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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

New Elementary School is Prepared for a Tsunami

By Kevin Wilcox

The robust engineering of a gymnasium wing is intended to provide students and nearby residents with a safe refuge if an earthquake creates a tsunami on the Cascadia Subduction Zone.

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The key design challenge was to make the school inviting, given the scale of the vertical safe refuge. © TCF Architecture

October 11, 2016—The new Ocosta Elementary School in Westport, Washington, sits about 4,000 ft inland from the Pacific Ocean. The school is handsome—a single-story earth-tone building, flanked by a taller gymnasium wing. It is part of a campus that accommodates 700 students, some attending the adjacent junior high and high school.

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 percent 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 Ocosta School District hired TCF Architecture, headquartered in Tacoma, Washington, to design the school, and the Seattle office of Degenkolb Engineers to conduct the engineering for the project, which included the classroom wing and the refuge.

Degenkolb was familiar with the project because it emerged during a community-based education effort known as Project Safe Haven. Degenkolb was working on the project with the Emergency Management Division of the Washington State Military, which aided coastal residents in evaluating and developing vertical escape solutions. Citizens identified the school as a good location for a safe refuge, according to Cale Ash, P.E., S.E., a principal of Degenkolb.

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The elementary school is essentially three separate buildings that appear and function as one. During a seismic event, they would react independently. © TCF Architecture

"That laid the groundwork for identifying the site," Ash says. "We participated as a technical advisor to provide preliminary design so they could develop cost estimates for these different options. The Ocosta project consists of a new classroom wing that joins to an existing classroom wing, and also connects to the safe refuge portion, which contains the gymnasium, cafeteria, and the music room."

The design that TCF Architecture developed is warm and inviting, taking cues from ranch and Craftsman residential styles. One of the key design goals, which was successfully achieved, was to seamlessly integrate the safe refuge portion of the project into the design of the new school. 

"[The] key to the design process was the identification of how a facility of this physical scale could appear inviting from a pedestrian vantage point, as well as how a structure of this magnitude would suit the villagelike feel of its community," said Brian Ho, a principal of TCF Architecture. Ho, who provided written responses to questions posed by Civil Engineering online, was the project's architectural project manager and the lead designer.

"The School District wanted to show its gratitude to local voters who supported the successful bond passage, so our team was also tasked with developing an interior aesthetic to honor the unique history of the region and its people," Ho said. "Inspiration provided by beach glass, sea grass, tidal waves, cranberry bogs, and a compass rose worked its way into the finished design, along with a large historic photograph and display niches [to] showcase objects made by local artisans."

The project included several engineering challenges. Understanding the full extent of the tsunami hazard was the crucial first step. The safe haven portion of the project would need to withstand not only a major earthquake, but also the forces of tsunami waves and the impact of the debris they would carry and the scour they would cause.

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Thick concrete shear walls are founded on auger-cast piles, 24 in. in diameter, the deepest extending approximately 50 ft. © Degenkolb Engineers

The geotechnical conditions at the site include sandy soils and a high water table. Had the project faced only seismic liquefaction, the engineers might have chosen a mat or grade-beam foundation. But the tsunami hazards led the engineering team to specify a deep foundation of auger-cast piles, 24 in. in diameter, the deepest extending approximately 50 ft.

"We [had] to consider earthquake-induced liquefaction. That was a significant driver for the foundation selection, in addition to considering the scouring affect that happens with tsunamis," Ash says. "The flowing water will erode those soils and can expose the foundations. [So] the foundation had to be designed to resist first the design earthquake and then what we call the design tsunami."

Both are significant events. Research indicates that the Cascadia Subduction Zone is capable of a magnitude 9 earthquake with a 2,500-year return period. Detailed modeling of the site indicated that an earthquake of that magnitude would likely create a 14 ft deep inundation zone around the school site. The fact that the site is 25 ft above sea level and slightly inland reduced the impact somewhat.

"The materials we chose were largely informed by how buildings performed in the 2011 Japanese earthquake and tsunami," Ash explains. "We saw that the concrete buildings, by far, performed the best, followed…by steel buildings. Concrete has an inherent combination of strength and ductility. Even if it does get a little bit damaged, it still has the ability to carry its gravity loads. That was pretty appealing to us for the primary lateral system."

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. Concerned about possible debris damage to those columns, the engineering team specified bolstering them in two ways. "First, we encased them in concrete to toughen the flanges," Ash says. "But then we also have a moment-resisting, beam-to-column connection above there so that if any one of those columns on either side of the building were damaged or destroyed, the framing above it would be able to bridge over that and carry the roof—without one of those columns in place. That's how we dealt with trying to avoid progressive collapse."

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.

The elementary school is essentially three separate buildings that appear and function as one. A small, existing wing of classrooms was maintained. A new, larger wing of classrooms was added to the north, and farther north of that is the gymnasium and safe haven. The new classroom wing is a much more typical, modern school structure that relies on a steel-braced frame. It is separated from the gymnasium by a seismic joint that enables the buildings to move independently in a seismic event.

The school is in operation now, following a ribbon cutting ceremony on June 11, at which it received positive feedback from the community. "[This project] has been very rewarding," Ash says. "I got involved in Project Safe Haven to do the right thing."

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