Tsunami shelter

In the event of a Cascadia subduction zone earthquake off the Washington coast, evacuation from the vicinity of the school would be by vehicle on single-lane State Route 105 and would have to get all the children to safety off the low-lying peninsula within a 30-minute window between National Tsunami Warning Center's alert and the arrival of a 40-foot (12 m) tsunami.^[10] Realistic travel time to high ground in an emergency has been estimated to be around 100 minutes, more than three times as long as the margin of safety.^[11]

To provide for vertical evacuation instead, the school's tsunami shelter is located on the roof of the gym, elevated 53 feet (16 m) above sea level to withstand surges that would follow such an earthquake. The gym is accessible via four staircases, including one from the outside, designed to be wide for quicker evacuation and built with 14-inch (360 mm) thick concrete and steel rebar. The shelter is designed to accommodate the students and staff of both the elementary school and the adjoining junior–senior high school, a total of 700 people, with room for some members of the public; the total capacity is estimated to be between 1,000 and 2,000 people.^[2][12]

The building and shelter are secured to the ground by 169 pilings driven 50 feet (15 m) into the ground. It is rated to withstand a 9.2-magnitude earthquake (which would be one of the largest ever recorded) and has a 6-foot (1.8 m) parapet to shield against tsunami surges.^[2][8] The pilings, which form the main structure for the shelter, were considered a cost-effective way to build new tsunami-resistant shelters; the shelter only added 20 percent to the Ocosta Elementary School's $13 million construction budget.^[13] Concrete and steel were chosen as the building's main structural materials after observations made during the 2011 Tōhoku earthquake and tsunami, where concrete buildings were the least damaged.^[7]