Pre-Engineered Timber Roof Trusses

[CBEngi]

Hello,
I am reviewing an existing roof structure for the installation of solar panels. I have the original design drawings from the pre-engineered timber roof truss designer. I have completed by own model of the truss system and have comparable results. Then I adjusted my model for the revised loading.

The truss-designer gives a maximum perlin spacing of about 4' OR continuous sheathing. I have the top chord in combined compression and bending at about 95% capacity with the original design loads, if I assume the unnbraced length of 4'. But in reality the installing contractor choose to install the roof sheathing directly to the top chord, thus basically making the top chord fully supported.

If I update my calculations with an unsupported length of say 300mm my top chord is now only at about 35% capacity in combined compression and bending for the original loading. Do the pre-eng truss people need to make some allowance for an unsupported top chord for varying installation conditions. For a system that is known to be designed to almost code minimum (or maximum capacity) it seems strange they would "leave" this additional capacity available.

I'm working my way through the other members and connections.

Thanks in advance.

 

[1503-44]

You are correct. Not much fat in the typical roof truss design. When I was designing them during my university days, now a very long time ago, we always assumed continuous lateral top chord support via sheathing. Standard O.C. spacing of the trusses was 24". The construction contractor was responsible for providing "adequate lateral support as required during installation", as was noted on all of our fabrication dwgs. For roof trusses dead load was normally 20 psf, live typically 20, or 30psf. As I recall, live load was not applied in conjunction with snow or wind load.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[XR250]

Not much fat in the typical roof truss design

Maybe not in the capacity side, but the demand side always seems to have some fat. The typical truss design in my neck of the woods has a top and bottom chord dead load of 10 psf each. Realistically, it is closer to 5 in practice. Also, as has been stated in previous threads, the solar panels block the live load in their footprint. Care needs to be taken, however, to distribute the rack feet point loads to the adjacent trusses.

 

[KootK]

Can you share the truss design drawing? The software is pretty foolproof as long as the input is good. And, at the same time, most conventionally trained EOR's will struggle to replicate the arcane modeling assumptions developed by the metal plate connected truss industry and expressed in the ANSI/TPI standard.

In short, if your looking at an Alpine, Mitek, or Truswal drawing that says that the truss is good for 4' oc, my money says that it is.

 

[BAretired]

I interpreted the OP as saying that the specified loads used only about 95% of the truss capacity, but with deck applied directly to the top chord, reducing the unbraced length, the truss was loaded to only 35% under full design load. That seemed a bit surprising to me, but b/L goes from 32 down to nearly zero, so it may be correct, depending on which code is used to check it.

I am assuming that the purlins and truss panel points were spaced at 4'-0" so bending was not a factor in the original design, but exists with deck bearing on the top chord.

BA

 

[1503-44]

If bending was a condition in the original design, it would have been common to entirely ignore the bending stress in the design of the elements themselves. In the typical cases, element loads were assumed as axial load only, no matter how the load was actually applied.

the truss was loaded to only 35% under full design load

It does seem low, however it is possible with certain combinations of member sizes and their spans within their shorter ranges. We don't know what the spans are in this case.


“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"