Researchers are redefining structural resilience this month by testing the first 10-story TallWood building made of mass timber and rocking walls on the University of California San Diego’s shake table.
By Montana Denton, Science Writer (@montana_denton)
Citation: Denton, M., 2023, TallWood Project tests earthquake-resistant structure, Temblor, http://doi.org/10.32858/temblor.309
Mass timber may just be the construction material of the future, increasing in popularity due to its anticipated sustainability and energy efficiency. But whether it holds up to the most strenuous structural test of all — earthquakes — remains to be seen. Colorado School of Mines associate professor of engineering Shiling Pei wants to find out.
To that end, the Natural Hazards Engineering Research Infrastructure (NHERI) TallWood Project, led by Pei and funded by the National Science Foundation, aims to investigate the resilience of residential and mixed use mass timber buildings this month by simulating a series of large earthquakes on a 10-story timber building — the tallest full-scale building ever tested by an earthquake simulator.
Timber not directly from trees
Mass timber buildings use solid or engineered wood as their primary building component. Unlike timber directly harvested from trees, mass timber is solid, panelized wood products that are often reinforced — in this case, with cross-laminated timber (CLT) — to optimize structural performance. The material is gaining popularity as a building component and can offer a more sustainably sourced alternative to traditional construction materials like steel and concrete.
“The marvel of this test is not just an improvement in our understanding and confidence in principles of seismic design, but its ability to open the door to entirely new possibilities for sustainable design,” says mass timber expert and structural engineer Greg Kingsley. Kingsley, CEO of the Colorado-based engineering firm KL&A Engineers and Builders, brought his industry expertise to School of Mines mass timber endeavors, reviewing the TallWood drawings for basic stability and completeness. To create more climate-friendly buildings, he explains, engineers should be focused on reducing the overall carbon footprint of the built environment. Proponents of mass timber construction argue that wood buildings have significantly less emissions than concrete and steel alternatives; the wood used in these buildings also stores carbon, offering a green, renewable, and often more cost-effective construction material.
Rocking walls
The TallWood test “is an unprecedented, full-scale seismic simulation test on a timber building,” Kingsley says, but it’s also a test of a unique rocking wall system.
The rocking wall system, which consists of a solid mass timber wood wall panel anchored to the ground using steel cables or rods with large tension forces, aims to minimize risk of collapse and reduce seismic damage, Pei explained in a press release. “When exposed to lateral forces, the wood wall panels will rock back and forth — which reduces earthquake impacts — and then the steel rods will pull the building back to plumb once the earthquake passes.”
This lateral load-bearing system is not currently incorporated into building codes, says Kingsley. Yet the load-bearing system has the potential to increase the benefits of constructing buildings of several stories with mass timber, called “tall wood” buildings (hence the name of the subject of this news story).
Such a system would also potentially protect nonstructural components within the building — like stairs — which are expected to experience notable motion during the seismic simulation. In the reality of an actual earthquake, these components must be protected by the outer envelope of the building to preserve the interior’s functionality, allowing for safe exit and efficient first responder access.
Seismic simulations
Earthquake-ready renovations can be put to the test with the University of California, San Diego’s NHERI Large High Performance Outdoor Shake Table (LHPOST), which tests the seismic resilience of structures constructed on it. Part of the National Science Foundation’s nationwide earthquake engineering collaboration efforts, the site shakes structures at scale under realistic seismic conditions. Capable of shaking structures up to 2,000 metric tons, the table has the largest payload capacity in the world compared to other earthquake simulators. Recent upgrades mean that it can synthesize the same three-dimensional ground motions that occur during earthquakes — when the ground undulates up and down, back and forth, and left to right, while also rotating (rotational motions are called roll, pitch and yaw).
LHPOST has contributed to updating building codes and validating new technologies, and can help institutions like CalTrans, FEMA and the Department of Energy collect crucial data for construction. And, it offers research groups like Pei’s the ability to build full-scale constructions to develop more earthquake-proof infrastructure.
In 2017, Pei’s team built a two-story mass timber building on the NHERI LHPOST. The team tested their construction by simulating shaking from the Northridge earthquake, a magnitude-6.7 quake that struck Los Angeles in 1994. The building experienced virtually zero structural damage after strenuous testing, providing evidence of mass timber buildings’ capacity for seismic resilience.
TallWood Test
The next step is putting a tall wood building to the test. Pei’s TallWood is the first structure to be tested on the new and improved shake table. The School of Mines-led project has been constructed onsite in San Diego, and began testing today, says Pei. Tests simulate earthquake motions recorded during prior earthquakes — including Northridge — and will cover a range of magnitudes, from magnitude 4 to magnitude 8. The subsurface simulated in the test is based as if the building were erected in Seattle’s Capitol Hill neighborhood.
During the weeks of testing, Pei’s team is starting with low levels of shaking and will gradually increase the intensity over the duration of the experiment. The TallWood building has been outfitted with 800 cabled sensors and approximately 60 video cameras to capture just how the seismic damage will be inflicted on the design and how it responds as tremors increase in strength. You can watch some of the tests here.
The unpredictability of earthquakes calls for constant human innovation — creating stronger, safer, more sustainable, and more stable structures. “The power and personality of earthquakes are inherently unknowable, and site conditions are quite variable all over the world, so the best we can do is decrease — hopefully very significantly — the likelihood of damage or collapse,” Kingsley says. Moreover, the TallWood test may potentially alter the future of building construction, he says. “[This test] could make design of tall wood buildings more economical and more resilient, which will then have the even broader impact of increasing the options for reducing the embodied carbon footprint of mid-rise buildings.”
References
Wichman, S., Berman, J.W., and Pei, S., 2022, Experimental investigation and numerical modeling of rocking cross laminated timber walls on a flexible foundation, Earthquake Engineering & Structural Dynamics, https://doi.org/10.1002/eqe.3634