STRENGTH REDUCTION FACTORS FOR MULTI-DEGREE-OF-FREEDOM SYSTEMS

摘要

Strength reduction factors of multi-degree-of-freedom (MDOF) structures are investigated. The study is based on the results of strength reduction factors computed for eight steel moment resisting-frame buildings undergoing different levels of inelastic deformation when subjected to 28 earthquake ground motions. The ground motions used in this study were recorded on different soil conditions corresponding to firm sites (site classes A, B, C and D according to the 1997 NEHRP Provisions). Four main variables are considered: a) the fundamental period of vibration of the building; b) the level of inelastic deformation in the building; c) number of stories; and d) soil conditions. Special emphasis is given to the ratio of the strength reduction factor that results in an adequate control of the maximum interstory displacement ductility ratio in a MDOF structure to the strength reduction factor corresponding to the same ground motion and same level of inelastic deformation in an equivalent single-degree-of-freedom (SDOF) system having a period of vibration equal to the fundamental period of the MDOF structure. Results indicate that strength reduction factors of MDOF structures are typically smaller than those corresponding to SDOF systems. Thus, the lateral strength of MDOF structures needs to be higher than the one obtained from SDOF systems. The ratio of the MDOF to SDOF strength reduction factor is primarily affected by the number of stories and the level of inelastic deformation. Finally, for the site conditions considered in this study, the soil type has only a small effect on MDOF reduction factors.