Analysis Of The Strength Reduction Factor For Multi-story Steel Frame Structures

As present, more and more countries adopt the design method based on the strength for the seismic design of structures, which has been being the main method in the engineering field for a long period of time. The design method based on the strength is to get the seismic design force by making the elastic force divide the strength reduction factor. So the strength reduction factor is the key to the design method based on the strength, which is directly related to that the seismic design of structures is whether safe or economic.Study on the strength reduction factors has already made some progress abroad, and some research results have been applied, such as NEHRP, the American seismic code, which is given the strength reduction factor R of different structure systems as well as the displacement amplification factor Cd combined with the engineering practice and the investigations of the seismic damage, the American code UBC、IBC206、European code EC8、Japanese code BSL、New Zealand code NZS4203and so on, which are given the corresponding strength reduction factors according to the different structure systems, while the Chinese seismic code adopts a single hidden strength reduction factor for all structure types, which is given according to the probability of earthquake occurrence, not reflecting the reduction functions of different structure ductility for the seismic load. For the steel structure system, that failure to make full use of its good ductility and energy dissipation capacity, to a great extent, leads to that the seismic design of the steel structure not reasonable.With the in-depth study of Chinese seismic theory, General rule for performance-based seismic design of buildings (CECS160:2004) has made a breakthrough. It is given different strength reduction factor and the displacement amplification coefficient with different structural styles considering the influence of the structure deformation on the seismic performance, but they are comprehending and adsorbing the foreign advanced seismic design concept with reference to the NEHRP, to put forward the strength reduction factor which is suitable for our Chinese seismic design also needs further study.Based on the research about the strength reduction factor at home and abroad, the research work is done in the following aspects:(1) review the method to determine the structural strength reduction factor and introduce the IDA method in detail; (2) Based on the theory of the strength reduction factor, this paper introduces two methods to determine the structural strength reduction factor, namely the direct method and indirect method, and the indirect method is improved, which can overcome the problem that according to the conventional method to determine the significant yield point to solving the strength reduction factor will bring bigger error, when the structure gets the limit value of the interlayer drift, it appears to yield but no significant yield steps.(3) This paper adopts the IDA analysis method to study the strength reduction factor of the steel frame. It respectively uses the direct method and the improved indirect method to get the strength reduction factor of the steel frame, and verifies the accuracy and efficiency of the improved indirect method. It also studies the influence of the number of floors as well as the site effect on the strength reduction factor.