new equipment pad on existing slab 2

[uasteelcrete]

My company is installing a new boiler on an existing slab where the old boiler was installed. I have been looking for examples of how to design an equipment pad that will be installed on an existing slab. I have searched my text books, the ACI and online sources but all I'm finding is elevated slabs, slabs on grade, and slabs on deep foundations. I'm hoping that someone on this forum can give me some advise on where to look for examples of how to design equipment pads on existing slabs. See attachment for a sketch.

The existing slab is a 9 in thick slab on grade with top and bottom rebar mats comprised of #4's @ 12" each way and top and bottom.
The new boiler is approximately the same size and weight as the original. The footprint is 256" x 101" and weight is 162,000 lbs, flooded.

I have installed many such equipment pads but I was not involved with the design. I expect for the new pad I will use #6 rebar dowels into the existing slab and a mat of #4 rebars at 12" o.c. I'm having some difficulty deciding how to properly check this scenario. Can anyone please give me some advice?

Thank you.

 

[racookpe1978]

Unless a 9 inch pad is thick enough to set the boiler on the (original) dirt/rock/compressed soil that is present in the area, you'll need more concrete. 9 inch isn't too thick, it can be demo'ed and removed, but it will be cheaper and faster to re-use if possible.

Murphy's Law not being ever repealed, the pads (and original boiler support steel) will be in the wrong place. Regardless, design a "ideal" slab assuming no concrete is in place now but that the soil under the slab is compacted "perfectly".

Then, see where you need your new pads and new steel supports and boiler legs, etc are expected to be. See if any of these new pads (deep parts of the slab or electric penetrations, etc.

My best guess of what you'll find? Chances are, most of the original slab can be re-used as-is. A few steel pads will need to be sawn out and re-built with deeper 2 to 3 feet deep bases. Let's say you find you need 12 inches or 14 inches for a completely "new" slab. If 9 inches are present, you would only need 4-6 inches added depth. If 10 or 11 inches slab was required, you'd still have to add at least 3 inches of new concrete, but that is less expensive than demo and pour 10 inches of new concrete. This assumes you could re-pour more concrete over top of the old slab: adding only enough cover depth to get an effective layer of rebar and enough roughness or tie-bolts into the old concrete surface to hook the whole assembly of double slabs together.

 

[BadgerPE]

This may be a valuable document, although you will have to purchase the full version through ACI. Also, depending on the boiler support, you may have to check punching shear of the slab. Design of Floor Slabs on Grade by Ringo and Anderson is a great reference for the available strength provided by the subgrade material.

 

[uasteelcrete]

racook,
The old boiler and the new boiler are both mounted on skids so the weight is and will be spread out through the skid. Also the old boiler was not bolted down. One side was sitting flat on the slab and the other side was shimmed up level (approximately 4 inches) because the existing slab was designed to drain and therefor not level. One entire side if the boiler was sitting on two stacks of shims, each approximately 4"x4"x4". There are no existing pads or support steel. Once the old boiler is demoed we will have a bare slab to start from.
I'm currently looking for a way to analyze the double slabs as you mentioned. I'm having some trouble finding examples of that. I will continue searching.

Crackerjack,
I have searched through the ACI, I have the entire volume available to me. So far I haven't found anything that stands out.