Research On Seismic Resistant Property Of Mixed Structure With Adding Light Steel Stories

The light steel structure is often used as the adding stories when multi-layered brick and frame structures are reformed to increase their layers. Thus, it is formed a mixed structure which is a kind of vertical irregular structure with flexible upper stories and rigid bottom. The seismic response of the structure will be bigger than the original structure.Therefor, the mixed structure is limited to be used in the code for seismic design. Seismic strengthening of the story-adding structure is needed because of some original structure's seismic fortification criteria not satisfying the current specification requirements, and some even without aseismatic design.The traditional seismic strengthening method is mainly to improve the bearing capacity of structural member or strengthen the wholeness of structure. Nonetheless, it can't fundamentally change the nature of structure, and the increaseing stiffness may even cause a bigger earthquake response.Seismic isolation is a technology which block the earthquake energy transfered to the upward by setting isolation story, and therefor to reduce the seismic response. It includes isolation system and story isolation system. The story isolation system is to place isolation story between two layers of superstructure. The purpose of it is to prolong the natural period of structure and increase damping. Thus, it can indirectly improve the seismic performance of structure by controlling the horizontal seismic response.In this paper, a mixed structure is studied using analysis software SAP2000. It consists of three-layers brick houses and one-layer light steel structure at the top. The structural seismic response analyses are carried out by means of response spectrum and time history methods for the original structure, the story-adding structure with isolation system and the direct story-adding structure.The results of the study show that:Compared with original structure, the lateral displacement increases about 26%under seldomly occurred earthquake and the inter-layer shear increases about 23%under frequently occurred earthquake. So the seismic response of direct story-adding structure are too big to satisfy the requirements of displacement and bearing capacity. Meanwhile, the scheme of story isolation system can reduce about 60%of the inter-layer shear under frequently occurred earthquake and about 55%of the lateral displacement under seldomly occurred earthquake, thus, to ensure the story-adding structure safely. The combined isolation system have the better damping effect than the lead-rubber bearings.