Multi Degree of Freedom Effects on Ductility Reduction Factor for Near Fault Ground Motions

Study on single degree of freedom (SDOF) structures provided information on seismic demand for elastic and inelastic systems. But this information needs to be modified to become of direct use for design of real structures, which are mostly multi-degree of freedom (MDOF) systems, governed by several modes. According to the near fault ground motions have cause much damage in the vicinity of seismic sources, this paper evaluate the modification that must applied to strength reduction factors derived from simplified SDOF models in order to account for MDOF structures in near fault zones. This proposed by estimation the ratio of strength in MDOF systems that result by limiting maximum story ductility ratio to the strength corresponding to the same ground motion and same level of ductility in an equivalent SDOF system having a period equal to the fundamental period of the MDOF structures. Nonlinear dynamic time history analysis were carried out on four steel moment resisting frames with two distinct behavior of yield mechanism, undergoing five level of ductility ratio when subjected to 15 near fault ground motions with forward directivity effects. Modification factors spectra were computed as a function of period and number of stories and were compared to those of corresponding spectra for far fault ground motions. The required modification factor for inelastic MDOF systems, for near fault motions was shown to be dependent on target ductility ratio and the type of yield mechanism and to a lesser degree, period of vibration and number of stories. The result demonstrate that in the low level of ductility ratio, during the short period range, the modification factor given from near the fault ground motions is less than those from the far fault ground motions, and this is true during all period range as the level of ductility is increased. Finally, since the equivalent pulse of near fault ground motions have significant effect on structural response, modification factors were proposed as a function of the ratio of structural period to equivalent pulse period and ductility ratios.