Pre-engineered wood trusses - self weight added to design loads?
Pre-engineered wood trusses - self weight added to design loads?
(OP)
I am doing a lot of roof evaluations to determine if existing structures have enough excess capacity for roof mounted solar arrays. I keep running into buildings with pre-manufactured wood trusses. Fortunately, I often have access to drawings that indicate the loads that the trusses were designed for. However, I am wondering, if the drawings indicate the truss is to be designed for 20 psf LL and 10 psf DL, does the truss manufacturer include the self weight of the truss in that 10 psf, or is the 10 psf considered a superimposed dead load and they add in the self weight of the truss separately? From my experience the type of trusses I am looking at usually weigh enough to eat up 3.5 psf of capacity so this is significant. Anyone know what is typically done when the drawings do no specifically say if the loads are superimposed or not?






RE: Pre-engineered wood trusses - self weight added to design loads?
Dik
RE: Pre-engineered wood trusses - self weight added to design loads?
RE: Pre-engineered wood trusses - self weight added to design loads?
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.
RE: Pre-engineered wood trusses - self weight added to design loads?
RE: Pre-engineered wood trusses - self weight added to design loads?
RE: Pre-engineered wood trusses - self weight added to design loads?
RE: Pre-engineered wood trusses - self weight added to design loads?
Add a 4x4 or similar "strongback" under the top chord to distribute the 4 ft. O.C. point load to all 2 ft. O.C. trusses.
Those are some skimpy design loads! Around here the trusses are designed for 20/10/10. The snow load is only about 15 psf but we are required to use 20 psf for maintenance etc (something that never happens). So there is always at least 5 psf reserve capacity (things may be different in your area)
You could certainly ask them to use a 1/4" Simpson SDS screw in lieu of a 1/2" lag.
RE: Pre-engineered wood trusses - self weight added to design loads?
RE: Pre-engineered wood trusses - self weight added to design loads?
If the top chord is in compression, I don't normally concern myself with the installation of lag screws... pre-drilling removes some of the material, but, this is replaced with a much stronger material when the lag screw is in place. I wouldn't install a lag screw larger than 1/2" in a 1-1/2" wide chord. I'm not sure of why the reduction in strength.
xr250:
I've not used the SDS screws and will look into them.
OP:
Can you ask the truss supplier if the self weight is separate from the stipulated loading? It usually is. If you have the shop drawings, then the supplier should be easy to contact if they are still in business.
Dik
RE: Pre-engineered wood trusses - self weight added to design loads?
Talk to VJ Tanner, I know you've likely spoken with him many times previously. That information on reduction in strength came from him through one of the local truss suppliers.