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Posted by Wayne Whitney on April 7, 2008, 11:42 am
>
> > It's not a matter of ripping off the T&G boards, it's a matter of
> > deforming the framing so that each rectangle formed by two joists and
> > two of the T&G boards becomes a non-rectangular quadrilateral.
>
> It's safe to assume that the guy's framing DD's house didn't use 32"
> long scrap to sheath the roof, so it's not just two _rafters_
> involved, probably more like five or six or more.
Right, I was referring to the whole system deforming, so that "each"
rectangle deforms, together.
> At the forces required to make a, what?, 30' roof start to rack
> appreciably, the whole roof would come off in a piece. Other
> failure modes would be involved way before roof racking became an
> issue.
The longer the ridge is, the lower the resistance to lateral forces
will be. As for uplift, that's a great point--increase the racking
resistance enough and you can be sure that the uplift failure mode
will dominate. So _if_ the OP is in a high wind or seismic area, and
_if_ the OP wishes to take the opportunity to strengthen the roof
structure, then it would also make sense to add hurricane clips to
resist uplift.
Yours, Wayne
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Posted by DerbyDad03 on April 7, 2008, 11:57 am
>
>
> > > It's not a matter of ripping off the T&G boards, it's a matter of
> > > deforming the framing so that each rectangle formed by two joists and
> > > two of the T&G boards becomes a non-rectangular quadrilateral. =A0
>
> > It's safe to assume that the guy's framing DD's house didn't use 32"
> > long scrap to sheath the roof, so it's not just two _rafters_
> > involved, probably more like five or six or more. =A0
>
> Right, I was referring to the whole system deforming, so that "each"
> rectangle deforms, together.
>
> > At the forces required to make a, what?, 30' roof start to rack
> > appreciably, the whole roof would come off in a piece. =A0Other
> > failure modes would be involved way before roof racking became an
> > issue.
>
> The longer the ridge is, the lower the resistance to lateral forces
> will be. =A0As for uplift, that's a great point--increase the racking
> resistance enough and you can be sure that the uplift failure mode
> will dominate. =A0So _if_ the OP is in a high wind or seismic area, and
> _if_ the OP wishes to take the opportunity to strengthen the roof
> structure, then it would also make sense to add hurricane clips to
> resist uplift.
>
> Yours, Wayne
OP here -
On the rare chance that a hurricane or earthquake hits western NY,
either one of them can have my house.
Thanks for some interesting reading, but I won't be paying anything
extra for hurricane clips or rack-prevention plywood.
Heck, I don't even want to pay for new shingles! <g>
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Posted by RicodJour on April 7, 2008, 12:00 pm
>
>
> > > It's not a matter of ripping off the T&G boards, it's a matter of
> > > deforming the framing so that each rectangle formed by two joists and
> > > two of the T&G boards becomes a non-rectangular quadrilateral.
>
> > It's safe to assume that the guy's framing DD's house didn't use 32"
> > long scrap to sheath the roof, so it's not just two _rafters_
> > involved, probably more like five or six or more.
>
> Right, I was referring to the whole system deforming, so that "each"
> rectangle deforms, together.
Which doesn't happen. Deformation happens locally and then spreads
when the initial resistance is overcome. By the time the whole roof
is involved you've got one hell of a big and airy skylight.
> > At the forces required to make a, what?, 30' roof start to rack
> > appreciably, the whole roof would come off in a piece. Other
> > failure modes would be involved way before roof racking became an
> > issue.
>
> The longer the ridge is, the lower the resistance to lateral forces
> will be.
Maybe you should review that sentence. How can the resistance be
lower with a longer ridge? More rafters are involved, more points of
attachment, more moments resisting racking. It can't be lower. If
you meant proportionally lower, well, I don't buy that either as the
roof is probably of a piece, but for argument's sake, exactly how
short/narrow of a house do you think DD lives in?
> As for uplift, that's a great point--increase the racking
> resistance enough and you can be sure that the uplift failure mode
> will dominate.
You can always reinforce something to make something else the weak
point. That's not the issue. The issue is whether it's money well
spent and whether it's a real risk.
> So _if_ the OP is in a high wind or seismic area, and
> _if_ the OP wishes to take the opportunity to strengthen the roof
> structure, then it would also make sense to add hurricane clips to
> resist uplift.
IFF (if and only if) that is the case, the hurricane clips should be
the first thing installed.
R
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Posted by Wayne Whitney on April 7, 2008, 1:07 pm
> Maybe you should review that sentence. How can the resistance be
> lower with a longer ridge? More rafters are involved, more points
> of attachment, more moments resisting racking. It can't be lower.
Engineering practice is to treat conventional roof rafter and ceiling
joist construction as if it were a simple flat diaphragm (joists plus
sheathing), as far as calculating its lateral load resistance. That
is certainly easier to visualize and analyze.
For lateral force applied in a given direction, the only part of the
structure that can carry that force to the foundation are the walls
parallel to the force. So the diaphragm has to carry the lateral
force to the end walls parallel to the force. The diaphragm is
modelled as a deep beam spanning between those end walls. The longer
the distance between those walls, the greater the span of this "beam",
and the less the resistance.
> IFF (if and only if) that is the case, the hurricane clips should be
> the first thing installed.
Absent an analysis of a conventionally framed roof with T&G board
sheathing, that seems quite reasonable, that the uplift capacity is
the weakest link. Uplift on conventional roof framing puts both the
sheathing-rafter nailing and the rafter-plate toe nailing in
withdrawal, so that probably won't work very well. :-)
Yours, Wayne
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Posted by Norminn on April 7, 2008, 7:34 am
Wayne Whitney wrote:
>
>
>
>>Well, if the winds and sheer force are great enough to rip off T&G,
>>a bit of ply over it ain't gonna stop it.
>>
>>
>
>It's not a matter of ripping off the T&G boards, it's a matter of
>deforming the framing so that each rectangle formed by two joists and
>two of the T&G boards becomes a non-rectangular quadrilateral. This
>only requires that the T&G boards slide relative to each other.
>Plywood would resist this deformation much more than the individual
>boards do.
>
>Yours, Wayne
>
>
Looks like an answer from an old engineering text. If the freeking
rectangles are deforming, something
else is happening to the house under it. Hurricanes take the roof OFF,
and take them apart after
they are off.
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