Things that go 'BOOM!' in NC State University lab
03/16/2014 9:28 AM
03/16/2014 12:32 PM
Here’s how you know when something is about to go “boom” at N.C. State’s Constructed Facilities Laboratory: when everybody stops what they’re doing and gathers around to watch. It’s an understandable impulse because, let’s face it, breaking stuff is cool and fun to witness. Unless, of course, you’re standing under it, which is why it happens in a lab under controlled conditions.
A recent morning found a cluster of engineering students testing how much force a large piece of concrete could withstand. It was a 40-foot-long T-beam you might find in a parking garage, about 10 tons in weight – something you’d probably drive across, over or under without much thought, assuming (or at least hoping) it won’t fall down.
One reason most parking garages don’t fall down is the testing that goes on at places like the CFL. Or as laboratory manager Greg Lucier puts it, “We push things to the breaking point here so they don’t break out there.”
Following weeks of pre-test number-crunching, the test team had fit the T-beam with pistons to apply force. To measure movement and cracks, sensors and lasers were in place, everything wired to computers.
Two students took their place at the computers that simultaneously controlled force application and measured the results. The computers had to be turned on at the same time; so Hamid Kazem, a Ph.D. student in construction engineering, nodded to his partner and counted it down.
Gradually, the piston force went to “seven kips” (engineering shorthand for 7,000 pounds of force), then up the scale to 13,500 pounds; followed by pauses at 21.8, 24.5 and 30 kips – weights where computer-model calculations determined that cracks might start to appear. Since the average American car weighs around two tons, the T-beam was holding up between seven and eight vehicles at 30 kips.
At each pause, the students bustled around the end of the beam like a NASCAR pit crew. Sharpie pens and cameras came out to mark and photograph hairline cracks as they emerged. Nearby, CFL director Sami Rizkalla watched his charges work.
“We’re looking to determine service load, factored load and ultimate load,” explained Rizkalla, who has been the CFL’s director for 14 years. “Service load is a normal load it might have to hold up; factored load is, we hope, at least 50 percent more than you’d ever expect it would have to hold – we’re there now, 30,000 pounds. And ultimate load is where failure happens. We expect that to happen around 40,000 pounds.”
Just like clockwork, students from elsewhere in the lab appeared and hovered around to watch as the force applied topped 35 kips.
Finally, right at 43,000 pounds of force, the concrete gave a muffled, rending SNAP from deep within as the steel girders inside were pushed past the breaking point. Failure. Or maybe success.
Satisfied, the crowd dispersed as the pistons powered down. But the work was just beginning.
‘A finite life span’
The CFL has been open since 1995, and you’ll find it just down the hill from Hunt Library on N.C. State’s Centennial Campus. From outside, it looks like a greenhouse. But inside, it’s more of a gearhead kind of place.
It has huge pistons that can be mounted on the floor or walls to push or pull something up, down, sideways – whatever it takes – to simulate massive weight or even an earthquake. The biggest piston in here can apply 2 million pounds of force. If it takes more than that to break something, well, they’ll just rig up more than one.
Just inside the front door is a wall of pictures of some of the CFL’s “greatest hits” from the past, large objects that were destroyed in the name of safety. There’s also a six-inch-thick cross-section of a steel girder salvaged from the World Trade Center towers after the 2001 terrorist attacks – a sobering reminder that, as Lucier says, “everything can be broken.”
The CFL is one of about a dozen such large-scale testing facilities in the country. Just as much creativity goes into how best to break things as how to make them – computer simulations to determine not just how much force it takes, but applied at what spot and what angle. Clients range from government agencies and materials manufacturers to the military. The CFL has tested everything from helicopter rotor blades to the gracefully sweeping beams holding up Terminal Two at RDU Airport.
Depending on the scale involved, these tests cost anywhere from a few hundred to a few million dollars, funded from grants or fees (with the money going toward the CFL’s operating expenses). And while breaking stuff is the dramatic and sexy part, the cracks that open up at the moment of failure are just the tip of the iceberg.
The other nine-tenths of the process is designing computer simulations, recording and collecting data and crunching numbers. That part of it can take weeks, months, even years.
“Everything has a finite life span,” Lucier said. “That’s a big issue in the civil and structural engineering world. Most states have a lot of highway bridges getting close to the end of their designed life. So do they pour in a lot of maintenance dollars, try to get a few more years? Or tear down and replace? Allowing them to collapse is not an acceptable option. When it comes to structural engineering, the tolerance for failure is zero.”
Full-scale safety testing
Students do most of the CFL’s work. Around 40 participate, most of them graduate students and three-quarters of them male. Far more women are in engineering today than a decade ago, but the gender split is still nowhere near 50-50.
About two dozen testing projects are in process at any given time, from the T-beam and other large-scale trials to small ones like the permeability test going on in one of the CFL’s smaller lab rooms, measuring how much water a concrete sample absorbs. Safety is the primary concern, obviously.
Everyone has to don a hard hat helmet as soon as they walk in – even big wheels like Paul Zia, now a retired professor, who founded the CFL and still comes around to watch.
“It’s like medicine, where you eventually have to test things on an actual person,” Zia said. “You can do small simulations, but ultimately, you have to try it out full-scale.”
As he spoke, Zia watched the next trial get started, testing a concrete ledge of the sort the just-broken T-beam might rest on. Again, at each pause, the students examined and marked the surface – gingerly, as the force went up and the failure point approached.
At 45,000 pounds, it still looked like it would hold. CFL director Rizkalla leaned in to look, pointed at a crack and asked, “Do we have another pen?”
Just at that moment, a loud BOOM reverberated through the building. As the concrete gave way and deep cracks spidered across the surface, everyone jumped.
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