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RE: WOOD: Wind Uplift on Rafters

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Okay, this is a good point. I don't think my question was clear enough, but you answered it anyway.
 
If the wind load goes from the sheathing fasteners into the hurricane clips, how does it get there? Seems to me since the sheathing is nailed to the rafters, and the rafter is tied to the top plate by the hurricane clip, the answer is that the force goes THROUGH THE RAFTERS, right?
 
So why don't we consider the rafters are seeing the wind load as a distributed load acting "outward" from the building?
 
Is it that it is considered to all go through the sheathing as an in-plane force? In that case, why worry about the force on the attachment to the rafter?
 
I'm still not getting this.


From: David Topete [mailto:davetopete(--nospam--at)yahoo.com]
Sent: Tuesday, September 14, 2004 11:48 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: WOOD: Wind Uplift on Rafters

Bill,
Attaching the sheathing to the rafters is just as important as attaching the rafters to the dbl top plate/beam...  Whethrer you rely on nail pullout values or screws, the easiest way to take out the uplift is to use hurricane ties (how appropo...) by Simpson Co. (or equiv.) 
Say you have 15 psf of uplift.  Then design the rafter conn for 15psf * 2ft trib * 1/2 of 12ft rafter span = 180# for rafters spanning 12ft, spaced at 24"oc.
HTH
David Topete, PE
SF, CA

Bill Polhemus <bill(--nospam--at)polhemus.cc> wrote:
-->

Okay, more dumb question time:

 

I had noticed in the tables in the 2000 IRC (and Im 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 Im looking at the newly-purchased WCFM, and the tables therein for Roof and Wall Sheathing Suction Loads (Table 2.4) states specifically that its 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


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