New conc floor slab on existing outdoor slab fro metal bldg 2

[mrnico]

We are putting up a 40x60 steel Mueller building on an existing outdoor slab 4"+ thick which was poured in the 1950s with no reinforcement. We are looking at doing a 24x24x24" footing at each of the six column feet with a rebar basket in it. The new slab will be 3000psi, 6" thick with #4 bars @16"o.c. each way. The loads I have are:

3x UPS 77"x38" @ 4990# each
12x batteries 74"x15.5" @ 3345# each
CLC Cabinet 77"x38" @ 4277#
transfer switch 36"x48" @ 1100#

These loads should be fine, but I wonder about the forklift wheels, 4"x6" contact patch each. Empty forklift weights 8000lbs and loaded weighs 12,000lbs. Sounds fine to me, but does anyone have any recommendations to back it up?

I am thinking that for the ramp going up is best to cut out the existing old 4" slab and pour in a thicker ramp up onto the new building with thickened edges and maybe more rebar in it, tied into the new 6" slab's steel. Thanks.

 

[Ron]

I'm not sure where you're located, but a 2x2x2 footing for a rigid frame bent is potentially weak, if you have any wind load to consider. Dowelling into an unreinforced slab won't help much with overturning moment.

As for the fork lift traffic, an 12000 lb load can be a problem with a 4" slab. I would core the slab in several locations to check the thickness. It is probably less than 4" and will likely vary a lot.

You need to do a little more investigation into the existing conditions before you do the design.

 

[msquared48]

I agree with Ron here. I think the footing would be more like a 3X3X3.

Mike McCann
MMC Engineering

 

[mrnico]

We were going to place the top of the 2x2x2 footings flush with the existing slab, then the 6" slab (with #4 bars at 16" o.c. both ways) will go on top of the existing 4" slab. I thought they might want to increase the footing size, but they have used that size in new work areas nearby successfully on an almost identical building. This building is fairly well sheltered, being only about 8 feet from a much taller and larger old aircraft hangar, with a lot of taller, much larger buildings all around. Going with a 100mph wind design. I am waiting to get the column reactions from the manufacturer for the final decision, as I do not have the sections used for the building design yet.

They have cut the existing slab at the six column locations and also trenched across about 40' in two areas, and the minimum thickness is at least 4". So the full floor will be 6" reinforced over 4" plain concrete.

 

[Ron]

mrnico...I'm not sure where you're located, but in the US, aerodynamic shading consideration is generally not allowed....the only thing the other buildings will do for you is help define the Exposure Category.

The tied reinforced slab will help with your overturning if the footings are keyed under it.

Be careful with that assumption that the existing footings are OK because a nearby building has the same and it's performing. That type of historical assessment works for soil bearing and settlement, but it doesn't work for uplift and overturning, simply because I doubt that any of the buildings there have been subjected to code mandated wind loads. You are required to design to code...not the owner's wishes.

 

[mrnico]

Thanks, Ron, I understand that. That's why I am waiting for the mfr's calcs to check for uplift. The contractor is trying to forge ahead though, and I am just trying to get the general consensus as to whether I should go ahead and tell them to enlarge the footings now, before they place any concrete. Although I do not think the client will allow them to place any mud until we turn in a submittal with the footings and design mix for approval anyway.