Study On The Seismic Performance Of Reinforced Concrete Frame Structures With Inserted Steel Storey

Adding of steel storeys is a common way for increasing storeys of a building in recent years due to its advanced technical,safety,economy,light weight,good seismic performance and fast construction as compared with concrete structures.However,there are no design and construction specifications issued for storeys adding of steel structure so far.The hybrid structure with added steel storeys exists the following problems: the stiffness and mass distribution in the vertical direction are not uniform,damping ratios the original concrete structure and steel structure are different,the difficult connection between the two structures and so on,all of which will have influence on the whole seismic performance of the hybrid structure.In order to study the seismic performance of the hybrid structure with added steel storeys,the following work has been done in this paper:First of all,combined with adding steel storey for a retrofitting project in Xi'an city,the models of original concrete structure and hybrid structure with added steel storey were built respectively by using PKPM design software.Modal analysis,vibration modal decomposition response spectrum analysis and dynamic time-history analysis under minor earthquake were conducted for the two structures.Then the finite element analysis software ETABS was used to model the original structure and the hybrid one,and the dynamic elastic-plastic time-history analysis of the two structures were carried out.The results of modal analysis show that the cycle of the original concrete frame decreases after the insertion of the steel storey due to the increase in the stiffness.The result of elastic analysis shows that the maximum horizontal storey displacements decrease,and the largest decrease occurs at the location of steel story insertion,i.e.,by about 34%.The variation of inter-storey displacement and inter-storey angular displacement is similar,that is,the overall decrease at the inserted steel storey is about 17.4%.The shear forces of the inserted steel storey and adjacent storeys increase by about 9%.Elastic-plastic analysis results show that: under the action of major earthquake of EL-Centro wave,Taft wave and Lanzhou artificial wave,the horizontal inserted steel storey displacement decreases on the whole,and under Lanzhou artificial wave,the reduction is the most,about 33.41%.The variation of the inter-storey displacement and angular displacement are similar,and less than the original structure,and under Lanzhou artificial wave the storey displacement and angular displacement are reduced most,by about 31.1%,after the insertion of steel storey.The inter-storey shear forces are enlarged after the insertion,under Lanzhou artificial wave action the increases is the most,about 44.85%.plastic hinges occurred first at the ends of beams then columns,which is in accordance with the criterion of strong beams and weak columns.Both elastic and elasto-plastic analysis show that after the insertion of steel storey,the overall stiffness the original structure is increased resulting from reduction of some columns heights and additional structural members.In the similar retrofitting projects this trend may cause the increase and internal forces both in the uppers structural members and foundations which may result in necessary strengthening.Meanwhile the main beams are usually connected to the original columns at their middle heights,special attention should be paid to their joints design and construction so as to meet the requirement of weak beams and strong columns.