Long-term friction increase effects on a combined base isolation/supplemental damping system

A two-step reassessment study is presented on a base-isolated building, twenty years after its construction. The structure hosts a non-profit association providing social services and serves as a civil protection emergency station in a small town near Florence, Italy. The building is equipped with a Base Isolation with Supplemental Damping (BISD) system, combining steel-Teflon (ST) flat sliding bearings with pressurized fluid viscous (PFV) spring-dampers. This installation, which represented the first practical application of a ten-years research activity carried out by the first and second author on this technology, made an absolute novelty at the time in Italy and Europe in the field of BISD strategies. The first step of this study involves the structural reassessment of the building—originally designed according to OPCM 3274 (2003)—based on the current Italian Building Code, NTC 2018. The second step investigates the potential impact of long-term environmental exposure, particularly moisture, on the ST isolators, which may alter their frictional behavior over time. Non-linear time-history analyses were conducted by varying the friction coefficient of the ST devices from the design value of 2%—determined through qualification testing—to 5.5%, the upper limit suggested in the technical literature and by EN 15129 under worst-case aging and environmental conditions. The results confirm the effectiveness of the BISD systems for medium-high earthquake levels (Basic Design Earthquake – BDE and Maximum Considered Earthquakes – MCE) also by considering post-ageing conditions.