...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...?
