Vertical load effect on seismic base isolation systems

Abstract

In areas with high seismicity, base isolation systems are used to prevent damage at structural elements and to maintain serviceability of nonstructural elements after destructive effect of earthquake. Application of seismic isolation method has become common in Turkey recently, especially at hospital structures. In terms of disaster management, it is very crucial that important buildings provide immediate occupancy after earthquakes. Therefore, changes were made in regulations and design methods and base isolation system were included in the codes. In the Turkish Building Earthquake Code (TEC 2018), there is no comprehensive provision regarding vertical dynamic earthquake effect and especially near field effect in seismically base isolated systems; therefore, there is no clear guidance on how to take these effects into account in any technical document. Increased popularity of the system created many concepts that need to be investigated. One of the most emphasized issues is the situation in which vertical load effect is applied dynamically (as time history record). However, vertical load is a key parameter for friction pendulum isolators so the vertical earthquake effect is generally studied for friction pendulum type in literature and there is insufficient research about effect of vertical dynamic load on elastomer (rubber) bearings. In this study, nonlinear time history analyses are performed for a hospital building with seismic base isolation, lead rubber bearings (LRB), considering the effect of vertical load. Isolators' stability problems, scaling methods of vertical component and p-delta effect of the isolator units in the design were studied by including vertical load effects. To determine effect of vertical earthquake in the near field to the design of lead rubber isolator, site-specific response spectrum is determined by performing site-specific seismic hazard analysis, and earthquake records compatible with the target spectrum were selected for dynamic analysis. The aim of this study is to utilize the results obtained to enhance the design of the isolator units with respect to vertical motion of the earthquakes.