Seismic Base Isolation in Reinforced Concrete Structures

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Seismic hazards are a major concern in many populous regions of the world. Performance-based seismic design has brought about new technological advances and introduced an innovative approach to constructing seismic-resistant buildings. Base isolation systems are increasingly utilized methods of advanced seismic resistance, and the effects of these systems on the seismic responses of structures are studied in this paper. In order to verify the effect of base isolation system, two different structures are presented (symmetrical and non-symmetrical school buildings) in which the seismic responses of the 'fixed-base' and 'base-isolated' conditions have been compared using SAP2000 (a well-known computer program). The high damping rubber isolation system has been used and devices have been installed at the foundation level. Time history analysis has been performed on three earthquakes: El Centro, Loma and Coyote. Comparing the results of the base-isolated condition with those obtained from the fixed-base condition has shown that the base isolation system reduces the base shear force and story drifts, whilst also increasing the displacement.

Earthquakes are one of nature’s greatest hazards; throughout historic time they have caused significant loss of life and severe damage to property, especially to man-made structures. On the other hand, earthquakes provide architects and engineers with a number of important design criteria foreign to the normal design process. From well established procedures reviewed by many researchers, seismic isolation may be used to provide an effective solution for a wide range of seismic design problems. The application of the base isolation techniques to protect structures against damage from earthquake attacks has been considered as one of the most effective approaches and has gained increasing acceptance during the last two decades. This is because base isolation limits the effects of the earthquake attack, a flexible base largely decoupling the structure from the ground motion, and the structural response accelerations are usually less than the ground acceleration [1]. Many comparative studies have revealed that the responses of the isolated structure are significantly smaller than the fixed base structure [2], [3], [4], [5], and [6]. Most of these studies compared the seismic demands (e.g. inter story drift, floor acceleration and base shear) for the two types of building structures, but only a limited number of studies investigated the responses of the isolated structure using high damping rubber (HDR) isolation with detailed procedures of the design of HDR. Skinner et al. [7] indicated that a base isolator with hysteretic force-displacement characteristics can provide the desired properties of isolator flexibility, high damping and force limitation under horizontal earthquake loads, together with high stiffness under smaller horizontal loads to limit wind-induced motions. Kelly [8] gave a brief introduction to the response mechanisms of base isolated buildings through two degrees of freedom linear system. The effectiveness of the isolation system to mitigate the seismic response is through its ability to shift the fundamental frequency of the system out of the range of frequencies where the earthquake is strongest. Also, Skinner et al. [7] demonstrated that the most important feature of seismic isolation is that its increased flexibility increases the natural period of the structure. Because the period is increased beyond that of the earthquake, resonance is avoided and the seismic acceleration response is reduced.
Han et al. [9] studied the seismic risk analysis for an old non-ductile RC frame building before and after retrofit with base isolation with LRBs. They found that that the viscous damping, concrete compressive strength, steel yield stress and the beam–column joint parameter that defining the elastic range of the joint has the most impact on the structural seismic demand for the un-retrofitted building; for the isolated buildings the temperature also have significant effects on the seismic demand. Base isolation was found to be effective in reducing seismic risk for higher damage levels. Acar [1] studied the effect of HDR isolation on the seismic responses of different structures using IBC2000 and FEMA design codes and concluded that the site condition where earthquake data is recorded has a great influence on the design parameters of the structure. That is as the soil becomes softer, the response of the structure increases. Therefore the selected ground motion data must have been recorded on similar soil condition with the site where the structure is located. In this paper, the effect of base isolation system on seismic responses of structures is studied. Two different structures are presented (regular and irregular 5-storey school buildings) in which the seismic responses of the fixed-base condition and HDR isolation condition have been compared using the well known computer program SAP2000 [10]. Time history analysis is performed using three earthquakes; El centro, Loma and Coyote.

 
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Seismic Base Isolation in Reinforced Concrete Structures

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