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Installing post footing in basement floor

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Installing post footing in basement floor Mark 11-02-2006
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Posted by RicodJour on November 2, 2006, 2:10 pm
Mark wrote:
> >
> > 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.

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


Posted by Bob Morrison on November 2, 2006, 2:46 pm
In a previous post RicodJour wrote...
> 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?
>

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.

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com

Posted by Mark on November 2, 2006, 4:54 pm

>
> 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.

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



Posted by Bobk207 on November 2, 2006, 10:15 pm

Mark wrote:
>
> >
> > 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.
>
> 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


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

>>>>> 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. <<<<<


7 gage materail is pretty thin (.179) doubt that much load will spread
beyond the width of the 4x6 post


cheers
Bob


Posted by M on November 3, 2006, 4:45 pm
> 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"

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.



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