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WOOD: Wind Uplift on Rafters

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Title: WOOD: Wind Uplift on Rafters

Bill,

Sorry I'm arriving late to the party, but I get the digest and was out of the office a couple of days.

In case this hasn't been answered, we do account for uplift of the rafters in the 2001 WFCM Footnote 2 to Table 2.14A and 3.26A-H. As you'll see in that footnote, the adjustments are dependent on roof slope and windspeed. The WFCM Commentary for Table 2.14A gives all the background calculations for the tabulated values.

Btw, for those of you new to the WFCM (or 2001 NDS), there's a FREE eCourse available for each at the following link that'll give you an overview of the documents:

http://www.awc.org/HelpOutreach/eCourses/index.html

Hope this helps.

Buddy


John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791
http://www.awc.org
The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

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The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.

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From: "Bill Polhemus" <bill(--nospam--at)polhemus.cc>
To: <seaint(--nospam--at)seaint.org>
Subject: WOOD: Wind Uplift on Rafters

Okay, more dumb question time:
I had noticed in the tables in the 2000 IRC (and I'm certain this is the same as in the earlier SBC, BOCA and UBC as well), the rafter loading considered is always DL & LL, Wind is never mentioned.

Now I'm looking at the newly-purchased WCFM, and the tables therein for "Roof and Wall Sheathing Suction Loads" (Table 2.4) states specifically that it's "for sheathing and sheathing attachment).

So where the heck does the wind go after it hits the attachment? Why does it APPEAR that the rafters never "see" the wind effects? Is this another of those arcane "residential design" things that we discuss here from time to time?

When I design a flat-roof with, say, steel joists, it is always understood that there is wind uplift transmitted to the joists, which often results in a NET uplift force. So why not the rafters in a sloped roof? Or am I missing something here?

Thanks for any insights you can give.
Bill Polhemus, P.E.
Polhemus Engineering Company
Katy, Texas USA