Free standing wood pergola in South Florida 2

[vlad1981]

I witnessed an argument today.
Contractor is tasked with building a free standing wood pergola in south florida, specifically, in Palm Beach county.
Initially it was going to be 12ftx20ft, engineer said for such pergola (Main beam spanning across 20 ft dimension) it would need a middle post (supports at 0-10-20 ft).
Engineer, further more, calculated it and reducing risk category, choosing the best possible wood etc and god knows what other little tricks, came down to a max. of 12x14 ft, maybe 12x15 ft.
It just hit my curiosity, and after running a couple numbers, the positive reactions on the beam from the roof cladding are simply a lot (150 MPH hurricane wind speeds), at 12x14ft beams and posts are OK, but pretty much close to the limits, and in all honestly, me personally haven't measure any, but most pergolas I've seen are give or take it, not any bigger than proposed (12x14 ft).
My question, for those with some expertise, have you ever designed a bigger pergola than 12x14ft (wood, not PVC nor aluminum), and if so, what are some major assumptions to make it work under hurricane winds?

 

[Aesur]

What size members were they trying to use? I would think you could easily design a larger structure, in fact commonly do 20x20 in lower wind areas. When it comes to wood, there are many different sizes and species avaialble, including glu-lam posts, so I find it strange that they can't get a 12x20 to work without there having been other reasons, such as limits to member sizes, etc..

 

[GC_Hopi]

I would be curious if there is a budget constraint. Contractor was probably asking to use only standard dimensional lumber, also going to Aesur's point.

 

[vlad1981]

@Aesur, purlins and rafter seem to work fine, beams 2x8 and posts 6x6, and it gets to fail under storm wind conditions.
As you said, 20x20 in lower wind areas may be doable (no additional support in the center???), in South Florida, with 150-170 MPH hurricane, could be an issue.
No idea what kind of wood, I'd say home depot, anything the contractor can get.
@GC_Hopi I'd say the same, maybe a budget constraint, things down here can get a bit crazy with contractors, pressure treated from Home Depot, is not a lot of info.
I think engineer designed using Southern Pine, select structural, not quite sure, I did run some quick numbers using southern pine select structural, and it all comes down to hurricane for things to get messy, for normal conditions it works.
That's why I got curious.

 

[Aesur]

The member sizes are the limiting issue, if they are wanting to stick to such small members then yes it's going to be limited in size. Under just D + Lr, I would think you would be hard pressed to get a single 2x8 to span even 9' with 6' trib, assuming a DL of 3 psf and Lr of 20 psf.

With a 20'x20' we would most likely be using 6x or even Glu-lam beams with probably 8x8 columns minimum and knee braces.