Equivalent Lateral Load Method vs. Response Spectrum Analysis Which Way is Forward

Samir H. Helou, Ibrahim Muhammad


In earthquake prone zones like the region in which Palestine is situated, structures have to resist added forces induced on them as a result of member distortions caused by the ground motion on which they rest. The response is attributed to the inherit properties of the structure itself, the nature of the exciting motion as well as the particular foundation system of the structure.

The present study aims at clarifying the prudent technical approach for the structural analysis and design for seismic action in compliance with pertinent code requirement; namely that of the IBC and the UBC. Of the recognized three dimensional standard methods of seismic analysis of multistory structures are the Equivalent Lateral Load method and the more sophisticated Response Spectrum Method. The later is better known as the Modal Analysis procedures both methods form the foci of the present study. Other methods are available but they are beyond the scope of the following discourse. Examples that illustrate the difference and the limitations of the two prime methods are presented in addition to the study of the effect of various seismic parameters that include, inter alias, height, topology and the vertical distribution of stiffness and mass on the overall structural response. Moreover, prevalent structural systems which include the locally popular yet essentially adverse practice of adding unreinforced facade masonry walls to framed reinforced concrete structures is briefly alluded to.

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