Pergola design/load help needed

Ladera05September 27, 2013

Hello there. I�m building a pergola ~13� square from post to post and would like to some help to see if it�s structurally sound. This is my first time building this and would like to ask for help from the experts out there. Our plan is to have either Wisteria grows on top of this structure or perhaps some table grape vines. This structure is located in Rocklin, CA (Sacramento). Ground is somewhat hard and have quite a lot of river rock in ground.

Already done:
Poured 12" concrete tub footing � at ~ 2 ft deep (5000 psi commercial concrete)
Bracket is embedded within concrete footing
It�s 3" off the ground from the base of the bracket

Planning to use the following lumbers but open for changes to ensure structure sounds and no sagging
Use Redwood for this structure( but considering Dougfir or treated fir if the expert recommend)
1. 6"x6" x 7� or 8� columns (4 columns) (I can either keep 8� height or can trim it to 7")
2. Stack on top of the columns are 2 cross beam of 6"X 8"X 16� (with ~13�.3" unsupported spanning)
3. Stack on top of the cross beams are 8 rafters of 4"x 6" x 16� (the intention is to have 2 ft spacing)�try to maximize the space without creating sag problem
4. 2"x3"x16 or 2"x2"x16� (32) for lattice � 4-5" spacing

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What holds the vertical members upright under a lateral wind load?

    Bookmark   September 28, 2013 at 11:24AM
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I'm very interested in following this, as I'm trying to find a contractor for a 28'x14' pergola.


    Bookmark   September 28, 2013 at 4:06PM
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I was going to make the same comment as pls8xx this morning, but saw his already out there. Hopefully, we'll hear.

This is only a gut reaction, but ...

... your posts seem too short. (Especially 7'.) If there is never a chance that vines will be grown on it, you could get away with 8' ht. If vines might ever happen, 9' ht. would be better.

... your cross beams seem overly bulky. They would be better to be narrower and taller... example: 4" x 10" (or 12") instead of 6" x 8".

    Bookmark   September 28, 2013 at 9:47PM
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deviant-deziner(Oh zone)

You might find The Sunset guide to Trellises and Arbors helpful.
It has a modestly good chapter on calculating spans that doesn't cover in detail the engineering requirements for designing live loads but it does give span recommendations based on a 5lb psf combined with using softwood class ( redwood classification).
You can probably find it at your local library.

    Bookmark   September 29, 2013 at 3:35PM
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littlebug5(z5 MO)

I don't know about California wisteria, but Missouri wisteria grows EXTREMELY large and heavy. The trunk of one that I recently destroyed was bigger than my thigh. With vigorous offshoots everywhere.

If you're going to plant wisteria, better plan for a very, very heavy load.

    Bookmark   September 29, 2013 at 4:11PM
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woodyoak zone 5 Canada(5b)

Also, if you're planting one of the wisterias with long flower racemes (e.g. a lot of the Japanese ones) you will need to ensure there is enough clearance between the flowers and the top of the heads of tall people so that people walking under the porgola don't get entangled in the (bee magnet!) flowers. At least a 9-10" height would be a very good idea. I'm not sure how long table grape clusters would be, but the same sort of consideration would apply there too I'm sure.

    Bookmark   September 29, 2013 at 6:02PM
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thanks for some of the comments above..I've done some additional research on the web, there're company sell pergola with the same size however they use 2 thin 2x6by 16 beam for each side and 2x6 rafters as well. they assure that its structurally sound given they have extra corner post/beam, post/rafter 45 deg support bracket ...still 2x6 seemed flimsy...

my revised design to see if it makes sense....use same post of 6", 2 2x8 or even 2x10x16 beam...and 2x8 rafters.

Perhaps because one uses 2 beam each side, I would use 2 sets of bolt and nut through holes to hold these on...

Any suggestions of using 2x8 or 2x10?, 2x10 seems bulky but don't know if 2x8 would be ok structurally.


    Bookmark   September 30, 2013 at 1:04AM
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Lad, I think you would be best served by paying attention to some of the comments already posted. You don't want to build the pergola and have it fail inspection. You don't want to undersize the beams, discounting the importance of using the proper size to ensure performance, and to have no probs with deflection of the structural members.

    Bookmark   September 30, 2013 at 6:58AM
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I get paid to design these so I'm not interested in a freebie for a random stranger, but I'll say this:

- per d-d's recommendation there are a number of load tables available, in print and online.

- it is really, really easy for the proportions to get weird. Free 3D programs like SketchUp can be a huge help for seeing what it looks like.

- I tend to err on the side of chunky. If you go with bigger members for beam and rafters and your posts look a little dainty, you can always box out the posts to have more mass

    Bookmark   September 30, 2013 at 8:07AM
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Ladera05 said:

".... they assure that its structurally sound given they have extra corner post/beam, post/rafter 45 deg support bracket ..."

In my opinion, support for the weight of the structure is being over emphasized while ignoring the needed bracing ( or other means) for keeping the structure upright. Those 45 degree brackets are one of the most important elements of the design, both in size, strength, and method of attachment to posts/beams.

    Bookmark   September 30, 2013 at 10:57AM
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