After two phases of testing and more than 100 simulated earthquakes, the 10-story TallWood building perched atop the largest shake table in the world remained intact.
By Fionna M. D. Samuels, Ph.D., Optimum Seismic Fellow (@Fairy__Hedgehog)
Citation: Samuels, Fionna M. D., 2024, Test results are in: TallWood building a resounding success, Temblor, http://doi.org/10.32858/temblor.336
Wooden buildings have recently been touted as eco-friendly solutions for future buildings in cities. Mass timber — engineered by bonding layers of wood together — promises more strength and fire safety than traditional wood-frame construction with the same climate resilience of carbon-sequestering wood. As building codes change to include multistory mass timber structures, people in seismically active regions may wonder how these new constructions will hold up in an earthquake. As it turns out, experiments suggest they’ll hold up surprisingly well. After three months of testing and more than 100 simulated earthquakes, one 10-story building has demonstrated the impressive seismic resilience of mass timber buildings.
TallWood recap
As previously reported, the TallWood building was designed and constructed by a team led by Colorado School of Mines associate professor Shiling Pei and dubbed the Natural Hazards Engineering Research Infrastructure (NHERI) TallWood Project. The National Science Foundation-funded project is managed by the University of California, San Diego (UCSD) where the tower was built. At 112 feet (34 meters) in height, the mass timber tower is the tallest full-scale building structure to ever be tested on a shake table.
The UCSD-operated shake table is the largest outdoor earthquake simulator in the world. Thanks to a recent upgrade, it can carry 4.5-million-pound structures through realistic, three-dimensional earthquake simulations with six ground motion components. Three of the six are translational — up and down, back and forth, and left to right — while the other three are rotational: pitching, yawing and rolling. “Basically, earthquakes on demand,” Pei says.
This allowed Pei and his team to test their design through a variety of simulated earthquakes equivalent to between magnitude 4 and magnitude 8 events. They started slowly, testing quakes smaller than those the building was designed to withstand based on design codes. They ramped up the magnitude until reaching several ground motions equivalent to earthquakes with a 2,500-year return period, a monster seen “every three to five centuries,” Pei says. In each case, more than 750 sensors interspersed throughout the building captured its movement.
The tower was built to withstand Seattle’s earthquake hazards. It features four mass timber rocking walls. Rocking walls, which are a type of self-centering design, are constructed to prevent significant damage during a quake. Each 10-foot (3-meter)-wide panel spans the height of the tower and is anchored to the shake table with steel rods. These rods control the motion of the walls, allowing them to lift on one edge and compress on the other while rocking. When the shaking stops, the rods bring the wall’s edge back to being flush with the shake table — equivalent to the foundation. Thus, the building returns to its original vertical position.
Rocking success
Although self-centering designs are not new, incorporating rocking walls into mass timber structures is fairly novel. In 2017, Pei led a previous project that tested rocking walls in a wooden two-story building on the same shake table. However, in addition to being the tallest wooden building ever tested on a shake table, TallWood is the tallest to test such walls. “Theoretically you can always say ‘I think that idea will work,’ but this is the first time to actually see it work in [a] ten-story wood building,” Pei says.
The rocking walls didn’t just work; they were a resounding success, Pei says. The experiment was almost disappointing. “If you’re looking for damage, this is a very boring test,” he says, because there was no structural damage despite the building “violently shaking.”
A second round of testing at UCSD in collaboration with the Japanese company Sumitomo Forestry Co., Ltd. saw the same results with regard to seismic damage despite slight structural modifications to accommodate Japanese building codes. Those codes require stiffer walls compared to codes in the United States, Pei explains. Ultimately, he says the lateral rocking wall system proved to be “adaptive to different code requirements.”
Beyond a resilient frame
Of course, many nonstructural components of a building are also important to its residents. For instance, stairs, windows and some exterior walls are not usually key to the integrity of a structure, but they’re vital to its livability.
“With the overall theme of designing the structure for resilience, we also wanted to consider resilience of the nonstructural components,” says Keri Ryan, engineering professor at the University of Nevada, Reno and lead investigator of TallWood’s nonstructural components and systems. “Even if they’re not really carrying any of the load, they’re attached to the building in some way and they have to go along for the ride,” she explains.
With that in mind, the team installed different external facades, each tweaked to isolate the wall from the movement of the building in slightly different ways. This was achieved by connecting the wall to the building’s frame with assemblies designed to allow movement between the two. Essentially when the structure begins to sway, as designed, the facade can slip past the moving frame rather than be bowed or deformed by the movement.
“The assemblies didn’t all work exactly as we expected,” says Ryan, “but they all were able to accommodate the movement without damage, which was huge.” Not only could this kind of technology ultimately protect people from falling facades during a quake, further research and development may also help keep the building more livable directly after an earthquake. This “functional recovery” aims to keep buildings safe and inhabitable immediately after a seismic event so that residents are not displaced while any minimal damage the building incurred is repaired.
Shaking can cause nearly invisible cracks in windows and walls that break the seal of a building envelope, but are not structurally significant, says Ryan. In fact, she says a preliminary study conducted during the TallWood tests, in collaboration with RJC Engineers, a Canadian engineering firm, found that although the installed windows appeared undamaged at the end of testing, calculations showed a 20 to 30 precent loss of their air tightness. A window’s movement will likely come down to its manufacturing, Ryan says. Linking the structural and functional performance of buildings’ facades during and after earthquakes will be a fertile ground for future studies.
Rocking walls in the real world
“We are in the process of getting all the data together to support putting this new rocking wall system into future building codes in the United States,” says Pei. This process is a prerequisite to its adoption into real-world construction, particularly after the International Code Council (ICC) implemented a new set of policies allowing up to 18-story wood buildings in the International Building Code 2021. In seismically active areas where mass timber use is attractive — like Washington and Oregon, which have a history of lumber extraction — new builds must be designed with earthquake resilience in mind.
As Senior Principal at engineering firm Estructure, Maryann Phipps says the company has advised many owners and architects with a vested interest in making resilient buildings with natural materials. However, because it is impossible to design any building to be completely earthquake proof, the company’s clients must understand and consider “acceptable damage,” she says.
“These full-scale tests are so valuable to all of us,” Phipps says, because they are the best tool to predict earthquake damage, especially for new systems. That said, as an emerging strategy, Phipps emphasizes that it will take time for rocking walls to be fully adopted and she’s not ready to make any recommendations. “It’s too early in, from my perspective,” she says. It’ll take more evidence from both researchers and early adopters to prove to her that “the benefit you think you’re designing is the one you’re getting.” But these tests help “make the [rocking wall] concept a reality for a lot of people because, while it might sound good and work on paper, we always learn something from a shake table test,” she says.
The future of TallWood
After Pei’s final tests were completed, the top four stories of the TallWood building were removed and the project was handed over to Andre Barbosa, an engineering professor at Oregon State University. He will lead the NHERI Converging Design Project, which will use the now six-story TallWood building to test three different lateral force-resisting systems. The first will be the currently installed rocking wall system. The next will be a new design that only has energy dissipation devices installed on the first floor of the building. The final system will involve adding a specially designed steel frame and brace system to the entire building.
This next project will both measure and validate new methods of resilient construction and demonstrate the versatility of mass timber structures, Barbosa says.
Ultimately, the data Barbosa’s team collects will be combined with that from both the two-story and ten-story building shake table tests directed by Pei. Together, these data will help to develop resilient mass timber design solutions across many building heights and types.
Fionna M. D. Samuels is Temblor’s Optimum Seismic Fellow. She is a science writer hailing from the Front Range of Colorado where she got her Ph.D. in Chemistry from Colorado State University. Her work has appeared in Eos, Scientific American and Symmetry. Optimum Seismic is sponsoring their first Temblor science writing fellow to cover important news about seismic resilience of the built environment.
References
Pei, S., van de Lindt, J. W., Barbosa, A. R., Berman, J. W., McDonnell, E., Daniel Dolan, J., … & Wichman, S. (2019). Experimental seismic response of a resilient 2-story mass-timber building with post-tensioned rocking walls. Journal of Structural Engineering, 145(11), 04019120.
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