Studying the efficiency of seismic isolation in buildings with different structural system
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Efficiency of Seismic Isolation in Buildings with Different Structural Systems
Introduction to Seismic Isolation Systems
Seismic isolation systems are designed to protect buildings from earthquake-induced forces by decoupling the structure from ground motion. These systems can significantly reduce seismic responses such as interstory drifts, base shear, and floor accelerations, thereby enhancing the overall resilience of buildings during earthquakes. Various types of seismic isolation systems have been developed and studied, each with unique characteristics and performance benefits.
Partial Mass Isolation (PMI) Systems
Partial Mass Isolation (PMI) systems involve isolating different portions of a building's story masses to create multiple inherent vibration suppressors. Research has shown that PMI systems with isolated mass ratios (IMRs) ranging from 0.25 to 0.50 can effectively mitigate seismic responses while integrating seamlessly into the building's architecture. This approach simplifies design and construction, especially when identical isolated components (ICs) are used at different stories, and is particularly effective when ICs are allocated to the upper stories of the building.
Base Isolation in Reinforced Concrete (RC) Buildings
Base isolation systems, including rubber-based and friction-based isolators, have been extensively studied for retrofitting existing RC frame buildings. These systems are effective in limiting damage beyond the design limit state level, although they show limited margins with respect to collapse prevention. Multi-stripe nonlinear time-history analysis has demonstrated that base isolation can significantly enhance the seismic performance of RC buildings by reducing overall damage and maintaining usability.
Hybrid Systems: Base Isolation with Tuned Mass Dampers (TMD)
A novel approach involves combining base isolation with a tuned mass damper (TMD) located at the basement level. This hybrid system, which includes low-damping rubber isolators and a large-mass TMD, has been shown to improve seismic performance by reducing displacements, interstory drifts, and base shear. The TMD is connected to the base isolation system via lead-core rubber isolators, which act as both dampers and springs. This configuration has proven effective in minimizing seismic responses through nonlinear time-history analyses.
Geotechnical Seismic Isolation (GSI) Systems
Geotechnical Seismic Isolation (GSI) systems use materials like Sand-Rubber Mixture (SRM) and geogrid reinforcement to create a base isolation layer below the foundation. This system is particularly suitable for low-rise buildings, as it reduces peak ground acceleration and seismic settlement. The addition of geogrid reinforcement enhances the bearing capacity and reduces lateral deformations, making GSI systems a viable option for improving seismic resilience in low-rise structures.
Impact of Adjacent Structures on Base-Isolated Buildings
The performance of base-isolated buildings can be affected by impacts with adjacent structures, such as retaining walls or entry bridges. Studies have shown that such impacts increase superstructure acceleration and decrease bearing displacement. Despite these effects, base isolation remains more effective than non-isolated structures. The severity of impact effects depends on factors like the isolation gap, stiffness of the impact element, and the flexibility of the superstructure.
Composite and Hybrid Isolation Systems
Composite isolation systems, which combine different isolation technologies, have been studied for their effectiveness in protecting high-rise buildings. For instance, a hybrid system incorporating both base isolation and additional damping mechanisms has shown significant reductions in displacement, velocity, and acceleration of the stories. This approach enhances the overall stiffness and seismic resilience of the structure.
Shear Building Representations in Seismic Isolation
Seismically isolated buildings are often modeled as idealized shear buildings for research purposes. This simplification involves assuming infinitely stiff beams and axially inextensible columns. While convenient for analysis, this idealization can influence the seismic responses of isolated buildings. Comparisons between shear building models and full three-dimensional models have highlighted the importance of accurate representation in predicting seismic performance.
Periodic Foundations and Material Damping
Periodic foundations, which exploit the filtering effect of bandgaps, have been proposed as a new type of seismic isolation system. These foundations can significantly mitigate seismic responses by filtering out specific frequency ranges. The inclusion of material damping further enhances the effectiveness of periodic foundations by reducing seismic responses within the pass bands. This approach has been validated through studies on multi-story shear-type buildings.
Conclusion
Seismic isolation systems, including PMI, base isolation, hybrid systems, GSI, and periodic foundations, offer various benefits for enhancing the seismic resilience of buildings. Each system has unique characteristics and performance advantages, making them suitable for different structural types and seismic conditions. Ongoing research and optimization continue to improve the effectiveness and applicability of these systems in protecting buildings from earthquake-induced forces.
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