Reinforcing Equipment Foundation Exposed to View

[Boiler106]

We have a 16 in thick x 40ft x 30ft equipment foundation in the interior of a plant. We are currently using #7@12" based on analysis.

For a mat this long, is it typical to provide 0.5% reinforcing in the top mat? We currently have enough to provide >0.5% reinforcing if we included the bottom mat, however, ACI 360r-10 recommends using only the top mat.

The equipment will pretty much occupy the entirety of the mat.

 

[OldDawgNewTricks]

ACI 360 is for slabs-on-ground, so I don't think it really applies here. I would include both mats to satisfy the 0.5%.

 

[GC_Hopi]

I agree with OldDawg. Your description would have me leaning toward calling this a structural slab ie mat foundation and design it that way without using ACI 360r. ACI 360r is more a design for unreinforced concrete slab and the reinf is only for crack control not for moment capacity.

 

[SlideRuleEra]

It's essentially the equivalent of "thick" continuously reinforced concrete pavement. Will be fine for your application.
I used the this approach often for large, heavily loaded pads. Totally avoids the issue of joints... none needed.

 

[TLHS]

Given the thickness, this is presumably a reasonably lightly loaded industrial foundation? This isn't a slab on grade, so I wouldn't be looking at that standard.

In my neck of the woods, we would typically start by throwing in minimum bending steel as if it were a beam type element. We'd mirror that top and bottom. If that was a reasonably looking level of reinforcement we would stop there. If it looked overly big you start justifying with the various reductions you can do for foundations or when you've got significantly more bending reinforcement than required by calculation (these are things in the Canadian code, don't know the equivalents in ACI). Or you drop a size in the top mat if there's not a calculated need for it. For a 16" foundation, though, it's not going to be a lot regardless.

Industrial plant stuff is an odd beast, though. You could theoretically optimize the heck out of it and not be wrong, but it isn't what's typically done. Practice varies in different places, though.

What you're saying for size and spacing is at least in the ballpark to what I would expect.

My big question is that this is a pretty large footprint. Does this footing need stiffness for dynamics or alignment or anything?