Reinforcing bar or wire for small equipment pads?

[StructureMan44]

We’re placing 4in thick equipment pads on an existing 6in thick slab on grade. The pads are small and range from 15-26ft^2 and the building will experience an interior temperature swing of 40°F-120°F. Which is more prudent choice in regards of longevity: placing #4s @ 16in (in some cases only two bars are placed in direction) or placing welded wire (D5) at a much closer interval?

 

[concretemasonry]

Don't worry about the temperature swings. Your temperatures are air temperatures and not the ambient temperature of slab or ground below the thin soil.

If in doubt, add another piece of rebar since it is cheap.

You should develop some way to bond/adhere the 4" slab to the existing slab.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[TLHS]

What's the equipment? If there's any reasonable rotating or recipricating mass I'd cut through the slab on grade locally, demo and pour a monolithic slab/footing. Same if it's a significant weight and I was worried that I might settle locally and crack the slab.

 

[StructureMan44]

concretemasonry: Thanks, the existing slab will be roughened/chipped before the new slab is poured. For anchoring into the existing slab I was planning on adding some L-bars that are drilled and epoxy doweled into the existing slab.

TLHS: Thanks, none of the equipment has a large rotating mass and the equipment ranges in weight from 500 to 4000lbf. Knocking out the existing slab I believe will be too costly.

 

[StructureMan44]

Are there any options if the existing slab is not deep enough to meet the required embedment depth for #3 post-installed reinforcement dowels?

 

[TLHS]

You don't need to fully develop your bars. Think about what your bars are doing if you're tying into an unreinforced slab? All they can do is transfer shear or develop the weight of the slab they're lifting up.

You'll have them at a pretty close spacing, relatively speaking. Say you're putting them at a 12" grid or something, each bar will only lift something in the ballpark of 12"x12"x6" of concrete. That's almost no weight. If you want to put a better number to it, figure out the moment capacity of your slab as plain concrete, figure out how far past the edge of your pad your slab would survive in moment if you were to hang it from the pad, and then use that to come up with a maximum load one of your pieces of rebar can see before the slab fails first.

Embed them in 4" of so, see what that capacity is and whether it's enough given the above information.

Note that you'll presumably have to justify any shear capacity using something other than shear friction, since you don't have full development. You'll normally have enough bars, though, that this isn't an issue.

 

[StructureMan44]

Thanks TLHS: If I understand correctly, you're suggesting placing dowels from the new slab into the existing reinforced slab every 12" O.C. in both directions (12" grid)? Is this typical for equipment pads?

I was considering only adding dowels at the edge of the new equipment pad making the concrete per dowel much greater than 12"x12"x4". These edge dowels would then lap with the pad reinforcement grid. Still, it will likely be 12"x12"x6' so it's still not that much?

The existing slab is reinforced with #4@12" and the new slab reinforcement is undetermined.

 

[KootK]

As a first step, I'd assume that the existing slab is the real structural supporting element and that the equipment pad is just filler, like a housekeeping pad. i.e. no horizontal shear transfer between upper and lower slabs. In that case, I would also go with dowels around the perimeter just to nominally keep things from sliding around. You can use the upper slab to assist with punching shear if that is a concern.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

I was just making up numbers for purposes of discussion. The spacing of the dowels depends on what you're trying to do. If you want to fully develop the weight of the slab on grade below, then yeah, a spacing of somewhere between 1.5 to 3 times the thickness seems about right if you don't want to do much analysis (assume it pulls out a cone of material and then space based on the angle that you've assumed for that cone). If it's a small pad, I'd go with that probably. I'm in a high seismic zone, though, and normally have overturning concerns with any type of equipment I'd be installing.

Options:

1. You need the weight of the lower slab for overturning or something similar - do generally what I said above. You can increase spacing if you are comfortable using the bending capacity of plain concrete.

2. Your housekeeping pad doesn't transfer significant shear or uplift/overturning - in this case, your dowels don't do anything. Throw a few in to make sure your housekeeping stays tight with your slab on grade in curing and operation

3. You have a large amount of shear somehow... but no overturning - Do the same as number 2, but double check that your dowels have sufficient breakout capacity. I've never had this govern if I have at least a handful of bars around the perimeter of the pad.

 

[StructureMan44]

TLHS: Thank you. This equipment pad is indoors (no wind = no overturning on equipment). "L" dowels around the perimeter with 12" of embedment provide plenty of capacity.