Installing post footing in basement floor

[Mark]

Good morning.

I am nearly ready to replace my 4x4 basement support post. As part of this project, I plan to cut a hole in the slab and pour a proper post footing using an 8" sonotube, A Simpson CB64 bracket, and a bag of 5000psi Sakrete.

I will try to keep the hole small so that the sonotube is a fairly snug fit. . .may an inch or even less clearance all around. My question is, how would I properly fill the gap between the concrete pour and the side of the hole? I have a couple of ideas:

- Pour concrete around the outside of the sonotube during the initial pour. The paper would be left in place in the "underground" portion, and removed where the footing projects through the slab.

- Remove all of the sonotube after the concrete sets up. Backfill around the footing using (what?)

Other suggestions?

Thanks

-Mark

[RicodJour]

Yes. Investigate the loads before you settle on an 8" sonotube. Normally there is a spread footing under the sonotube to increase the bearing area.

That 8" sonotube only has about .35 SF of bearing area. Guessing at a soil bearing capacity of 2000 LBS/SF that sonotube will only be able to bear about 700 pounds of load.

I think you are way undersized. Don't design a solution until you know the loads involved.

R

[Mark]

Now I'm wondering if I shouldn't just rest the new 4x6 post on top of the slab, which is how the original 4x4 post was installed. I have no idea what the load is, but it was enough for the 4x4 to exceed the crush load of a doubled two-by joist. That's what is motivating me to upgrade it.

[RicodJour]

As an alternative, you could sawcut a 18" square in the slab - essentially making a control joint, and build a wood form, on top of the cut slab and pour a "footing" on top of the slab. The size I mentioned is a total guess, you still must determine your loads. You may also want to install an adjustable steel post instead of a wood post.

R

[Mark]

I think I'll just install the post on the slab using a Simpson ABA46 bracket, with some grout packed inside for good measure. The house stood for 75 years with a 4x4 post resting on the slab, so this should suffice. My main concern is to fasten the post to the slab, and spread the load at the top with a steel post cap so the joist isn't being crushed.

Incidentally, the soil on my lot is compacted glacial till. Having had the misfortune of needing to dig in it, I can say with confidence that its bearing capacity is greater than 2000 psf. But I have no idea what it actually is.

-Mark

[tonyg]

If you dig a tidy hole you can pour right in the hole and skip the tube.However one bag of concrete is not enough for much except a small fence post. tonyg

[RicodJour]

You're doing a structural repair due to someone else failing to pay attention to or understand the involved loads and material strengths. I just checked your original post replacement thread - hadn't read it. In one of your posts you pointed out that you felt a particular post base would be insufficient as it only had a rated capacity of something like 3500 pounds, yet your respond to my post about loads with an "I have no idea".

You seem to selectively address and ignore load information. Why is that?

R

[Bob Morrison]

I second Rico's concerns here. It should be no big deal to cut a 18"-24" square hole in the slab, excavate it to 10"-12" deep. Put (3) # bars in the bottom (3" above the dirt please), fill the hole with concrete back flush with the top of the slab. If you insist on using Quikrete or a similar product then add one shovel full of portland cement to each bag of Quikrete.

[Mark]

My assertion that the EPB64 post base with a 3500 lb load capacity would be insufficient was strictly "from the gut." Having said that, I don't think it was an unreasonable concern. This is an indoor application, so I shouldn't need the standoff that the EPB series brackets provide. The ABA46 that I propose using is specifically intended for installation on a basement slab, and provides about 1" of standoff for good measure. It is also rated for about 10,000 lbs downward force. Simpson specifies that it may be filled with grout to "increase its capacity," although they don't specify what the higher capacity might be.

Without a strain gauge, I cannot tell what the actual loading is on the existing post. Perhaps it is less than 3500 lbs, but I am more comfortable with

10,000 lbs capacity.

The existing 4x4 post has stood for 75 years without any obvious distress in the slab, so I have no reason to believe that it won't serve as a suitable base for a 4x6 replacement column. Of course there _may_ be an integral footing in the slab at that point. Unlikely, but possible.

I plan to use a CC3-1/4-6 post cap, which is made from 7 gauge steel and spreads the load over 11 inches. I hope that will be sufficient so that the post no longer exceeds the crush strength of the paired two-by joist.

-Mark

[Mark]

I will think it over for a few days before I start into this. My main concern would be how to support the structure while I am doing this; digging a 24" hole would put my shoring posts a fair distance from where the existing column is located.

I'm happy to mix my own concrete. Is there a formula that you can recommend for a high-strength requirement like this?

Thanks

-Mark

[RicodJour]

You seem to be a methodical guy. Which makes it a little difficult to follow your reasoning in basing your design on a failed installation that almost assuredly doesn't meet code (code being the minimum acceptable construction).

In the amount of time it took you to investigate the post bases you could have determined exactly what the design load is so you wouldn't be guessing. Unless you have a large amount of experience in structures, "from the gut" equates to "wild assed guess". If you need a little assistance in figuring out how to calculate the loads, just say so. I assure you that even someone as experienced as Bob Morrison, engineer extraordinaire, has had to ask some questions - and I've been happy to answer them! ;)

R

[Bob Morrison]

A strain gage won't do you much good unless you can unload the post first.

A 10,000 pound load equates to about 5 square feet of footing. The 24" square x 12" deep I suggested earlier will give about 8000 pounds capacity.

Put in a temporary post on either side of the hole about 12 inches away from the edge of the hole.

I don't do field mix design other than to say if you use typical bagged concrete mix you should add some portland cement.

[Bobk207]

for a high-strength requirement like this?

Mark-

Not to "pile on" but........

if you're asking about concrete mix design & how to shore the structure while you're doing the repair

then your "from the gut" evavluation of the post base might be coming from another part of your anatomy..... :)

I didn't go back & read the old thread (I thought you'd be done with this repair already)

Are you sure you've diagnosed the problem correctly? Why are you replacing the post? What caused the original crushing?

If you're going to repair thsi properly you've got the understadn the load path......just beefing up the post, post base & post cap may or may not solve the real problem. A stronger post & post base are really pretty useless if the slab / soil cannot take the load.

Obviously you have some technical skills (strain gage) but there's more to structural repair then you currently know. Just making some of the components arbitarily stronger, not knowing (or at least an educated estimate) the load means you're pretty much guessing wildly.

If you're going through the trouble of doing the repair, take the advice from the experienced guys in the ng......otherwise, why bother?

this statement pretty much sums up your approach

spreads the load over 11 inches.

[carolyn]

What are you using as a footing for the post? The post needs to sit on something that spreads the weight out. If your post is directly on bedrock, a footing should not be needed. Otherwise check the local regs and verify the size of footing you will need.

There is a product available at big-box stores, that is basically an inverted funnel. The bottom of the tube is the correct diameter for a footing, and over about a four foot rise it narrows to a 6" or 8" inch diameter. It is made of plastic and meant to be left in the ground. Dig your hole, drop in the cone, fill with concrete, add any bolts out the top, backfill and wait for a day. Modern additives for the concrete could have the concrete set and ready to use, before you have the hole backfilled.

If you are able to have you post land on bedrock, then you can use sand, gravel, or almost anything to backfill the hole. You can even use the dirt you pulled out in the first place.

If you leave the paper in place, as it rots it will shrink and you will get settling around it. That in itself isn't much of an issue on a single column, but around a foundation wall, it can lead to drainage problems if not watched and repaired when the time comes. The other issue is the rotting material - will any of the mold and whatever else grows on it, be able to get into the house? If so, how will you deal with it?

A shovel? :) Backfill with the original material from the hole.

Carolyn

[Bob Morrison]

Carolyn:

I believe you may be making more out of this project than may be necessary. The OP indicated that is soil is glacial till. This material will stand with a vertical cut for a few days. More than enough time to simply cut a hole in the slab, excavate the material, then backfill the hole with concrete flush with the original slab. This kind of work gets done in my area all the time. It is simple and easy to do. The hard part is getting the slab cut and removing the concrete in a neat and orderly way.

For a repair involving a major support for the house it certainly would not be out of line to spend the money on a concrete sawing company to give a neat finished appearance to the project.

[M]

I did something like this but not exactly. I did have to unload a house and re-load it. It is a one story house. I used a 20 ton bottle jack on 2-

2x12 cut slabs.. ap two feet log. then a cinder block two more 2x 12s on top of the cinder block an 8x8 post and put the bottle jack on top of the 8x8. I was bale to jack the house well over 3/4 of an inch. i also did the same thing with a 12 ton bottle jack on the other side. The two jacks only cost a few hundred and was able to buy them at Napa. When I was done I had to re-fit the house with a different sill and had to shim the load points so having the bottle jacks helped me get the right fit by jacking and releasing and putting in more or taking away shims until it was right. None the less I think a couple of 12 ton jacks would do the job for you.

[carolyn]

Bob,

You are probably right. I missed the 'glacial till' bit, and even now I don't know what it is. Off to google in a moment. Thanks!

Carolyn

[david ashworth]

[Mark]

However I go about this, I _had_ planned to install an adjustable shoring post for a few days, to (hopefully) work out any settling in the new footing. Anything wrong with this strategy?

Thanks

-Mark

[RicodJour]

Why do you feel that the settling would occur in a few days? If you see the benefits of an adjustable post, why wouldn't you make the adjustable post permanent? Installing a temporary adjustable post would be making extra work for yourself and basing it on the assumption that all settlement would happen within your theoretical time frame. Nature and structures rarely cooperate with arbitrary time frames.

R