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Re: WOOD: Wind Uplift on Rafters

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"So why don't we consider the rafters are seeing the wind load as a distributed load acting "outward" from the building?"
 
We do...You should design loading on wood exactly the same as you do for steel, concrete or masonry. The rafter has several load cases to consider...those acting upward and those acting down. Incidently wind acts in both directions depending on roof pitch and that has to be considered.
 
 
HTH,
 
Rand
 
 
----- Original Message -----
From: "Bill Polhemus" <bill(--nospam--at)polhemus.cc>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, September 14, 2004 12:04 PM
Subject: RE: WOOD: Wind Uplift on Rafters

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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