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Wood Roof Diaphragm Chord

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abusementpark

Structural
Joined
Dec 23, 2007
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For typical wood structures with load bearing walls, either the top plates of exterior walls or a continuous fascia board is used as the diaphragm chord member. How do you guys normally handle wall offsets or other discontinuities that disturb the force transfer?

For example, see the attachment for a hypothetical one story wood structures with load bearing walls and either trusses or joists framing the roof. For lateral load applied in the direction indicated, the two available shear walls are the left and right exterior walls. So, we need continuous chord members between these shear walls.

How would you handle the condition shown along the bottom wall if you were using either the wall top plates or a fascia board as your chord member? There would be a discontinuity near the center of the diaphragm span.
 
Using a fascia board as a diaphragm chord sounds like poor practice to me. How do you splice it? Fascia boards are subject to deterioration and sometimes need replacing. The repairer wouldn't consider this to be a structural element.

As the plywood itself gets wider at the offset, maybe it can be the chord member. Otherwise, the plates may need to extend across, giving a bit of a bump in the ceiling, but no big deal.
 
Personally, due to the re-entrant corners and the vertical hard point at the ends of two shear walls, I would ues a beam in line with the two walls that served to both control vertical deflections of the roof diaphragm at those points, plus serve as a drag strut between the two shear walls.

As an alternative to the beam, use blocking and strapping between the shear walls.

Mike McCann
MMC Engineering

 
I personally do not use facia boards as chord members. Douple top plates are fine. In the case that you show, I would use a line of blocking with a tension strap cont. Through the opening. Lap the tension strap onto the top plate far enough to develop the forces.

The link below is a techincal bulleting from Simpson covering this issue.


 
Using a fascia board as a diaphragm chord sounds like poor practice to me. How do you splice it? Fascia boards are subject to deterioration and sometimes need replacing. The repairer wouldn't consider this to be a structural element.

I would call for a tension strap at all splices.

As the plywood itself gets wider at the offset, maybe it can be the chord member.

I suspect that is what happens in real life. I've seen a lot wood structures built without any attention to diaphragm chords.


 
Personally, due to the re-entrant corners and the vertical hard point at the ends of two shear walls, I would ues a beam in line with the two walls that served to both control vertical deflections of the roof diaphragm at those points, plus serve as a drag strut between the two shear walls.
What kind of beam? Like a drag truss diaphragm in lieu of using the roof sheathing?

I'm not sure if I follow.
 
I agree too to never use a facia board as the chord member. The double top plate of the shear wall properly spliced should be able to suffice in most cases.

Mike McCann
MMC Engineering

 
In the case that you show, I would use a line of blocking with a tension strap cont. Through the opening. Lap the tension strap onto the top plate far enough to develop the forces.

So the strap would have to be placed before the roof joists are put in place?
 
I agree too to never use a facia board as the chord member. The double top plate of the shear wall properly spliced should be able to suffice in most cases.

What about the case where you have a very significant bearing depth on the roof truss (for example, 2-3 feet)? Do you provide blocking trusses in between the main trusses along the entire length of the chord to ensure the top plates can be engaged?
 
Do you provide blocking trusses in between the main trusses along the entire length of the chord to ensure the top plates can be engaged?

Not necessarily, although you can. For example, 4X members could serve as the blocking, hung from a hanger off the top chord of the joist.

Mike McCann
MMC Engineering

 
Do you provide blocking trusses in between the main trusses along the entire length of the chord to ensure the top plates can be engaged?

Not necessarily, although you can. For example, 4X members could serve as the blocking, hung from a hanger off the top chord of the joist.

How else does the load transfer from the diaphragm to the top plates without the use of blocking?
 
You only need the full depth blocking if the shear wall is directly below the diaphragm. No full depth blocking is needed under a drag strut with no shear wall below.

Mike McCann
MMC Engineering

 
You only need the full depth blocking if the shear wall is directly below the diaphragm. No full depth blocking is needed under a drag strut with no shear wall below.

I'm referring to the gap between the roof diaphragm and the top plates. How does the diaphragm engage the top plates of the wall to act as a chord member?
 
Full depth solid blocking, or premanufactured truss blocking. Diaphragm is nailed to the blocking and the blocking to the top plate with Simpson A34's, A35's, LPT4's, etc., at the appropriate spacing.

Mike McCann
MMC Engineering

 
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