Shake, Burn and Learn
|Researchers tested a six-story steel-frame building on the world's largest outdoor shake table.|
San Diego, Calif., Dec. 20, 2016 -- On a recent afternoon, two Jacobs School engineers equipped with 3D glasses stood in front of a towering 12’ digital reproduction of a six-story building, projected onto a curved wall of screens. They had tested the building a few weeks before, putting its light-weight steel frame through a series of increasingly powerful earthquake and fire tests on the world’s largest outdoor shake table at UC San Diego. Now researchers were zooming in and out of the building’s digital twin to assess damage.
This virtual inspection was made possible by data gathered by small drones, which flew around the building, first before the tests to help create a digital, 3D map of the structure, and then during the tests to capture the impact of the earthquake. Finally, after the tests, they took to the sky to look for damage. Researchers used powerful visualization algorithms to turn the data the drones collected into an immersive 3D environment. The combination of these unique technologies — a one-of-a-kind shake table and powerful data visualization methods —allows structural engineers at the Jacobs School to get an incredibly detailed digital model of the structures they test. This in turn allows them to make recommendations to improve design methods and building codes around the nation and the world.
"This series of tests is particularly important because, for the first time, we simulated on a shake table an aftershock and main shock occurring after a live fire,” said principal investigator and UC San Diego structural engineering professor Tara Hutchinson.
In the past 12 years, many projects have been put to the test on the shake table at the NSF-funded Englekirk Structural Engineering Center — everything from wind turbines, to parking garages, to one-story masonry structures and six-story concrete buildings.
Back in 2011, Hutchinson assembled a team of engineers to test the safety of a five-story building's nonstructural components —elevators, stairs and facades. Those tests resulted in new building design requirements and new construction methods — and will likely yield more insights in coming years.
More recently, researchers tested different ways to retrofit wood-frame soft-story structures, which feature large open spaces on the first floor. The series of tests was critical to an initiative to seismically retrofit about 6,000 of these buildings in California alone.
“Earthquake engineering is very important for society because engineers help lessen the number of deaths in earthquakes and decrease disruptions on society,” said Joel Conte, a professor of structural engineering and the principal investigator on the Jacobs School’s shake table. The facility has made a significant impact on building design and codes, he said.
Hutchinson’s latest $1.5 million project is a good example of the facility's mission. It is supported by a coalition of government agencies, foundations and industry partners including the U.S. Department of Housing and Urban Development, the California Seismic Safety Commission and partners from the steel industry and insurance companies. The researchers received approximately $1 million in in-kind donations from industry partners to build and outfit the structure.
|Professor Joel Conte (left) and Falko Kuester examined a digital reconstruction of the building.|
The building’s architectural layout was designed to replicate a multi-family residential condominium or apartment building at full scale. But in this case, engineers were pushing the limit of structural height, erecting the building 64 feet above the shake table. The largest building of this construction type tested before was a two-story residential structure. Testing a building at full scale is important because certain failure mechanisms can’t be reproduced when structures are scaled down, said Conte. In this recent experiment, engineers were trying to find out how a mid-rise building made of cold-formed steel framing would perform during an earthquake and a post-earthquake fire. The answer is that it did well after going through the seismic tests. “The building could have been easily repaired,” Hutchinson said. “The occupants would have gotten out safely.”
Hutchinson believes that it’s likely because the structure is lighter than a concrete building of the same height and as a result has less mass to generate damaging forces.
The fire tests however were less kind to the structure, which was equipped with few fireproof materials and fire-stop systems. Researchers led by Brian Meacham, a professor at the Worcester Polytechnic Institute, ignited pans of heptane, a liquid fuel, in eight rooms on the building’s second and sixth floors to achieve temperatures as high as 1000 degrees Celsius (almost 1800 degrees Fahrenheit) within the seismically damaged rooms. Doors fell off their plastic hinges, which melted. Several of the researchers’ video cameras, installed to record the fire’s progression, suffered a similar fate. Finishing materials detached from ceilings and walls. The simulated earthquakes occurring after the fire tests further weakened some of the structure's floors, bringing them close to collapse.
The building’s performance was captured by an extensive array of more than 250 analog sensors, as well as digital cameras — and of course, by drones. The drones, operated by the research group of professor Falko Kuester, took 4K video footage of the building during each of the tests as well as hundreds of high-resolution images, before and after.
“We used the drones to get the right sensor to the right location at the right time,” Kuester said. He and his team then compiled the data into high-resolution 3D models consisting of billions of data points, called point clouds.
Hutchinson, Conte and colleagues navigated through the point clouds together using the tall, curved display environment called the WAVE (for Wide-Angle Virtual Environment). The WAVE is a Holodeck-like environment that allows the viewer to step into a virtual recreation of the research. An array of 35 monitors that ends in a crest above viewers’ heads and a trough at their feet, the WAVE fills a person’s field of view for 180 degrees at the vertical and 160 degrees at the horizontal. If you are wearing special 3D glasses, the display can also tell where you are looking and adjust the images it shows accordingly. Users can step back to study the entire structure and then zoom in to see the tiniest details, such as cracks and changes in shape and color. “This is big VR for big data and big science," Kuester said.
The WAVE and its software allow re-searchers to travel through space and time to explore how structures have changed and how they have been damaged by natural disasters such as earthquakes.
At the same time, the shake table allows researchers to gather baseline data before and right after an extreme event, an opportunity that they wouldn’t have in real life.
“We’ll then have high definition data from before, during and after the test, which was never available before,” Kuester said.
The next step for engineers is to look closely at this data and produce a series of technical reports and scientific papers — a process that could take a year or so.
UC San Diego's shake table is funded by a $5.2 million grant from the National Science Foundation. The facility is part of the NSF's Natural Hazards Engineering Research Infrastructure — a network of state-of-the-art research facilities and tools designed to better understand and prevent earthquakes as well as wind and water hazards.