SEISMIC RESPONSE ASSESSMENT OF AN EXISTING RC STRUCTURE RETROFITTED WITH STEEL EXOSKELETONS

Several reinforced concrete (RC) structures built in Europe in the first decades after WWII do not meet the current safety standards, as those older buildings were built without any regard to the seismic actions and the structural details for ductility. Moreover, to reduce land use and environmental impact due to the construction of new buildings, governments are encour aging the upgrading and the reuse of older ones, rather than their demolition. Consequently, nowadays seismic retrofitting of existing structures is a very frequent request that civil engi neers must handle. In general, such a challenge could be addressed combining member- and structure-level techniques. In principle, global (or structure-level) techniques may represent a feasible retrofit solution not only for structures characterized by low lateral stiffness, but also for buildings exhibiting a significant number of under-designed members with respect to the design seismic action suggested by modern Codes. Furthermore, these techniques could rep resent a more cost-effective strategy rather than the upgrading of existing members, especial ly when the potential disruption of occupancy and the replacement of non-structural elements are considered in the design process. More specifically, this paper aims to show how existing RC buildings can be seismically upgraded through retrofitting by adding external steel brac ing systems referred to as “exoskeletons” as they are placed along the outer surface of the building. Although such a technique has a significant impact on the structural dissipation ca pacity and allows avoiding soft-storey mechanisms, its effectiveness is generally affected by the detailing of the braces and connections against buckling and post-buckling fracture. Con sequently, an accurate model of the nonlinear response of the exoskeleton is as essential as representing the existing RC structure response in case of rare or very demanding earth quakes. In this context, this paper describes the modelling and analysis of an existing RC frame structure for which an exoskeleton was designed according to the current Italian code.