Pergola Help

[EddieWalker]

Two ways to do the posts. Set them in the ground several feet and let the backfill, either compacted dirt, or concrete lock the posts into position. This will solve most racking issues on freestanding structures. Then the diagnal, knee bracing can easily handle the movement at the top of the posts.

Attaching the posts to the top of the ground now requires additional bracing. It will never be as solid as setting the posts in the ground, and depending on spacing of the posts, size of materials, height of posts and what's used to brace it, you can get it fairly stiff.

Knee bracing is mandatory. You absolutely have to have it at every post. Corner posts need it going both way. Line posts can get away with it in both directions, but a third brace will make a huge difference in stiffining it up.

Attaching the bracing to the posts requires that it does not move. Never use nails. Screws are better, and the bigger the screws and the more you use, the stronger it will be. For really severe jobs, lag bolts and even threaded rod all the way through and nuts on either side is needed.

Attach the tops of the bracing on as many peices of wood as you can. If there is just one joist or beam, attach it to that, but if there are boards going in both directions, tie into both of them. Again, the more you tie into it, the stronger you attach it and the bigger it is, the stronger it will be.

As for how bit to make the knee brace, I usually cut mine at two feet. That seems to work pretty good, but sometims I go bigger if I can get it up into more wood in the ceiling or trellis work.

For added strength, bolt on a larger beam to your posts. Think of a dinner table that is free standing on the floor. At the top of the legs, and where the table rests, is a beam that holds the legs in place. This has to be big enough to hold the legs from racking. In your case, I'd have used 2x10's or 2x12'd depending on the span and if I was doubling them up on either side of the posts or not. I would bolt this onto the posts with three bolts per post. Never just two bolts, that wont' stop the rackng like three will.

You can also add a railing along the base of the posts. If you did that on one side, it would increast the stiffness of those posts tremendously. If you did it on three sides, you would be very hard pressed to find any visable movement. The railing locks the posts into position and stop them from moving around.

Look at gazebo kits. A lot of that frill and fancy wood work is actually structural to stop racking.

Good luck, it sounds like you might have to undo some of what's been done before moving forward again.

Eddie

 

[anojones]

Thanks for everyones help. If you look at the photos one can see the 4X6 beams are resting on top of the posts with the Simpson ties and knee braces holding it in place- is this a problem? When I look at most other pergolas the beam is made up of two pieces of 2X stock bolted thru the post. Our current plan is to do as suggested, add a double 2X6 beam perpendicular and below the 4X6 beam with 4x4 knee braces. The contractor is concerned about running two bolts (only wants to use one) per 2X6 beam thru the posts as he feels this will weaken the post which is holding the 4X6 beam. Also in the photo, the beam isn't long enough to add knee braces to both sides.

 

[bstrom]

Thanks for everyones help. If you look at the photos one can see the 4X6 beams are resting on top of the posts with the Simpson ties and knee braces holding it in place- is this a problem? When I look at most other pergolas the beam is made up of two pieces of 2X stock bolted thru the post. Our current plan is to do as suggested, add a double 2X6 beam perpendicular and below the 4X6 beam with 4x4 knee braces. The contractor is concerned about running two bolts (only wants to use one) per 2X6 beam thru the posts as he feels this will weaken the post which is holding the 4X6 beam. Also in the photo, the beam isn't long enough to add knee braces to both sides.

Yeah, the wimpy Simpson tie thing isn't doing much (its for uplift mostly) like a mortise an tenon or through bolts. It's just not a good design without using real timber frame joinery but what you are suggesting (if I understand it) should stiffen it up without having to tear it apart. He should use two through bolts at least, spaced diagonally. The posts will be fine. Two smaller bolts diagonally will give more rack strength than one big one (plus the knee braces). It may not look beautiful but it should work. Next time get him to do a simple drawing first or explain what he is going to do. Good luck.

 

[anojones]

Well- the contractor is gone. He added the 2X6 beams and knee braces and we had him dress out the footings. It's marginally better but still racks in both directions quite a bit and without using much force. We're to the point of fixing the remaining issues ourselves. It seems we have three options:
1) Beef up the post mounts- currently the post mounts have four sleeved nails into the concrete and attached to the posts with screws (see picture). We could add new beefier stirrup mounts bolted to the post with an anchor drilled and glued into the concrete. We could also add a bolt to the current mounts through the post. I'm not sure beefing up the surface mounted post is going to make that much of a difference and maybe we should make the structure stable without having to rely on the footings.
2) Add a second set of knee braces both directions at both corners. Longer ones to add stability. We could probably test this first by trying it with 1X4 stock and deck screws to see if it will work.
3) Add a rail down low to one or two sides (like Eddie mentioned). This is not ideal as it adds barriers to an open structure- but we could do it one side without it effecting the overall flow/appearance and could maybe even incorporate a bench or something.
Thanks for all the help, this whole project has me down.

 

[EddieWalker]

You won't gain anything with larger bracketry at the base. For that to work, you would have to use something that would be too massive to be practical, and it would have to be anchored to the concrete in such a way that it couldn't move at all.

The thing that really jumps out at me is how small your angle braces are. They seem way too short to really accomplish anything.

If it was mine, I would remove them one at a time and replace them with new ones that were twice as long at the very least. I might even go longer.

Eddie

 

[bstrom]

Another suggestion to try is to temporary tack nail diagonal cross pieces (maybe a 1x4) across the top members (from about the corner to the opposite diagonal corner on both sets of cross members. So you would have sort of an x on top. This would work better on the legs but kind of defeats the purpose of the structure. If that stiffens it up then you could use something more permanent and better looking.

It's just a bad design from a structural standpoint. If you experiment with diagonal lines, using temp 1 x 4's you may be able to salvage it. You could even try that to get the length of your longer Knee braces. Remember the golden diagonal rule! Have fun!