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Shake Weighted: Simulating a 6.7 Earthquake

by Rachel Swaby
The big one at the push of a button.

This summer, 140 earthquakes jiggled, jolted and bounced two buildings in western New York. This wasn’t bad luck or some freak occurrence; the buildings were having a challenging July and August by design. See, the series of seismic events were produced not by tectonic plates, but by a pair of massive shake tables bridged over to create the largest earthquake simulation platform in the country. The University of Buffalo lab performs these powerful events for an audience of scientists and industry folks—all in order to test the mettle (and in this case metal) of a wide variety materials and systems.

The most recent, massive show: Cold-formed steel, presented in two acts. The first cold-formed structure ready to rumble was a 50 foot long, 20 foot wide, and 20 foot tall two-story naked frame; the second was identical in size and shape, but dressed with all sorts of non-structure-sustaining elements, like drywall, a staircase—and eleven 2000-pound concrete blocks to simulate the weight of everything else typically inside an occupied building (furniture, air conditioning units, water heaters).

Three years in the making, the tests drew researchers from six universities, steel industry design consultants, and nearly $1 million from the National Science Foundation. The goal was to gain a better understanding of how cold-formed steel stands up to extreme conditions. Better understanding leads to more informed computer models and building codes, so engineers and construction companies can hit that sweet spot of efficiency and safety.

The purpose of the tests was to evaluate the integrity of cold-formed steel.

Cold-formed steel is exactly what it sounds like. Where traditional steel is poured into a mold or rolled into a sheet while in a hot, liquid form, the cold-formed variety is rolled into a thin sheet at room temperature, which can be folded to improve the material’s strength. It’s not appropriate for towering skyscrapers (think of it as an alternative to wood), but because its strength-to-weight ratio is high, it’s weatherproof, and below a certain building height it’s more cost-efficient than traditional steel, cold-formed is ideal for low to mid-sized buildings.

What we know about the stuff comes from “subsystem” testing. A researcher will test one section and computer models will extrapolate how the rest of the building will fare. But without data about how all the components work together—the walls, the roof, the windows—engineers can only get so close in their predictions.

So they set out to test two buildings at the University of Buffalo earthquake lab by running them through a series of tremors that varied in intensity, time, and speed. Up until August 16th, researchers from Johns Hopkins University had only brought the buildings, which were tested in succession, up to an earthquake intensity that computer models predicted they could withstand. But that Friday, the researchers turned up the heat. The final staged shakedown would be akin to California’s disastrous 1994 Northridge earthquake—a 6.7 magnitude-equivalent.

As she awaited the final test from the lab’s second story control room, Kara Peterman recalls, “I was feeling terrible. Just awful.” Peterman is a civil engineering Ph.D. candidate at Johns Hopkins University, and she has been overseeing the project in Buffalo since April. “I wanted to throw up out of nervousness.”

"If our building falls one way, it might take the lab down. If it falls another way, that’s millions of dollars worth of testing equipment."

Benjamin Schafer, Peterman’s faculty advisor and the lead on the project, wasn’t feeling all that relaxed either. Before the final show began, Schafer chitchatted with the project’s design engineer. The design engineer had been sent to LA after the Northridge earthquake to assess damage 19 years ago, and it wasn’t good; parking garages and mid-rise buildings, he recalled, were rife with structural fractures. Schafer’s anxiety spiked. “We were about to play this exact same thing in the lab. If our building falls one way, it might take the lab down. If it falls another way, that’s millions of dollars worth of testing equipment.”

In 1994 in Los Angeles, the 6.7 magnitude quake damaged 40,000 buildings; In Buffalo, New York, the cold-formed steel two-story building suffered very minor structural damage.

It was a great moment for cold-formed steel—not to mention the spared equipment in the earthquake lab that housed it. But the real value of the performance came not from what the engineers could observe by looking at the building, but from the all data streaming from it.

During each simulated earthquake—all 140 of them—eight video cameras armed for observation and over 150 sensors closely monitored the building’s health. The cameras helped researchers observe what wasn’t safe to see in person: the structure’s insides and other areas hidden from the lab’s control room. The researchers also deployed four types of sensors to offer them feedback about the event. The engineers rigged the construction with accelerometers, which measured building’s movement, load cells, which measured forces on certain walls, string tensiometers, which measured displacement, and strain gauges, which measured (you got it) the strain in the steel. Together these sensors recorded how dramatically their experiment wiggled, where the quakes caused pressure, how the materials responded under strain, and where the movement and forces dominoed from one section to the next.

“It’s very easy to find interesting things on the surface,” says Peterman. “We’ve already found 10 things we didn’t know, but it will take awhile for the full impact of the data to be made.” The analysis will power Peterman’s Ph.D. and keep Schafer busy for at least the next decade. “This data, as the first of its kind, will be a benchmark for nearly all we do going forward,” says Schafer. The team is already busy tweaking and testing computer models to get them closer the real world results.

What’s immediately apparent is that the cold-formed steel building far exceeded their expectations—perhaps by a factor or two or three, says Schafer. Thanks to the testing in Buffalo, the building codes that govern the material could be drastically revised within the next five years. Cold-formed steel structures are looking at a more streamlined—and therefore more efficient—future.

Sometimes, the only way to get there is to shake things up.

Photos courtesy Kara Peterman

Rachel Swaby is a New York-based writer. She has previously written about lock-picking communities.