New Research Focuses on Hurricane-Driven Rainwater Penetration
The first priority in the wake of the devastating 2004 and 2005 hurricane seasons along the U.S. Gulf and Atlantic Coasts has been to upgrade construction requirements to prevent damage from excessive winds and wind-borne debris. Stronger code requirements for impact-resistant windows have since been adopted in many jurisdictions, ranging from those that reference current International Code requirements and ASTM standards E 1886 and E 1996, up to those in defined High Velocity Hurricane Zones that take it up a notch—mainly Miami-Dade County standards TAS-201, TAS-202 and TAS-203 or AAMA 506.
Building codes typically require either structural panels providing glazed opening protection or impact-resistant windows, the latter being comprised of laminated glass in high-strength or reinforced frames.
The tests for determining compliance with these impact requirements are rigorous. For example, for windows to be located less than 30 feet above ground level, the impact of large missiles is simulated by thrusting a 2x4 stud into the product at 50 feet per second, equivalent to 34 mph. For windows located more than 30 feet above ground, the impact of roof gravel and other small objects is simulated by firing a shotgun-like pattern of two-gram ball bearings into the window at a speed of 130 fps, or, 88 mph. To pass these tests, there can be no through-penetration upon impact and no opening formed larger than 3 inches in diameter or tear longer than 5 inches.
All considered, the infrastructure is largely in place to ensure that windows in new construction can offer greatly improved resistance to winds and flying debris.
The next level of improvement is focusing on reducing water penetration. While many might count themselves lucky if protected from serious wind damage and storm surge-induced flooding, water penetration through or around otherwise intact openings from wind-driven rain can result in a significant amount of physical damage, cause occupant displacement and lead to extensive restoration expenses.
At the urging of the Florida Building Commission, the AAMA Southeast Region organization undertook a project in late 2005 to assess test methods and develop a standard of performance for windows capable of resisting water penetration under hurricane conditions. A task group was formed to review existing test methods and develop a rating system, with the caveat that it is impractical to expect any cost-effective building component to completely prevent water leakage when a downpour is slammed against a building at some 100 mph—conditions well in excess of those assumed for code requirements. But, the task group did seek to determine what limits should be imposed on such water penetration and how best to test and rate windows designed to meet such limits.
RATING SEVERE RAIN RESISTANCE
The task group’s first attempt to answer these questions is the most recent draft of the Voluntary Specification for Rating the Severe Wind-Driven Rain Resistance of Windows, Doors and Unit Skylights. It references the ASTM E 2268 Standard Test Method for Water Penetration of Exterior Windows, Skylights and Doors by Rapid Pulsed Air Pressure Difference. The test setup is similar to that used for water penetration testing of windows for compliance with the AAMA/WDMA/CSA/101/I.S. 2/A440-05 window and door standard and its predecessors, which calls for a water spray rate of five gallons per hour per square foot (and simulates a rainfall rate of 8 inches per hour). However, ASTM E 2268 adds a pulsing sequence to better simulate storm conditions and applies water spray at higher wind pressures (up to 42 psf). Test data is evaluated according to a scale of performance levels, indicating successively higher ranges (lower to upper limits) of the required 300 pressure pulsation cycles, with each cycle lasting two seconds. At all performance levels, the amount of water penetration cannot exceed 15 milliliters per meter of sill length.
But the process of drafting this specification has uncovered more questions. For example:
* Do the most intense rains occur in the highest winds, when pressure loading is the most severe? What design pressure best simulates typical hurricane conditions?
* How are raindrop sizes affected by wind speed, terrain and topography? What happens to locations near the shore that are subjected to ocean spray?
* How much of the wind-driven rain actually wets the building’s façade, especially at the corners where wind pressure effects are often enhanced? Should a corner window be designed differently to resist water penetration better than one located in the center expanse of the wall?
BRINGING THE STORM TO THE LAB
To answer these questions, AAMA turned to the University of Florida, where Forrest Masters, an assistant professor of civil and coastal engineering, has gained a national reputation for his bold steps to take some of the mystery out of hurricane effects. Research underway at UF is designed to get a handle on actual wind and rain effects, and then to devise a way to simulate those conditions on demand.
To accomplish the first part of this mission, Masters was a major player in launching the Florida Coastal Monitoring Program. He and his team chase hurricanes, planting six specially instrumented, 33-foot portable towers in the path of hurricanes as they make landfall. So far, the mobile monitors have been deployed into the teeth of some 20 hurricanes since the late 1990s. Capable of withstanding wind gusts up to 200 mph, the towers record and relay information to meteorologists at the National Oceanic and Atmospheric Administration at 15-minute intervals.
In 2007, the AAMA board of directors approved $60,000 to fund the acquisition of what amounts to a very high-tech rain gauge, which will enhance the measurement of hurricane rainfall parameters in Masters’ towers. Known as a precipitation-imaging probe, the instrument will permit real-time measurement of actual hurricane rainfall, including the size of the raindrops and rainfall intensity. Data from the probes collected during this and upcoming Atlantic hurricane seasons will be used to establish a catalog of “wind-driven rain scenarios” for different storm intensities impacting various terrains. The information will be used to calibrate the rain fields produced by UF’s mobile windstorm simulator to recreate hurricane-force winds and wind-driven rain at sufficient scale to test low-rise components and cladding systems.
The windstorm simulator is comprised of eight large diameter industrial-grade fans, coupled through an innovative hydraulic drive system to four 700-horsepower marine diesel engines. A specially designed duct accelerates the fan output to hurricane force, while steering vanes can be manipulated to induce turbulence. Water jets imbedded in the vanes simulate rainfall rates as high as 35 inches per hour. A demonstration of this equipment can be seen on AAMA’s Web site via the following link:
Masters plans to use the wind and rain data from the monitoring towers to calibrate the simulator to accurately reproduce hurricane conditions in the laboratory, enabling realistic testing of hurricane-resistant building products. This will allow the industry to develop more accurate testing methodology, sufficient to conduct meaningful leak-resistance tests and evaluations of windows, doors and wall systems.
Thankfully, there were few hurricanes to monitor during the last hurricane season, so we will continue to work with UF to gather data and answer our questions about water infiltration.
All of these efforts are paving the way for the improvement of hurricane-resistant products–something that should interest code officials, architects, builders and insurance companies who serve the 53 percent of Americans (153 million in all) that NOAA reports as living within the narrow coastal regions. With this many structures susceptible to water damage, this research and its correlation to product testing is vitally important.