This study attempts to investigate the higher-mode effects on the constant-ductility inelastic displacement factors of multi-degree-of-freedom (MDOF) systems considering soil-structure interaction. These factors were computed for 12,600 two-dimensional superstructure models of shear buildings and their corresponding equivalent single-degree-of-freedom (ESDOF) systems under 26 ground motions recorded on alluvium and soft soil. An intensive parametric study was carried out for a wide range of non-dimensional parameters, which completely define the problem. The underlying soil is considered as a homogeneous half-space based on the concept of cone model. The higher-mode effects were then investigated through defining the ratio of inelastic displacement factor of MDOF system to that of the corresponding ESDOF one. The influence of soil-structure interaction key parameters, fundamental period, ductility ratio, the number of stories, and dispersion of the results are evaluated and discussed. Results indicate that as the base becomes very flexible, unlike to the fixed-base systems, in which the defined ratios are greater than unity, using the inelastic displacement factors of ESDOF models for MDOF ones would result in a remarkable overestimation of maximum inter-story displacement demand. A new expression is proposed to estimate the ratio of inelastic displacement factor of MDOF soil-structure systems to that of SDOF counterpart.
Summary
The effects soil‐structure interaction (SSI) and lateral design load‐pattern are investigated on the seismic response of steel moment‐resisting frames (SMRFs) designed with a performance‐based plastic design (PBPD) method through a comprehensive analytical study on a series of 4‐, 8‐, 12‐, 14‐, and 16‐story models. The cone model is adopted to simulate SSI effects. A set of 20 strong earthquake records are used to examine the effects of different design parameters including fundamental period, design load‐pattern, target ductility, and base flexibility. It is shown that the lateral design load pattern can considerably affect the inelastic strength demands of SSI systems. The best design load patterns are then identified for the selected frames. Although SSI effects are usually ignored in the design of conventional structures, the results indicate that SSI can considerably influence the seismic performance of SMRFs. By increasing the base flexibility, the ductility demand in lower story levels decreases and the maximum demand shifts to the higher stories. The strength reduction factor of SMRFs also reduces by increasing the SSI effects, which implies the fixed‐base assumption may lead to underestimated designs for SSI systems. To address this issue, new ductility‐dependent strength reduction factors are proposed for multistory SMRFs with flexible base conditions.
Summary
In the present paper, a new algorithm to achieve optimum lateral load pattern for inelastic steel shear‐building structures is proposed and the efficiency of the proposed algorithm in terms of convergence speed and stability is investigated. Then, by conducting this algorithm on 28,800 elastic and inelastic steel shear‐building structures having different dynamic characteristics subjected to 40 design compatible earthquakes, a new seismic force pattern incorporating higher modes effect is proposed, and the adequacy of the proposed load pattern is parametrically investigated on the basis of the concepts of structural weight and dissipated energy and cumulative damage index. Through conducting numerous nonlinear dynamic analyses, the effects of uncertainties in story shear strength, structural fundamental period, and damping ratio by using the Monte Carlo simulation are parametrically investigated. Result indicates that optimum structures are in general more sensitive to the random variation of fundamental period and story lateral strength, whereas up to 40% of damping ratio variation does not affect the seismic performance of the optimum design frames. Finally, compared with recently proposed optimum structures without consideration of higher modes and code‐complaint design structures, the proposed mean optimum designed buildings of this study exhibit up to 42% less cumulative damage under the design earthquakes.
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