Seismic response of base isolated buildings considering pounding to moat walls
Masroor Shalmani, Armin
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Seismic isolation offers a simple and direct opportunity to control or even eliminate damage to structures subjected to ground shaking by simultaneously reducing deformations and acceleration demands. A base isolation system decouples the superstructure from the ground resulting in elongation of fundamental period of the structure and reducing the accelerations transferred to superstructure during ground shaking. However, increasing the fundamental period of the structure is mostly accompanied by increased displacement demands. In base isolated structures, this large displacement is concentrated at base level where seismic isolation devices are installed and designed to handle these large deformations without damage. A typical base isolated basement design requires a space in which the building is free to move sideways without hitting the surrounding structure. This space is commonly referred to as the "moat". Structural design codes such as ASCE 7-05 that regulate the design of buildings incorporating seismic base isolation systems require the minimum moat wall clearance distance equal to the maximum displacement at the base of the structure under the Maximum Considered Earthquake (MCE), although the superstructure is designed for design basis earthquake (DBE) level. Despite the cautious regulation for moat wall gap distance, pounding of base isolated buildings to moat walls has been reported in previous earthquakes. In conventional structures, the pounding problem between adjacent structures of buildings and highway bridges has been a major cause of seismic damage, even collapse, during earthquakes in the past several decades. Current design specifications may not adequately account for the large forces generated during impact in base isolated buildings. This study investigates the pounding phenomenon in base isolated buildings from both experimental and analytical perspectives by conducting shake table pounding experiments, developing effective models for impact to moat walls and evaluating the adequacy of code specifications for the gap distance of moat walls. A series of prototype base isolated moment and braced buildings designed by professional engineers for the purpose of this project is presented and one of the models was selected for a quarter scale shake table test with moat walls. The pounding experiments indicate that the contact forces generated during pounding can induce yielding in the superstructure and amplify the response acceleration at all stories of the building. The response amplification and damage depends on the gap distance, moat wall properties, and impact velocity. A detailed finite element model of the test setup is developed in OpenSees. An analytical study on the dynamic behavior of the moat walls resulted in proposing a new impact element. Numerical simulation using the proposed impact element compares well with experimental results. A series of collapse studies using the Methodology in FEMA P695 was conducted for both prototype models at various gap distances. The collapse probability of base isolated models used in this study and the effect of moat wall gap distance on the probability of collapse for base isolated structures is investigated. These studies verify that pounding to moat walls at the required gap distance by ASCE7-05 result in acceptable probability of collapse for the flexible and ductile moment frame models examined. However, the braced frame shows a notable drop in collapse margin ratio because of pounding to moat wall at the required gap distance and requires increasing the gap distance by 17%. to have an acceptable collapse probability.