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

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