Modification of the Design Response Spectra in Iranian Standard 2800 (Fourth Version) Taking in to Account the Directivity Effects

Many populated urban areas of Iran are threatened by near source problems; forward directivity effects and fling step, which may cause huge catastrophic. The major reason is that this country is surrounded by the two huge mountain chains, Alborz and Zagross. As an example, Tehran is a city faced around with four near active faults, Mosha, North, Ray, and Karaj faults, which are potentially hazardous. This is under the condition that the design response spectra [A*B(T)] in Iranian Standard No. 2800, which is established on the basis of ten percent chance in fifty years, does not illustratively account for the directivity effects for sites located at near sources. This article is intended to propose a technique to descriptively modify the far field response spectra taking into account such problems for sites 20 Km far away from the active faults. The proposed modification factors are developed based on a limited number of near source data with and without directivity effects (58 recorded data) using three attenuation relationships. The proposed coefficients for four site soil conditions are implemented to the existing far field design response spectra presented in the fourth version of the response spectra. A comparison is made with those of UBC-97 and ASCE-7-2005 corresponding to two seismicity cities in the United State aimed at understanding how to assess their differences. The recommended technique may be interpreted as a start for developing a series of design response spectra having the potentiality of more accurately accounting for the near source problems.