Prefab wood truss analysis for solar panels

[Jc67roch]

I am modeling some prefab wood trusses for a client to determine if there is sufficient capacity to install solar panels on the roof. I am comfortable modeling the trusses and all their members. But these, like most prefab wood trusses have the toothed, pressed metal connector plates at all connections/intersections. Is it relatively reasonable (safe) to presume that these plates will develop the full strength of the truss members (2x4s)? IE that the capacity of the connections will match that of the members framing in?

 

[BAretired]

No, that would not be a safe conclusion. In fact, it is rarely if ever, true.

BA

 

[WeezerMike1]

I agree with BA. Those connectors rarely develop member capacities, and getting any reliable capacities for the gangnail plates is usually impossible.

My standard approach for justifying new solar panels on a roof was to rely on the allowance for roof live load. Even long span trusses need at least 12 PSF of live load, and the panel effectively prevent that live load from being applied. You can usually present a valid argument to show that the solar panel dead load, plus positive wind pressure on the panels will still be less than the live loads for which the trusses were designed. You should scale those forces up and down to normalize for the applicable duration factor.

 

[Jc67roch]

Thanks guys! I like your approach WeezerMike1!

 

[BAretired]

But snow falls over the whole roof, whether or not there are solar panels, so the weight of solar panels should be added to the design dead load. Probably not much snow in California, but quite a bit in New York, I believe.

BA

 

[Aesur]

When the panels are flush to the roof and the region has no snow loading the method WeezerMike1 mentioned usually works, however please note that the IBC now had provisions for when you still have to apply live load on the roof. When you have a system that is tilted you may still be require to apply live load under the panels and the wind forces are typically significantly higher, sometimes a factor of 3 per ASCE 7-16 and SEAOC PV documents. If the system is on rails, it may not be distributing the load to each truss and skip a few intermediate, in which case you may have be applying more loading to specific trusses and you should consider this in the check.

Another good method to use/try is the 5% rule for existing structures.

 

[l______40mm______l]

Make sure you check wind reversal. If the solar panels are elevated then they can attract a decent amount of uplift.

 

[Triangled]

and watch your layout, as
if laid out with all edges aligned, not staggered, every 2nd or 3rd truss may receive all the dead load of the PVC, whereas the intermediate trusses won't even know they are there.
and similar with the snow load.