Dowels Needed: Large Pedestal to Mat Foundation 1

[Billdings314]

I am going to be supporting a piece of equipment (dynamic considerations need not be considered) on a large 17.5' x 10.5' concrete pedestal (more like a mat). That pedestal will be sitting on top of a large concrete mat foundation.

I am needing to prescribe the amount/size of dowels that will be used to secure this pedestal to my foundation.

I feel as though I should not be using ACI's requirement of 0.005*Ag in this case because that would yield roughly 130 in^2 worth of dowels (17.5*10.5*144*0.005).

Has anyone had some experience with this type of situation before? What should I be considering when prescribing the dowels area of steel needed. TIA!

 

[SlideRuleEra]

Assuming the pedestal is "thick", consider where the dowels should go. IMHO, with no dynamic loading, the perimeter of the pedestal is the only location dowels are needed.
Assume the dowels are located 3" inside the pedestal faces.
Dowel location perimeter is 17'+10'+17'+10' = 54'
Say, a dowel is located every 6" along the dowel perimeter - therefore 108 dowels are needed. 130 in² / 108 dowels = 1.2 in² / dowel. This gives #10 bars for dowels @ 6".

I would temper that math with engineering judgement. Consider the size and spacing of rebar used for the pedestal - dowels most likely should be similar size and spacing. So, maybe #6, #7, or #8 dowels with spacing to match pedestal rebar.

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[JStephen]

If you designed the pedestal and the slab below as one composite slab, then you'd need reinforcing for whatever shear force you calculate at the interface.

 

[WARose]

I don't know how tall the "pedestal" is (you made it sound like it's more like a mat, or perhaps a house keeping pad)......however, for vertical steel in such a situation (with "tall" pedestals) I've always considered a effective perimeter acting as a wall. (And reinforced as such.) Maybe about a foot of width. It depends on how much area you need for headed stud/dowel anchorage. (And also crack control.)

 

[KootK]

I feel as though I should not be using ACI's requirement of 0.005*Ag

I agree. The minimum reinforcing quantity originated as a means of preventing concrete creep from passively yielding the reinforcement in compression members. For that creep to occur, you'd need some minimum amount of compression stress in the concrete. With a 170 SF "pedestal", I doubt that you're there. Consequently, I don't feel that the ACI minimum applies here.

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.

 

[Billdings314]

@JStephen This equipment has little to no shear forces. 20kip self weight only.

@KootK thank you for shining some light on how the provision came about. I feel better about prescribing a lesser steel amount now.

@SlideRuleEra I like your comments on an effective area. This piece of equipment sits on a skid. The skid runs 2 main beams down the length of the equipment. I am thinking about considering the effective area as the area beneath the 2beams at a 45 deg angle down to the mat because more or less this would be the area which is being engaged.