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Guardrail Post Forces

Guardrail Post Forces

Guardrail Post Forces

Hello everyone,

Attached is the drawing with guardrail post forces distribution. Can you tell me if my calculation make sense? In addition, is there a difference in force magnitude when the post is notched as shown on the option B.

Also, does anyone know if there is some good literature explaining guardrail post (wood, metal, aluminum) design with examples?
Thanks everyone.

RE: Guardrail Post Forces

At first glance, this looks correct to me. The Notch would reduce the member and connection capacity

RE: Guardrail Post Forces

Your calculations look reasonable if bolt “a” is in fact the pivot point. However, you don’t really know so a moment about “b” should be checked too. The worst case is normally taken. For this, taking moments about “a” and “b” would lead to a maximum design load of 3,200 lbs for both bolts.

Some folks might take the moment in between the two as well. Again, you are trying to maximize the effects.

The notched section will impact the timber design aspects of the post.

AASHTO has pedestrian railing load requirements. OSHA might be helpful as well as general building codes.

Most systems I see for construction sites have to be designed for 50 lbs/ft applied to the rails combined with I think a 150 lb concentrated load at the top of the post so the loading changes based on the post spacing. Again, check codes for appropriate loading requirements.

For vehicles most guide rail is crash tested and DOT (if in the U.S.) standards apply. There are methods in AASHTO illustrating concepts.

RE: Guardrail Post Forces

The AASHTO loading is 50 plf along the rail + 200 lbs, but this presumably isn't a highway bridge, so that shouldn't matter.

The calcs appear to be somewhat conservative to me. The moment arm for the connection would be from the top bolt to near the bottom of the joist. I would think of it as analogous to reinforced concrete, with a compression zone calculated using the allowable compression perpendicular to the grain for the species of wood used. Then the moment arm is 5" minus 1/2 the depth of the compression zone. I'm guessing that will give you a moment arm of more than 4", which means at least a 25% reduction in bolt load. That's ignoring the contribution of the bottom bolt, which will also carry some of the tension, but with deformation of wood being non-linear, it would seem difficult to quantify.

Unless your bolts are tiny, the tension on the bolt shouldn't be a problem, anyway. I'd be more concerned with:

A) Are the bearing areas under the bolt head and nut adequate to prevent crushing of the wood (are the washers big and thick enough?).
B) What's preventing the rim joists from twisting? (Are the connections to the joists framing into the doubled rim joist adequate to keep the doubled rim joists vertical when the load is applied to the post?)

RE: Guardrail Post Forces

Thank you for all your answers. I agree with HotRod10 that the pivot point is located near the bottom of the post and my approach (pivot point at bottom bolt) is conservative. Let me continue the thread and ask few more questions:
1. When designing notched post, is the pivot point located at the top of the notch?
2. What is the moment magnitude for the post design? Should I assume a cantilevered post and it is going to be simply 200# * arm (distance to top of joist, where post potentially breaks)?
3. Is the only difference between the design of not notched and notched wood post would be the reduced area of the post (reduced Sx)?

Thank you very much for all your input!

RE: Guardrail Post Forces

The moment arm to use for the post is to the top bolt. That is your critical section, notched or not, since either way it will have the minimum cross section because of the bolt. You would only go to the top of the notch if you have some way to transfer significant shear to the joists at the top of the notch, which wouldn't be typical, and would be suspect in my mind due to the potential for splitting of the post.

RE: Guardrail Post Forces

By my numbers, there's no way a notched timber post will be strong enough for the bending moment (over 3800psi bending stress, without deducting area for the bolt hole). The full-sized post may or may not be adequate, depending on the wood species (over 1200psi, same assumptions).

RE: Guardrail Post Forces

Buleek -

I do it the same what you show, where I assume the moment is entirely resisted by the bolt force couple. However, I don't notch the post. And, yes, I assume a cantilever.

RE: Guardrail Post Forces

I agree, a notched post is not going to work in this case. But, unfortunately, this is existing and something I (we all?) have to deal with on a daily basis.
Thank you all once again for your help!

RE: Guardrail Post Forces

Attached is an article where they specifically talk about notched wood posts. The authors are in agreement that you should NOT notch the post.......but in their testing, none of the notched posts failed at the the notch.

Also, here is another source:

American Wood Council document DC6, which is available for free on their website. There are code compliant prescribed wood connections in the document.

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