Design considerations for new "sunken" pad within a pad? 1

[jeffmoor]

Hello All,

This concerns cutting a hole in an existing concrete floor, then pouring a new sunken pad in the hole. The application is an automotive shop with a new semi-portable hoist that the owner would like to have sit flush with the floor level when down.

The existing floor is a 4" concrete pad, roughly 20' x 20', reinforced with steel mesh. It was poured on a compacted gravel bed, on top of clay soil.

What we envision is a rectangular hole, roughly 52" (4.33') x 112" (9.33'). The finished surface would be 5.5" below the existing floor surface.

The hoist is a scissor type, rated to lift 6000 lbs. All the load weight is across the width on a steel bar at one end, and split between two large steel casters at the other. Our intent is to embed 1/2" (or better) steel pads on the surface of the new concrete both so add strength and to prevent wearing of the floor at the load points.

I'm an electronics guy, so well out of my element on this. I know enough not to start digging and pouring blindly, though. I'd appreciate any advice on any of the questions below, or anything else folks might think of...

1. The new surface must be lower than the bottom of the existing pad. Should the pads be tied together? Does this require diging out under the old pad's edges and filling the space between? Making the hole larger and pouring "walls" tied to the old slab?

2. The new pad will be on the same hard clay, of course, with compacted gravel underlayment. Anything foolish about this?

3. Assuming 3500 psi concrete, steel mesh reinforced, and taking the 1/2" steel in to consideration, what seems a reasonable thickness for the pad? Over 6"?

Thanks again for any advice you all may be able to provide.

Regards,
Jeff

 

[csd72]

Get a structural or Civil engineer involved in this, this forum is not the place to get a free design.

 

[jeffmoor]

My apologies, but I don't believe I asked for a free design. I was hoping to be enlightened on best practices or warned of blind foolishness. In either case, having read through these forums, the post didn't seem at all inappropriate to me. Live and learn. Sorry to waste your time.

 

[csd72]

Sorry Jeff, but there are too many unanswered questions on that one to even start giving you an answer.

 

[RememberTheAlamo]

Hi Jeff,

Have a similar project underway, with an elevator pit 5x5 sunk 6" below the surface of the slab foundation.

On #1 - Water is the subject and no matter how hard we discussed the "bonding" of the sunken pad with the slab . . . we could not get around the potential for water to seep into the sunken pad. Discovered a technique which we added to the design . . . fiberglass mat. We are overlapping the sunken pad (4+ inches) on all sides by 10" with the 6" slab. The pours will be a couple hours apart to improve the liklihood of the two surfaces bonding better. We are pouring 11 trucks that day so we will pour the elevator pad 1st and then adjust to pour the slab over top. No mistake . . . we aren't intending for the two pours to be "tight" . . . so we will be using fiberglass mat to cover sunken pit and up over the top of the slab. This will keep water out and we were surprised at how low the cost is. Apparently this is some kind of residential elevator tip!

#2 & #3 - It isn't the gravel under the slab that supports the loads . . . but rather the support strength of the clay surface the stone rests upon. Ours was measured at 2,000 per sq ft. Yours is likely 1,500 - 2,000 psf ? If your steel pads were 4" by 4" then, on a 4" slab, each pad would be resisted by 1 sq ft of clay surface or 2,000 lbs . . . 6" pit slab would support ~3,200 lbs . . . and you need at least 7,500 lbs of support (6,000 applied and a safety factor of 25%+)

Hope this helps a little.

Do reinforce both the sides and pit slab . . . a little steel here would help spread the loads.

 

[Bobber1]

I think the fiberglass mat is a good idea. It's becoming quite common to reinforce old concrete beams and slabs with carbon fiber mats and epoxy resin, so you should easily find a contractor to do this work. They usuually work with the carbon fiber fabric and epoxy resin but I see no reason they couldn't use glass fiber fabric and poly resin (boat fiberglass) if it's just for sealing.

Bob G.