Question about condenser foundation pad???

[structuralnuke]

I work at an electrical utility and have been mainly into structural/pipe support analysis but have recently been asked to help maintenance with replacement of a outdoor a/c condensing unit.
There is currently an outdoor slab-on-grade which supports existing condensing unit that will need to be enlarged to support the new unit. I have attached a pdf showing a layout sketch of the existing pad with proposed addition; along with a scan of the weight/weight distribution of both the new and existing units. The new unit is lighter but also has the weight distributed over a smaller area. We have no design information on the pad (depth, reinforcing, etc..).
There seem to be two options:
1. Demo the current pad and then design and construct a new pad.
2. Dowel into the existing pad and add reinforcement similar to what is expected in the existing pad. We could excavate ground around pad and determine pad depth.
What would you guys recommend? Also, if you guys recommend doweling have you had any success with the hilti adhesive systems? Also, are design calculation warranted if pad is added onto?

 

[SlideRuleEra]

IMHO, as a former electrical utility employee also, demo the existing pad and replace it with a new, properly designed one. The old one is too small to waste time & money modifying.

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[structuralnuke]

Thanks. And i agree with your recommendation, however i have learned maintenance management are set on leaving the existing pad and pouring the pad addition. Looks like i will begin research on concrete pad retrofits.

 

[SlideRuleEra]

In that case, sounds like you really have one option, not the two that you stated. Anyway...Start by finding out the thickness of the pad. If it is thick enough to use Hilti adhesive bolts as anchor bolts (they are a good product for this type application) then you don't have a problem. Weight of the units (both old and new) are trivial for a pad that is 8" thick, or so.

If the pad is "thin", say 4", then keep maintenance "happy" by leaving it in place (while you ignore it structurally). Then design an independent new pad that sits on top of the old pad (but is not bonded to it). The old pad really becomes just very good backfill.

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[SlideRuleEra]

... and be sure to raise the grade around the perimeter of the new pad (completely covering the existing pad), slope the grade uniformly away from the new pad in all directions... and tell maintenance you did this (constructed the new pad on top of the old) to "improve drainage".

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[structuralnuke]

anyone have any experience with design of the dowels for this type of application? There will be 2 concentrated loads on the new slab addition and I am unsure of the proper design method to ensure adequacy of the installed dowels to transfer load. Any suggestions?

 

[Ron]

Your loads are minimal. A 1/2" epoxy dowel can handle almost your entire load alone. Place your dowels at 8 to 12 inches on center and don't worry about it. A 1/2" dowel is easy to get and easy to install.

 

[DaveAtkins]

I don't think adhesive anchors will work. You need to develop the reinforcing bars in the existing pad, so you need to lap the new adhesive anchors with the existing bars. Drilling deep holes into the existing pad could get expensive.

DaveAtkins