Posts Embedded in Concrete 1

[Splitrings]

thread507-8985
I have seen several threads on the design of posts embedded in concrete but none seem to list a design procedure or method of analysis. The post could be wood or steel. ASABE EP486 has a procedure for posts embedded in the ground. Seems this procedurre would be reasonable to use if there was a value listed, or one you could assume for the lateral soil reaction for concrete. My geotechnical background is rather weak so I don't have a feel for these values or what they mean. Has anyone found a good design procedure? Lina posted this same question a couple of times.

 

[youngstructural]

A post in concrete is only like a post in soil in appearance, at least in my opinion. The soil will yield plastically and the loads are reasonably distributed; Your concrete is not going to yield, it will crack. You need to consider the problem elastically. Those nice smooth semi-circular and peak, or flat rectangular soil stress approximations really aren't representative of your problem.

The good news is that it's actually much simpler than a post in soil: You have the advantage of being able to consider your post as imparting triangular pressure distributions onto the concrete. Then you ensure that you have the steel to close any potential cracks. Simple.

Cheers,

YS

P.S. Sorry, no references that I can think of, other than ones about embedded items in concrete and preventing possible deterioration/interactions. I'm happy to post a sketch if it will help...

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[csd72]

Splitrings,

For lateral loads the material of your post makes no difference to the soil behaviour. It is the width/deth that matters as this determines if it should be treated as a stiff pile or a flexible one.

 

[youngstructural]

csd72: Not talking about into soil... Did you misunderstand or am I in line to learn something new here?

Looking forward to your reply,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[Splitrings]

You are correct YS. I am trying to analyze a post in a mass of concrete.

I am in agreement with you that the stress distribution will be triangular. The bigger question I think is where is the neutral axis? There is a point where increasing the embeddment depth no longer is effective. This must be governed by the stiffness of the post compared to the stiffness of the effective concrete section. I think I can make a reasonable assumption on a effective width of concrete to use. The effective length, in the direction of the load might be controlled by the ties or reinforcing you mention. ASABE uses an equation for dmax or the maximum embedment based on the EI of the post. I need to do the same...somehow.

 

[Ron]

The issue here is fixity. The intent is to achieve fixity of the post. In soil, that requires significant embedment; whereas, with concrete that can be achieved relatively easily. Be sure to check the mass of the concrete for overturning from the moment induced by the post. Once that is satisfied, it just becomes a triangular stress distribution against the concrete and the post.

 

[miecz]

Try the PCI Design Handbook (5th Ed.) Article 6.9, "Structural Steel Haunches."

 

[Splitrings]

Thanks Miecz,
That is just what I was looking for. The PCI Handbook was sitting right on the bookshelf in front of me.

 

[csd72]

Sorry splitrings I misread your post.