The new Ocosta Elementary School in Westport, Washington, sits about 4,000 ft inland from the Pacific Ocean. Not far to the west of the picturesque site, however, lies the Cascadia Subduction Zone. Once thought to pose a minimal threat to coastal residents, alarm has grown for decades as researchers come to better understand the subduction zone and its true potential. The scientific community now estimates there is as much as a 37% chance the subduction zone will trigger a magnitude 8.0 or greater earthquake sometime in the next 50 years. Even more concerning is the possibility that such an earthquake would trigger a disastrous tsunami.
So when voters in the school district approved a $13.8-million bond issue in 2013 to replace the elementary school, the plans called for the creation of a vertical safe refuge to which students, staff, and nearby residents could climb to safety in the approximately 20 minutes between the onset of such an earthquake and the arrival of the tsunami it would create.
The key structural element for the gymnasium and safe refuge are four massive concrete stairwell towers at the corners, leading to the safe zone on the roof. The concrete shear walls of the towers are 14 in. thick, reinforced concrete, and they form lateral force-resisting system for the structure. A series of 10 steel columns support the roof. The one-story gymnasium wing didn’t need to be raised to accommodate the required elevation for a safe refuge on the roof; the height that was already required to house the functions was sufficient. The structure is designed to accommodate 1,000 people, and has the capacity to handle live loads of 100 psf via a concrete slab atop a metal deck, supported by steel, wide-flange beams.
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