Addressing the near-fault directivity effects for their implementation to design spectrum

Moghimi, Saed
Near-Fault Forward-Directivity (NFFD) ground motions are highly polarized and they have the potential to impose larger seismic demands on the structures. This is due to the presence of impulsive signals in the beginning of their velocity waveforms, which amplifies the response spectrum in periods close to pulse period. Different directivity models proposed recently can be used together with Ground Motion Prediction Equations (GMPEs) to estimate the response spectrum exposed to pulse-type ground motions. This study utilizes two directivity models to investigate the effect of different seismological and geometrical parameters on the amplification level that the directivity effect imposes on the response spectrum. It is shown that in Shahi and Baker (2011) (the first directivity model utilized in this study) slip rate, fault characteristic magnitude, hazard level and source-site geometric parameters play important role, on the response spectrum amplification. In Chiou and Spudich (2013) (the second directivity model), the characteristic magnitude and source-site geometry are the determining parameters. The observations from the case studies are used to set some simple rules for reflecting the forward-directivity effects on design spectra at the 475-year and 2475-year return periods. The concept of ground motion polarization (directionality) is also utilized in the determination of maximum rotated component (RotD100) for NFFD ground motions. For this purpose RotD100 is calculated for the near fault ground motions with and without forward-directivity effect and a conversion factor is proposed by taking the ratios of spectral demands of RotD100 horizontal component between pulselike and non-pulse recordings.