleration and minimization of the top-floor displacement. This multiobjective optimization yields a set of trade-off optimal solutions (the so-called Pareto optimal designs) that can be post-processed with tools such as hierarchical clustering or decision-making algorithms and further analyzed. The purpose of this analysis is to identify similarities within data sets or choose a final optimal solution based on pre-defined priority criteria applied to the optimization objectives. We compare the base-isolated structure results with the solutions found for the same building with a traditional foundation fixed to the ground.