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• To: <seaint(--nospam--at)seaint.org>
• From: "Andrew Kester" <akester(--nospam--at)bbma.com>
• Date: Thu, 27 May 2004 12:17:52 -0400

Title: Message

Lloyd:

I don't know the old UBC provisions, but it sounds like you need to forget about that and you'll be fine.

You have it right, with a roof slope less then 20 degrees you do have a negative pressure on the leeward side of the roof (Zones E & G, B & D). In MWFRS design, in determining the overall horizontal force, Fig 6-2, note #7 basically says use zero for the windward pressure if it is less then zero. As the slope goes past 20 degrees the roof behaves more like a wall and at 25 degrees the leeward pressure become positive. You had it right when you mentioned Bernoulli, just think about how the wind swirls around the wall and roof interface when the roof is flat or less then 20 degrees and imagine that having a suction force.   I suppose it would be nice if it were a positive pressure as that would take care of a lot of uplift problems but the people at Simpson wouldn't be too happy!

And the wind pressure is most definitely a function of height, so qz goes up with h, as you ascertained....

We have the older version of this and it is very useful :Guide to the Use of the Wind

Thanks for making my dig into ASCE 7-02, we just got it and there is quite a lot of new wind info, tables, methods, etc. Definitely more detailed and better explained and the figures have vastly improved. I will take more complicated any day to not enough info!

Andrew Kester, PE

Longwood, FL

You wrote:

the negative pressures in the horizontal zones B & D. I'm

looking at the 85 mph Basic wind speed and a 10=B0 Roof Slope. So, When

I read the numbers in Fig. 6-2 and Note 7 of that same figure, I get the i= mpression

that I should be neglecting the negative or outward forces in zones B & D

so that the loading isn't less than if those zones were zero.

I get an increase in pressure

as I ascend the building. The ASCE 7-02 seems to take the upper or roof