| Historic building is the carrier through which the descendants can learn about architectural styles and national culture of old times. However, considering the bearing capacity as well as seismic resistance, most existing historic buildings have structural safety problems. Therefore, the seismic retrofitting of them is very necessary. Traditional ways of seismic retrofitting always results in the increase of structural seismic response and can not effectively solve the contradiction between protecting and retrofitting. Seismic isolation retrofitting,through retrofitting the foundation, is an effective way to decrease seismic response of upper structure and simultaneously protect the original appearance of historic building. Obviously, this is an effective seismic retrofitting way for such buildings.With the consideration of the structural characteristics of historic buildings, research on seismic isolation technology is carried out from the points of performance of isolation layer, performance of isolated structures and underpinning of structure.To study internal force and deformation of isolated structures, the relationship between kb/k1 and vibration modes of structure is analyzed, where kb is the stiffness of isolation layer and kx is the stiffness of every storey. So is the relationship between kb/k1 and modal participating mass ratios. Through analysis, the following conclusions can be drawn. (1) Rubber hardening of bearings can increase the ratio of kb/k1, which lead to the increase of seismic response of isolated structure. (2) For isolated multi-storey historic buildings, when the value of kb/k1 is smaller than 0.85, the modal participating mass ratios of first vibration mode is higher than 90%. Hence, upper structure can only consider the influence of first vibration mode, which means upper structure and isolation layer can approximately be regarded as two-particle system. (3) For two-particle system, ranges of kb/k1 under different site conditions are discussed in order to gain good isolation effect. Under the site of class Ⅰ, the figure of kb/k1 should be smaller than 0.09-0.23; under the site of class Ⅱ, the figure of kb/k1 should be smaller than 0.06-0.09; under the site of class Ⅲ, the figure of kb/k1 should be smaller than 0.03-0.06; under the site of class IV, the figure of kb/k1 should be smaller than 0.03.What’s more, the software Etabs is used to analyze an isolated historic building of a practical project. The effects of site condition, stiffness degradation of upper structure in the earthquake and rubber hardening of bearings on seismic response of isolated structure are discussed. According to analysis, conclusions can be drawn as following:(1) The historic building, treated by isolation retrofitting technology, has good isolation effect under different site conditions. (2) Rubber hardening of bearings has a non-negligible effect on the seismic response of isolated structure. In order to avoid the occurrence of rubber hardening under strong earthquake, it is better to use isolation bearing with bigger diameter to ensure that the horizontal displacement of bearings is under 250% y. (3) Isolation retrofitting is not suitable for all of the historic buildings. For historic building in the region of 7 or over 7 intensity that do not have ring beams and structural concrete columns, it is suitable to apply isolation retrofitting technology when its mortar strength is enough. In the region of 7 intensity (a=0.1g), the mortar strength should be higher than 0.5MPa; in the region of 8 intensity (a=0.2g), the mortar strength should be higher than 2.0MPa; in the region of 9 intensity (a=0.4g), the mortar strength should be higher than 5.0MPa.In the last, local underpinning of a project is introduced, which had been carried out before the underpinning folder beams were formed. The influence of opening holes in the walls of upper structure is analyzed and calculated, and it can be a reference for the design of underpinning members in such underpinning process. |
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