An Investigation of the strength reduction factor considering soil-structure Interaction

Document Type : Original Research

Authors
M. Hajian
R. Attarnejad
School of civil Engineering, University of Tehran
10.22034/25.2.53
Abstract
This study presented an evaluation of the response modification factor of structures considering soil-structure interaction (SSI). During an earthquake, the soil-structure interaction plays an important role in the response of the structure and affects the dynamic characteristics of the structure. In current structural design codes, due to the complexity of soil medium behavior, it is assumed that the structure is fixed at the base and the important effects of soil-structure interaction are disregarded. Thus, dynamic responses of structures will not be simulated properly. One of the major seismic parameter is the strength reduction factor, which represents the ratio of the elastic lateral yielding strength in system to the lateral yielding strength to maintain the displacement ductility ratio demand, less or equal to the target ductility ratio. Previous studies were often investigated the strength reduction factor of single degree of freedom (SDOF) systems, which cannot be an accurate estimation of real structurers. The models were also based on the simple method of spring and dashpot simulations of soil-foundation system. Thus, there is a need to conduct new studies on steel multi degree of freedom structures (MDOF) resting on soil medium. In this regard, a wide variety of numerical analyses was considered to evaluate parameters affecting the strength reduction factor.

For this purpose, first, a brief review of the literature related to the response modification factor regarding soil-structure interaction was presented. Then, several numerical structures with different number of stories, natural fixed base periods and displacement ductility ratios were modeled in OpenSees software. The supporting soil was modeled as a continuous and two-dimensional continuum medium using 2D plane strain shell elements. The structures were 3storey, 5storey, 10story and 15storey buildings, with periods between 0.1 and 3 s, subjected to 22 strong ground motions on NEHRP site classes D and E, on the basis of the ASCE7-16 (Soil type D, with shear wave velocity between 180 m/s and 360 m/s, and soil type E, with shear wave velocity lower than 180m/s). The displacement ductility ratios used in this research were assumed 2, 4 and 6. The results were demonstrated according to the natural fixed base periods of structures. It was observed that for flexible and fixed base systems, increasing the number of stories reduced the values of Rm. However, for soil–structure systems, the Rm become less sensitive to the number of stories. Rm was sensitive to aspect ratio (h/r) of structures. As h/r increases, Rm experienced higher values. On the other hand, strength reduction factor was mostly affected by displacement ductility ratios of the whole soil-structure system. Nevertheless, generally, soil-structure interaction reduced the values of Rm. The values of strength reduction factor were sensitive to soil shear velocity. As soil shear velocity decreased, the SSI effect was considerable and therefore Rm was decreased. Thus it can be concluded that the common seismic design approaches which does not considered soil-foundation flexibility, can result in higher strength reduction factor compared to the real situations. Using higher strength reduction factor leads to non-conservative dynamic structural outputs.

Keywords

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Modares Civil Engineering journal
Volume 25, Issue 2 - Serial Number 82
May and June 2025
Pages 53-66