Analysis Of Seismic Resistance Of Existing Frame Structures After Storer Adding

In the 1980 s, the majority of residences and civil buildings in China were of multi-storey buildings. Most of them are inadequate in utility space, yet in reality they cannot be demolished nor their use cannot be stopped. Usually for such buildings they are retrofitted by adding stories to meet the new functional requirements.Directly adding concrete stories on the top of an existing building is characterized with low cost, ease and simplicity of construction. In this paper, an actual project was taken as an example, by using the finite element software SAP2000 three-dimensional structure model of the project building was built to carry out the modal analysis of the building before and after floor adding. Comparison between the fundamental periods and modes of the original structure with that after storey adding shows that storey adding has made the structure more flexible, for its fundamental periods become longer, e.g., its first three periods are prolonged by 48.03%, 46.87 and 46.31% respectively.Because the project is originally a four-storey building, two stories are to be added, and the number of added storied is large, time-history analysis of the structure was conducted to observe the elastic and elastoplastic dynamic responses of the structure to frequent earthquakes and rarely met ones. By the analysis of Elastic time history on the comparison of the lateral drifts at the top, inter-storey angular displacements before and after storey adding shows that under excitation of Lanzhou, Tangshan or Elcentro seismic waves, the displacements at the top of the building increase considerably, with the largest increase of up to 51%; and there is also a tremendous increase in inter-storey angular displacements, but the peak value is still with the allowable one specified in the code; and the second storey get the largest earthquake response as compared with the other stories.Through the elastoplastic time history analysis of the building before and after storey adding, it has been found that storey adding makes plastic hinges appear significantly earlier,and more plastic hinges develop in the second and third stoery; the building is more susceptible to the seismic waves in the Y direction for plastic hinges develop more significantly and occur in more structural components along this direction; all plastic hingesoccur only at the ends of beams but not columns, indicating the design has followed "strong column-weak beam" principle.