Simplified modal analysis of multi-storey RC buildings for application in seismic retrofitting

This paper presents a novel simplified approach for determining the fundamental vibration characteristics of reinforced concrete (RC) buildings for application in the seismic retrofitting of substandard multi-storey RC building structures. Even when retrofitted, such structures would likely respond inelastically under a major earthquake, where the seismic response is most realistically represented by computationally expensive nonlinear dynamic analysis. Towards reducing such computational demands, the capacity spectrum method (CSM) offers a practical design and assessment approach which utilises nonlinear static pushover analysis in conjunction with the dynamic characteristics of a structure to establish whether the seismic capacity of the structure meets the demands of seismic ground motion. In this respect, the dynamic characteristics of a structure can be ordinarily obtained by performing conventional direct modal analysis which requires the assembly of mass and stiffness matrices to solve the eigenvalue problem. This paper presents the development of a simplified numerical approach that determines the translational modes of vibration and the associated periods for a retrofitted building structure in both horizontal directions in a simplistic manner without assembling the mass and stiffness matrices. The accuracy of the proposed simplified approach is verified for realistic structures via comparisons against direct eigenvalue analysis. This, along with planned research on simplified pushover analysis, paves the way for the practical seismic retrofitting of substandard reinforced concrete buildings within the CSM framework.