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Pressure Ratings: Design Pressure Ratings and Windspeeds

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Good discussion, I am going to offer a few comments on Design Pressure.

DP ratings are established by testing the window to pressures equal to 1.5 times the DP requirement. In other words, if a window is rated to DP40, then it is actually tested to 60psf. If a window is rated to DP20, then it is tested to 30psf.

One might assume that there is a direct linear correlation between DP rating and windspeed. One might also assume that a DP of 40 is twice as good as a DP of 20 - or that a DP of 80 must be twice as good as a 40 and four-times better than a 20.

Actually, this isn't the case in the real world.

While a specific DP does equate to a specific windspeed, there isn't a direct linear relationship between increases in windspeed and increases in design pressure. In other words, increasing the DP a certain amount does not result in an equal increase in the window's ability to resist a specific windspeed or pressure.

For example, a window with a lowly DP9 is actually rated for a 60mph wind and it is actually tested at 13.5psf which equates to a windspeed of about 72mph. A rating of DP9 is pretty darn low, but a 60mph wind can be a pretty significant gust.

Imagine that we now look at a window rated to DP15. Sounds darn low and no one in their right mind would want a DP15? Well, 15psf actually equates to about an 80mph wind. Now, as mentioned, the window is actually tested to a level 1.5 times higher than the rating, which for a DP15 equals 22.5psf, and that equals an approximate 95mph windspeed. So while a 15DP doesn't look like much, it really isn't bad at all and it is probably acceptable for any application in the right environment.

And btw, exceeding a window’s DP rating does not mean that suddenly the window will explode or come crashing into your home killing everyone inside; it simply means that the "performance values" of the window are rated to a particular pressure. Can windows "fail" when subjected to windspeeds above the rating? Of course they can, but failure generally means cracked glass or warped frames or something similar and not necessarily catastrophic collapse. However, on the flip side, there are very good reasons why hurricane-prone regions are now mandating minimum DP requirements when building new construction or renovating.

While back at the numbers a design pressure of 45psf actually equates to a windspeed of 135mph while a design pressure of 60psf by comparison equals a windspeed of 155mph and at the tested pressure of 90psf it is approximately 190mph. What you may now have noticed is that as the DP increases the percentage difference between design pressure and actual windspeed has decreased. In fact, you may also have already noticed that the ratios of the design pressures are the square of the ratios of the wind-speeds.

So while 60psf equals four times 15psf, the equivalent windspeed difference has only doubled.

My point being that while DP ratings can be very important, even crucial, in some environments, the idea that a person in an area that rarely or never experiences 100mph wind gusts is worried about the difference between a DP45 versus a DP60 is worrying over something that really isn’t a significant factor in their situation window or door with a DP15 or DP20 can be every bit as good in the correct environment as a window or door with a DP80.

Source Post: Air Infiltration

Posted: Sun Jan 08, 2006 2:46 pm