Balancing needed bending strength of a wood railing post in concrete with the lifespan due to rot

Question

I am installing a wood railing post for an exterior railing and had planned to use the approach for fastening to a [new] concrete footing that is commonly used for deck posts. The post sits on a bracket that is elevated a couple of inches/few centimers above the concrete by a hefty steel screw that is embedded into the concrete when poured.

The tradesman working with me vehemently disagrees with this approach pointing out that a railing must withstand quite different - in particular shearing as a heavier person pulls on it - versus a deck post that must withstand primarily compression stress. The tradesman is concerned that a single bolt would not be sufficient to withstand the pulling on it.

I am still leaning towards the bolt embedded into the concrete due to the concern of rot when embedding the pressure treated lumber into concrete. It will rot.

So then - does the embedded bolt possess sufficient shear strength? I believe that to be the case but do not have a reference to back that up.

Note that there are mixed results when doing a web search for:

should I encase the railing stair post into concrete

The first two results I found resulted in a "split decision" on the choice.

So, is my preferred approach of using the bolt going to be safe for that pro football player grabbing onto the railing? Or do we sacrifice some longevity of the structure by burying the post into concrete so that it's definitely not going anywhere today.. but conversely will definitely be suspect in a couple to several decades?

Answer 1

Here is a different concept, based on the vinyl fencing options where gates are going to be in the fence: at the gate posts they have metal inserts that are the same full length of the plastic post, go inside the plastic, and provide the real strength for the gate.

Use a steel post as a skeleton for a hollow wood cover post. The wood would only be the length of the visible railing. The steel post would go as deep as required into the concrete, and be almost as tall as the wood railing post.

Embed the steel post in the concrete instead, and treat it for anti-rust protection. Then set the hollow wood post over the steel post and fasten it towards the top, to not weaken the bottom of the steel post.

No football players will bend the underlying steel, and the wood cover completes the deck's look.

(picture from: https://www.doityourself.com/forum/fences-posts-railings-gates/520417-wood-sleeves-over-metal-posts.html)

Answer 2

You could try using a different type of bracket, like the ones shown below.

The bracket is attached to anchor bolts, either set in the concrete, or with expansion sleeves in drilled holes. The wooden post is inserted into the bracket and secured with screws or bolts. You could leave it floating an inch above the base plate to prevent rot.

There are similar ones that can be cast in place. See for instance this video at 1:20.

Answer 3

After further thought I do want to either bury the post in the concrete or at least have it laid flat on the surface.

In this case how can we retard the rot? Some recommendations I have seen include:

• placing six inches of gravel at the bottom of the post hole before pouring the concrete. Allows seepage of water How to Protect a Wooden Post from Rotting in the Ground

• further treating the lumber

• Using a sonatube and raising the footing several inches above grade. I am going to use an 8-inch diameter tube and do a 20 inch pour: 4 inches above grade and 16 inches below.

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Older Answer

This is only a qualitative answer but I'm likely to go with it. I'm fairly convinced by this J-Bolt for which the J provides significant additional shear strength.

Taken from this video: Options for Fastening Deck Posts to Footings

Answer 4

Use a concrete spur post

Wood will definitely rot at ground level - there's no question about that. So take steps to not only keep the structure intact when it does rot, but also make subsequent repairs easier.

Attach a spur post to the lower section of your wooden post, with 3 bolts through the holes as shown. Both the wooden post and concrete spur then go in the hole, and are concreted in place. You end up with about 50cm of spur below-ground and 50cm (the half with the bolt holes) above-ground.

Eventually the wood will rot at ground level, but it's firmly bolted to the spur so it still can't go anywhere. The wood above ground level remains sound, the spur is strong enough to stop a truck, and 3x M10 steel bolts will hold a lot, so the structure remains solid. And when (eventually) the post completely rots and needs replacing, all you need to do is bolt the new post to the concrete spur. The rotting remnants of the old post are still set in concrete, of course, but you can ignore them.

If you really wanted then you could even start with the wooden post slightly above-ground so that it doesn't rot off at ground level in the first place. This tends to cause problems with balancing the post-and-spur assembly when you're setting them in the ground though, so generally it's easier just to add 50cm to the post length you want and have the bases of the post and spur flush with each other.

Hint: You're much better using hex-head bolts instead of coach bolts into the wood, because you can't ever do up a coach bolt tight. Also use washers both sides to spread the load on the concrete and on the wood.