When navigating through the world of metal buildings, it is important to educate yourself about the various so-called safety ratings thrown out by manufacturers and sales people. Know also that these wind uplift ratings are determined primarily for standing seam roofs rather than through-fastened roofs.
The most important thing to understand is that no rating system is perfect and none of the test procedures can adequately recreate real world conditions. In fact, in most if not all cases, uplift testing only uses static wind forces rather than the variable forces found in nature. By this we mean that the testing is performed with the production of a steady wind pressure.
Wind uplift is created by the suction forces that occur when air moves rapidly over a surface. This is the same effect that lifts airplane wings and causes birds to be sucked into the sides of large moving vehicles. Wind passing over a roof of any material causes the same type of pressure and if the wind is strong enough it can create enough suction to pull roof panels off of buildings during events such as hurricanes.
Attempts have been made to provide a standardized rating system that denotes how well a roof will withstand wind uplift. While none of the ratings systems or test procedures is fool-proof, they do offer a way to compare roofing construction and the probability of failure.
Underwriter Laboratories UL 580 Rating
This is a classic test that has been in use since 1973. A 10 foot X 10 foot sample of roofing material is installed onto a test platform according to the manufacturer’s instructions. The edges are sealed with closely spaced fasteners and two purlins in the interior.
The sample is then subjected to a static uplift pressure for a 5-minute period and an oscillating pressure in 10 second intervals over a 60-minute period. This determines the class of roofing.
- Class 30: Resisted a nominal static pressure of 30 psf (pounds per square foot) and a range of oscillating pressure between 22 and 42 psf.
- Class 60: Resisted a nominal static pressure of 60 psf (pounds per square foot) and a range of oscillating pressure between 44 and 83 psf.
- Class 90: Resisted a nominal static pressure of 90 psf (pounds per square foot) and a range of oscillating pressure between 66 and 90 psf.
Unfortunately this test does not determine failure rate in winds that rapidly change speed and direction. In addition, it does not cover the failure rate of the anchors that will be used in actual construction.
This test covers both panels and anchors. A 5-panel wide sample (10 feet) by 25 foot length with intermediate purlin support at varied intervals and covers several spans is subjected to pressure from underneath to imitate wind load. The panels are clipped at the supports and panel ends with no other fasteners along the edge, leaving the panel to move freely at those points.
The pressure is applied in such a way as to determine slowly developing failures such as seam separations.
This is not a Pass/Fail test; it merely shows how a roof performs under uniform and unvaried wind pressure. The test is run to failure to find the ultimate uplift load capacity.
FM Global Standard 4471
FM Global is an insurer that came up with its own test for roof panel failure ratings. It uses a 12 X 24 foot section including the connecting fasteners and clips used in the field. The panels are subjected to higher and higher wind pressures from the underside until the assembly fails or can sustain a particular pressure for one minute.
FM Global panel ratings are stated as 1-60, 1-90, 1-120, and so on. This does not refer to wind speed; it refers to wind pressure in pounds per square foot (psf). A rating of 1-60 states that the roof was able to withstand 60psf of pressure for 1 minute.
The rating is used to apply a classification to roof panels. Class 1 roof panels are rated at 1-90. A safety factor of 2 means the maximum allowable design load is 45 psf. FM Global ratings also take into account other variables such as building height, terrain, type of roof and other considerations.
U.S. Corps of Engineers Testing Procedure
The U.S. Corps of Engineers developed a test procedure in response to the fact that the other test systems did not accurately measure uplift resistance. Unfortunately, this procedure is not really used. It required that the roof sample be installed exactly as it would be in the field. Perimeter clamping was not allowed. In addition, the test required supervision by an independent professional engineer.
Wind uplift ratings do not accurately reflect what will happen in actual use. They do, however, offer a way to compare roof panels and are a sincere and ongoing attempt to determine better ways to build roofs to respond to whatever nature dishes out. Just be clear on what the ratings actually tell you and keep in mind whether they are represent forces by wind speed or wind pressure.
For more information on designing buildings to withstand heavy winds, contact Whirlwind Steel today.