DETERMINATION OF MODAL DAMPING RATIOS FOR NON-CLASSICALLY DAMPED REHABILITATED STEEL STRUCTURES

Abstract

There are many reasons for rehabilitation of existing buildings. Adding stories is one ofthe most common reasons. When a steel building is retrofitted by concrete jacketing for addingstories, this system contains several structural systems. These systems are composite concrete andsteel systems in initial stories, welded steel system in middle stories and cold-formed steel framesin upper stories. Dynamic analysis of hybrid structures is usually a complex procedure due tovarious dynamic characteristics of each part, i.e. stiffness, mass and especially damping.Availability of different damping factors causes a higher degree of complication for evaluatingseismic responses of hybrid systems. Due to using several structural systems, an existing buildingis changed to hybrid system. Damping matrix of these structures is non-classical. Also, thenonlinear software is not able to analyze these structures precisely. In this study, a method andgraphs have been proposed to determine the equivalent modal damping ratios for rehabilitatedexisting steel buildings for adding stories.