Pre-engineered wood trusses - self weight added to design loads?

[mbenjami1]

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?

 

[dik]

It depends if they select that feature in their program. The only factory fabricated truss software I've used has an option to include self weight... I suspect all programs are similar. If evaluating roofs, you should try to obtain shop drawings for the trusses. This has the design loading stipulated.

Dik

 

[mbenjami1]

I do have the shop drawings for one of the buildings. It says the truss was designed for a top chord dead load of 5 psf and a bottom chord dead load of 5 psf. Do you think those numbers are in addition to the self weight?

 

[KootK]

I funded my college tenure and a couple years of indecision as a designer of these kind of trusses. That was a while back but, then, the design loads indicated on the drawings included the weight of the truss. So the truss weight would need to subtracted from the listed values in order to work out reserve capacity. A single 2X chord should be coming in under 0.75 PSF for 2' spacing.

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.

 

[jayrod12]

Keep in mind when installing the solar arrays that they typically mount them at 4 ft centres, i.e. every second truss, and they usually specify a 1/2" lag screw with over 1 1/2" embedment into the chord. The truss guys I've talked to around here indicate that the lag screw connection reduces the top chord capacity in excess of 20%, they may be being conservative but it's their product so I tend to take them at their word.

 

[mbenjami1]

Thanks, I have seen a couple drawings that specified an area load and indicated the self weight of the truss as a separate additional load, but most of them do not get that specific. I guess I'll just have to assume that the self weight must be subtracted from the design loads in order to be safe. Thanks for the input.

 

[BadgerPE]

Many of the truss drawings I have seen list the maximum end reactions. With a little back calculating, you may be able to figure out if it is included or not. As a caution, I have looked at a couple wood truss drawings, for solar panel installs on houses, that made no mention of unbalanced snow loading. The base snow load for the area was applied, but I couldn't locate any mention of unbalanced in the drawings. Maybe it's not required by the IRC, but both projects went dead pretty quickly so I never fully investigated.

 

[XR250]

A couple of things...

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.

 

[mbenjami1]

That's interesting that you mention the 5 psf reserve capacity due to roof live being 20 psf and snow being about 15 psf. We have the same snow load here. And I have seen that used by another engineer as justification for solar panels on a roof before. He stated in his letter, and I'm paraphrasing here, that the roof live load could be reduced in the area of the panels since maintenance would be limited just by the fact that you wouldn't have a bunch of people walking around over the panels. This does make sense to me but I see no legit way to justify this line of reasoning in the code. ASCE 7-10 says "The live loads used in the design of buildings and other structures shall be the maximum loads expected....but shall in no case be less than the minimum uniform distributed unit loads required by Table 4-1". Am I being too conservative by insisting that the roof be capable of supporting 20 psf live load in addition to all existing dead loads and the dead loads of the solar panels?

 

[dik]

jayrod:
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

 

[jayrod12]

dik,

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.