Seismic Design of Los Angeles Underground Transit Stations

The Los Angeles Metro transit system has been undergoing a rapid expansion since the 1980s. The expansion includes extensive segments with tunnels and underground stations. The Los Angeles area is subject to a significant seismic hazard, the greatest impact being ground shaking, which affects a large area compared to more localized earthquake effects such as fault rupture. Therefore, all underground stations in the Metro system are designed according to high standards of analysis and design to accommodate ground shaking. With the restraint provided by the ground, underground subway stations are inherently more resilient when subject to ground shaking than above-ground building structures. Nevertheless, Metro provides Supplemental Seismic Design Criteria as part of its Design Criteria to ensure reliable and robust design of new underground structures for resilience to ground shaking. The Supplemental Criteria requires designing for the fundamental racking (side sway) impact of ground shaking due to vertically propagating shear waves. However, some subway stations are adjacent to large building structures with deep basements that modify the racking deformations that otherwise would occur in the free field without major adjacent structures. The change in racking deformations results from inertial effects of the adjacent structure mass during shaking. A further complication is the reflection of shaking energy between the two structures (station and adjacent structure), with an associated period of vibration altered by the relative stiffness and mass of the adjacent affected structure. Therefore, computational seismic analyses of subway station response must include the impacts of nearby structures where such structures are present in the overall structural model as simplified single- or multi-degree-of- freedom (SDOF and MDOF, respectively) systems.