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.