The modification of strength reduction factors for MDOF effect

This paper focuses on the modification of SDOF system strength reduction factors to suit MDOF systems. Firstly, an approach to calculation of the modification factors is presented and a method of determining storey ductility of structures with incremental dynamic analysis is introduced. Then, the modifications of the strength reduction factors to take account of MDOF effect are investigated for four RC frame structures strictly designed according to the Chinese seismic code subjected to 80 earthquake ground motions. The ductility level, structural period and site conditions effects on the modification factors are discussed and a simplified expression to estimate the modification factors is presented, which is a function of the structural period and the ductility level. It is concluded that the modification factors are primarily affected by the ductility level and structural periods and the inverse of the modification factors increases apparently with increasing ductility level and structural period. It also appears that the site conditions have little effect on the modification factors.