Wind Load on Pergola - Shielding
Wind Load on Pergola - Shielding
(OP)
How do you typically design for wind load on pergolas. Do you apply full wind load (say for signs, lattice framework, or maybe trussed tower) on each 'rafter'. At some point I would think the rafters would be close enough that there is some shielding. Does anyone have any information on this?






RE: Wind Load on Pergola - Shielding
My own personal case study, my M-I-L wanted a pergola and wanted my input. I denied any involvement because I would require significant knee-bracing, likely upsized roof members from what she wanted, and a proper foundation. I told her this because I expected that she would put a roof on it, and then progress to sides. She's a smoker so I figured it would be her way of escaping the winter weather to smoke. A long battle later, I never ended up helping with the design (I was a good S-I-L and helped construct it from my F-I-L's plans) and what happened less than 2 months later..... Ta Da a covered roof because she was tired of getting rained on. A year later, there's walls on two sides to stop the prevailing winds from howling through.
I'm waiting for the day I find out it blew over in the wind, or at a minimum walked itself down her patio (because currently it relies on gravity and friction to keep it in place on top of the patio blocks).
RE: Wind Load on Pergola - Shielding
Since the 'rafters' likely don't get braced by sheathing or blocking, I've used opposing H3 holddowns on each side of each rafter at both bearings to keep it from rocking over.
You should be able to do a fencepost style foundation and not do knees or kickers---6x6 posts, 24" deep sonotube piers or so. I'd use galv through bolts for the 'beams' to add a bit of rigidity.
RE: Wind Load on Pergola - Shielding
I'd love for someone to be on board with building it correctly, i.e. as Calvin pointed out cantilever columns or braced posts. But my experience with these has been poor, to build them correctly costs money that no-one wants to spend. Even if they actually get a design from any engineer it's always classified as way over-designed and too expensive.
So what I've started doing, with the client's consent, is providing a foundation adequate of resisting the loading assuming a fully closed in structure and then saying pergola designed and detailed by others. Give maximum reactions capable of being supported by the foundations and then wash my hands of it.
RE: Wind Load on Pergola - Shielding
I'm talking about a pergola:
Specifically the wind load applied perpendicular to the rafters. Each rafter somewhat shields the one behind it. Is there any information out there as to how much shielding there is?
Another similar situation would be a wood deck with exposed floor joists (difference being that there is a diaphragm on top of it which probably effects the air floor around the joists).
Thanks again!
EIT
www.HowToEngineer.com
RE: Wind Load on Pergola - Shielding
Account for shielding if you'd like. The NBCC has some literature on shielding in Commentary I. Check out clause 65 and Figure I-32. It references a Wind engineering conference paper by Georgiou and Vickey, Wind Loads on Building Frames July 1979.
I'd still be assuming it gets closed in, especially depending on location. Year round hot weather, maybe not, real winter, definitely consider it.
RE: Wind Load on Pergola - Shielding
However it might be unconservative to assume an enclosed structure with a solid roof. Here's what I mean - if you don't have any shielding, say your rafter are 2x12's, well if you have (12) of them, then it would be like having a 24' tall wall (12' of trib applied to the top of your posts).
I wouldn't disagree with designing enclosed, but it would depend on the circumstance. Let's for now assume it is open.
Thanks again for the info, I'll look into it.
EIT
www.HowToEngineer.com
RE: Wind Load on Pergola - Shielding
RE: Wind Load on Pergola - Shielding
- is there public access to this document? I also found some information in Wind Loads for Petrochemical buildings by ASCE.
EIT
www.HowToEngineer.com
RE: Wind Load on Pergola - Shielding
"It is imperative Cunth doesn't get his hands on those codes."
RE: Wind Load on Pergola - Shielding
I think that the general notes and a good warning to the owner cover the future enclosed scenario. There is only so much you can do to prevent stupid.
Robert Hale, PE