Seismic Behavior Analysis Of Multi-story Steel Frame Structures Under The Vertical Seismic Effect Of Near-field Earthquake

Our Code for seismic design of buildings was based on far-field earthquakes, so only the influence of far-field should be considered when structure was designed. The vertical ground motions were so weak in far-field earthquake that they had little effect on common structures. However, near-field earthquakes are often accompanied with strong vertical ground motions which may cause great effect on the seismic performance of structures. It's necessary to explore the vertical ground motion of the near-field earthquake on seismic performance of structures. Since steel structures have excellent seismic performance and its domestic and foreign research on near-field earthquake was little. This paper selected steel frame structure as the research object, and analyzed its seismic performance in near-field earthquake which has strong vertical ground motion.Firstly, this paper discusses the performance characteristics of vertical ground motions in the near-field zone, using the data analysis software like MATLAB and Excel to discuss the effect on vertical earthquake from magnitude, fault distance, site conditions and focal mechanism, in order to verify that the near-filed earthquake has a strong vertical effect. According to the present code, a multi-story steel frame structure was designed. One of the frames was taken as an analysis model, and dynamic nonlinear time history analysis was proceeded by SAP2000. 8 group seismic waves which have significant vertical effects were chosen by SeismoMatch. One case is that only the horizontal seismic waves were input,the other is that both horizontal seismic waves and vertical waves were input. A total of eight group were compared, to discuss the effect on horizon displacement and vertical displacement of the structure, as well as the internal forces of beams and columns.After research and analysis, we find that the vertical ground motion generally performed significantly in near-field zone, and the peak acceleration ratio is bigger than the proposed value of 2/3 in code. In the near-field zone, vertical seismic effect on soft soil foundation was significantly severer than hard soil and bedrock foundation. After modeling analysis, vertical ground motions have little effect on the horizontal displacement of the structure, but have significant effect on the vertical displacement of the structure. The bigger peak acceleration ratio is, and the less time difference between the large pulse occurring in vertical ground motions and horizontal ground motions is, the moe significant effect has on the vertical displacement of the structure. Vertical ground motion has little effect on the internal forces of frame beams, but a great effect on the axial force of frame columns. In addition, the amplification effect of middle columns are much greater than the side columns. What's more, the vertical earthquake may cause tension in columns, so steel frame structure's seismic performs better than RC frame structure's.