Installing post footing in basement floor

Posted by Mark on November 2, 2006, 10:11 am

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

Posted by RicodJour on November 2, 2006, 10:16 am

Mark wrote:

> 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?

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

Posted by Mark on November 2, 2006, 11:10 am

> 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.

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.

Posted by RicodJour on November 2, 2006, 11:22 am

Mark wrote:

> > 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.

> 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.

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

Posted by Mark on November 2, 2006, 12:19 pm

> 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.

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

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