فایل ورد کامل ارزیابی اثرات متقابل خاک – فونداسیون – سازه بر روی تقاضاهای پاسخ لرزه ای ساختمانهای MRF چند طبقه بر روی شالوده های گسترده

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تعداد صفحات این فایل: ۲۷ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Evaluation of soil–foundation–structure interaction effects on seismic response demands of multi-story MRF buildings on raft foundations~~en~~

Abstract

Soil conditions have a great deal to do with damage to structures during earthquakes. Hence the investigation on the energy transfer mechanism from soils to buildings during earthquakes is critical for the seismic design of multi-story buildings and for upgrading existing structures. Thus, the need for research into soil–structure interaction (SSI) problems is greater than ever. Moreover, recent studies show that the effects of SSI may be detrimental to the seismic response of structure and neglecting SSI in analysis may lead to un-conservative design. Despite this, the conventional design procedure usually involves assumption of fixity at the base of foundation neglecting the flexibility of the foundation, the compressibility of the underneath soil and, consequently, the effect of foundation settlement on further redistribution of bending moment and shear force demands. Hence the SSI analysis of multi-story buildings is the main focus of this research; the effects of SSI are analyzed for typical multi-story building resting on raft foundation. Three methods of analysis are used for seismic demands evaluation of the target momentresistant frame buildings: equivalent static load; response spectrum methods and nonlinear time history analysis with suit of nine time history records. Three-dimensional FE model is constructed to investigate the effects of different soil conditions and number of stories on the vibration characteristics and seismic response demands of building structures. Numerical results obtained using SSI model with different soil conditions are compared to those corresponding to fixed-base support modeling assumption. The peak responses of story shear, story moment, story displacement, story drift, moments at beam ends, as well as force of inner columns are analyzed. The results of different analysis approaches are used to evaluate the advantages, limitations, and ease of application of each approach for seismic analysis.

Introduction

Over the past 40 years, considerable progress has been made in understanding the nature of earthquakes and how they could cause structure damages, and in improving the seismic performance of the built environment. However, much remains unknown regarding the prevention or mitigation of earthquake damage in worldwide, leaving room for further studies. During the past and recent earthquakes, it is realized that the soil–structure interaction (SSI) effects play an important role in determining the behavior of building structures. The seismic excitation experienced can be considered a function of the fault rupture mechanism, travel path effects, local site effects, and SSI effects (Gu 2008). Irrespective of the structure, the local soil conditions can dramatically influence the earthquake motion from the bedrock level to the ground surface, through their dynamic filtering effects. One example is the 1985 Mexico City earthquake where deep soft soils amplified the ground motion and modified the frequency of ground shaking. Similar behavior was observed during the 1989 Loma Prieta earthquake, in which the sections of the Cypress freeway in Oakland collapsed due to the soil-related motion amplification. Common practice of analysis and design of buildings is to assume the base of building to be fixed, whereas in reality supporting soil influences the structural response by permitting movement to some extent due to its natural ability to deform. The seismic SSI of multi-story buildings becomes very important after the destruction of recent major earthquakes. For the structure founded on the soil, the motion of the base of the structure will be different from the case of fixed base, because of the coupling of the structure–soil system. It is true that taking the soil into account when calculating the seismic response of the structure does complicate the analysis considerably. It also makes it necessary to estimate additional key parameters, which are difficult to determine, such as the properties of the soil, foundation and their interaction. The seismic response of the bridge with SSI under bi-directional earthquake excitations considering different soil flexibilities is investigated by many researchers (Abdel Raheem et al. 2002, 2003; Abdel Raheem and Hayashikawa 2003; Hayashikawa et al. 2004; Soneji and Jangid 2008; Shah et al. 2011). The emphasis has been placed on assessing the significance of soil behavior that affects the response of the system and identifies the circumstances under which it is necessary to include the SSI effects in the design of bridge structures. In seismic design of buildings, the consequences of soil flexibility are generally ignored. Mylonakis et al. (1997) and Roy (2001) showed the possible severities of neglecting the effects of the SSI in their studies. Similar study on implication of neglecting the SSI in ensuring the structural safety by conventional elastic and inelastic design procedure of moment-resisting building frames was shown by Tabatabaiefar et al. (2013). This research aims to study the SSI for multi-story buildings on raft foundation, evaluate the approach of Egyptian Code seismic provisions for analysis methods during the seismic design of buildings, discuss the alternative solutions for cases wherein existing provisions do not lead to satisfactory results and to quantify the effect SSI on the structural response so that designers can be aware of the likely impact of their decisions. Time history analysis (TH) has been performed to evaluate equivalent static load (ESL) and the response spectrum (RS) analysis methods; a set of time history records has been used. A parametric study with different approaches of analysis, design parameters of the underneath soil conditions and number of stories is carried out to evaluate the SSI effects on the building vibration characteristics and seismic demands including the fundamental period, total base shear, story displacements, story drifts, moment at beams ends and force of inner columns. The results show that SSI has a significant influence on the seismic response demands.

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