PERFORMANCE OF HIGH-RISE BUILDINGS WITH TRANSFER FLOOR UNDER SEISMIC LOADS

Abstract

In many high-rise buildings, architectural requirements may result in a variable configuration for the vertical structural elements between the stories of the building. To accommodate such vertical elements discontinuity, a "transfer" floor conveying vertical and lateral loads between upper and lower stories must be introduced. A drawback of the transfer floor is the sudden change in the building's lateral stiffness at its level: the structure becomes susceptible to the formation of a soft-story mechanism under moderate to severe earthquakes. These buildings generally showed conventional elastic behavior for minor earthquakes but suffer extensive crack in the vicinity of transfer floors for major earthquakes. However, for design purposes current numerical modeling of high-rise building adopts reduced stiffness for the vertical elements for strength analysis and full stiffness for serviceability and drift analysis: a tradition that needs to be verified. A 3-D numerical model is built-up for a high-rise building with such vertical irregularities and analyzed using lateral equivalent static, elastic response spectrum, linear and nonlinear time-history analysis techniques. The effect of transfer floors on the buildings drift is investigated where judgment for adopting a full or reduced stiffness for the vertical elements is scrutinized. Another aspect that basically affect the analysis and design of high-rise building with transfer floor is the construction stages analysis where engineers generally determine the behavior of structures by adopting a linear elastic numerical analysis including summations of vertical column loads. For high-rise buildings with multiple construction phases, the structural responses may significantly diverge from such a typical linear analysis results. Furthermore, time-dependent long-term deformation resulting from construction sequence can cause a redistribution of the structural responses that cannot be estimated using conventional methods. The existence of a transfer floor within the building height adds to the complexity of the structural system which also cannot be analyzed using typical linear analysis methods. Fortunately, recent advancement of the finite element method facilitated the nonlinear analysis and provided tools for proper design of complex structures, especially high-rise buildings. Thus, construction sequential analysis which is now essential for high-rise buildings with transfer slabs can be easily included in the analysis. However, this nonlinear analysis is not so popular because of lack of knowledge about its necessity and scope. In this research, a nonlinear numerical analysis for high rise buildings with transfer floor is presented, considering the effect of construction stages. The analysis results are compared to the results of a conventional analysis for these types of buildings. The main objective of this work is to assess the effect of construction stages analysis and time dependent material properties of various structural elements for a high-rise building with a transfer floor on the performance and design of these buildings.