LRFD based on Drift Ratio for Serviceability Limit State of SMRF

Abstract eng:
In reliability-based limit state design, randomness and uncertainties existing in structural design are taken into account rationally by applying probability and statistical theory, and the structural performance level is expressed quantitatively using failure probability. The difference in the performance level and the length of service life can be considered explicitly. However, the performance level of only structural members can be estimated in the first generation of reliability-based design. The objectives of this research is to develop a practical reliability-based seismic design format for ordinary steel moment resisting frames (SMRF) taking the seismic activities around the site, structural characteristics, and accuracy of the estimation of structural responses into account. As the damage level of a structure under a strong ground motion can be directly related to the interstory drift ratio, the limit state is defined as the response being beyond the tolerable interstory drift ratio of the structure. As the first step, a load and resistance factor design (LRFD) format for serviceability limit state is proposed in this paper. Assuming that the probabilistic model of spectral response at the site is available, the elastic response of the structure is estimated using SRSS. Expressing the design criteria by a linear function, LR-factors are estimated by applying the practical method for estimating the factors developed for a combination of load processes. Because the applicatility of the existing practical method for LR-factors is in question, a modefied method is proposed and the accuracy of the method is investigated using numerical examples. Although the original method is comparable with AFOSM, it provides optimistic estimates when more than one modal responses are dominant. On the contrary, the proposed practical method provides accurate estimate even for such cases. The proposed design format is applied to serviceability limit state design of a ninestory SMRF. While the means of required tolerable interstory drift ratio at 8th and 9th stories are overestimated by AFOSM, it is appropriately estimated by using the proposed practical method. For designing a low-rise SMRF, of which the 1st modal response is dominant, it is also proposed to use interstory drift instead of the first modal vector. Then an eigen value analysis can be avoided. The accuracy is investigated using four-story SMRF. The research is being continured for ultimate limit state design, in which the change in modal vector after yielding should be considered.

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