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The deck is nearly finished but I have a small structural problem with the pergola. A left or right force near the top of the pergola (such a a tall person leaning on the post) leads to a bit of noticeable wobble. Everything is rock solid at the deck level, and there is no wobble in the in/out direction. The span is 12' from post to post. The beam is 2x6 cedar, with 2" x 2" notches where the pergola joists connect with a six inch screw coming in from the top.

I'm thinking up putting 2x6 cedar diagonals at the posts to the beam, connecting at the first pergola joist on each side (20" from post) at a 45 degree angle. Does this sound like the right thing to do? Is 20" enough? The next joist would be at 40" from the post, and would come down too low.

[Newly Completed pergola(http://i.stack.imgur.com/rstJD.jpg)

This is an exaggerated view of the flex. Flexing

RESOLUTION: I've added one 2x6 45 degree brace in the upper right corner. It stabilized it significantly. I'll put up the other side when time allows.

FINISHED: Rock Solid now. enter image description here

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Are the post in the ground, or just through the deck? Are they anchored to the deck (carriage, etc)? Are your joints tight, or is there slack in them (did you need a mallet to slide them in)? –  Tester101 Apr 16 '12 at 16:30
    
Posts are saddle mounted to existing concrete footers. The carriage is tightly bolted to the posts. It's just the natural flexing of the 4x4's that's causing the wobble. There's no diagonals to absorb any right-left forces. –  Chris Cudmore Apr 16 '12 at 16:35
    
Also, I'd be connecting the diagonals to the beam, not the joist. The joist is just where I'd like to connect. (edited to clarify) –  Chris Cudmore Apr 16 '12 at 16:37
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You might consider some decorative bracing like this –  Tester101 Apr 16 '12 at 16:38
    
Yeah, that was the basic idea. But I'm not cutting anything like that. I don't have the saw, nor the skills to do that scrollwork. –  Chris Cudmore Apr 16 '12 at 16:40

4 Answers 4

up vote 10 down vote accepted

The pergola is much like a stick framed house--the framing itself resists vertical forces (gravity) but in and of itself, has no resistance to shear forces (side to side). For a house to stand up on its own, the sides need to be braced against shear forces...typically that's done with plywood sheathing. Barring that, diagonal bracing can be used.

On the pergola, since it's attached to the house, the house is acting as shear resistance on 3 sides of your pergola, but the front is the weak spot, as you've found out.

Some potential options:

  • sink the front two posts into the ground several feet and surround with concrete (typically you'd want 1/3 of the post in the ground

  • add diagonal bracing. 45 degrees is the strongest but for the pergola, likely not a requirement. If 45 degrees brings the braces too low, you could consider a different angle. Or make it an architectural detail...perhaps bringing the braces on the outside of the posts (extending the top beam)

  • metal reinforcement. Pick up some metal L-plates. The bigger the better and attach them to the post and beam.

  • add a railing to that side of the deck and diagonal brace that (tensioned cables, wood panels, diagonal rails, etc.)

  • add tensioned metal cables diagonally between the tops of the two posts. Not sure you could enough of an angle to make that work or not, though.

  • add diagonal bracing to the TOP of the trellis. Create a giant 'x' using wood beams or, again, possibly tensioned cables. That will transfer the horizontal shear forces on the front back to the ledger and house.

You will have to do something, though. Even a 'wiggle' of a couple of inches will cause problems over time as the wind rocks it back and forth. The hardware may not fail but eventually the wood where the hardware connects the members will fail with wear over time.

Of the options I listed, I'd probably start by getting some large L plates. It's likely the least expensive option and you can hide them fairly easy on the inside of the posts and beams. Alternatively, you might be able to use some heavy duty metal shelf brackets. As a bonus, you might find some with some architectural detail that will look nice.

PS: After re-reading your question, I have a few structural concerns:

  • you state your beam is a 2x6 with 2" notches. The notches means your beam is really just a 2x4. While there isn't much of a load (just the joists) a 2x4 can't really support any force without noticeable defection over time. I'd be worried that in a decent storm, there could be enough force to eventually snap it. That would be worse-case. Best case, though, I think it'll just start sagging after a season or two and look ugly. I'd consider under-mounting an un-notched 2x8 or even 2x10. As a bonus, that can help with resisting some shear force.

  • I can't tell if the house-side is actually connected to the house. You do have 2 posts back there so now I'm thinking this is maybe a free-standing pergola. If that is the case, then you have no shear force resistance on any of the 4 sides--so you'll need to apply whatever solution to all four sides if that is the case. [EDIT: I just saw your comment in the other answer, the back is, indeed, a ledger. So disregard this one]

  • you mentioned that you attached the beam to the posts with a screw. Note that a) screws have very little shear force resistance (compared to bolts, plates or even nails) and b) one screw isn't going to cut it. If it's truly being held with one screw on both ends, they could snap quite easily with minimal force. Simpson hardware and/or through- or lag- bolts should be used to make those connections.

Again, these are all assumptions so apologies if you've already accounted for those issues--but if not, please do consult local codes. I'm concerned that the structure is way under-built and a good wind storm can send large pieces of wood flying in every direction--dangerous to your home and anyone in the vicinity.

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Yes, I thought through the notching. Notches are above the neutral axis, where the beam is in compression. Any sagging will compress against the joist, so no beam material was actually removed. It is weaker in shear, but in bending it's as if no material is removed. (Although, an upwards force is not supported) Also, it's not one screw. It's actually 2 lag screws. –  Chris Cudmore Apr 18 '12 at 15:12
    
Well, I'm not an engineer, but the issue is that it's made out of wood. Wood expands/contracts/compresses and those notches won't necessarily stay tight after a few seasons. So you really don't have the compression force there. Also note that once you notch a beam, you've effectively moved the neutral axis. –  DA01 Apr 18 '12 at 15:23
    
Also keep in mind that the beam is attached to house, not spanning the entire 12 feet unsupported. –  Chris Cudmore Apr 18 '12 at 15:29
    
OH...I might be confused. Which beam was notched? I assumed it was the one spanning the front (holding up the ends of the joists). –  DA01 Apr 18 '12 at 15:31
    
Accepted for bracing. Tester101 also suggested the same thing in comments, but he doesn't need the rep. –  Chris Cudmore Apr 19 '12 at 19:15

I'm not sure how the cross beams are attached to the support beam near the house, but it seems to me that would be the point you need to focus on. Any small amount of flex in the outer most posts can be translated into movement, because the cross beams are able to torque at the ends.

Top View of Pergola

You may be able to eliminate this torquing, by adding in an additional support beam near the house. Like this.

Top View of Pergola

Due to the length of the cross beams, this may not eliminate all of the movement. Any flexibility in the cross beams span could still translate into movement at the outer posts, but it should be much less than you are seeing now.

In most pergola construction, support beams are doubled up at the posts to limit torquing. Tight fitting joints will also eliminate some movement. If you don't need a mallet to fit the joints, your notches are too loose.

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You've mis-interpreted the loads. The load is (on your diagram) coming up from the bottom at the post. Joints are tight. The rear beam is a ledger, so it's not going anywhere. The rear vertical posts are purely cosmetic, they do nothing (other than support the ledger during attachment.) –  Chris Cudmore Apr 17 '12 at 18:30
    
@ChrisCudmore I know the rear beam in a ledger, but how are the "joists" attached to it? If they can torque, you'll see flex at the outer end. –  Tester101 Apr 18 '12 at 2:01

The simplest solution would be to run tensioned cables diagonally across the top of the trellis. Since it is tied to the (hopefully? ;-) ) solid brick wall, it will resist torsion along the diagonals. Then the posts will be relegated to their proper job of keeping things up.

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  1. How is the ledger attached to the house? I suggest 3/8 lag bolts with washers at 24" o.c. These should extend through the brick into wood framing members instead of masonry anchors.
  2. I suggest adding joist hangers where the joists meet the ledger. Toe nailing or end nailing is weak.
  3. A 12' span for a 2x6 is weak, and the notching makes it worse. Add a 2x10 to each side of the 4x4
  4. Add 4x4 braces at 45 degrees. Nail to the 2x10 from each side. The 4x4 braces should be at least 2 feet long.

I recommend using spiral galvanized nails.

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