# Pier Foundation on top of concrete floor?



## Paul_NJ (Mar 1, 2007)

I'm planning to replace the tree trunk posts supporting the main beams in a 200 yr old farmhouse, with steel columns, as well as replacing some of the beams that are badly devoured by powder post beetles. I plan to pour 24x24x12 foundations for each new column, and originally figured I'd saw thru the 4 inch concrete floor and dig/form/rebar/pour each pier foundation. Here's my question: if the purpose of the 24x24 pad is to spread the load, why can't I form and pour a pad right on top of the existing concrete floor and not cut and dig through it? Sure would be easier. In fact all I'd need is 8 inches above the floor. Sounds like it might not be allowed, but I don't see why not?


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## superseal (Feb 4, 2009)

you need to verify sound, virgin soil for proper footings - how do you know there's not a sinkhole underneath or crappy soil in place.

Plus you want the footing 12" thick not 8" on top of 4".

This is an amateur question and I'm suspicious of your activity :nuke:


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## WarnerConstInc. (Jan 30, 2008)

Might as well just sit them on the concrete floor and skip all that work.


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## greg24k (May 19, 2007)

Concrete footing designed to provide support under the basement concrete floor. They designed to distribute LL and DL to undisturbed soil... if you have a typical concrete floor which is designed to support a regular floor load, the floor could be poured over backfill dirt. Therefor, when you go to install new columns, you will transfer the point load over the unsupported floor area and without proper reinforcement under it, it will sink, especially if your area has expansive soil or you live in a high seismic activity zone. You doing structural repair/modification, you should consult with a structural engineer, because you need a permit for the work you trying to do.


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## dakzaag (Jan 6, 2009)

WarnerConstInc. said:


> Might as well just sit them on the concrete floor and skip all that work.


I second the motion. 4" concrete can support a bunch of weight, everyone knows that.:whistling


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## stuart45 (Oct 7, 2009)

All of our ground floor load bearing masonry partition walls have proper foundations. We wouldn't be allowed to build them straight off the slab.
Even when a re-inforced concrete raft foundation is used, the slab is thicker under the load bearing walls.
Saying al that I have still seen many old houses that have things that defy the laws of physics.


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## concretemasonry (Dec 1, 2006)

How do you even know the slab is 4"?

How strong is the hand mixed concrete?

How do you know there is not a void under the slab?

I think Warner was just making a joke.


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## jomama (Oct 25, 2008)

I think most contractors would be surprised just how many floors are hollow underneath, with the floor only being picked up on the footings and a few spots of dirt here & there.

We replaced a large section of our shop floor years ago. Due to the poor material they placed under the floor, and apparently no compaction, most all of the floor has a 2-3" void under it. Couldn't believe the floor spanned so well, especially when considering I've had my loaded dumptruck in there numerous times, along with plenty of other heavy equipment. That being said though, the basement floor NEEDS to be sawn out for the new concrete post pads to ensure they'r eon stable soil..........


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## Paul_NJ (Mar 1, 2007)

jomama said:


> I think most contractors would be surprised just how many floors are hollow underneath, with the floor only being picked up on the footings and a few spots of dirt here & there.
> 
> We replaced a large section of our shop floor years ago. Due to the poor material they placed under the floor, and apparently no compaction, most all of the floor has a 2-3" void under it. Couldn't believe the floor spanned so well, especially when considering I've had my loaded dumptruck in there numerous times, along with plenty of other heavy equipment. That being said though, the basement floor NEEDS to be sawn out for the new concrete post pads to ensure they'r eon stable soil..........


Thanks for the explanations. I had thought since I was only replacing wood posts with steel in the same spots, and that the underlying soil had been in place for almost 200 years, that a rebar reinforced 24x24x12 pad on top spreading the load would be if anything an improvement. But the comments about possible voids and/or questionable fill beneath the old floor make a lot of sense.


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## Sar-Con (Jun 23, 2010)

There's a 4" concrete sog under a 200 yr old farm house?


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## nrnik (Apr 18, 2015)

*Raft slab*

Hi
I am new in this great site.
I need to know 
How can I calculate raft slab excavation volume?
Thanks


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## ScipioAfricanus (Sep 13, 2008)

nrnik said:


> Hi
> I am new in this great site.
> I need to know
> How can I calculate raft slab excavation volume?
> Thanks


You would look at the plans provided, dig out the calculator, tally up the volume of dirt you need to take out that the concrete of the raft slab will replace...yeah...that is pretty much it.

I think you should start a new thread on this.


Andy.


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## JBM (Mar 31, 2011)

nrnik said:


> Hi
> I am new in this great site.
> I need to know
> How can I calculate raft slab excavation volume?
> Thanks


y=mc2


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## Fouthgeneration (Jan 7, 2014)

NRNIK:
Volume is read at the stereo amplifier knob....

Or just touch the TV remote to see the #....:whistling

Or maybe height(in Units) x Length(in units) x depth(units again) = Units(cubed) hunh!

next reincarnation, stay awake in fourth grade math.....:blink:

OPer: "i am replacing termite eaten piers....." Where did/do the termites nest? Your SOG has many tunnels and voids under it....

Why did the 200 year old structural system fail NOW? someone has changed the moisture level to where the wood piers were at 18% moisture, so termites could eat them..... Fix the water issue before sinking( the Punster strikes Again)any more funds in a wet hole.


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