Concrete Slab Question

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Andy Asberry - 28 Sep 2005 00:47 GMT

This is on a farm; no inspections of any type. I have a 8'x8'x6" slab
that I had a feed bin on (now gone). The leg anchors (4) of the bin
were 6 feet on a side. In other words, one foot in from each edge of
the slab. Where the anchor bolts for the legs are there is an 8" pier
4' deep. There is no perimeter beam. #3 rebar on 12".

What I want to do is build a block building 8' high, pour a flat 4"
roof and place a 1500 gallon water storage tank on top. The building
would house the pressure pump and tank. I would fill the voids in each
corner and midway each wall. Top course would be U lintels with rebar.

I'm guessing the total weight at about 20,000#. The feed bin held
24,000# carried over the piers. The 8" blocks will be about flush with
the edge of the piers. Am I safe with the cantilever/shear of a 6"
slab with that weight?

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Bobk207 - 28 Sep 2005 08:20 GMT

Where is this structure located?

I would dowel in to the slab with steel for the wall & consider
grouting more often than 48".

You should get a civil to take a look at it but IMO the feed bin
condition was worse for the slab than your proposed pump / storage
building

cheers
Bob

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Andy Asberry - 28 Sep 2005 21:26 GMT

>Where is this structure located?
>
[quoted text clipped - 7 lines]
>cheers
>Bob

Central Texas. I don't even insulate the water valves in the barn
lots. It gets cold occasionally but not for long. Bury water lines 8"
deep. No frost line.

The feed bin had been there over 20 years. No cracks or tilting of the
slab.

Thanks.

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Bob Morrison - 28 Sep 2005 15:43 GMT

In a previous post Andy Asberry wrote...

> I'm guessing the total weight at about 20,000#. The feed bin held
> 24,000# carried over the piers. The 8" blocks will be about flush with
> the edge of the piers. Am I safe with the cantilever/shear of a 6"
> slab with that weight?

The answer depends on your location. If in a high seismic zone or an area
where the frost depth is high then the 6" slab may not be sufficient.

AS Bob K suggested, you might want to have an engineer take a quick look
at the issue just to satisfy yourself that the structure will be safe.

Signature

Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA

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Andy Asberry - 28 Sep 2005 21:58 GMT

>In a previous post Andy Asberry wrote...
>> I'm guessing the total weight at about 20,000#. The feed bin held
[quoted text clipped - 7 lines]
>AS Bob K suggested, you might want to have an engineer take a quick look
>at the issue just to satisfy yourself that the structure will be safe.

No seismic or frost line here. There are many larger tanks around here
on 3" pipe stands (local description) with the legs in concrete. Some
a hundred years old.

Optional ideas:
I could reduce the size to put the walls over the piers. Tank
diameter is 78".
Maybe u lintels for the first course to create a beam above the
slab.
Set the tank on the slab and place the pump on an adjacent new
slab.
Demo this one and start over. Not likely. That would be a bear.
Build a pipe stand over/around the building to support the tank.

Bob, I want to take this opportunity to express the appreciation I
feel the group owes you. Sometimes, I wonder why you continue to try
to help so many who only want someone to agree with them. Unlike those
dummies, I will heed your advice. Thanks.

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Bob Morrison - 29 Sep 2005 01:44 GMT

In a previous post Andy Asberry wrote...

> No seismic or frost line here. There are many larger tanks around here
> on 3" pipe stands (local description) with the legs in concrete. Some
[quoted text clipped - 14 lines]
> to help so many who only want someone to agree with them. Unlike those
> dummies, I will heed your advice. Thanks.

Andy:

Thank for the compliment. I'm thinking that if could locate the center of
your new CMU walls at the outside edge of the piers then you should be
okay. I would be inclined to suggest that you add some vertical bars
doweled into the slab at 48" o/c and (2) #4 horizontal in a bond beam
blocks at every 48" o/c. You may want to grout the bottom two courses of
CMU solid and of course you would grout the cells that have vertical and
horizontal bars.

Your top slab should be at least 6" thick and have #3 or #4 12" o/c each
way.

Does that help? I still think a quick review by a local engineer would be
the best.

Signature

Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA

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Homeowner Forum / Construction / September 2005

Concrete Slab Question