Effects of soil-structure interaction on the seismic performance of reinforced concrete building

In assessing the earthquake performance of the structures, usually the maximum displacement is used. In addition, it is known that post-earthquake residual displacements that occur after an earthquake also affect the functionality of structures. Reliable estimation of displacements is important in determining the earthquake performance. In determining the performance of the structures, taking into account the soil-structure interaction is an issue that should not be overlooked, especially for structures to be built on soft soils. In this case, the displacement values are found to be more realistic when soil-structure interaction effects are properly accounted for. In structures subjected to earthquake ground motion, finding the maximum non-linear displacements in the time history analysis requires long and comprehensive analyzes. The calculation of displacements of equivalent linear systems is relatively simpler than performing nonlinear dynamic analysis. For the structures in which the first vibration period dominates the response, simplified methods can be utilized. For such systems, rather than dealing with long dynamic analyzes in the time domain, simpler static pushover based methods are preferred. One of these simplified methods is the displacement coefficients method, which allows nonlinear displacements to be found based on linear displacements of equivalent simpler systems. The inelastic displacement ratio, which is used for finding the maximum nonlinear displacement demand of structures, forms the basis of displacements coefficients method. While nonlinear displacement demands are calculated by the coefficients method, the yield strength of the system is determined by using the static pushover analysis method. The strength reduction coefficient is found by dividing the elastic demand of the considered earthquake ground motion and the yield strength found with the help of static pushover analysis. Nonlinear displacement demand can be estimated using the strength reduction factor and the inelastic displacement coefficients. The maximum and permanent displacements can be accurately predicted only if the utilized model is realistic. It is known that the soil-structure interaction significantly influences the displacement demands of buildings, especially for those constructed on soft grounds. In the analysis model used in structural analysis, taking into account the soil-structure interaction allows for more realistic performance evaluation. In the performance evaluation of soil-structure interaction systems, the nonlinear behavior of soil is generally taken into account by means of equivalent linear approximations. In this case, the cyclic behavior of the soil is not taken into consideration, only the situation which corresponds to a certain peak strain value is taken into account. Today, however, there are models that take into account the nonlinear behavior of the soil. Modeling both the superstructure and the soil using nonlinear models allows for a more realistic calculation of nonlinear displacements. Reliable estimation of inelastic displacements leads to a realistic performance evaluation. In this study, both superstructure and soil analysis models are established using nonlinear methods. Superstructure models are considered as equivalent single-degree-of-freedom systems. Plastic hinges with bilinear behavior are used at the junction of the foundation and the supperstructure. The period range of the fix-base systems is chosen to cover the period range of 1 to 10 storey buildings. Fixed system periods are found using the existing building height and building period relationships. The considered period range varies from 0.067 s to 0.99 s. The foundation is modeled as a beam on nonlinear soil springs. In this study, surface foundations are considered. Clay and sandy soils are considered as the soil types. For both clay and sandy soils considered in the study, certain strength and shear wave velocity ranges were estimated using existing recommendations. In the scope of this study, medium clayey soils were studied. In the clayey soils, unconfined compression steength is considered to be between 75 and 300 kPa and shear wave velocities are considered to be between 171 and 401 m/s. Secondly sandy soils are considered. The standard penetration (SPT) value is assumed to be between 5 and 30 on sandy soils. Soil strengths and shear wave velocities corresponding to SPT values in this range are estimated. According to the SPT values taken into account for the sandy soils, the soil resistance being between 180 and 1080 kPa and the shear wave velocity being between 180 and 365 m / s is taken into consideration. The earthquake ground motion records used in the study were selected taking into account the range of shear wave velocities of sandy and clayey soils. In total, analyzes were made for 54 acceleration records using the components in both two directions of the records obtained from 27 stations. Earthquakes with magnitudes 6.5 and higher were considered. The soil-structure interaction systems that are created by using the non-linear models of the superstructure and the soil are solved by nonlinear time history analysis method. Structural displacements and displacements resulting from the foundation rotation have been investigated. The elongation of the period of the soil-structure interaction systems compared to their fixed based counterparts, was examined. The maximum inelstic displacement ratios for soil-structure interaction systems were calculated by dividing the maximum inelstic displacements obtained from the non-linear time history analysis by spectral displacements. The change of the inelastic displacement ratio, according to the soil and foundation parameters, was investigated. The dispersion of the results of the maximum inelastic displacement ratio was examined. Using nonlinear regression analysis, equations were proposed that can be used to estimate the mean value of the maximum inelastic displacement ratio. The inelastic displacement ratios were obtained by using displacements of soil-structure interaction systems. While inelastic displacement ratio is calculated, it has not been calculated separately, either as the displacement from the foundation or from the structure. Because, past earthquakes have shown that even if there is no significant damage to the structure, the structure can no longer be used, as a result of foundation rotation. Taking this into consideration, the inelastic displacement ratio was calculated, taking into account the displacement of the total system, without leaving the displacements from foundation or displacement from structure. The equations proposed for estimating the inelastic displacement of soil-structure interaction systems, also take into account the total displacement ratio, both from the foundation and from the superstructure. The main equation proposed for predicting the inelastic displacement ratio of structures to be built on clay and sand soils was the same form. However, the regression coefficients for clay and sand were different. It has also been observed that these coefficients vary, depending on the soil parameters. Therefore, a second regression analysis was performed. For the calculation of the regression coefficients of the main equation, new equations were proposed depending on the soil and founadtion parameters. Since the regression coefficients of clay and sand soils are different, separate equations have been developed to calculate the regression coefficients of both sand and clay soils. Two types of residual displacement ratio have been investigated when residual displacements are examined. The first is the direct residual displacement ratio. The second is indirect residual displacement ratio. Direct residual displacement ratio is obtained by dividing the residual displacement of the soil-structure interaction system by the elastic spectral displacement. Residual displacemnts of soil-structure interaction systems is derived from nonlinear time history analysis. The indirect ratio of residual displacement is obtained by dividing the residual displacements of soil structure interaction systems into maximum inelastic displacements. While calculating the indirect residual displacement ratio of the soil-structure interaction systems, both residual displacement and maximum inelastic displacements were achieved with nonlinear time history analysis. The variation of both direct residual displacement ratio and indirect residual displacement ratio with respect to soil parameters was investigated. As the average of the results obtained from the 54 acceleration records is taken, the dispersion of the residual displacement ratio is examined according to the acceleration records. Equations have been proposed to estimate the mean value of direct residual displacement ratio. For the indirect residual displacement ratio, only changes of results were given according to the soil parameters. For predicting the mean value of the residual displacement ratio, the main equation proposed for clay and sand soils is the same form. However, it has been found that the regression coefficients of main equation obtained for clay and sand soils were different. Also, it was observed that the regression coefficients of the main equation were changed according to the foundation and soil parameters. Therefore, a second regression analysis was performed to develop new equations for the calculation of the regression coefficients of the main equation. This new equation has been developed based on foundation and soil parameters. Since the regression coefficients were different in clay and sand soils, separate equations were proposed for calculating the regression coefficients of both clay and sandy soils. The equations proposed for estimating the maximum inelastic displacement ratio and residual displacement ratio, were obtained for the estimation of mean values. Standard deviation equations for both inelastic displacement ratio and residual displacement ratio, have been proposed in order to be able to take into account variability between acceleration records. As in the equations proposed for the mean values, the standard deviation equations also were obtained based on the soil and the foundation parameters. Standard deviation equations for inelastic displacement ratio and residual displacement ratio, were proposed for both clay and sand soils. The equation proposed for the standard deviation of the inelastic displacement ratio of clay and sand soils is in the same form. However, since the regression coefficients of the main equation proposed for clay and sand soils were different, separate equations for sand and clay soils were proposed, for the calculation of the regression coefficients.The same is true for residual displacement ratios. Equations proposed for the standard deviation of the residual displacement ratios of clay and sand soils were obtained by considering the regression coefficients, clay and sand grounds. Using the mean and standard deviation equations of inelastic displacement ratio and residual displacement ratio, a probabilistic evaluation was made. For a probabilistic assessment, a specific site has been considered where the hazard curves are already available. However, hazard curves are usually given for cases where the shear wave velocity is greater than 760 m/s. The hazard curves were revised with the help of attenuation relationships, taking into account the shear wave velocity used in the study. Under certain periods and soil conditions, the annual frequency of exceedings for both the inelastic displacements and the residual displacements were calculated. Results are compared for different foundation and soil conditions.