Inside a state-of-the-art testing facility less than an hour south of Charlotte, it’s raining man-made hailstones and possibilities.
Research scientist Ian Giammanco clicks his mouse to unleash the hands-over-your-ears destruction: about a dozen compressed air guns – or “hail cannons” – hovering 60 feet in the rafters, firing about 9,000 hailstones at speeds of up to 80 mph at a building that’s rotating on a turntable.
Four minutes later, researchers at the Insurance Institute for Business & Home Safety Research Center in Richburg, S.C., begin assessing the damage. They’re on their way to learning more about residential construction building materials and systems in an effort to reduce the $1 billion in property damage caused by hail in the U.S. each year.
Despite an April 2011 storm that caused massive roof damage in the Charlotte region, lead researcher Dr. Tanya Brown concedes that North Carolina “is definitely not a hail hot spot. That’s more in the Central Plains area of the U.S.” Nonetheless, she says, IIBHSRC wanted the testing site to be away from the storm dangers of the Atlantic coast, and in a location where the weather is generally more conducive to field research.
“North Carolina does get more hail than Georgia and Florida,” she said. “Part of that could be the influence of the Appalachian Mountains, which can trap moisture into certain areas.”
Making a new dent
The research center isn’t the first to test building materials’ resistance, or even simulate or make hail. But its first full-scale test, conducted in late February, broke ground in at least a couple of ways while paving the way for other advances.
“We have the capability to do full-scale testing, where we’re doing thousands and thousands of hailstones on an individual structure – where you’re basically looking at the whole system, as opposed to just a piece of roof,” Brown said. “The other aspect is the ice balls themselves.”
According to Brown, roofing products have been tested for impact resistance for a number of years by Underwriters Laboratory – the longtime safety testing and certification organization – and Factory Mutual, an insurance and research conglomerate. The UL test standard measures impact resistance on the drop of 1-inch to-2-inch steel balls; Factory Mutual uses cannon-propelled ice balls.
A key innovation in the IIBHSRC model is its mixture of 80 percent seltzer and 20 percent tap water, which is injected with a syringe into a mold and stored in a deep freezer.
“The FM standard basically takes distilled water, pours it in a mold, and puts it in a freezer,” Brown said. “What happens is that pure ice is actually more dense than what real hailstones are, so what we try to do by adding the seltzer water is decrease the density of the hailstones by having some of those bubbles in the seltzer water get captured in the freezing process.”
The pre-storm work is tedious and expensive. For the late-February demo, Brown said, she and two other staff members made ice every day for five days a week, for about eight weeks – just for testing on the one house. “It’s one of those things that’s kind of fun at first, but by Day 15 it can get a little taxing,” she said.
She estimates that each hailstone in the demo cost about $2,000 – “just a total swag off the top of my head. Each of our stones is made by hand, and we can make about 300 a day. So that’s six hours a day between three people. If you calculate all of our salary time, all of the equipment time in terms of getting a seltzer machine and getting all the freezers in and getting all the molds in and our use of the space there,” that’s how she arrived at the $2,000.
With an eye on reducing the labor-intensive nature of the prep work, “one of the things our board of directors has done is given us a small sum of money to look at automating that process,” Brown said. “It’s really not a difficult process; it just takes a long, long time.”
Speeds and angles
The blast from the hail cannons is intended to help offset the height limitations of the test chamber or any building, which prevent hail from raining down from thousands of feet in the air. The goal of replicating Mother Nature as much as possible also includes considerations such as velocity and inclines.
Brown says the smaller hailstones were propelled at about 51 mph, the largest ones (2-inchers) at 76 mph. “The reason we do that is to match the terminal velocity that these size stones would experience in the real world,” she said.
Brown says the angle for the demo was “pretty fixed, but the system is flexible so that we can change them from about straight vertical to about a 20-degree incline or so. We haven’t really investigated that (the different impacts of different angles) because this was our first full-scale test. But in a theoretical sense, what you might expect at a nonvertical angle is, you would start to see more potential wall damage.
“The reason we have the capability to fire at an angle is to simulate the effects of windblown stones,” she said. “If we want to simulate that effect, we should also increase the firing speed just a little bit to account for the extra momentum that’s provided by the wind.”
Giammanco calls the cannons “glorified potato guns,” that use basic PVC fittings and tubing, but Brown notes a key feature in the triggering mechanism.
“Within each barrel is a trigger that was developed by our machinist and constructed here on site to hold the hail ball in place and not let it fall out until the blast of air hits it,” she said.
“In a small laboratory, the testing we do has one ice stone in there, basically, and the cannon is in a relatively horizontal position so the stone is not going to fall out. When we’re talking about being 60 feet above the ground, with the cannon, gravity is not our friend there.”
Materials and aging
The house spinning on the turntable gets battered from all points. “We want to make sure that everything got a fair fight there because we had different materials,” Brown said.
For this event, she says, the team had three different kinds of roofing material. Two of them were asphalt shingles – one that was impact-rated according to the UL standards, the other a regular, standard product. The other side of the roof had various metals in two different installation methods.
Fiber cement siding was on two walls and vinyl siding on two other walls, along with aluminum gutters, vinyl- and aluminum-framed windows and a standard door – “materials you typically see on houses in this area,” she said.
So which materials fared best?
“That depends on your definition of resistant,” Brown said. “If you look at the metal roof, hundreds of dents will be showing. But a dent is not going to prevent that roof from protecting your house. It’s not going to let water penetrate into your attic and into your living space.
“If you flip over to the asphalt side, there were less number of impacts by quite a bit, but those impacts were a lot more severe, with punctures of shingles that could allow water to get in.”
All materials used in the February demo were new, so there’s much to be learned about the effects of aging on them. “Nobody has looked at aging much in a laboratory sense ... and getting data from the real world is very difficult,” she said. “So one of the things that we’ve been charged with doing this year is setting up a program that will allow us to naturally age specimens on our facility for up to 20 years. We’re in the planning stages of that.”
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