The hurricane season is now well under way, and as the buffer of El Niño rapidly retreats, the forecast is looking windy, with an estimated 17 “named storms” on the horizon of the Atlantic basin.
While hurricanes seldom hit here in these parts of Southwestern Ontario (the exception being the Category 4 Hurricane Hazel in 1954, in which a 7-year-old boy drowned), a team of wind engineers at the University of Western Ontario is nonetheless preparing for the worst: a Category 5 storm.
In a hurricane simulation study called the Three Little Pigs Project, a full-scale two-story red brick house will be hit with the equivalent of 186-mile-per-hour winds and sprayed with water until it is on the brink of collapse.
Seeing the chandelier swinging from the ceiling is not acceptable.
“We really expect that we are going to tear the roof off,” said Dr. Michael Bartlett, a civil and environmental engineer at the university who is delivering a talk on the experiment at the 12th International Conference on Wind Engineering this week in Cairns, Australia.
“It’s not just that we can simulate a Category 5,” Dr. Bartlett said, “but we can simulate a Category 5 and see the unsatisfactory performance, and then do something that we think will improve the performance” of the structure.
Alan Davenport, an authority on windblown buildings who in 1965 founded the Boundary Layer Wind Tunnel Laboratory at the university, conceived of the Three Pigs project in 1999, at the end of the United Nations’ International Decade for Natural Disaster Reduction.
Despite the good intentions, “it was a bad decade,” said Dr. Davenport. In 1992, Hurricane Andrew caused $26.5 billion in damage in the United States (Hurricane Katrina’s bill in 2005 was $81.2 billion), and Hurricane Mitch killed more than 11,000 people in Central America in 1998. But the disaster reduction initiative “did get people thinking in new terms,” he said, “and led to a more organized measurement of wind turbulence.”
The Three Pigs house is anchored to a three-foot-deep concrete floor in a hangar exposed to the prevailing southwesterly winds at the airport in London. The experiment’s turbulent winds, however, depend not on Mother Nature, nor massive Hollywood special-effects fans. Rather, the wind is replicated by a series of 100 “pressure boxes” bolted to the house and braced against a galvanized steel cocoon.
The boxes are equipped with “pressure load actuators,” which are essentially sophisticated vacuum cleaners. Activated in unison they sound like an out-of-tune choir belting out a low- to high-C drone of 100 decibels, and blast out pulsating pressures that mimic the airflow of turbulent gusts as they dart around and through a house during a hurricane.
“When wind blows over a structure, it creates pressures which act on the surfaces of the building,” said Gregg Kopp, a civil and environmental engineer the university and an associate research director at the wind tunnel laboratory.The effect, Dr. Kopp said, is similar to the pressure differences between the upper and lower surfaces of an airplane’s wing that cause it to lift into the air.
“The same thing happens on a house — as the wind moves over it, low pressures are created which want to lift it,” he said. “These pressures are less than atmospheric pressure, so we call them suctions. A suction is like a vacuum, or the principle that allows a vacuum cleaner to work. So, instead of blowing wind at the house, we replicate the pressures and suctions. This is more efficient and less expensive.”
The ultimate goal of the project is to gather enough information on damage caused by hurricane winds to recommend changes in building codes. The trick is striking a balance between what consumers are willing to pay and the amount of acceptable risk set out in the building codes.
“The answer isn’t making a house so reliable that nothing fails, because nobody wants to pay for that,” said Dr. Kopp. “People would rather pay for the nice marble counter top, the things they can show off.”
“We’re trying to improve the evidence for changing the way we design and build light frame houses,” Dr. Bartlett said, noting that although Florida building codes were improved after Hurricane Andrew, most North American houses could not withstand the 95-m.p.h. winds of a Category 1 storm.
“Code committees don’t like to change things unless there is an obvious reason,” said Dr. Bartlett. “Whenever you change a line in the code, then every single designer or engineer who uses that code has to learn something new, and there is a tremendous cost associated with that.”
In gathering the evidence for proposed changes, the Three Pigs experiment first measures the impact of a hurricane on a miniature model of a house in the confines of the Wind Tunnel Laboratory. That process translates general data on hurricane wind speeds into specific data on how those winds “excite,” or act upon, the geometry of a certain structure. The model is wired with ports that record the pressure on every square inch, producing what is called a “time history” of the turbulent wind pressure accurate to the second.
The results of the tests are used in the Three Pigs house. As the mechanical vacuums replay the recorded measurements, the effects of the storm are measured by 35 “load cells” beneath the house and recorded by 17 cameras within.
“That becomes the evidence that we take to building code committees,” Dr. Bartlett said. If engineers compare traditional building methods with those proven in their testing, “we think that building code committees will respond positively to that. They’ll say, ‘Yes, that’s sensible. That’s a change worth making.’”
The project will also include educational videos distributed to insurance companies and homebuilders’ associations.
“There are 235,000 new homes built in Canada every year,” said Dr. Bartlett, “and for the United States we usually multiply by a factor of 10. So to have meaningful impact on the quality of such a large range of construction means that somehow we have to communicate through channels that are unconventional, to really get the word out.”
The raw data generated will also be used to refine computer modeling, which in turn will be applied to other domestic structures — a bungalow with siding, for instance, or a four-story apartment complex — thus eliminating the need for such extreme testing to be repeated.
Towers of condominiums, Dr. Kopp said, would be exempt. They already undergo rigorous testing and are engineered with redundant structural safeties.
“What governs the standards in high-rises is the comfort of the occupant,” he said. “Seeing the ground moving beneath, or the chandelier swinging from the ceiling, is not acceptable. The top corner in a condo building is the worst environment — and also where you have the wealthiest tenant, with the best access to lawyers.”