Rigorous Tests Show Resilience of Tall Mass Timber Buildings
Members of the NHERI Converging Design team atop the mass timber structure at the University of California San Diego.
Rigorous Tests Show Resilience of Tall Mass Timber Buildings
KEY TAKEAWAYS
Whenever people can walk out of a building, unharmed, following a major earthquake, it means the structure has done its job to protect occupants. Better still would be if they could walk back in and get on with their lives as soon as possible instead of abandoning the building to demolition or waiting for prolonged, expensive repairs.
Recently, dramatic advances in mass timber construction, specifically of mid-rise buildings, have brought that day considerably closer, and Oregon State University is at the vanguard of the progress.
“If there’s a big earthquake, we want buildings to protect people, but the buildings themselves are often structurally compromised and have to be torn down,” said Andre Barbosa, the Cecil and Sally Drinkward Professor in Structural Engineering at Oregon State. “That’s neither sustainable nor economically sound.”
Ideally, buildings in earthquake-prone areas should be capable of functional recovery, a concept that describes a quick return to pre-earthquake functions after limited downtime, interruption, or repair costs. (This somewhat new design philosophy can apply to all critical infrastructure, and even social systems.) Mass timber construction, Barbosa observes, holds the potential to meet the criteria for functional recovery.
What is mass timber?
Mass timber refers to a category of building products in which layers of wood are bonded together to create large panels, beams, and other structural elements. The prefabricated components are typically delivered to construction sites where they’re fastened into place. The technology is being used with increasing frequency to replace components traditionally made of concrete or steel, like floors, walls, beams, and columns.
A growing number of buildings in the U.S. and across the globe are built entirely from mass timber, but the practice is not widespread. (According to a report to Congress, as of March 2023, 1,753 mass timber projects had been built or were in design in the United States. For context, 5.9 million commercial buildings existed in the country in 2019 alone.) Research conducted by Barbosa and his colleagues aims to increase mass timber’s impact.
Tallest structure ever to be seismic tested
From August 2023 through April 2024, the NHERI Converging Design team, a research group led by Barbosa, tested the seismic resilience of a six-story mass timber building. (NHERI is the Natural Hazards Engineering Research Infrastructure, a national network of test facilities funded by the National Science Foundation.) The team included researchers from Oregon State’s colleges of Engineering and Forestry, as well as several other universities. Work was funded by the NSF, the U.S. Department of Agriculture, private industry, and the TallWood Design Institute, a collaboration among the College of Engineering, the College of Forestry, and the University of Oregon College of Design.
The test building actually began with 10 stories. Months before Barbosa’s team began testing, the taller structure was erected by the NHERI TallWood Project (an NSF-funded group headed by researchers from the Colorado School of Mines) atop the world’s largest outdoor shake table, located at the University of California, San Diego. Oregon State was among the collaborators. Standing at 112 feet, it ranks as the tallest full-scale structure ever to undergo seismic testing.
A variety of mass timber products were used in the building (including cross-laminated timber, glue-laminated timber, mass-ply panels, and others) so that all of them could be evaluated simultaneously. Barbosa notes that Oregon State researchers, working closely with the timber industry, have played a prominent role in the initial development and testing of several mass timber products. All of the mass timber elements, as well as other crucial structural components installed in the test building, were contributed by Oregon State’s industry partners, including Freres Lumber, Boise Cascade, Smartlam, StructureCraft, Mass Timber Services, and Simpson Strong-Tie.
After being subjected to 150 simulated earthquakes, ranging from magnitude 4.0 to magnitude 8.0, the building suffered virtually no structural damage.
The key to resilience
Key to its resilience were four mass timber rocking walls – one on each side, extending the full height of the structure. The walls were designed to absorb and dissipate the shaking forces, localize any damage in easily replaceable components, and then pull the building back to plumb.
After the NHERI TallWood Project finished up, Barbosa’s team readied the structure for their own investigation. They started by removing the top four floors so that the building’s height matched their research parameters. In keeping with the sustainability promise of mass timber, the materials that once formed the top four floors are being repurposed by the UC San Diego Center on Global Justice to build refugee housing in Tijuana, Mexico.
“If this had been a reinforced concrete building and we wanted to remove floors, we would have had to demolish them and send the material to a landfill,” Barbosa said.
Three configurations of the now-six-story structure underwent additional seismic testing. In the first phase, the rocking wall system used in the prior tests remained in place. For the second phase, two of the walls were swapped out for a new type of rocking wall system that included energy dissipation devices installed on the building’s first story. In the final phase, the rocking walls were replaced with a steel frame-and-brace system equipped with an advanced energy dissipation system.
The building was subjected to another 94 simulated earthquakes; once again, little or no damage was observed.
“Overall, we hit it with about 240 earthquakes, and after each one we could walk right back into the building,” Barbosa said. “We also showed that if any of the lateral force-resisting systems had been damaged — which they weren’t — we could quickly replace six-story walls and still have a resilient structure that could sustain another 20, 40, or 60 earthquakes.”
The test results, he added, demonstrate the functional recovery potential for tall mass timber buildings.
Rapid functional recovery
“To see no damage after so many high-intensity earthquakes was remarkable and is a testament to the great engineering strides in mass timber design that have enabled rapid functional recovery,” said research team member Steven Kontra, who earned dual master’s degrees in civil engineering and wood science in June. “While resiliency of the mass timber construction was self-evident, this was not our only design criteria. In the new ‘converging’ design methodology that was used for these tests, we combined resiliency — a well-established design target in structural engineering — with sustainability, which, historically, has not been a top priority for designers.”
After the shaking was finished and the building was completely deconstructed, Barbosa arranged for the material to be reused.
“I don’t know exactly where it’s headed or what it will be used for, but I know it won’t end up in a landfill,” he said.
According to Barbosa, the results offer solid evidence to support updating building codes to allow greater latitude in mass timber construction, such as greater height. He also hopes that it instills confidence in self-centering, mass timber rocking walls, whose resilience has been established through these rigorous experiments.
“We’re hoping that our work removes some of the barriers to faster adoption of mass timber construction,” Barbosa said. “Interest is growing and it’s becoming far more common. More people trust the technology, and it’s beginning to take off as a solution to a number of engineering and architectural challenges.”
“By putting this information out there, engineers can get a better understanding of how these systems perform under real earthquake loading conditions,” Kontra added. “That reduces uncertainty, and hopefully it will lead to more frequent consideration of mass timber design solutions as an option for taller buildings.”
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