ESTIMATION OF AGGRAVATING EFFECTS DUE TO THE COINCIDENCE BETWEEN SOIL AND STRUCTURE: NUMERICAL STUDY FOR MDOF SYSTEMS

Abstract eng:
Post-seismic investigations repeatedly indicate that structures having frequencies close to foundation soil frequencies exhibit significantly heavier damages (Caracas 1967, Mexico 1985, Pujili, Ecuador 1996; L’Aquila 2009). However, these observations are generally not taken directly into account, neither in present-day seismic regulations (small scale), nor at large-scale seismic risk analysis. A comprehensive numerical analysis to investigate the effect of coincidence between soil and building frequencies is performed. A total of 887 realistic soil profiles are coupled with a set of 141 SDOF elastoplastic oscillators, and their combined (non-linear) response are computed both for linear and non-linear soil behavior, for a large number (60) of input signals of various PGA levels and frequency contents. The associated damage is quantified on the basis of the maximum displacement as compared to both yield and ultimate post-elastic displacements, according to the RISK-UE project recommendations (Lagomarsino and Giovinazzi, 2006), and compared with the damage obtained in the case of a similar building located on rock. The correlation between this soil/rock damage increment and a number of simplified mechanical and loading parameters is then analyzed using a neural network approach. The results emphasize the key role played by the building/soil frequency ratio in the damage estimation; other important parameters are the PGA level, the soil/rock impedance contrast and the building ductility. Multi degree-of-freedom (MDOF) elastoplastic oscillators are then investigated. One of the main outcomes is that even if the existence of harmonic frequencies for MDOF somewhat modifies the results, particularly in the linear range (low PGA), the concept of spectral coincidence remains the key contribution in rock to soil damage increase.

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