Modern trends in base isolation applications for seismic protection of historic buildings

Historical buildings have relatively low height, are usually massive and their natural vibration period is rather low. Hence if such buildings are located in a seismically active region, using base isolation systems is a very effective way for improving their dynamic response. The idea of base isolation was known hundreds years ago, however practical implementation of modem base isolators started in the last few decades. The main advantage of a base isolation system is that no elements should be added to an isolated building, which is especially important for heritage architecture. A traditional base isolation system is obtained by using two main base isolator types - elastomeric (neoprene) bearings or friction pendulums. Hundreds of buildings all over the world are provided with such isolators and have proved enhanced dynamic behaviour. Design of base isolation systems and selection of their properties usually depend on dynamic characteristics of the isolated building. The latter can be obtained by non-destructive impulse testing of the structure before its protection. An appropriate method for structural dynamic parameters experimental estimation was developed by the authors. For this reason an impulse test was carried out on a three-story structural part of a building designed for a seismic region. In some cases the displacements at the base isolation level are rather big and exceed the allowed limits. In such cases it is recommended to add dampers to the base isolation system. Effective variable friction dampers were proposed and tested by the authors. These dampers significantly reduce the displacements at the isolator level (between the first floor column and foundation) and also yield further improvement in the seismic response of a building.