1.33 in dia steel post epoxied into wood. Guardrail design

[shacked]

I have a condition where the guardrail sub contractor wants to use square solid steel posts for the guardrail posts and turn the ends round so that he can come in after all of the flooring is installed, use a hole saw and drill into the wood block or support that I provide at each post location and epoxy the 1.33" dia steel post into the wood.

I am trying to see if this will work out but I think that the limiting issue would be the wood edge distance to the center of the post. See attached pdf.

This is loaded a little differently than a machine bolt laterally loaded so I don't think that the NDS code required 4d edge dist is applicable.

I am trying to justify this connection since the epoxy bonds with the wood fibers and the steel creating a semi-fixed condition(for purposes of 200Lb live load at the top of the guardrail post). I could resolve the moment into a couple based on an assumed embedment then determine the compression perpendicular to grain.

Since the wood grain will also be resisting this how would I determine the required wood side thickness?

I realize that the wood should be dried or engineered lumber in order to avoid shrinkage and I still have to resist the block rotation, but that is a different issue.

Thanks for your input.

 

[jayrod12]

I'd be concerned with cross-grain bending personally. The portion of wood outside of the post will want to just split away when the load acts outward.

 

[OldBldgGuy]

Cross grain bending or perpendicular to grain tension, same thing, it's a problem here. You can quantify perp to grain from CWC or AWC tables.

 

[shacked]

Ahhh, thanks for the insight. Yes I agree, all that I initially checked was embedment to ensure that the wood does not crush perp to grain.
I guess if you were to secure a steel side plate with machine bolts located at the closest side and centered on the post that could ensure the tension parallel to grain within the immediate area around the post wouldn't shear, and that the only mode of failure would be the wood crushing. Assuming that the steel will resist the design moment.

 

[jayrod12]

Technically yes, you'd want those bolts to be thru bolts and even then, you've still got the potential for cross grain bending as the post will want to twist the wood in torsion essentially.

 

[wannabeSE]

Before you go too far, have you checked the steel post bending? I doubt 1.33” diameter will come close to working with a 200# load at 42”

 

[dhengr]

Shacked:
It seems reasonable to assume that properly placed epoxy would improve the post bearing stress (and crushing) in a hole in a piece of wood. All the better if you could apply the epoxy under pressure, for some period of time, so it actually impregnated the wood around the hole. But, you still have other issues which you must resolve. Splitting, cross grain bending or tension, and the prospect that your wooden blocking must be adequately secured to the structure so that it can’t just be rolled out of the structure when the post is loaded. Can the 1.33” dia. post take the load at the transition to the sq.bar? Then, you also have the 200lb. load applied, at 3.5’, in any direction, so edge distance and cross grain tension are a problem at the top and bot. canti. reaction points/zones down near the deck surface. Although, the inward guardrail failure us usually less likely to be fatal. I think it’s becoming a crazy world out there when the guardrail sub- contractor really starts dictating the structural engineers responsibilities and obligations. He wants you to work as hard as you can to make his installation as easy as possible, and at what remuneration? He really wants max. flexibility so his railing can be +/- an inch long. It’s his responsibility to provide the hardware which will work and meet the code, and his responsibility to engineer that properly and cooperatively w.r.t. other trades, and it is your responsibility to provide sufficient structure and blocking to help him meet that obligation.

 

[shacked]

Thanks for the replies. The solid posts work:

Based on Fy = 36000psi
Reqd plastic sec mod = 0.39in^3 (PER AISC 360-16 CHAPTER 16.1) Even though it may not fully apply to stair guardrail design since stair guardrail design is outside the scope of AISC, even though it is steel)

Sq solid post Plastic sec mod provided = 0.488in^3
Rnd solid post plastic sec mod provided = 0.392in^3.

Since the square post will be put on a lathe and turned down to 1.33" dia there is no connection between the sq to round portion of the post, therefore I am not concerned with stress concentrations for only a guardrail.

The only part that I am concerned is with the securing into the wood. As I stated previously the block rotation is a different issue and that can easily be remedied with another full depth block placed perpendicular to it with a simpson "dtt2z" holdown placed at the top and an allthread through the top of the block with the post.

dhengr I semi agree with the last portion of your post, but according to him the owner wants to see the post just disappear into the floor without an escutcheon placed at the bottom. I asked why the post can't be installed with a baseplate and lag screws into blocking below, then the flooring guys place the finish material around it. He said that they hate to do this and it never looks right when it is completed.

So I decided to try and give it a go to see if I could make his method work, but I am having some serious doubts the more I look into it. I asked him if he ever has had an engineer provide calculations to a City for this connection and his response was priceless, "I haven't, but every one I ve done is sure beefy."

Ok then I will just explain that to the lawyer when I get sued because someone fell. I'm sure I will be golden.