Wind Load on Pergola - Shielding 1

[RFreund]

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?

EIT

http://www.HowToEngineer.com

 

[jayrod12]

A lot of people hate my response on this but I've seen it happen more than once(same goes for screen rooms), I design as if they close it in.

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).

 

[calvinandhobbes10]

I'd definitely assume the roof gets clad. Walls---your call, I would probably assume not closed in, with due respects to jayrod. Adding walls should theoretically add some shearwall stiffness, so if they clad it in, that ought to give someone the opportunity to do proper lateral check. That does, of course, depend on proper post connection, as he pointed out.

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.

 

[jayrod12]

Except the walls are always full of windows because they want natural light so they're "shearwall" capability is next to zero. And if you think they ever get it re-analyzed prior to doing any work to it, you're a dreamer.

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.

 

[RFreund]

Thanks for the responses, however, I think we might be discussing different things here...
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

http://www.HowToEngineer.com

 

[jayrod12]

We know exactly what you're talking about. As I indicated, people notoriously realize that they're a lot better as a gazebo. so they close it in after the fact. I would give a go at designing it as if it's closed in. If you can't get that to work, then feel free to trim it down.

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.

 

[RFreund]

I see now, I thought you were referring to a screen porch scenario.
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

http://www.HowToEngineer.com

 

[Liam Nesson]

As Jayrod12 has already stated Figure I-32 in the NBC 2010 Structural commentaries deals with shielding. From the table, for solid members where the spacing is less than 6 times the depth of the member, significant shielding may occur.

 

[RFreund]

Thanks again
- is there public access to this document? I also found some information in Wind Loads for Petrochemical buildings by ASCE.

EIT

http://www.HowToEngineer.com

 

[MacGruber22]

The way I see it, someone is likely to grow vines all over a pergola. After a couple years, during the summer they are solid with greens. After a few more years, the vine stems are so thick that they allow snow to pile up in the winter. I have seen this in action.

"It is imperative Cunth doesn't get his hands on those codes."

 

[RobertHale]

For MWFRS considerations I have treated these like a solid roof with a liberal factor on the lateral load area (2x typically). I designed each rafter (not just the outboard rafter) for the perpendicular c&c load. I used the free standing monosloped roof values.
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