In-plane and out-of-plane nonlinear seismic response of masonry infills for hospitals retrofitted with hysteretic damped braces

The present work studies the combined in-plane (IP) and out-of-plane (OOP) collapse of unreinforced masonry infills (MIs) of strategic infrastructures such as hospitals retrofitted by hysteretic damped braces (HYDBs), where immediate occupancy (IO) and structural safety (SS, e.g. life-safety or collapse prevention) are the performance levels required after low-to-high amplitude earthquakes. Despite many advances in the development of new supplementary energy dissipation systems, few investigations have evaluated their effectiveness in improving the IP-OOP response of MIs. Because of the complex articulation of medical centres, reference is made to a five-storey pavilion of the hospital campus of Avellino in Campania (Italy). Bare and infilled structural models of this reinforced concrete pavilion are designed in a medium-risk seismic zone, in order to fulfill the corresponding IP drift ratio thresholds provided by a former Italian code for limiting nonstructural damage of MIs at the serviceability limit state. The IO and SS performance levels are checked in a displacement-based design procedure of the HYDBs at the Serviceability (SDE) and Basic (BDE) Design Earthquakes. An optimization procedure is considered, taking into account that an anticipated (delayed) activation of the yielding devices may give rise to concerns about their suitability with respect to high (low) intensity seismic loads. The demolition and reconstruction of MIs, which generally goes hand in hand with the introduction of damped braces in existing frames, is avoided. To this end, exterior, with diagonal damped braces in the window bays, and interior, with less-intrusive chrevron damped braces, retrofitting systems are adopted. Two aspect ratios of MIs are analysed assuming corner arrangements along the perimeter. Nonlinear time-history analyses for the four reference seismic levels assumed in the current Italian code highlight that IP and OOP failures of MIs can be avoided when the activation of the HYDBs at the SDE can be attained without jeopardising structural safety at the Maximum Considered Earthquake (MCE).