Help with embedding wood post into concrete footing

[jweisbo]

Hi Folks ... I am not a structural engineer thought posting to this site could get me in right direction. Looking to support a deck in Michigan, hired a local structural engineering firm to do the post/beam and footing sizings and although most of work seemed okay, was advised to embed a 6x6 wood post into the bottom of a 36 inch Bigfoot form with a 14 inch Sonotube sleeve over the top.

This seemed to not pass the smell test ... I suspect putting a 6x6 post to the bottom of the form severely weakens the structure. Also, lots of people around here feel embedding posts (even pressure treated) is less robust than posts anchored to footings above grade. I contacted the manufacturer of the Bigfoot form system and they advised against putting the post to the bottom of the form.

Looking thru the calculations of engineer hired for job, vertical load on the footing is 16682 lbs. I found a round footing sizing chart (google) and for 2500 PSF soil conditions a 36 inch footing is needed. My suspicion is this table assumes 2500 psi homogenous concrete footing with centered axial loading (straight vertical load).

I cannot seem to find any info on how to design a bell shaped concrete footing with an embedded wooded post. Is there such a thing? Perhaps a simpler footing design is in order (like a rectangular shape with steel reinforcement). Since this project is for my home and I plan to live here until I die, I would like to construct these footings in a robust manner. I should add footing depth is 42 inches minimum for this area and that the height of the posts above grade will be about 11 feet. Soil has been tested (silty clay) and will withstand 2500 PSF. Deck will attach to structure of house.

Where can I find info on proper design techniques?

 

[KootK]

I agree with your assessment. I feel that there are two durability issues:

1) The post being in direct contact with the ground.
2) Potential for water to get in to the system where the wood enters the concrete.

Can you just ask the structural engineer to revisit this? It's hard to imagine that they'd object.

What kind of a structure is this? The only reason that I can think of for embedding the post is to attempt to create a moment connection at the bottom of the post which is rare.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[graybeach]

I personally would not embed such a post in concrete because the wood will alternately expand and crack the concrete, and contract, which will allow water intrusion. For pole/barn type structures the recommended foundation detail shown in ANSI/ASAE EP486.1 is to put a footing in the bottom of a hole that is deep enough to be below the frost line and to provide lateral/uplift resistance. A wood collar is attached to the bottom of the column, and the hole is backfilled and compacted. The soil will eventually deteriorate the column so a pressure treatment is required. But, if you do not need moment connections, it seems like you should be able anchor your columns to the top of poured pier using a commercial steel post base.

 

[Brad805]

Have you looked at the bigfoot website (

http://www.bigfootsystems.com/include/docs/gravityLoadTable_US.pdf)?

What you describe sounds very common and I believe it would be covered by their typical details. As for embedding the column, I do not follow the logic. If the post is embedded to the bottom of the form, it would defeat the purpose of the form.

 

[jweisbo]

This is a deck addition project. Vertical loading on these posts is close to 17000 lbs so a 6x6 inch post bearing on 2500 PSF soil will sink, hence the idea to use a spread footing (bell shaped at bottom) like Bigfoot.

Interestingly enough there seem to be lots of differing opinions on viability of a pressure treated wood post embedded in a concrete footing on this website. Assuming for a minute I wanted to accept the shortened life, is there some approved way to do that? What is the publication where this is covered?

Also, if I don't embed the posts, I think the connection has to be treated as "pinned" so it cannot react any moments only axial loads? I am guessing that with the height of the posts being 11 feet and the fact they have eccentric loading at the top (triple 2x12's bolted to the knotched post) there may be a need to either increase post size above 6x6 or embed to avoid buckling of the posts (column).

If there is anything else that you could suggest I read that would help me I would appreciate it. Intuitively this seems like a very common design detail for residential so I cannot believe I have to invent a new wheel here. Thanks again for your comments. My idea here is to try to look at all the alternatives and have a couple that I like before I go back and challenge the firm doing this work.

 

[jweisbo]

Brad805 ... I did look at Bigfoot website and had them send me their gravity load table/chart. The 36 inch Bigfoot form solves the problem of 17000 lbs bearing on 2500 PSF soil unless you put the post all the way to the bottom of the form.

Is there a way to spread the load at base of the footing and still embed the post or is that just not done?

 

[KootK]

I am guessing that with the height of the posts being 11 feet and the fact they have eccentric loading at the top (triple 2x12's bolted to the knotched post) there may be a need to either increase post size above 6x6 or embed to avoid buckling of the posts (column).

I doubt it. This would be pretty atypical.

My idea here is to try to look at all the alternatives and have a couple that I like before I go back and challenge the firm doing this work.

I wouldn't waste your time. Ask for the top mounted detail or an explanation for why the embedded detail is required. The top mount detail should be cheaper to build and more durable. It's a no brainer in my opinion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[nackra]

you might consider embedding a column base in the concrete and bolting the wood post in the column base

http://www.strongtie.com/literature/c-c-2015.html

 

[bridgebuster]

It's not a bell shaped concrete footing, it's a drilled pier (or shaft) with a bell. The bell gives you more bearing area and it also provides uplift resistance.

The attached may be of some help. There are some foundation details.

I agree with graybeach; I would attach the column at the top of the pier. Suppose the posts have to be replaced some day? If you embed the posts you would have to tear out the foundation to replace them. If your columns are pinned, no big deal, that's why we have bracing.

Assuming there's no local law requiring PT wood it's your property; just consider the future if you're planning to sell the house.

 

[manstrom]

Embedding the post in the concrete is done, but it isn't exactly best practice.

I would recommend using a Simpson baseplate. The tradeoff is that you lose some fixity at the base, but the post will last much longer.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller