WHEN WIND HITS THE WALL
By Joyce Laird
Airboats are the watercraft that zip through the Everglades. The propellers designed for those boats have found another use in Florida, to simulate hurricane winds that test building techniques.
James Erwin of the Laboratory for Wind Engineering Research at Florida International University, is a key figure in the wind tests. He works with a team of faculty, students, and two other full-time researchers at the lab, which is part of the university’s International Hurricane Research Center.
“Our work is geared toward enhancing our knowledge of how extreme winds interact with buildings,” Erwin said. “The ultimate goal is the development of effective mitigation techniques to reduce storm-related damages, and to make improvements to local building codes in hurricane prone regions.”
One of the tools they use is called the Wall of Wind. For a recent test of the connections of a roof to walls, the WoW consisted of six marine grade Chevrolet 502 crate engines of 400-plus hp each connected to airboat propellers. Together the fans could simulate category 1 hurricane winds across a 16- x 22-foot wind field.
The team assembled a residential building model using common wood construction techniques, and installed sensors where trusses connected to the walls.
“Our experimental setup used a total of six multi-axis load cells and allowed us to measure the force distribution across the roof,” Erwin said. “We were able to simultaneously record the forces and moments at each roof-to-wall connection point in order to understand the overall load path created as the wind was applied at different angles on the building.”
The supplier of the load cells, JR3 Inc. of Woodland, Calif., adapted them for the application. The large sensors needed for the tests are typically made of aluminum, but to stand up to 85 mph winds, they had to be of stainless steel. The sensors were also given special waterproofing.
The sensors were 7 1/2 inches in diameter and 2 inches thick with a little over a thousand pound rating on the horizontal forces. Each sensor weighed 15 to 20 pounds.
HURRICANE FORCE: This array of large airboat fans (above) can
generate winds of more than 75 miles per hour. Sensors attached
to trusses on a model building (below) measure the stresses
hurricanes place on typical roofs.
According to Erwin, “What we are doing isn’t simply blowing wind on a model building. We attempt to recreate the natural characteristics of atmospheric flows around buildings, based on information available in published literature, and through access to real-world data collected by meteorological towers deployed by the Florida Coastal Monitoring Program during hurricanes. We also have the ability to inject wind-driven rain into the flow.”
To protect the sensors, JR3 incorporated O-rings on the canister so Erwin could seal them tightly when mounted to his hardware. An off-the-shelf pre-sealed connector assembly prevented water getting into the connection. “It’s a standard for the industry but it’s the first time we had to use it,” said John Ramming, president of JR3.
The company also included an indicator card behind a little glass window. The card changes from blue to pink with increasing humidity. If the indicator changes color after a test, the sensor is shut down and sent back to JR3 for repair and recalibration.
Erwin said the wind loading data collected from the WoW’s category 1 winds—speeds too low to separate the roof and walls—played a role when the university’s Structures and Construction Laboratory used hydraulic lifting machines to stress the connections to failure. Data from the WoW experiments allowed researchers to apply realistic loads to the connections.
A 12-fan Wall of Wind is in the works and will be able to generate 140 mph, or category 4, winds. A turntable will be able to rotate building models to simulate wind from any direction.
For this application, JR3 will supply three load cells that each weigh about 150 pounds. “Erwin is talking about 25-, 50,000 pounds of payload, 24,000 foot-pounds of torque or more,” Ramming said. “So we are almost a factor of 10 bigger in some aspects than the last load cells we sent him.”
The author is a freelance business and technical writer based in Arleta, Calif.