Comparison of preformance analysis for eight and four story reinforced concrete structure with fixed end and base isolation systems

This thesis compares the seismic performance of 4 and 8 story reinforced concrete frame type buildings with connecting to foundation as fixed base and base-isolated. Lead rubber bearings are used as the seismic isolation system. Within the scope of strength-based design and deformation-based design, linear and nonlinear analysis methods were applied to these buildings using the SAP2000 program. All design and analyses were conducted in accordance with the criteria of the Turkish Earthquake Code (TBDY-2018). Initially, the design of the fixed-base buildings based on strength was carried out using the equivalent earthquake load and mode superposition methods. After determining the dimensions and reinforcements of the structural elements, analyses were performed using the single-mode push method and the time history analysis method as nonlinear analysis. For the base-isolated buildings, preliminary design was carried out using equivalent earthquake load and mode superposition methods. After determining the diameters of the lead rubber bearings, the diameters of the lead cores, and the dimensions and reinforcement layouts of the beams and columns, the final design of the isolated systems were completed using the nonlinear time history analysis method. In linear analyses, critical parameters evaluated in terms of earthquake demands included story shear forces, story displacements, and relative story drifts. For the four-story building, the ratios of the base shear force, maximum story displacement, and maximum relative story drift values obtained with the equivalent earthquake load to the results of the fixed-base system were calculated as 132%, 36%, and 33.3%, respectively. For the eight-story buildings, similar proportional values were found to be 158.8%, 52%, and 39%. Despite the higher period of the seismic isolation system and the corresponding much lower elastic spectral acceleration values compared to the fixed-base system, the base shear force in the base-isolated systems was not reduced. This is due to the behavior factor (R) used in the fixed-base systems being 8, whereas it was 1.2 in the base-isolated buildings, along with the upper limit values used for isolators in the superstructure calculation of base-isolated buildings. However, relative story drifts and story displacements were significantly lower in the base-isolated systems. In the nonlinear time history analyses (NLTHA), critical demand parameters such as story shear forces, story displacements, relative story drifts, absolute accelerations in buildings, relative velocities of stories, deformations in selected columns, and plastic rotations in selected beam sections were considered, and the seismic performance of fixed-base and base-isolated buildings was compared based on these parameters. The comparison was based on the averages of 22 NLTHA's results conducted with 11 acceleration records. When average values are expressed as the ratio of base-isolated to fixed-base building results, these ratios for the four-story buildings were 45%, 21%, 21%, 28%, and 83% for base shear force, maximum story displacements, maximum relative story drifts, maximum absolute story accelerations, and maximum relative velocities, respectively. For the eight-story buildings, the ratios were 45%, 42%, 44%, 33%, and 83%. Deformations in the columns of the four-story fixed-base system exceeded the controlled damage performance limit, whereas deformations in the columns of the isolated system remained below the limited damage performance limit. In terms of plastic rotations in beam sections, both systems met the "Controlled Damage" level of TBDY-2018. Although the columns of the eight-story base-isolated system were below the limited damage level, the columns of the fixed-base system were at the pre-collapse performance level. The beams of the base-isolated system were at the limited damage level, while the beams of the fixed-base system remained below the controlled damage limit. In nonlinear analyses, all critical parameters in the isolated structural system were found to be less than those in the fixed-base system, but story displacement in the isolated system was higher due to the low horizontal stiffness of the elastomers. Based on these findings, it can be concluded that under severe earthquake effects, the isolated structural system is more resilient than the fixed-base structural systemThe main purpose of this thesis is to study the performance of fixed-end structural system and the base isolated structural system using led rubber bearings, considering low and mid high reinforced concrete building to determine which of the two structural system has the superior performance and durability under the effect of a strong earthquake by comparing the results of linear and non-linear analysis of the two structural systems considering some critical parameters such as base shear force, floor displacement and relative drift ratio in the manner of linear analysis, in addition to these parameters the building acceleration and relative speed as well as the strain of columns and the rotation of beams sections will be in the consideration through the non-linear analysis of two structural systems. Two types of buildings will be considered in this research, the four and the eight stories as a low and mid high reinforced concrete building, on the other hand in light of linear analysis and design the equivalent earthquake force method has been used, while the time history analysis method was considered as a non-linear approach of analysis, however, all the analysis and the procedure of application will take a place under the recommendations of Turkish Building and Earthquake Code which was published in 2018. All the analysis results considering the critical parameters will be expressed in terms of ratios for the results of base isolation structural system to the results of fixed-end structural system. In the light of four stories building and linear analysis, the ratios of base shear force, floor displacement and relative drift ratio were found to be 130%, 71% and 26.5% respectively, however for the non-linear time history analysis the ratios of base shear force, floor displacement, relative drift ratio, absolute acceleration and relative speed were found to be 47%, 194%, 21%, 28% and 83% respectively. For the eight stories building as the results of linear analysis the ratios of base shear force, floor displacement and relative drift ratio have been calculated as 158.8%, 72% and 31% respectively. For the same building type taking in to consideration the non-linear time history analysis results in the manner of ratios for the two types of structural systems, the ratios of base shear force, floor displacement, relative drift ratio, absolute acceleration and relative speed have been determined as 48%, 151%, 44%, 33% and 83% respectively. By observing the strain values of columns during the time history analysis of the two structural systems, the final result was the same considering the four and the eight stories buildings in which the strain values of fixed-end structural system columns have exceeds the life safety limit according to the TBEC-2018 recommendations while the values of strain considering the columns of base isolation structural system were sustained under the limited damage level, on the other considering the rotation values of beams sections, all the resulted values for the two structural system considering the two types of building have accomplished to be under the recommended limits provided by the TBEC-2018 which are the life safety limit for fixes-end structural system and limited damage for base isolation system. In the light of previous information and results of the two structural systems for the two types of building, considering the linear analysis results even though the base shear force determined in the base isolation system is bigger then what is in the fixed-end system, the performance of base isolation system is better taking in to consideration the results of floor displacement and relative drift ratio, on the other side with the consideration of the non-linear analysis results regarding the higher value of floor displacement in the base isolation system which is caused by the low horizontal rigidity of the isolators, the other critical parameters results clarify a higher performance and durability for the base isolation system, so at the end we can say that the base isolation system has superior performance and it's more durable under the effect of strong earthquakes than the fixed-end structural system in the case of low and mid high reinforced concrete buildings. The occurrence of low relative story drifts and accelerations in the seismic isolation system will allow the reuse of important machines in the hospitals. The seismically isolated building will continue to provide uninterrupted service.