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Huffing and puffing to blow a house down

Researchers are studying the effect of hurricane-force winds on homes to learn how structures might be made safer through inexpensive but strategically placed reinforcements.

A big bad wolf of a “hurricane” hasn’t exactly blown down the roof or walls of a brick house at the University of Western Ontario. Even so, the university’s “Three Little Pigs” project is offering a revealing look at how winds from storms such as Hurricane Ike, which buffeted the Gulf Coast of Texas earlier this month, can easily destroy unsecured homes.

The eventual goal of all the simulated huffing and puffing, researchers say, is new insight into how those homes might be made safer through inexpensive but strategically placed reinforcements.

“We could make a house that could withstand even an F-5 tornado,” said Greg Kopp, a professor of civil engineering at the university. “But no one’s going to want to pay for that.”

Initial tests, conducted in August at the university’s one-of-a-kind Insurance Research Lab for Better Homes in London, Ontario, simulated the wildly fluctuating pressure loads associated with a storm as strong as a Category 4 hurricane, with wind speeds of more than 130 miles per hour. Instead of a huge wind tunnel, however, the lab used dozens of vacuum-like pressure hoses to simulate a storm’s push-and-pull forces on a two-story home, with monitors and video cameras capturing the results.

“The experiment was disappointingly un-dramatic,” concedes Kopp. “We tell people that we destroyed a house and they’re imagining just a slab staying there by the end of it. And that’s not what happened.”

Despite the deceptive outward appearances, though, the four-bedroom home had lost all of its roof-to-wall connections by the end of the test, so that only the weight of the roof was keeping it in place atop the house.

“People think more about the videos that have come from tornadoes, where there’s just an explosive loading of the structure,” Kopp said. “For hurricanes, the buildup of damage is exactly that —a buildup.”

Beyond wanting to understand how that gradual process works, he said, his team is wondering whether many homes that appear to have withstood storms relatively unscathed actually may have significant structural failures.

The university’s Boundary Layer Wind Tunnel Laboratory has long focused on pressure loads for engineering high-rise buildings and bridges. Kopp said that focus began to shift about six years ago when researchers noticed that residential structures were getting hit much harder than commercial buildings by major hurricanes. “We realized that in order to get a good handle on it, we needed to test a full-scale house, so we started to think about how to do that efficiently,” he said.

Building a huge wind tunnel to accommodate a house would have required $50 million. Instead, the university received $7 million in grants to construct a facility where researchers could simulate hurricane-force pressure loads on a home’s exterior. The hard part, Kopp said, was designing a system capable of replicating a storm’s rapid fluctuations and huge variations.

“Wind tends to want to find the weakest link and break that,” he said. “And individual shingles or tiles could see much, much higher loads than, say, the load average over the entire roof.”

For a hurricane blowing straight at the corner of a house, he said, “the pressure can be five to six times higher right near the edges than on the leeward side.” Turbulent airflow further complicates the effects of nearly horizontal wind movement.

His team worked toward a faithful simulation by measuring the pressure with a small-scale wind tunnel model. In collaboration with the Cambridge, England-based design firm Cambridge Consultants Ltd., the researchers scaled up the results with fans deployed more like vacuum cleaners.

“Imagine you’re vacuuming your carpet and you want to get that pesky pebble on it and you put the vacuum right over it to suck it up,” Kopp said. In the same way, the team used 58 box-enclosed fans rigged to a framework surrounding the house to exert sucking or blowing pressure simulating the variable effects of wind.

Meanwhile, 200 sensors were measuring the resulting displacement and 20 video cameras were documenting the damage. “We got into a higher Category 2 or lower Category 3 before there was substantial damage to the house,” he said. “The failure was very much progressive rather than the roof all failing at one time, until we had broken essentially all of the roof-to-wall connections, and that was at a Category 4.”

The critical analysis was on how the roof’s trusses joined the walls through toenailed connections, where nails are angled down from each truss to the top of the wall. The researchers had expected the roof to eventually move about four inches away from the wall — a shift that likely would have flung a real home’s roof onto the lawn. Instead, individual toenails moved about an inch at a time until all the connections had been completely compromised, beginning with the leeward side of the roof.

Kopp recalls hearing a colleague describe the aftermath of a tropical cyclone in Australia and a house with a curtain hanging out between the roof and a wall. “They couldn’t figure out what had happened,” he said, but his test demonstrated that a roof can catastrophically fail yet settle back down on the wall after the winds stop. “This really surprised us.”

Blasting homes with 120 mph winds
Nicholas P. Jones, dean of the Whiting School of Engineering at Johns Hopkins University in Baltimore, said he wouldn’t have considered the progressive failure of the roof’s leeward side as a surprise, given that pressure there can be strongly negative in a storm. A weak link exposed by the wind, he said, can lead to a domino effect of failures. Every time a truss’s connection is compromised, for example, its neighbors are forced to shoulder higher loads and the likelihood increases that they, too, will fail.

Nevertheless, Jones said, “there is still a lot to be learned about the interactions of airflow in the lower part of the atmospheric boundary layer, where buildings are located.”

To that end, he views the University of Western Ontario’s unique facility as a good complement to other efforts like the Wind Science and Engineering Research Center at Texas Tech University in Lubbock, where members of the Hurricane Research Team also chase major storms to collect real-time data. At the University of Florida in Gainesville, scientists have used a cannon to blast storm shutters with ceramic roof tiles to simulate the power of hurricane-force winds and deployed the world’s largest portable hurricane wind and rain simulator to blast test homes with wind speeds up to 120 miles per hour.

In the town of Southern Shores on North Carolina’s Outer Banks, Jones helped outfit a two-story community center dubbed the “Hurricane House” with a suite of sensors to remotely monitor its response to hurricane-force winds. Although a lack of funding prematurely killed the project after six years, Jones said a few passing tropical storms and hurricanes provided some useful data on how the house — built to showcase a new wind-resistant design — handled wind speeds in the 50 to 60 mph range.

Ultimately, he said, a collaborative effort that adopts multiple approaches — including full-scale wind measurements and simulations with wind tunnels, pressure hoses and computational fluid dynamics — may have the best chance of successfully filling in the knowledge gaps. As president-elect of the American Association for Wind Engineering, Jones said he plans to use his tenure to push for exactly that.

As for Kopp, one next step is to use embedded sensors to measure the force exerted on the nails at each truss connection. Beyond those tests, he said, his team may start adding on some hurricane mitigation measures while removing other parts of the structure that don’t face relevant pressure loads.

Though current tests have focused entirely on the roof, the institute has previously analyzed the role of windows and glass in maintaining a home’s integrity during a storm. A window shattered by flying debris can act like a funnel for high winds, quickly increasing the pressure within a house by up to 70 percent, Kopp said. With that pressure pushing upward on the roof, in addition to the exterior pressure buffeting it from the side, the roof can fail at far lower wind speeds, sometimes explosively.

An effective deterrent is to install storm shutters or at least board up all windows with plywood — one fairly simple lesson that Kopp said could make all the difference.