...The more things change, the more they stay the same....
So i got inspired to inquire about details and framing conditions relating to the use of pre-fab roof trusses that span across or parallel to "non-bearing" walls below.
The state of play, is that our STD details indicate an STC clip that allows for something like 1/4" to 1/2" of deflection.
We also have a note section the typically calls out deflections that are in excess of the code minimum for deflections for the roof trusses.
Taken together, these two elements work fairly well when one considers that the clips have about a 3/4" max delfection capacity, and the live load deflections for typical truss in the 30ft range yield about the same deflection.
The implication of the above is that the non-bearing walls should be constructed low by the deflection amount to allow the truss to deflect under loaded conditions. That is NOT the case in the field. Our experience points to the idea that ALL walls are constructed to the same height at the top-plate.
So how are we to handle this situation - where trusses are routinely being installed with "intermediate bearing points" that are not part of the truss calc, and could potentially damage the bottom chord or "overload" walls below that are not designed for these bearing conditions.
The truth appears to be that we are (almost) powerless to change the construction industry in this area without attracting significant criticism and "push-back" from contractors and owners alike - the gain would appear to be fairly minimal... trusses may deflect more freely - but then there may be other problems cropping up relating to gypboard stress/bending and other corollary cosmetic issues...
So it appears that our approach then moves to one of accommodation, where we appreciate the truss deflection issue; note an adequate solution on our plans, but then "overlook" the issue and implementation in the field when we observe... as do others in the inspection community...
So, amazingly, if our truss conditions are in the "normal" (say up to about 30ft) range, we use our typical details as noted above, and our plans should pass muster as relates to the theory.
As mentioned, ...the changes... stay the same...
Does anyone have a comeback on this...?
So i got inspired to inquire about details and framing conditions relating to the use of pre-fab roof trusses that span across or parallel to "non-bearing" walls below.
The state of play, is that our STD details indicate an STC clip that allows for something like 1/4" to 1/2" of deflection.
Taken together, these two elements work fairly well when one considers that the clips have about a 3/4" max delfection capacity, and the live load deflections for typical truss in the 30ft range yield about the same deflection.
The implication of the above is that the non-bearing walls should be constructed low by the deflection amount to allow the truss to deflect under loaded conditions. That is NOT the case in the field. Our experience points to the idea that ALL walls are constructed to the same height at the top-plate.
So how are we to handle this situation - where trusses are routinely being installed with "intermediate bearing points" that are not part of the truss calc, and could potentially damage the bottom chord or "overload" walls below that are not designed for these bearing conditions.
The truth appears to be that we are (almost) powerless to change the construction industry in this area without attracting significant criticism and "push-back" from contractors and owners alike - the gain would appear to be fairly minimal... trusses may deflect more freely - but then there may be other problems cropping up relating to gypboard stress/bending and other corollary cosmetic issues...
So it appears that our approach then moves to one of accommodation, where we appreciate the truss deflection issue; note an adequate solution on our plans, but then "overlook" the issue and implementation in the field when we observe... as do others in the inspection community...
So, amazingly, if our truss conditions are in the "normal" (say up to about 30ft) range, we use our typical details as noted above, and our plans should pass muster as relates to the theory.
As mentioned, ...the changes... stay the same...
Does anyone have a comeback on this...?
2 comments:
Industry Standard & Drywall:
I spoke with a superintendent at W.G. Clark. They always frame non-bearing and bearing wall plate heights the same at all levels. Depending on the truss manufacturer, sometimes their truss bottom chords have up to a 1/2" camber. After the ceiling drywall is hung(1/8" shy of the framed wall face), it is then decided whether to leave a gap at the top of the wall drywall for future roof live loading. Adding a slotted clip at non-bearing walls is typically required on their projects.
I think we do still need to detail the rare, onerous non-bearing, shearwall connections if we do elect to avoid detailing the other non-bearing connections. Thoughts?
I concur. I would also say that providing the detail exclusively at shearwalls would reduce the chances of them making mistakes that harm the structure; if they routinely ignore the detail for partition walls, why should we assume they pay any attention to the non-bearing shear wall connection?
My viewpoint is that even if the bottom chord of the truss ends up bearing on the wall, the wall itself will deflect, or the joists under the wall will deflect. The only time I become concerned about the wall is when a partition wall is parallel to and over a bearing wall or large beam; at that point I designate it on the plans as a mid-point bearing wall and have the truss manufacturer shop-install a vertical/diagonal member.
Of course, interior bearing walls that align properly are almost invariably shear walls as well, so the "non-structural wall above a structural wall or stiff beam" situation doesn't come up as often as you'd think.
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