The Use of the Dynamic Magnification Factor in the Dynamic Analysis of Framed Structures

Abstract

This paper examined the use of the dynamic magnification factor in the analysis of framed structures. It is a method of practice in dynamic analysis of structures to magnify static response by a dynamic magnification factor in order to obtain the equivalent dynamic response. This method has been applied to the dynamic analysis of many structures including bridges and some country’s codes of practice made specifications in respect of the dynamic magnification factor for the analysis and design of various types of structures subjected to dynamic excitation. The suitability of this method to the dynamic analysis of frames was investigated in this paper by carrying out static and dynamic analysis of four frames using the flexible frame model and the stiffness formulation. Dynamic responses were first obtained by direct analysis as solutions to the set of equations governing the motions of the frames and secondly by the magnification of the static responses using the dynamic magnification factors. By comparing the results obtained in both methods it was inferred that the practice of magnifying static responses to obtain their dynamic equivalents in frame analysis gives correct results only in the case of deflections and not in stresses. Finally, this practice should de discouraged or limited only to the case of deflections in the dynamic analysis of framed structures.