Research On Design And Construction Of Adding Storeys On Building By Outer-Jacketing Megaframe

As a method to add stories on the building, using the outer-jacketing structure to add stories is more and more widely used in reconstructing the existing buildings, for using this method, the added stories are not restrained, and the existing build-ings are not seriously affected. The main frame of the megaframe structure carries the whole vertical load and the most of horizontal load. The secondary frame only carries it vertical load and transmits it to the main frame, and it carries few horizon-tal load. If the secondary frame in the first storey of the mega structure is removed, the distribution of the structure's stiffness varies a little. Owing to this characteris-tics, using the outer-jacketing structure to add stories around the existing building can be adopted.There are some problems needed to be urgently solved in using the outer-jacketing megaframe to add stories, such as how to meet the requirement in the seismic code"no damage in the frequent earthquake, mendable in the medium earthquake and no collapse in the strong earthquake". So it is necessary to make deeper research on the mechanism of collapse. The roof of the existing building can not carry the construction load during the construction of the megaframe floor. Therefore, the added megaframe can not be constructed floor by floor. In this case, the construction technics of no support self bearing floor is urgently needed. The design methods and the detailed construction measures corresponding to the con-struction method and some detailed construction measures relating to the long span prestress technology are needed. In order to solve the above problems, we made some research in this paper.Based on the planar linear element model, using the nonlinear analysis pro-gram IDARC-2D, the seismic performance of 375 outer-jacketing megaframes are investigated. These megaframes are different in stories of megaframe and stories of secondary frame, and they are built in site-class I, II and III and the seismic group of the first, the second and the third. After the using scope of new concrete mega-frame structures and the seismic construction measures are given, the conclusion is: these cases meet the limit of elastic-plastic drift ratio and the transformable system does not occur, which meet the requirement of current seismic code"no collapse in the strong earthquake". They are: the main frame of outer-jacketing megaframe in the zone of seismic fortification intensity 8 conforming to measure grade 1-st, the height of megaframe under 50m conforming to measure grade 2-nd and that over 50m conforming to measure grade 1-st in zone of seismic fortification intensity 7. The seismic performance of the same model are analyzed by using the static elastic-plastic analysis method recommended in the current seismic code using two kinds of distribution of transverse load, inverse triangle and uniform. Capacity spectrum method, modified capacity spectrum method andα-T method are used to analyze the frames. The same conclusions are got with the elastic-plastic time history me-thod.The outer-jacketing megaframe has the transform floor. The influence of dif-ferent construction methods to the internal force of structural member are analyzed such as floor by floor, main-secondary floor and the self bearing floor of the mega-structure floor. The noteworthy problems during construction are presented. The load of the mega structure floor is too big, and the roof of the existing building can not carry the construction load of the first floor of the mega structure. Considering this, the construction methods and measures of no support self bearing floor is dis-cussed, which can not affect the service of the structure and ensure the safe con-struction.Start with the team work of the main frame and the secondary frame, whether the simplified method of neglecting the stiffness of the secondary frame can be used or not is analyzed. Based on the design proposal got from the elastic-plastic analy-sis of the outer-jacketing megaframe, the design procedure is compiled, which not only provides measures for the scheme selection of this kind of structure, but also facilitates its design.At last, combined with a reconstruction project of a test building in a collage in Harbin, the design progress of an outer-jacketing megaframe and the correspond-ing construction methods were given. The seismic performance of this frame under rarely earthquake was analyzed, and the elastic-plastic analysis results in this paper were verified. All these provided reference for the detailed design of this kind of frame.