Estimating the Higher-Mode Response of Ductile Structures

While the importance of higher-mode actions is appreciated within the engineering community, the affect that ductile nonlinear response has on higher-mode characteristics and the subsequent implications this has for design has received little attention. In this article, the manner in which the higher-mode response of frame-wall structures is affected by inelastic behavior is closely examined and a means of accounting for this in design is proposed. The work focuses firstly on the characteristics of the higher modes present at the development of peak response and then considers how these characteristics would affect the total forces in the building. The study utilizes a series of nonlinear time-history analyses of two different groups of RC frame-wall structures subject to a suite of real records. It is shown that a new modal analysis approach that incorporates transitory inelastic modal characteristics gives significantly improved predictions of peak base shear in frame-wall structures than more traditional modal analysis methods which use elastic higher-mode characteristics. The issues associated with the use of transitory inelastic modal characteristics are discussed and various challenges that would need addressing for the prediction of other response parameters and structural types are identified.