Researchers at the University at Buffalo put a retrofitted brick-and-mortar building to the test. Though the building stood up to the “earthquakes” it’s designed for, stronger shaking caused bricks to tumble.
By Fionna M. D. Samuels, Ph.D., Optimum Seismic Fellow (@Fairy__Hedgehog)
Citation: Samuels, F. M. D., 2023, Largest US-based shake-table test of masonry buildings completes its first phase, Temblor, http://doi.org/10.32858/temblor.320
Unreinforced masonry buildings — classic brick-and-mortar builds — are often the oldest buildings in a city or town. These structures give places character and are frequently preserved as historical sites. Unfortunately, they are also some of the least resilient to earthquakes.
“I’ve traveled around the world after big earthquakes,” says structural engineering professor Andreas Stavridis of the University at Buffalo, New York, “Unreinforced masonry structures are the first ones to get damaged; they look good aesthetically, but they were not built to resist earthquakes.”
The buildings are so vulnerable that constructing new unreinforced masonry structures was banned in California almost a hundred years ago and codes were developed to retrofit historic buildings. Other states followed suit in subsequent years, some banning unreinforced masonry buildings and developing their own retrofitting ordinances, often looking to California as an example. Some cities, like Seattle, are currently in the process of developing these ordinances.
Now, a collection of researchers led by Stavridis is putting retrofit codes to the test with the largest shake-table test of unreinforced masonry in the United States. The team consists of three University at Buffalo faculty members, three graduate and three undergraduate students, who are advised by a panel of six practicing engineers. After four years, their first phase of shake-table testing is complete.
Retrofitting the building
Fifty-three years after the devastating 1933 Long Beach earthquake prompted the California government to ban new construction of unreinforced masonry structures, the state legislature deemed the hazard of existing unreinforced brick structures too great and passed the 1986 Unreinforced Masonry Building Law. The new law required many cities to identify and retrofit brick buildings in their communities by 1990. The new codes proved themselves when far fewer retrofitted buildings were severely damaged in the 1994 Northridge earthquake than nonretrofitted ones.
The test structure in Buffalo was retrofitted with steel anchors and strongbacks — vertical beams that resist out-of-plane movement — in accordance with the current guidelines that many seismically active U.S. cities follow. More information about engineering for earthquakes can be found in the American Society of Civil Engineers (ASCE) standard 41-17.
“We think that there might be room for improvement in the guidelines,” Stavridis says, simply because the code has only been “tested” by real earthquakes. Nevertheless, the team went into the experiment expecting the code to work, based on California’s history of retrofitting unreinforced masonry buildings.
This is just the kind of study Amanda Hertzfeld, Seattle’s unreinforced masonry retrofit program manager, is interested in as the city works to design a cost-effective retrofit ordinance. “Most shake table studies are looking at new buildings; very few focus on existing structures,” she says. As Seattle finalizes a new ordinance mandating the retrofit of more than 1,100 unreinforced masonry buildings, the city’s leaders want to “make sure that we’re proceeding with the best available science.”
Building a big experiment
This first University at Buffalo experiment is the largest dynamic test of a retrofitted structure in the U.S. to date. It took a month and a half to build and retrofit the 20,000-pound rectangular brick structure, says Stavridis. At 12 feet high, 9 feet wide and 21 feet long, the building pushed the limits of the table, he says, but it was important to make a structure similar to those found in real cities.
“I went through multiple design iterations until I found one that the team was happy with,” says Greg Congdon, one of three doctoral students working on the project and the lead student for this test. Initially, the group discussed retrofitting the building according to original California codes enacted in the 1980s, but ultimately decided to work with practicing structural engineers to design a test structure that would directly mirror today’s real-world retrofits. Once the entire team of scientists and collaborators signed off on Congdon’s design, construction started.
“We actually partnered with the local chapter of the International Masonry Institute,” says Congdon. The partnership worked well for both parties: The research group was able to build a high-quality building on their shake table and the apprentice masons had a unique training space to refine their skills.
After the building was constructed, Congdon says he put on the final touches: adding a simple roof, painting the walls so that hairline cracks would be more visible, installing steel supports throughout the building in line with current retrofitting codes and carefully placing a collection of nearly 200 accelerometers, potentiometers, and strain gauges around the structure to precisely measure how the walls responded to shaking.
The shakedown
After the structure was built, it was time to shake it down. Instead of reporting the level of shaking as a magnitude, Stavridis says that they think about the movement in terms of the ground acceleration.
The magnitude of an earthquake is determined by how much energy a fault rupture releases, but the amount of force a building feels from the earthquake — the ground acceleration — is dependent on a multitude of factors, from its relative location to the earthquake’s epicenter, the depth of the fault and the substrate on which the building sits. By considering ground acceleration instead of magnitude, Stavridis says the test results are more meaningful to engineers and can be applied more broadly.
In March of this year, the team began testing. The plan was to slowly build up from minimal ground acceleration to design-level ground acceleration — the maximum that the retrofit was designed to withstand. As the shaking ramped up, the team was thrilled to find the retrofitted building remained remarkably intact with no obvious cracks.
“It survived up to the design level,” says Congdon, “but as soon as we went past that, it failed.”
You can watch the last moments of the building on YouTube. With just a few large shakes, massive cracks appear in the walls and a corner parapet crashes to the floor.
After the final test, the structure was precariously stacked such that the only way to safely demolish it was “brick-by-brick,” Stavridis says.
Finding the building’s faults
It took a month to safely demolish the structure, then more time for Congdon to run thorough tests to determine the natural strength of the building materials. “I’ve tested the brick, the mortar and the concrete in the foundation of our test structure,” says Congdon. “Those are the materials with the most uncertainty.”
Knowing the physical properties of the construction materials, which can vary slightly, will help the team interpret the data collected by the accelerometers, potentiometers and strain gauges installed throughout the structure, which Congdon is currently analyzing. These data can reveal damage inside the walls, invisible to the naked eye, and help Congdon determine exactly how powerful the final quake was.
After he’s finished with this analysis, Congdon says that he’ll bring the results to the team to review and ultimately publish. Only then will they be able to comment on the reliability of the current codes for earthquakes above design level. The team’s goal was to increase the ground acceleration by 15 percent, but it is possible that the incredible mass of the building changed how the table accelerated, potentially overshooting their target.
“We hope that we were right there,” says Stavridis. “If that’s the case, this test is eye opening, but at this point I cannot say for sure.”
While they wait for the final results of Congdon’s data analysis, the group has started testing individual structural components, like single walls and individual corners on the cleared shake table. After this suite of comprehensive testing is complete, they will build a second full structure for a final shake test. They plan to tweak this final retrofit based on what they learn from the first test to see if they can improve its performance.
And that’s not all. “Much further down the road, after all testing is done, we’re going to try to tie in some risk modeling for these buildings that are already in people’s portfolios,” says Congdon.
Retrofit codes in the real world
Back in Seattle, Hertzfeld plans to talk to Stavridis about the potential implications of this work on the planned retrofitting ordinance. The Seattle Department of Construction and Inspections recently published their first draft technical standard, which lays out options people will have for retrofitting their buildings. The policy’s minimum standard of retrofitting must ensure that the building will be safe for people inside and outside during a quake, though not necessarily damage-free afterward.
“We’re really interested in seeing the results of their testing,” says Hertzfeld. Retrofitting is not cheap and she’s seen other cities demolish buildings instead of putting money into retrofits. Many of the buildings in Seattle that need retrofits are historic or being used as affordable housing, so demolishing them would negatively impact the community.
It will be great if the study finds that a basic retrofit — what Hertzfeld classifies as “minimally acceptable” — works. But if the study doesn’t, she says her team might need to revisit the current draft policy.
“Through research and science and engineering, we know how to reduce the loss of life and reduce damages post-earthquake,” says Hertzfeld. “It’s just a matter of creating the policy and implementing it.”
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.
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