Wood Roof Diaphragm Chord
Wood Roof Diaphragm Chord
(OP)
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.
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.






RE: Wood Roof Diaphragm Chord
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.
RE: Wood Roof Diaphragm Chord
As an alternative to the beam, use blocking and strapping between the shear walls.
Mike McCann
MMC Engineering
RE: Wood Roof Diaphragm Chord
The link below is a techincal bulleting from Simpson covering this issue.
h
RE: Wood Roof Diaphragm Chord
I would call for a tension strap at all splices.
I suspect that is what happens in real life. I've seen a lot wood structures built without any attention to diaphragm chords.
RE: Wood Roof Diaphragm Chord
What kind of beam? Like a drag truss diaphragm in lieu of using the roof sheathing?
I'm not sure if I follow.
RE: Wood Roof Diaphragm Chord
Mike McCann
MMC Engineering
RE: Wood Roof Diaphragm Chord
So the strap would have to be placed before the roof joists are put in place?
RE: Wood Roof Diaphragm Chord
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?
RE: Wood Roof Diaphragm Chord
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
RE: Wood Roof Diaphragm Chord
How else does the load transfer from the diaphragm to the top plates without the use of blocking?
RE: Wood Roof Diaphragm Chord
Mike McCann
MMC Engineering
RE: Wood Roof Diaphragm Chord
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?
RE: Wood Roof Diaphragm Chord
Mike McCann
MMC Engineering