Seismic Performance Of Existing Buildings With Added Large-spanned Steel Structure Under Rare Earthquakes

With the acceleration of urbanization in China, its city scale is expanding rapidly. Theexisting facilities and space can no longer satisfy the requirements of people’s life.Development of urbanization in China is a long process, which cannot be realized over onenight. The existing buildings in major cities are either newly built or have been used for manyyears. To increase the building areas either by means of rebuilding after demolition at thesame places or by relocating on other sites would increase the cost and waste the landresources. Therefore, to expand the service areas and utility rate of the existing buildings,priority is given to renovating or storey adding. For such kind of buildings, due to the lowlevel of the original design, some are without earthquake fortification, others are of lowerstandard owing to the lack of consideration of future development. Accordingly, research onthe design of such buildings for storey adding or renovating is of particular importance.Because of its high strength, light weight, short construction period, occupation of smallland area during construction and convenience of construction, steel structure is a preferableand favorite choice in the renovation of old buildings by means of storey adding, which hasbeen well accepted in the trade.In this paper, an actual project was taken as an example. According to the characteristicsof RC structure, the rational design scheme of storey adding was proposed. Then by using thefinite element analysis software SAP2000, three-dimensional models were built. The modalanalysis was conducted for the original structure before and after storey adding and the steelstructure itself so as to investigate their changes in the natural vibration periods and modes.Dynamic elastoplastic time history analysis was also carried out for the structures with andwithout added storey and the steel structure as well to study their structural responses underintensities of rare earthquakes.The analysis shows that after storey adding, considerable increases in the maximumsways, the maximum angles of inter-story drift and the maximum inter-story shear forcesoccur to both the RC structure and steel structure, and the adding of steel structure intensifiesthe seismic response of the original RC structure; and for different seismic waves imposed atdifferent directions, the extent of increase also differs. The RC structure exerts greatamplification effect on the seismic response of the added steel structure, which cannot be taken into account in the simplified design so that the model of overall structure after storeyadding should be built to conduct the analysis of structural responses.From the observation of the time, sequence and process of the appearance of plastichinges in the structures with and without the added storey, the added storey makes the plastichinges occur earlier, more and develop more extensively; the lower ends of columns in thefirst floor, the ends of beams and upper ends of columns in the second floor are all the weakparts of the structure, to which much attention should be paid in storey adding, andcorresponding measures of strengthening be taken when necessary; under rare earthquakes,some columns may stop working prematurely, which greatly lowers the seismic performanceof structure and effective strengthening measures should be adopted in this case.