Combination of different types of damped braces for two-level seismic control of rc framed buildings

Hysteretic damped braces (HDBs) represent an effective and low-cost solution for the seismic retrofitting of reinforced concrete (RC) framed structures. However, notable damage to nonstructural elements can be observed at the serviceability design earthquake (SDE), when activation forces of the HDBs are set too high in order to attain structural safety (SS) performance level at the basic design (BDE) and/or maximum considered (MCE) earthquakes. Moreover, HDBs designed at the immediate occupancy (IO) performance level prevent nonstructural damage of MIs under SDE but may collapse and/or induce structural damage for high intensity seismic loads, due to early activation which renders them unable to develop large energy dissipation. On the other hand, as regards significantly increasing dissipation without extra stiffness, nonlinear fluid viscous damped braces (VDBs) offer a promising solution provided that limit states of the VDs are not exceeded. This work proposes a two-level displacement-based design procedure, where HDBs, ensuring SS at the MCE, are combined with HDBs and VDBs, calculated for IO at the SDE. Gap-hook elements attached in series or in parallel to the HDBs are also examined, with the aim of preventing their deformation at MCE and delay their activation at SDE, respectively. A six-storey RC framed structure designed in L'Aquila (Italy) for moderate seismic loads is to be retrofitted in a high risk-seismic region. The OpenSEES platform is considered for the numerical modelling of the original and retrofitted structures. Nonlinear multi-stripe analyses are carried out with reference to actual real records scaled to the SDE, BDE and MCE seismic intensity levels assumed in the current Italian code.