Consequences of the Recently Modification of Iran Seismic Design Code (No. 2800) (Due to A*B Form) and Proposing Forward Directivity Effects Coefficients (Na and Nv)

This article intends to discuss and assess the weaknesses of Iran seismic design code (No. 2800, 4th version) including spectral ordinates at the constant maximum acceleration and velocity ranges, starting point at response spectra. The dependency of corner period (Ts) on the strength of strong motion rather than the spectral ordinates at constant maximum acceleration as it is in the existing forms is discussed. A simple methodology for predicting the period dependent forward directivity effects parameter, N(T) is proposed. The real spectral ordinates data at 120 near source sites on the four types of soil types from the two cities in the United State are used to develop the proposed forward directivity factor N(T). The average values of the obtained design response spectra are compared with those of the 2800’s third and fourth versions and the differences are assessed. It is concluded that modifying the currently used design response spectra seems to be inevitable due. Further, the parameter (S) in the existing spectral shape form (S+1)B1(T) seems to be the cause of ambiguity to the users and needs to be changed in current status. Finally, four new forms of design response spectra are proposed meanwhile a conscious solution for the forward directivity effects parameter representative N(T) is recommended.