Assessment and vulnerability reduction of under-designed existing structures: Traditional vs innovative strategy.

• The behavior of under-designed existing structures is investigated. • Traditional and innovative retrofitting are compared by a finite element analysis. • The innovative strategy provides a better nonlinear performance of the structure. • The presented strategy reduces the dynamic vulnerability of the existing structure. In the present paper the structural behavior of under-designed existing structures is investigated. The analyzed existing structure is assumed as originally designed in the past with respect to old codes and with respect to gravitational loads only. The under-designed structure is considered to be located in Italy in an area now classified by the new codes as high seismicity zone. In the investigation traditional and innovative steel braces are modeled and compared for the retrofitting of the structure by using a finite element analysis with a displacement based approach. With respect to a traditional retrofitting strategy the adopted retrofitting strategy with innovative steel braces allows a localization of the input seismic energy into the innovative steel braces which are endowed with dissipative properties and thus a better performance of the original existing structure is noted in the elastoplastic range. With respect to a traditional retrofitting system the innovative steel braces applied to under-designed existing structures show to provide a more robust nonlinear behavior of the under-designed structure and a significant reduction of the plasticization in the original existing structural frames when they are subject to dynamic events. These features are associated to a vulnerability reduction of the under-designed existing structure also at the ultimate limit state. [ABSTRACT FROM AUTHOR]