Effect of Soil-Structure-Interaction on Inelastic Displacement Ratios of Existing Structures

Abstract eng:
The computation of Inelastic Displacement Ratios (IDRs) in different structural types and site conditions has been attracted numerous studies in the past. Those studies usually were performed through a nonlinear time history analysis of a SDOF oscillator, which is assumed to be fixed at its base. There are multitude relations for calculating this ratio denoted by C R in cases with known values of strength reduction factor or Cμ in cases with known ductility ratios. However, all of them exclude Soil Structure Interaction (SSI) effects. In fact, it should be mentioned that although numerous investigations have been carried out to consider (SSI) in elastic structures, the aforementioned effect in inelastic structures is not enough clear yet. If the structure with nonlinear hysteretic behavior is located on a flexible foundation, the inelastic behavior of the structure will be completely different to from that of fixed at its base. To evaluate this, multiple levels of strength reduction factor or target ductility as well as diverse values of SSI indexes are considered to provide a comprehensive parametric study. It should be noted that the results could be implemented in Coefficient Method of FEMA-356, newly modified in FEMA-440 where the effect of SSI in such inelastic structure is under investigation. In fact, a different approach than to FEMA-440 is explained here that exerts SSI effects on IDRs, while the concept of such documents is to convert Soil-Structure System to an equivalent fixed base oscillator and then use IDRs determined for fixed base assumption. The SSI model used in this paper is fundamental lumped parameters based on the concepts of cone models representing the soil with a 3-DOF system, which is capable of involving frequency dependency of soil. All of the analyses are performed in response spectra format. That is, it can be implemented for MDOF systems responding through their fundamental mode of vibration for a wide range of ordinary structures and soil conditions.

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