Is there a minimum size for a post and beam floor concrete pier?
In a basement I have a 4×4 post holding beam under 2×6 floor, about 16 sq ft total (1/2 of area). This post rests on a concrete floor of unknown thickness perhaps 50 years old. I would like to break up the concrete floor and pour a concrete base for an Simpson post base. Whats a typical size for this post base?
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Replies
There's nothing typical. The variables are primarily the load on the post and the bearing capacity of the soils.
But it is fairly common to find that done with an 18" x 6" round concrete doughnut pad or to see a 24" square poured in place done.
If I understand your situation right, and assume a few things, I might not bother cuttin the concrete out and just use a steel plate 6" x 6" to spread load over the slab you have now. It has been tested all these years.
Or is it failing in some way?
A 4x4 post can only hold about 5-6,000 pounds without buckling. If you've got old concrete under it which isn't cracking or failing, as the venerable Piffin stated, the concrete is probably good for 5,000 psi. The only thing I might do if I was worried about it is put a steel plate about 1/2x12x12 under the post to spread the weight out some. A reference for you, I put 8x8 posts which are designed to carry 15,000 pounds on 8x36x36 concrete rebar-reinforced pads.
A little correction or clarification.
I doubt that the old concrete is cured to 5000PSI. We pour new slabs at 2500, sometimes 3000PSI. You probably meant to say 5000lbs total.
Let's suppose that the 4x4post is actually holding up 8000 lbs. 3.5" x3.5" gives you 12.25 sq in. Eight thousand pounds spread over that is only about 650 Psi
But if you replace that wood post with steel (good and less likely to bow) on a 4" diameter round column with steel 1/8" thick, you now only have about one and a half inches of bearing, increasing your loading psi to 5095, enough to punch through concrete with a design strength of 2500. That is the reason for a flat shoe plate welded under it. Thick is good so it doesn't deflect but 1/2x12x12 might be overkill. 1/4" x 6"x6" should suffice.Excellence is its own reward!