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Hexagonal Foundation Pedestal for Vessels, Stacks, Etc.

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This is something that has bothered me for years now, and I wonder if anyone 
 here, who might have some experience with design of foundations for process 
 equipment, has an opinion.

Typically, you have a shallow foundation, or pile cap with deep (pile) 
foundations, one which a vessel or stack is situated. The bulk of the concrete 
 is, of course, simply for the purpose of placing the equipment. The foundation 
 or cap is typically octagonal in plan.

Often, you will see an octagonal “pedestal” on top of a largre octagonal 
 (or rarely, square) foundation. The typical way it’s detailed is the 
foundation has mats of rebar as if it were a slab, and the octagonal 
“pedestal” has verticals as if it were a very large column. This always 
 seemed kind of silly to me. What makes the pedestal “know” that it’s 
 supposed to be a compressive element and the foundation underneath “know” 
 it’s supposed to be flexural?

Typically, the “pedestal” will have vertical reinforcement with “hoops” 
 of what are supposed to be ties. It always seems to me to be a case of 
trying to force some sort of paradigm of a compressive column/pedestal onto 
what’s more likely just a large haunch.

I have an example in front of me with a “pedestal” that would enclose a 
 circle of 15’-3” diameter. The total area of the pedestal is about 
28,000 square inches. If I were to take provide the minimum 0.5% of steel 
area to concrete area of the pedestal in accordance with ACI 318-11 10.8.4 
& 10.9.1, that would be equivalent to #8 bars at 3.5” all around the 
perimeter. This is just nuts.

Add to that, you should actually have hooked ties every other bar developed 
 into the “pedestal” if you were to abide by the letter of ACI 318 7.10.5. 
 Even more nuts.

It just seems to me that this “typical” detail is a hand-wave that someone 
 came up with long years ago, and people just started following out of habit. 

I suppose one proper way to analyze this would be some sort of strut-and-tie 
 mechanism (which I guess you could do as “2D” since it’s a radial 
arrangement, but I’m not expert in strut-and-tie). You could also do a FEM 
 analysis to try to figure out what’s going on in that mass.

OR…you could provide enough area of steel across the interface between the 
 foundation and pedestal to make sure you’ve got direct shear taken care 
of, and otherwise fuggedabaddit. Maybe some polyethylene fiber reinforcement, 
 and make sure the concrete mix is sufficient for the mass of the concrete 
(in this case, about 22 cu. Yard pour. Not that bad.

Any thoughts?

William L. Polhemus, Jr. P.E.


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