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

Question

My soon to be constructed deck will have beams made from three 2x12s sandwiching ½" plywood, sitting on 6x6 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?

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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.

Answer 1

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.

Answer 2

What makes the commercial product better than just purchasing steel plates and bolts?

The commercial product doesn't rely on the screws in wood to connect the two sides. A sideways force would be spread over the area of the bracket, compressing a wide area of wood.

With the plates, the same force would lever the screws out of the wood.

Answer 3

The most dangerous thing I hear on construction sites is, "Aw, that ain't goin' nowhere." I can't tell you how many things I've seen go wrong because someone did some eyeball engineering and guessed wrong.

I would not substitute plates for post caps. The connector has to secure the beam and post in position, resist lift-up / shear / twisting forces. The post cap has been designed and tested so you can count on it to transfer these forces to the metal without compromising the integrity of the wood structural members

[R402.1.2]. The beam shall be attached to the post by notching the 6x6 as shown in Figure 8 or by providing an approved post cap to connect the beam and post as shown in Figure 10. All 3-ply beams shall be connected to the post by a post cap.

Your two plates are definitely not an approved post cap. (The Simpson products you're comparing are approved, you can follow the links to the approval reports on the Simpson web site.) What you're making isn't even a homemade post cap, it's a homemade mending plate.

Since mistakes in deck construction really do cause a lot of injuries, it's not something I'd get creative with, comply fully with the code, and when in doubt, err towards caution.

You can see the figures mentioned in the quoted text above in the American Wood Council's Design for Code Acceptance (DCA) 6 - Prescriptive Residential Wood Deck Construction Guide - 2015 IRC Version

https://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62015-DeckGuide-1804.pdf

Answer 4

That is made for structural building members, e.g. this is designed to support a composite main beam holding up a 2-story building. Home Depot doesn't even stock it.

What you're actually after is this guy, which does way more stuff in a more complicated way than what you're trying to make. If drilling holes in a piece of flat stock was good enough, why would Simpson sell this one?

I gather you just want to mount these vertically and run bolts through, and use what, clamping force? Bolt shear strength? to hold everything together??? That is different than what Simpson is doing with these.

End of the day, you need to mind who you are. You're not a structural engineer, you're just a layman with an opinion. Products like these, and the application book that comes with them, define a "gold standard" way to build a deck that has been broadly approved in a variety of arrangements. You either need to stick to those approved methods... or go get the stamp of a structural engineer for your method.

To do otherwise is to take the position that the engineer is not important. That's fine to say while you're in a barber's chair, but not something you want to say when arguing with your insurance company or on the witness stand.

Answer 5

What is the folly of this method?

If the "only" thing that were to happen was that one of these DIY plates failed with you on the deck (and nobody else), you aren't likely to sue yourself, but the medical bills could easily more than overwhelm the difference in price between the DIY solution and the approved solution.

Is it dangerous?

You do not have the structural engineering skills/knowledge to design this junction properly and to code specifications (if you did, you wouldn't be asking a bunch of strangers on the internet), therefore, it's quite likely that it will be underdesigned and that could be quite dangerous.

What makes the commercial product better than just purchasing steel plates and bolts?

The commercial product has behind it a team of lawyers who will understand and interpret the legal aspects of the building code and a team of structural engineers who will interpret the mechanical and structural aspects of the code. They will work together to design a product that will meet or exceed the minimum requirements. They then have a team of manufacturing engineers who will ensure that the product is built according to specification and a team of quality engineers to ensure that the output of the manufacturing process actually meets the design specification.

You are purchasing some steel plates and bolts, then asking some strangers if that'll do.

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Additionally, you'll find that while sandwiching a piece of ply between 2x material is common practice for building indoor headers above doors and windows since it provides an even surface on both sides for applying finishing material, it's not particularly common in outdoor deck construction. At a minimum, you'll be exposing the end/edge grain of the plywood to moisture and trapping it there. It will sit there until it evaporates giving it maximum time to penetrate into the most vulnerable part of the wood, increasing the rate at which the plywood will fail. As the plywood fails, your joists will become loose and begin to fail. As a matter of fact, you probably won't find commercially manufactured post caps (designed for holding the beams on top of the posts) that will accept your dimensional/plywood sandwich - it will be too thick.