Seismic response assessment of a reinforced concrete structure using different types of seismic isolators

Abstract

Throughout human history, earthquakes have been among the most destructive natural disasters. The dynamic behavior of structures is directly impacted by the energy produced as a result of abrupt cracks in the Earth's crust, which travel to the surface as seismic waves.Turkey's location on active fault lines makes earthquake hazard a constant and unavoidable reality. The inability to precisely predict the time, magnitude, and impact of earthquakes necessitates that structures be designed not only with the objective of preventing collapse, but also in accordance with performance targets that prioritize life safety, reduction of economic losses, and post-earthquake functionality. Major earthquakes occurring particularly in densely populated areas have revealed the limitations of conventional design approaches and clearly demonstrated the need for more effective engineering solutions. In order to diffuse earthquake energy within the structural system, the traditional seismic design technique is predicated on making structural parts stronger and more ductile. In this method, seismic energy is directly transmitted to the superstructure, and certain levels of damage are accepted. Although ensuring life safety is the primary objective, preserving the functionality of the structure after an earthquake cannot be guaranteed. Particularly for buildings that need to continue functioning following an earthquake, such as hospitals, emergency response centers, fire stations, data centers, and key infrastructure sites, this strategy is inadequate.For this reason, structural control methods, particularly seismic isolation systems classified as passive control systems, have come to the forefront. By separating the structure from its foundation, seismic isolation systems work on the basic tenet of minimizing the transfer of seismic energy to the superstructure. The isolators used in these systems are designed to provide high vertical stiffness and low horizontal stiffness. This lengthens the structure's natural vibration period and moves the system away from the ground motion's dominating frequency range. Story accelerations, velocities, and internal force demands all drastically diminish as a result of the period elongation, which also lessens base shear forces and bending moments pressing on the structure. However, greater displacements at the isolation level could result from the period's extension. Therefore, the damping capacity of the isolators is of critical importance. Because of its great damping capacity, the structure's post-earthquake functionality is maintained while maximum and residual displacements are kept under control. In the first stage of this study, previous research was reviewed and seismic isolation systems were introduced. The literature indicates that elastomer-based isolators are widely preferred due to their high energy dissipation capacity and ease of application. The performance evaluation of two distinct elastomer-based seismic isolation systems lead rubber bearing (LRB) and high damping rubber bearing (HDRB) systems is the primary emphasis of in this context. While the lead core's yielding behavior in the LRB system results in a hysteretic mechanism for energy dissipation, specifically compounded rubber with a high inherent damping capacity provides continuous and consistent energy dissipation in the HDRB system. A model of a seven-story reinforced concrete building was created as part of the study. SAP2000 software was used to quantitatively assess how base isolation affected structural behavior. The analysis method used was nonlinear time history analysis. This method enables the direct application of time-dependent acceleration records of real earthquakes to the model and allows realistic investigation of the inelastic behavior of the structure. The 1999 Kocaeli and 1992 Landers earthquake records obtained from the PEER database were used as ground motion inputs. The selected records were scaled in accordance with the target design spectra defined by AFAD, and spectrum matching was performed using the SeismoMatch program. As a result, it was guaranteed that the ground motions utilized in the analysis accurately reflected the level of seismic threat. The performance of fixed-base structures was compared to that of LRB- and HDRB-isolated structures under a variety of earthquake records. Additionally, models with varying levels of material strength were developed for the same structural geometry in order to examine how differences in structural material strength affect the efficacy of seismic isolation systems. In the first model, C30 concrete and B420C reinforcing steel representing new design conditions were used. The low strength levels commonly found in the existing building stock were represented in the second model by using C15 concrete and BC1 reinforcing steel. Thus, the effectiveness of seismic isolation systems was evaluated not only for newly designed structures but also for existing structures with limited material capacity. Base shear force, base bending moment, maximum displacement, story accelerations, story velocities, and natural vibration periods were taken into consideration as the main criteria in the performance evaluation. The first natural vibration period of the fixed-base construction was found to be 0.60 seconds based on the analytical results. When the LRB system was used, this period increased to 1.77 s, corresponding to an increase of approximately 195%. In the HDRB system, the fundamental period reached 2.38 s, representing an increase of approximately 297%. This notable period elongation suggests that the structure's effective horizontal stiffness has decreased and that the superstructure receives earthquake energy at lower frequencies. Both isolation methods offered notable decreases in base shear force and base moment values when compared to the fixed-base model. Because of its strong inherent damping capability, the HDRB system was specifically found to be more successful than the LRB system in lowering force demands. In the analyses conducted under the Kocaeli and Landers earthquake records, the fixed-base model produced the highest base shear force and moment values, whereas these values were significantly reduced in the LRB and HDRB models. Because of the energy dissipation mechanism based on the lead core yielding, the LRB system demonstrated noticeable hysteretic behavior and helped to manage the consumption of earthquake energy. The performance of the structure was positively impacted by this attribute, especially when there were prolonged ground motions. The HDRB system, on the other hand, demonstrated a more balanced and continuous energy dissipation behavior, reducing internal force demands in the superstructure more effectively. However, in some analysis cases, it produced lower residual displacement values compared to the LRB system. Analysis of story velocities and accelerations revealed that acceleration values were focused in the upper stories and rose with the fixed-base model's height. In the isolated models, accelerations were significantly reduced and seismic demands were concentrated at the isolation level. This provides a significant advantage in terms of reducing the risk of damage in superstructure elements. In the C15–BC1 model with reduced material strength, significant increases were observed in force and deformation demands in the fixed-base system. In contrast, in the LRB- and HDRB-isolated models, the reduction in material strength had a limited effect on structural response. It was observed that in isolated systems, behavior was largely controlled by the properties of the isolators, and the decrease in superstructure material capacity did not dramatically affect overall performance. According to this research, seismic isolation techniques show a great deal of promise for fortifying the existing stock of buildings. Overall, the seismic performance of the structure was shown to be greatly enhanced by both isolation technologies. While the LRB system demonstrated controlled and steady behavior due to its hysteretic energy dissipation capabilities, the HDRB system produced more effective outcomes in terms of force reduction and acceleration control. The system should be selected according to project-specific performance objectives, including force reduction targets, energy dissipation mechanism, displacement capacity, maintenance requirements, and expected performance criteria. This study demonstrates that seismic isolation systems provide an effective solution for both newly designed structures and existing building stock with low material strength. In future studies, consideration of different earthquake records, soil–structure interaction, temperature variations, aging effects, and economic analyses, as well as comparison of different isolation types, will contribute to a more comprehensive evaluation of seismic isolation systems.