Unusual housekeeping pad situation

[bkewok]

Project Overview:
We’re installing TWO Large commercial extractors (1,100 lbs each, empty) on a reinforced foundation designed for significant dynamic loads.

Housekeeping Pad: 5ft x 10ft elevated 4-5" / #4 bar at 12" oc E.W. Two rebar mats (2" above the base, 2" below the top).

Concrete Mix: Includes water reducer, superplasticizer, and corrosion-resistant steel fibers. Strength: 3,000 PSI in 3 days, final 4,000-5,000 PSI.

Dynamic Forces:

Static Load: 1,350 lbs

Max Dynamic Load: 2,690 lbs

Frequency: 8.0 Hz (Low Speed), 15.9 Hz (High Speed).

Issue:
We found an unexpected 4-inch floating layer bove the 10-12in foundation slab, impacting our reinforcement placement and possibly load dynamics.

The pad is going to have to be tied into the structural foundation beneath it to ensure full structural continuity, better load transfer, and reduced risk under vibratory stress.

But the question is what to do with the the 4-in overlap between the original floating slab (unreinforced and not bonded) and what will be the bottom 4 inches of the new Housekeeping Pad. For reference, I made a hasty sketch..

Possible course of action:
We had discussed isolating the pad from the floating slab like an expansion joint around the perimeter but it has also been brought up that we should use epoxy grout instead.

Standard Install:
Of course the manufacturer specs have us tying it in on the sides but that's when you have compacted fill on the bottom and a reinforced slab surrounding the pad. I must also add that this area could possibly be exposed to your standard water and liquid chemical detergent.

Any advice for input and or feedback will be appreciated.

Thanks and Merry Christmas

 

[Lomarandil]

I would isolate the pad from the floating slab. If you dowel into that slab, you're just as likely to cause cracking (differential settlement, dynamic load transmitted to the unreinforced section) as you are to solve anything. Not to mention that doweling is a lot of extra effort.

And while you should get someone who is more familiar with dynamic equipment foundations to confirm this, my understanding is that your pad has enough mass relative to the dynamic force to handle the equipment dynamic loading by itself.

 

[jhnblgr]

Seems like the only real issue is that the vibration could crack the floating layer. Vibration damping seems like a better option to prevent the transfer in the first place.