Valley set roof trusses bearing on existing trusses

[TroyD]

The attached PDF shows a construction detail of new valley roof trusses bearing on existing trusses. This is for a small addition to an existing one-story commercial building. The structural plans reviewer is asking if the existing roof trusses can handle the additional weight from the new trusses. On small projects like this, there typically is not budget to evaluate the existing roof structure to this extent. I'm curious how others might respond to this concern from a building official without in-depth analysis? There are no existing building plans or truss drawings to refer to. Is there some generalization that can be made regarding the additional roof dead loads?

 

[jayrod12]

Generally I've been told that during design of new roof truss systems, there is no additional dead load taken into account for areas where valley sets or overbuilds occur on the trusses. I guess you could do a quick check how much of an increase it is in PSF. If you have the valley set shop drawings you should be able to determine the dead load of the framing, I'd bet it's borderline negligible.

 

[gte447f]

TroyD, I just wrapped up a set of drawings with similar overbuild on an existing roof. My overbuild was stick framed over existing trusses. I detailed all of the new framing and connection to the existing roof without any consideration to the capacity of the existing trusses. After sending the drawings out the door, this question occurred to me. Like jayrod12, I believe it is negligible, but by the numbers I do not believe the distributed psf load increase is small enough to ignore per chapter 34 of IBC. If original design load was 20psf(D)+20psf(L) and new load is 5psf (D), that is a 12.5% increase, right?

 

[XR250]

I would not worry about that. The design dead loads (and live loads) are typically high on trusses. Remove the shingles underneath and call it a day.

 

[KootK]

- agree that this isn't usually given much attention and probably doesn't deserve it.

- agree that you may have trouble demonstrating that PSF load is less than or equal to design load.

- you'll benefit from the fact that the extra load is probably at the end of the affected trusses. It's a less than proportional impact on chord bending, and therefore chord axial load, that way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.