Will these DIY steel plates for post to beam connections work as well as the commercially available ones

deckinspectionsteelstructural

My soon to be constructed deck will have beams made from three 2x12s sandwiching ½" plywood, sitting on 6×6 posts. This gives both the beams and the posts a width of 5½ inches.

To ensure the beams stay put, I'd like to purchase 3/16" x 5" x 8" steel plates (SS 304 7ga). I would use 2 steel plates per post/beam connection (one on each side), plus (qty 4) SS 304 ½" hex bolts / washers / nuts.

Using these plates is vastly cheaper than the ready made solutions from Simpson and other companies, of which at least some number are also made with 7ga steel. Even increasing the steel plates to ¼" thick is still vastly less expensive than the ready made connectors.

Here's a diagram of the DIY connectors plus hex bolts:

.-------------,
|             |
|   O     O   |
|             |
|             |
|             |
|             |
|             |
|   O     O   |
|             |
'-------------'
Note: Perhaps but not necessarily to scale

What is the folly of this method? Is it dangerous? What makes the commercial product better than just purchasing steel plates and bolts?

Update

Just wanted to add that this question may have been a bit misleading. The actual code approved connectors for this structure will be something more like these. The beam will be completely supported on top of the posts. This is a 12' x 20' ft deck. The premise was that the DIY plates seem significantly stronger than the code approved connectors. I made this unclear by originally linking to the 7 gauge Simpson connectors.

Best Answer

First of all, there are typically two ways of making sure your construction is safe:

  • Use an off-the-shelf approved product and install it exactly in the approved way.
  • Design your own solution and do a real load calculation to prove that all the loads and possible failure modes are accounted for.

Your question sounds like you're trying to pick the latter option, but handwave away the expensive part of it. Perhaps you can get away with it if the design is indeed sound, but without analyzing the loads, you can never be sure. Various authorities take a dim view of people who say "actually dunno, but my gut feeling is that this will hold".

Regarding the loads, you likely realized that the primary load (compression of the posts) is not really the reason for these connectors. If it were, you wouldn't need any plates at all, you could just place the beam on top of the post and that would do just fine.

Instead, the post caps are there to carry the horizontal loads in both axes (along the beam and perpendicular to it). That's why the post cap looks the way it does. There will always be such loads, because of:

  • Posts not being exactly vertical, not having exactly the same height, sinking by different amounts over time, etc.
  • Beams not being exactly horizontal (including some sagging).
  • Non-uniform loading of the deck, dynamic loads (people, wind).

These forces are going to be significant. If the post ends up being inclined 1% away from the vertical (and that's just a tiny bit), 1% of the weight of the deck is going to act on the post sideways, trying to topple it. All the other forces mentioned above just add to that.

Your two plates could perhaps easily handle the loads perpendicular to them. However, the other direction is more tricky. You would likely have just the clamping force multiplied by the friction coefficient between smooth steel and wood (not a lot). With a lot of luck, the shear loads on the bolts could also help, but good luck drilling those four holes exactly so that you hit all four holes on either side. (More likely, the bolts will be loose in their holes and thus not active.)

Also, relying on the clamping force means that you'll have to:

  • Ensure that the beam is exactly as wide as the post, so the plates rest flat against both the post and the beam.
  • Tighten the nuts sufficiently to apply the necessary clamping force.
  • Periodically check and re-tighten, because the weather and all the wiggling from the dynamic loads is going to make the wood give way over time.

So to summarize, your design surely can work if designed properly, but there's a whole lot of factors that need to be taken into account.

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