September 22, 2009
Japanese Quake Test
A massive shake table test in Japan could lead to safer wood-frame buildings in earthquake zones
In the end, it swayed but didn't fall. Perched atop the largest shake table in the world and subjected to a massive simulated earthquake, this six-story, wood-frame condominium stood tall, and the success of the test may lead to safer, taller wood-frame building construction in earthquake zones.
"The earthquake that's being simulated here is effectively an earthquake that would occur on average only once every 2,500 years," said Colorado State University engineer John van de Lindt. "So, it's a very, very strong earthquake, much stronger than we currently design for in the U.S."
Van de Lindt leads a team that's developing new ways to build mid-rise, wood-frame buildings that can stand up to earthquakes. In many earthquake-prone areas of the U.S., wood-frame buildings taller than three stories are uncommon--and building codes generally limit such construction to just four stories. Van de Lindt hopes validation of the new design techniques could safely push those building codes to six stories, or even taller.
Standing up to earthquakes
"The detailing that we've incorporated into the structure is not difficult, it's not especially expensive, it's just an engineered system," said team member David Rosowsky, an engineer from Texas A&M University, talking about the building constructed for the shake test. "We can certainly put this into all of our structures built in high seismic regions and expect that these structures will perform well in earthquakes and, indeed, lives can be saved."
To test their new design, van de Lindt's team turned to E-Defense, a remote shake table facility in Miki City, Japan. The event was the grand finale in a series of shake tests on wood structures funded by the National Science Foundation's (NSF) Network for Earthquake Engineering Simulation (NEES). The shake table is not only the largest in the world, but it's "tri-axial," meaning it can move in three directions at once. That's key for simulating an earthquake as realistically as possible.
"When a test is held, it can be felt in the surrounding area, so they decided to build it where there are no people around, and that is why it is isolated," E-Defense researcher Hidemaru Shimizu said. "They actually flattened a mountain top, and dug a crater, and put 22-foot-thick concrete around it to try to limit the effect on the surroundings."
The building is outfitted with over 300 sensors that provide researchers with exhaustive data on how it holds up to the simulated seismic forces. One major design innovation is a backbone of steel rods called an "Anchor Tie-down System."
"A steel rod provides the overturning restraint for the entire building, and it goes up from story to story, effectively making a continuous steel rod from the first level, all the way to the roof," explained van de Lindt. "We have strain gauges and we're measuring the strain in the steel, so we know how much it's elongated, and what this does is, this system prevents the overturning of the entire building."
Another innovation they are testing is called "performance-based seismic design," which is a nail pattern technique that distributes stiffness throughout the construction and makes it stronger.
"Most of the walls in this building are regular light-frame wood walls. What we see here is something called a double mid-ply wall," continued van de Lindt. "The double mid-ply wall has sheathing inside of the studs and then the nail goes through the sheathing and through a stud on each side. This particular wall actually has the capacity of about four wood shear walls, and there are two in this building, one on each side."
Shaken, not stirred?
Just getting the building materials to Japan and erecting the condo proved to be a huge logistical challenge. And, as always, the devil was in the details.
"There were also some smaller challenges like language barriers with the carpenters--going both directions--trying to explain the difference in construction methods to them," said van de Lindt.
At last came the moment of truth--the simulated 7.5 magnitude earthquake.
"What we learned from this test is that a six-story, light-frame wood building, if designed correctly, and in this case, designed using Performance-based Seismic Design, you can use that philosophy and you can design a six-story structure and, honestly, looking at that, I think we can do an eight-story," van de Lindt said afterward.
Post-shake inspections confirmed there was no significant structural damage to the building. It's a success story that may pave the way for a new era in wood-frame construction in earthquake zones.
Any opinions, findings, conclusions or recommendations presented in this material are only those of the presenter grantee/researcher, author, or agency employee; and do not necessarily reflect the views of the National Science Foundation.