Seismic reliability assessment of RC tunnel-form structures with geometric irregularities using a combined system approach

Tunnel-form structures represent a new type of structural systems with enhanced earthquake resistance and considerably reduced construction times, if compared to conventional reinforced concrete frames and dual systems. Due to the limited information about the seismic performance of tunnel-form buildings in the presence of vertical and horizontal irregularities, seismic design standards generally prevent such irregularities and therefore impose significant architectural limitations. To address this issue, a liability assessment is here conducted on irregular 5- and 10-storey tunnel-form buildings subjected to 12 different earthquakes, representing a design spectrum. The structural response of these buildings is obtained under both Design Basis Earthquake (DBE) and Maximum Considered Earthquake (MCE) hazard levels by using time-history and nonlinear static (pushover) analyses. The reliability of the buildings is then assessed by using a novel combined system approach, in which the structural effects of walls and coupling beams at each storey are modeled as series and parallel subsystems. The results of this study show that, despite the geometric irregularities, all the structural elements could satisfy the Immediate Occupancy (IO) performance level under both DBE and MCE scenarios with over 95% reliability. Therefore, enforcing the regularity for tunnel-form structures in current seismic design guidelines appears to be too conservative; the results of this study can then prove useful for a more efficient design of irregular tunnel-form structures in seismic regions.