Multiobjective sizing optimization of seismic isolated reinforced concrete structures

Numa Leger (SIGMA Clermont), Luca Rizzian, Mariapia Marchi (ESTECO)

CHALLENGE - 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.

SOLUTION - In this work we performed a multiobjective sizing optimization of two simple buildings under seismic actions and with elastomeric isolators at the base. While the superstructure and the isolation system are generally designed separately, here we optimized at the same time the structural elements (superstructure column and beam sections and reinforcements) and the isolator parameters (rubber type, maximum allowed displacement and elastomer size). We considered three objectives: minimization of the superstructure material cost, minimization of the top-floor acceleration and minimization of the top-floor displacement.​ We performed optimizations with the genetic algorithm MOGA-II​. We considered an initial population of 100 candidate designs and a directional crossover and mutation probabilities of 90% and 10% respectively. To achieve a reasonable convergence of results, 160 generations were executed. All simulations were performed with the multi disciplinary optimization platform modeFRONTIER​ integrated with SAP2000 by means of an Excel file​.

BENEFITS - Once the optimal solutions are found, a final design should be chosen based on decision maker’s preferences, like e.g. smaller initial costs (for the superstructure and/or isolating devices, in the BI case), or smaller actions and costs expected for maintaining the structure during its life cycle and/or after seismic events. Several post-processing decision making and data analysis tools are available in modeFRONTIER.​ Here we present a clustering analysis​. Fig. 4 presents the optimal solutions found for the BI structures [A] (top panels) and [B] (bottom panels) classified with a single-linkage clustering on the isolator parameter values. Different color shades correspond to different clusters.​