Solar carport design

[cmbyrd77]

I am designing a structural steel carport that will be supporting solar panels on the top of it, therefore I have no real diaphragm action because I am not counting on the solar panels and their connections to form any sort of lateral rigidity. I wanted to frame the purlins into the side of the man supporting members that articulate of the columns to avoid it from rolling. However, the client wants the purlins to run over the top (see attached sketch). Should I be worried about the torsional stability of the main supporting beam. I feel like the purlins framing on top will brace the top flange as long as I make the columns and their base connections. However, there's another part of me that thinks that the wind loads out on the ends of the main member won't get back to the columns without the main member rolling. The other option I thought about was just adding stiffeners at every purlin location. Thank you in advance for you help.

 

[JAE]

I would think some type of stiffener/brace at each purlin - such that it would keep your main girder from twisting - would be necessary.
You have downward force conditions (gravity) and upward force conditions (wind) so your main beam flanges can be in either tension or compression.

 

[msquared48]

OOOOOO!!!!!!! Fly, fly away!

Construct the column and main supporting beam(s) out of HSS sections. That should help with any torsion. Don't forget to check for unbalanced snow loads too.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[grantstructure]

Your purlins will brace the girder against rotation. I don't think you need stiffeners unless these are very flimsy sections--and since you're designing the purlins as unbraced the full length, they won't be little slender sections. Even if the calcs say you can get away without it I'd pick a pretty wide flange, which will help with your bracing of the girder as well.

For cantilevers you brace for rotational buckling on the top flange, so your purlin bracing is on the best side to be effective. What you are counting on for your brace is having to put the purlin into double curvature to buckle the two girders simultaneously. Therefore the connection of purlin to girder should have some stiffness and capacity as if you needed to restrain bending moment at the end of the purlin. If you're worried that the girder section is too flexible you could add half-height stiffeners at the purlin locations, but I'd just pick a solid enough section that it's not an issue, since the design is pretty clean without them.

 

[JAE]

Well...depending on the direction of loading, the tension flange at the ends of the cantilevers may not be rotationally braced enough.
With upward wind forces, the tension flange isn't braced...unless you suppose the beam is stiff enough to no rotate due to a top flange connection and a stiff-enough girder web.

I wouldn't want to count on that myself.

 

[grantstructure]

Yep, that's true. Uplift would be an issue.

Probably need either web stiffeners in the girder or breakaway solar panels.