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RE: Fw: roof truss daiphragm forces

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Saturday, May 19, 2007

It's not that he is looking too deep, he seems to be unfamiliar with
manufactured roof trusses. You bring up a couple of interesting points. The
ceiling (if vaulted) has to be greater than 1/2 the roof slope. So in a 5:12
roof slope, the ceiling can not exceed 2-1/2: 12. 2:12 is fine, but the
engineer must design the chord at the top of the wall if there is an outward
thrust at the bearing between truss and wall. Generally, the calculations
you have done based on tributary distribution of wind and/or seismic to see
what governs will be enough to justify the wall chord or nailing of the
double top plate and any compensation for discontinuities that may be in the
design (although it doesn't sound like it).
The truss manufacturer is responsible for all connections to accommodate
internal stresses in the plated truss.

Now I think we get to your real questions of shear parallel to the ridge
line. I assume that there is a ridge board to keep the trusses perpendicular
until the sheathing is added.

If this is designed assuming tributary area, then the shear at the ridge
should be zero. Considering that roof to be an unblocked diaphragm, I don't
see any reason to provide edge blocking at the panels when there is no shear
to transfer at this point. The blocking should only be added at the
perimeter walls where shear is transferred by tributary width to the walls.
It is a simply supported beam example. 

Now for reality - you can hold up the project arguing if there is shear to
be transferred because the sheathing is not staggered over the ridge, or you
can provide the blocking that he asks for and move on. I would think it is
easier to block it than argue the point. The top of the blocking can be
ripped to the 5:12 slope to get a solid nailing surface, and the blocks can
be face nailed through the ridgeboard to provide connection to the ridge.
Beyond this, the panels are oriented so that the panel ends land on a truss
and the panels may act as a drag without adding straps. Since it becomes the
job of the chord below or attached to the gabled end truss to resist
deflection caused by the location of maximum bending in the diaphragm (again
a simply supported beam analogy).

Give him what he wants even if it overkill or let the owner pay for you to
fight it out with him :>)
 
 
 
Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant
C-41250 (Exp. 3/31/09)
dennis.wish(--nospam--at)verizon.net
http://structuralist.spaces.live.com
http://www.structuralist.net (new site and services coming soon)
 
 
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-----Original Message-----
Pinyon Engineering wrote:
>  
>  
>
> Dennis ,
>  
> This is a 40x36 2-story garage  with a 5:12 roof slope and 2:12 
> ceiling vault with gable end balloon framed to the roof sheathing. 
> They are mfg 2x light wood trusses.  I talked with him an he was 
> worried about the tension forces across the ridge in the truss members 
> due to internal stresses caused by diaphragm action.  I called out for 
> 3x blocking at the ridge so both roof plywood planes could be nailed 
> together. 
>  
> seems like this plan checker is looking too deep
>  
> Thanks
>  
> Tim

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