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Investigating the Behavior of 3D Irregular Steel Structures with Seismic Base Isolation under Multi-Directional Excitations and Predicting Results by Using the Endurance Time Method

Zarbilinezhad, Milad | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54189 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Esmail Pourestekanchi, Homayoon
  7. Abstract:
  8. This research has been performed with the aim of gaining familiarity with various types of seismic isolation systems, their unique features and numerous advantages, factors affecting their performance and finally their modelling and design. Despite the advantages of seismic isolation systems as control instruments, but where there is not enough information on the involved parameters in the performance of these systems, there is possibility of unpredictable behavior of the structures. Hence, considering the extent and influence of each parameter involved in the performance of the isolators, a parametric study is performed utilizing the 3D models under multi-directional excitations, using the Endurance Time Method (ET) in Seismic Isolated Structures in which Lead Rubber Bearings (LRB) are used as isolation systems. The study is performed to assess the effective parameters on the performance of isolators including the stiffness of isolation system and supplemental damping of the isolated superstructure. For this purpose, a number of steel structures with lateral bearing system and special bending moment frame were modelled and designed with 3, 7 and 12 stories and 4 spans in both directions and in two groups of regular and irregular buildings along their height. The structures were first modelled and designed with fixed bases and then for each group of structures a number of isolators were implemented based on two design approaches. The first approach is applied for the group of structures with a variable design period of isolator (variable TD). In this approach per each effective equivalent damping ratio of 15% (ξ_eff=15%), three values are designated for the design period of the isolation system (TD=2.5, 3.5 and 4.5 seconds) and the supplemental superstructure damping per design periods of the seismic isolation system is taken constant. In the second approach, by assuming the system stiffness to be constant, the supplemental superstructure damping per different levels ( ξ_eff=0%,5%,10% and 15%) is examined so that one could investigate the impacts of using different supplemental superstructure damping levels and horizontal stiffness of isolators on the performance of entire isolation system. From the obtained results it is found that reduction in the stiffness of isolation system and increase of the supplemental superstructure damping both improve the superstructure performance in terms of controlled maximum drift, absolute displacement, maximum acceleration especially at the upper stories, torsion and base shear of the stories. Also, it was known that different levels of the supplemental superstructure damping and stiffness of the isolators have different effects on the corresponding hysteresis curves. It is observed that reduction in the stiffness of isolator only causes reduction in the maximum induced force in the isolator, whereas it not only causes excessive increase in the maximum displacement induced in the isolator but also causes unpredictable behavior of the maximum dissipated energy during a cycle in the isolator. On the other hand, increase in the supplemental superstructure damping (ξ^* ) causes both decrease in the maximum displacement and decrease in the maximum force induced at the isolator level, but it is regarded as an undesirable factor which reduces the maximum dissipated energy in a cycle
  9. Keywords:
  10. Endurance Time Method ; Lead Rubber Bearing (LBR) ; Supplemental Damping ; Three Dimensional Model ; Multi-Directional Excitation ; Irregular Structures ; Regular Buildings

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