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We built a pergola last fall and are noticing some wiggle in the structure.

The columns are bolted into deepened edges of a concrete slab, But we did not want traditional brackets at the column to roof connection, so unfortunately the structure is not designed well for shear.

The structure is 8' to bottom of roof on the low side of the 1:12 roof. I wonder if we can add tension rods as an x at top of the columns. The bottom of the x would be at 6'8". Or maybe we could add trellis on two sides of the structure?

Any other ideas?

pergola low side

pergola high side

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  • Is the wiggle the same in both directions?
    – Ack
    Commented Apr 17, 2020 at 16:39

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I'd add a knee brace that matches your post size. Install (2) Simpson Strong Tie KBS1Z at the base of each brace and provide a single thru bolt at the top of the brace, centered at the mid depth of the beam. The thru bolt should be similar to those used at the top of the posts, just make sure you penetrate all 3 members and it's at least 1/4" in diameter. Make sure the brace is angled somewhere between 30 and 60 degrees and you're in the clear.

enter image description here http://embed.widencdn.net/pdf/plus/ssttoolbox/qyjicced3n/C-C-2019-p296.pdf

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    Yes, Knee brace. You do not have to use the ugly Simpson brackets, Simpson makes a black decorative line of some brackets. If your knee brace is the same thickness as your post you can bolt in between the beams and then use structural bolts/hardware to secure it to the post. strongtie.com/decorativehardware_woodconnectors/category
    – Alaska Man
    Commented Apr 15, 2020 at 21:10
  • I would prefer an alternative to knee braces. I understand they are the most obvious choice, but we don't like them. Is there nothing we can do with tension rods?
    – Ziggystardog
    Commented Apr 15, 2020 at 21:47
  • McMaster Carr is probably your best bet for purchasing an internally threaded rod and rod end bolts. It's pricey but once you get into steel connections like this, that ends up being the case. Make sure you get something galvanized or coated in zinc.mcmaster.com/turnbuckles/clevis-to-eye-turnbuckles
    – represton
    Commented Apr 15, 2020 at 23:53
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    Some attractive metal braces would help. Like cast iron ornate vine pattern. The ones I saw online were shelf brackets. My step dad used to make them out of wrought iron with scrolls , mom always hung plants on them. They looked nicer than a knee brace but did the same job.
    – Ed Beal
    Commented Apr 16, 2020 at 14:31
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I wonder if we can add tension rods as an x at top of the columns.

Yes, that will work. The deeper the 'x' the better of course, but any depth will improve the situation. I would be willing to be that 1' of depth will do what you need.

For clarity, you mean resistance to lateral loads, not shear. Shear as in shear walls (I'm guessing that is where you got that term) is something different. Currently what is holding up your structure laterally is a 'moment' connection at the post to roof connection. The bolts are very close together and wood is relatively 'bendy' which is why you have the issue. The use of the 'x' will in affect increase the depth of the moment connection. The individual rods of the 'x' work in tension but not compression. So one works for one direction and the other for the other direction.

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  • Thank you for clarifying the terms. I will look into tension rods to increase the depth of the moment connection. The framer who built it for us, tells me he used 2 1/2" redheads drilled 4 1/2" into the concrete. I wonder if that connection is also a bit weak. The concrete was thickened at all four corners of the slab, although not under the two middle columns that we added in order to decrease the joist depth. Anyway, I do hope adding some tension rods will solve our problem. Thanks for the advice
    – Ziggystardog
    Commented Apr 16, 2020 at 0:55
  • The footing connection is fine for it's purpose which is to hold the post in place and resist against uplift. While the base does create a bit of a moment connection (the post would likely stand up on it's own after installation) there is not really enough to consider it as part of the lateral resistance. There are some really nice looking architectural / structural rod / cable items out there and something will fit nice with your beautiful pergola and house. For example: cableartinc.com/archRodSystem_index.php
    – Ack
    Commented Apr 16, 2020 at 1:04
  • Also, as an even more unobtrusive lateral bracing solution, I wonder if we could reinforce the existing two bolt connection between the column and beams by adding custom steel brackets or angles? I see all sorts of modern wood pergolas online that do not have knee braces or tension rods. Our framer probably could have used a different connection and we'd be fine, right? Is there any way to solve this in a minimal way?
    – Ziggystardog
    Commented Apr 16, 2020 at 1:23
  • Yes, but it's a complected yes. Moment joints in wood just don't work very well for many reasons. Putting in a steel corner bracket will likely help, if nothing less then you are effectively taking a two bolt connection and making a four bolt connection (in a given direction) with the related benefits. Note that the bracket can be internal or external, the bolts could be pins etc. Note also that there will ALWAYS be movement, we're simply looking to reduce it below a level that is acceptable to you or even unseen by eye; it helps to think of it that way, there is no 100%
    – Ack
    Commented Apr 16, 2020 at 1:30
  • My framer who built the pergola is suggesting adding steel brackets (an angle with a fin), 2 each at the base of the four corner columns. Would that work? or does the lateral bracing need to be at the column to roof connection?
    – Ziggystardog
    Commented Apr 17, 2020 at 16:05

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