Precast concrete offers solutions for protecting people and places from nature’s worst weather events.
By Deborah R. Huso
The people of Southeast Asia are no strangers to tsunamis, but no one was prepared for the one that followed the 9.0-magnitude earthquake just off the eastern coast of Japan in March 2011. In some places, waves reached well over 100 ft. Tens of thousands of people lost their lives despite the fact that Japan, when compared with the rest of the developed world, was relatively well-prepared for such an event. Many coastal structures survived the disaster, and many of those structures were concrete.
“Even older precast structures withstood the tsunami,” says Clay Naito, Ph.D., associate professor with the Department of Civil and Environmental Engineering at Lehigh University in Bethlehem, Pa. He took a two-week trip to Japan in the wake of the natural disasters to study the damage from debris and to look at the country’s precast buildings.
Naito says both precast and cast-in-place concrete survive these types of disasters better than most other structural forms, and indeed he saw a 40-story, total-precast building that survived the earthquake.
An Oregon community prepares for the worst
In Cannon Beach, Ore., engineers have designed a 9,800-sq-ft precast concrete Tsunami Evacuation Building (TEB) and city hall, which if constructed will cost the local government around $4 million – twice the cost of a conventional building. But the 1,700 residents of Cannon Beach aren’t balking at the price tag, because they know a tsunami is coming – they just don’t know when.
Geologists at Oregon State University (OSU) in Newport say a massive earthquake is likely to occur in the Cascadia subduction zone off the Pacific Northwest coast within the next 50 years. “Cannon Beach was affected by the 1964 Alaska earthquake and a tsunami generated by it,” says Kent Yu, Ph.D., principal branch manager with Degenkolb Engineers in Portland, Ore. He is among the members of a team, including professors from OSU, working pro bono to design a TEB for Cannon Beach.
Using OSU’s Tsunami Wave Basin, the most advanced testing facility of its kind in the world, engineers have been evaluating the design of a proposed concrete TEB/city hall. “We want to make sure the building can sustain the shaking from a 9.2 earthquake,” Yu explains. They also want to ensure the structure can bear the impact load from water and debris. That includes making sure the building can withstand storm surge water pounding into it and pushing upward against the floors.
The structure Yu’s team of engineers has designed is rectangular in shape with a concrete frame, structural walls that run parallel to the water flow, and breakaway walls that will let loose to allow water and debris to flow through the building to prevent a damming up of debris at the front of the building. The proposed structure would be built on a deep-pile foundation with concrete beams. “I believe precast will perform better,” says Yu, who points out that precast concrete performed well in seismic testing simulating a magnitude 8 earthquake in San Diego in 2008. The testing was a joint venture among the University of California, San Diego, the University of Arizona and Lehigh University.
The proposed TEB/city hall would be 30-ft tall, thus allowing for vertical evacuation of city residents, as geologists have estimated water depth from an incoming tsunami would be about 15 ft. Engineers have also proposed putting a sea wall in front of the building to further reduce the impact of tsunami waves.
“The key component is to know the inundation depth and how fast the water will come through the building,” Yu explains. And that’s where OSU’s Department of Geology and Mineral Industries has come in, working up an inundation map to help engineers calculate water force to size the columns for the TEB.
Multiple risks of tsunamis
Cannon Beach isn’t the only community in the Pacific Northwest at risk, of course. Every community from Northern California up to Canada’s Vancouver Island could be impacted by an earthquake in the Cascadia subduction zone. The city of Cannon Beach is currently seeking funding from the Federal Emergency Management Agency (FEMA) to help build the proposed TEB.
But a manmade TEB isn’t always necessary. “An evacuation shelter is something you’d only put in where you have no higher ground to go to,” says Lehigh University’s Naito. He says in low-lying areas, he could see precast concrete as a cost-effective way to build elevated tsunami evacuation platforms at various distances along a whole beachfront, pointing out that precast concrete also has better durability than steel in areas that are persistently exposed to moisture and salt-laden air, like seaside towns.
The challenge with an earthquake-driven tsunami, however, is keeping the structure together. “You have to have failure mechanisms in place,” Naito explains. “You have to allow for large joint openings to allow the structure to move in an earthquake.” The challenge with precast, however, is its mass. “That hurts you in an earthquake, but it’s good in a tsunami,” Naito adds, noting that engineers face major challenges in designing buildings to withstand multiple natural hazards.
FEMA’s P646, “Guidelines for Design of Structures for Vertical Evacuation from Tsunamis,” promotes building strong structural systems that have the ability to resist extreme force; open systems that let water flow through without much resistance; ductile systems that resist failure in extreme weather conditions; and redundant construction systems that allow for partial failure without complete building collapse.
Defying deadly tornadoes and hurricanes
Tsunamis aren’t the only severe weather events engineers are concerned about. In the wake of the deadly tornado that swept through Joplin, Mo., in May, more communities in Tornado Alley are looking at precast concrete as an option for building structures with high wind resistance (See “Precast Concrete Safe Rooms: Shelter from the Storm” on page 24).
This is an area in which David Bloxom, president of Speed Fab-Crete, a design-build contractor of precast building systems in Fort Worth, Texas, has been working for a long time. His company has been involved in precast concrete since 1963 and recently completed a precast concrete community center in Emory, Texas, designed to withstand an F5 tornado. That designation means winds in excess of 250 mph, similar to what residents of Joplin experienced.
The newly built, 6,000-sq-ft community and evacuation center was designed to meet the guidelines of FEMA’s 361, “Design and Construction Guidance for Community Safe Rooms,” for extreme wind events. The building’s walls and roof are 8-in. thick. “The walls are staggered to withstand pressure from a severe storm,” Bloxom explains. “One wall goes a little behind the previous wall.” The precast wall panels are 18-ft tall and 15-ft wide, and the panels’ overlap of 6-to-8-in. provides additional strength. Precast panels are welded to each other as well as to the building’s foundation. The roof system consists of cap panels that go from a support panel in the building’s center to the structure’s outside walls.
Completed in the summer of 2011, the shelter hasn’t had an opportunity yet to demonstrate its mettle, but Bloxom is confident it will survive an F5 tornado event – his company has designed countless buildings that have already withstood tornadoes and hurricanes from Brownsville, Texas, to central Oklahoma. A 10-story precast condominium Speed Fab-Crete built on South Padre Island on the Gulf of Mexico more than 20 years ago has weathered multiple hurricanes. “The building itself has never sustained structural damage,” Bloxom says. In 2000, Fort Worth experienced an F2 tornado that tore through the city’s downtown and devastated many buildings, including twisting a steel-frame high-rise building, but the five precast structures in the tornado’s path built by Speed Fab-Crete had no structural damage. One of their projects, Evans Brake Service, sustained a direct hit without structural damage, even though a building next to it collapsed.
“One of the biggest problems in a tornado is flying debris,” Bloxom says, “like flying bricks hitting people.” His company has worked on several school construction projects. All of them, he says, have solid precast concrete walls with a concrete cap over hallways where children and faculty can take shelter in a tornado without worrying about structural collapse.
The goal for extreme weather-resistant precast structure isn’t necessarily to avoid damage entirely but to avoid structural failure, thus saving property and people. “The trick is helping the whole building stay together,” says Naito. “If designed correctly, precast is much more resilient to that purpose.”
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.
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