Comprehensive parametric investigation of two base isolated structures with common isolation plane

Rubber bearings are used to prevent vibrations in the buildings and to allow the bearings to be displaced due to thermal expansion in the bridges. The first use of rubber bearings in order to protect constructions against earthquake effects, occurred in Pestalozzi primary school in Skopje, Yugoslavia in 1969. The same horizontal and vertical stiffness of the rubber supports applied as a single block caused the bulge to occur due to the weight of the building on the side surfaces. The French engineer Eugène Freyssinet, who discovered that the axial loading capacities of the rubber layers were inversely proportional to their height, suggested strengthening the rubber layers by adding thin steel plates in the vertical direction. Here the bond between the layers is provided due to the friction force. Thanks to the vulcanization method used to ensure that the thin steel plates and rubber layers adhere to each other, studies and applications of modern seismic isolators have begun to increase. Seismic isolation systems are frequently used to prevent damage to structures in areas with high seismicity due to strong ground motions. Base isolated structures consist of superstructure, isolation plane and base isolators. These systems are single structures such as residential, data center building, hospital, liquid tank and offshore oil platform which are assumed to displace relatively rigid over the base isolators. The architectural details to be solved in the case of using separate base isolated structures together, are complex and expensive. For this reason, independent superstructures are designed in common isolation plane. More particularly, constructed hospital buildings in recent years in Turkey, meets this definition. Linear design methods, which are determined in consequence of carried out studies in the past, are consider the base isolated systems as consist of single or multi degree of freedom systems. Therefore, design procedures in standards are prepared for base isolated structures with independent isolation plane. There is no design criteria in standards for dynamic interaction of base isolated structures with common isolation plane. Each of the base isolated structures which are designed in common isolation plane are considered in practice as if structures with independent isolation plane. As a consequence of this assumption causes the higher mode effect due to the hysteretic behavior of the isolators and therefore the dynamic interaction of superstructures to be unevaluated. Purpose of this study is to determine amplification of the base shear coefficient due to interaction of superstructures by performing comprehensive parametric investigation for the two base isolated structures with common isolation plane. Within the scope of this study, dynamic interaction of two base isolated structures with common isolation plane is investigated through the systems changes parametrically. In addition to this sample analyses are carried out for three and four base isolated structures with common isolation plane. In this case, number of story of superstructures, equivalent period and equivalent damping ratio of base isolators are considered as parameter. Analyses are carried out for the cases where the number of story of two base isolated structures with common isolation plane changes from one to ten. All analyses are repeated for 1.5, 2.5, 4.0s equivalent period and %10, %20, %30 equivalent damping ratio of base isolators. The hysteretic properties of the base isolators are normalized to obtain constant equivalent period and equivalent damping ratios as a result of nonlinear analysis versus varying superstructure mass. This process is performed by determining the yielded stiffness and characteristic strength by determining the isolator displacements obtained from the linear time-history analysis with the stiffness corresponding intended equivalent period and the equivalent damping values. Analyses are carried out only for the direction in which the structures are located in the common insulation plane. The effects that occur other directions and the bi-directional interactions of the isolators are out of scope of this study. Story mass and story stiffnesses of superstructures are fixed for all analyses. Mass of the isolation plane is fixed for each structure and is calculated by multiplying the number of structures in the common isolation plane. Only linear elastic shear springs are considered in superstructure models which are defined as multi degree of freedom system. Hysteretic behavior of isolators are represented by a bilinear element. Only hysteretic damping which is caused by nonlinear behavior is considered in base isolators. The ground motion records used in the analyses are matched in accordance with the design spectrum generated according to earthquake ground motion level with probability of exceeding %2 in 50 years. All analyzes were carried out by means of the MSBIS program, which was developed within the scope of this study, which allows nonlinear analysis of base isolated structures with independent and common isolation plane to be solved parametrically. Numerical solution of equation of motions is carried out with Newmark-β method. Dynamical balance is provided by Newton-Raphson method doing iteration in every time step. The accuracy of the developed program is demonstrated by comparison with SAP2000, the generally accepted structural analysis program. Resultant shear forces, story accelerations and story drifts of the first structure is determined by means of nonlinear time history analyses according to changing angular frequency of the second structure and effective period and damping values of base isolators. In addition, for the case of first structure have ten stories, the variation of story shear force coefficients for the changing number of story of the second structure is examined. Results are normalized according to values which obtain from the inspected base isolated structures with independent isolation plane. Thus, amplification in base shear forces of the structures can be determined depending on the angular frequency of the superstructures, equivalent period and equivalent damping of base isolator. Base shear force coefficients of the structures with common isolation plane are obtained and the results are compared according to the total shear force coefficient obtained by vector summation in the dynamic analysis. This rate of error is increasing in direct proportion to the order of the dynamic interaction of the structures. Additionally, necessary joint spacing of two base isolated structures with common isolation plane is calculated as per related standard and compared to results obtained from the nonlinear dynamic analysis. As a result of this study, it is determined that dynamic interaction of two base isolated structures with common isolation plane due to increasing damping ratio of base isolators, causes to increase shear coefficients, story drifts and maximum story accelerations significantly. This increment is determined to increase with the separation of the angular frequencies of the two structures. Therefore, if two structures have the same angular frequency, the resulting internal forces and displacements are equal for the same equivalent period and equivalent damping ratios base isolators. Base shear force of one story structure inspected from two base isolated structures with common isolation plane, increase with respect to vary other structure number of story according to situation that inspected structure with independent isolation plane. When the ten-story structure is examined, it is determined that the story shear forces increase in the lower stories and decrease in the upper stories. As a result of the analyses carried out for the three and four base isolated structure with common isolation plane, it is determined that the base shear force coefficients increase with the increase of the number of buildings having different angular frequency from the examined structure. Moreover, increasing in hysteretic damping causes to change lateral distribution of base shear forces to superstructure. Base shear force of structure is evenly distributed to every story for an equivalent damping value of \%1, which is assumed to be linear of the isolators. Nevertheless, base shear force of structure is distributed in the form of an inverted triangle for increasing damping ratios.