Assessment of Spectrum Modification Factor for Design of Steel Moment Frames in Near Fault Regions

Iranian Seismic design code (standard No. 2800) increases the design spectrum ordinates by a modification factor (N factor), to consider the damaging effects of near fault ground motions in design procedure of structures. Accordingly, the spectrum modification factor depends on fundamental period of vibration and seismicity level of the region. In this paper, the preciseness of the expression given by Iranian seismic design code for calculation of spectrum modification factor is examined and estimations are suggested for N factor by evaluating the near fault effects on inelastic seismic response of steel moment-resisting frames. For this purpose, the response spectrum shape and nonlinear seismic response of sample structures are evaluated for one set of far fault and three different sets of near fault ground motions. Incremental dynamic analysis is utilized to obtain the seismic response of the sample frames (including five steel frames with 3 to 15 stories) in various intensity levels. The results of the study demonstrate that modification factors provided by standard No. 2800 does not precisely take account the effects of near fault excitations on inelastic seismic response of steel moment-resisting frame structures. Also, it was found that there is no meaningful relationship between spectrum modification factor and structural period.