slab on grade

[ajk1]

There is a small area (about 50 square feet of an existing slab on grade that is only 2" thick and is underlain by sand base which seems reasonably well compacted. The slab on grade in the remaining area of the building (which was built in 1966) is 6" thick. I believe the 2" thick slab was placed about 25 years ago, probably unknown to the architect at the time, when underground ducts were re-routed when an addition was built onto the building. The room in which this 50 square feet of 2" slab occurs is used as an accounting office, the office is about 20 feet square and had vinyl tile on it. It has performed satisfactorily to-date. A recent flood required removal of the vinyl tile, and when I chain dragged the concrete surface I identified this area of thinner slab by the very hollow sound when the chain passes over it (as well as by the change in appearance from the adjacent slab). However when we drilled an exploratory core hole, we did not find any signifcant void under the slab, but we did find that the slab was only 2" thick.

Questions:

1) To-date there has been only a desk on the 2" slab area, but the owner wants to have the flexibility to place filing cabinets anywhere in this room. Should we take out the 2" slab at this time, or do you think that a 2" slab is sufficient to support filing cabinets? I have no confidence that the Westergaard equations work in this case.

The cost of replacing it with a 6" slab matching the remainder of the slab in the building, including some dowels, is $3000.

I figure that the filing cabinets contents probably would not exceed about 20 pounds per foot. So 4 drawers, each 24" deep would be about 160 pounds plus the weight of the cabinet would be about say another 100 pounds = 260 pounds. Does not seem like that much weight.

2) Are there other reasons to replace the 2" slab. I have never heard of a 2" slab on grade.

 

[JAE]

A 2" slab is a mud slab in my view.

The performance of such a thin slab would be based upon the stiffness of the underlying subbase. The weight of the proposed cabinets may or may not be reflected in cracks depending on the actual subgrade modulus.

Even if the subbase was good, I'd still want to warn the owner that heavier loads could result in settlements and flexure/cracking in the slab.

 

[ajk1]

JAE - thanks. That is my feeling too. However, after 25 years like this, there are no cracks. The owner has deferred to me as to whether to replace it with a proper slab or not. I am having a hard time making up my mind whether to replace or not (cost is $3000). I am leaning to replacing now while the offices are closed because of the flood damage repairs. I am interested in whether that is what you would most probably do in this situation.

 

[JAE]

$60/sf. seems high - but it is a small area.

For such a small area I'd recommend it to the owner.

 

[DST148]

@ajk1: Typically, the minimum thickness is governed by the building codes. As per IBC 2003, section 1911.1, the thickness of concrete floor slabs supported directly on the ground shall not be less than 3 1/2". A 6 mil vapour barrier...........
If required, you may also refer to ACI 360 - Guide to design of slabs on ground

 

[dik]

The area is small, but for setting up, demolition, removal of debris, new concrete, permits and to make a profit, I don't think the $3000 is too bad.

Dik

 

[woodman88]

If the owner wants my professional insurance to cover the cost (or some of the cost) for replacing the existing slab if it does not continue to work in the future. I would do calculations to show that the slab was done per the building code that it was poured under or require that the slab be brought up to the current building code.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[MiketheEngineer]

Kind of a contrary opinion - but most soils are good to about 1500-2000 psf.

That said it will probably be OK - BUT maybe NOT. Sit down and explain it to the owner and tell him that it HIS call - and get it in writing.

Or tell him it is OK for "light office loads - BUT he can try anything he wants.

What is the worst that can happen - he replaces it now or maybe later?? Same price

 

[Ron]

As JAE noted, shear is probably not an issue (typical filing cabinet would be less than 5psi at perimeter for shear); however, depending on the subgrade modulus, the bending resulting from the load would be an issue.

For $3k, I'd replace it.

 

[ajk1]

If we replace iot now when the room is not in use due to the flood damage repairs in progress, it will be much less cost and disruption than if we repalce it later after the office staff is moved back into the room.

I have told them they have a choice, but it would be nicest to replace it and they agree, and things are now proceeding that way.

Thanks for all the comments that helped me make up my mind. It is a place of worship and I am a member there, so it is not a question of them suing me for anything (LOL), as I have been quite open on it all. Thanks again.

 

[Ron]

ajk1...just to keep your mind from being at ease...one of the more common claims in professional liability is an engineer providing pro bono services to a church!!

 

[ToadJones]

$3k for that job is very high in my opinion.
For 50 sq ft, the contractor could easily do the job in one day even without a jackhammer.
There isn't even a yard of concrete there for a new 6" slab.
So literally there is a few hundred dollars in materials.

Get some more quotes and fix the floor for the owner.

I poured a 1400 sq foot portion of my driveway for less!!!! (only 5" thick).

 

[ajk1]

The first quote was for much more. There are restrictions on hours of noisy work due to the school there and so on. I think it is a reasonable price, especially given the current hot Canadian economy.