ABSTRACT
ABSTRACT: A method is presented for the determination of the maximum structural response through a linear elastic spectrum analysis using equivalent modal damping values instead of the crude strength reduction factor. Geometrical and material non – linear structural effects are converted into equivalent time – invariant modal damping values. These equivalent damping values for the first few modes of the structure are numerically computed by first iteratively forming a frequency response transfer function until certain smoothness criteria are satisfied and then by solving a set of non – linear algebraic equations. A design – oriented scheme is developed in order to apply the equivalent modal damping concept to the seismic response analysis of multi-degree-of-freedom (MDOF) building structures. This scheme involves a) the quantification of equivalent modal damping of a structure for predefined deformation limits and b) the use of spectrum analysis and modal synthesis for the calculation of the design base shear of the structure. For illustration purposes, curves providing equivalent damping as functions of period for the first few modes as well as design acceleration versus period for given equivalent damping are constructed using a large number of steel moment resisting frames excited by various seismic motions. The whole design procedure is illustrated by means of a steel moment resisting framed structure. It is concluded that the proposed design scheme can be viewed as an improvement to the force based method of current seismic codes with equivalent modal damping values playing the role of the strength reduction factor.
