vertical vessel foundation design

huangyhg

vertical vessel foundation design
i am designing the foundation for a vertical storage tank.  it's a sizeable tank.  i am being asked to design a foundation where the tank is supported directly on the pile cap? i am not sure i understand the function of the pedestal separate from the footing/pile cap.  all designs i have done for vertical vessels included an octagonal pedestal with a sqaure or octagonal footing.  
additionally, what is the reason for not extending the anchor bolts into the footing?

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not enough information.
the tank is 45ft tall with an od = 11'-11".  it is being designed for an oil and gas plant in wyoming.  i am not sure what the design wind speed or eq loads are yet.  it will be supported on either steel h piles or helical piles.  
   
i mean the details at the foundation level.  what anchor bolts are you talking about?  a sketch would help.  pedestal?  pile cap?  what do they look like?
it hasnt been designed yet.  i have been asked to design the foundation without a pedestal.  so i am trying to understand the use of a pedestal versus designing a foundation without a pedestal.  i understand how to design the foundation.  i do not know how to make a professional assesment to design a foundtion with a pedestal or to design a foundation without a pedestal.  
the pip standard for vertical vessel foundation design includes a statement about it being preferrable to design the pedestal deep enough to contain the anchor bolts and to keep them out of the footing.  that was my question in regards to anchor bolts.   
if the soil is good and/or overturning loads are small, the footing may need to be only the size of the tank (perhaps about 14' square, circular, or octagonal in your example).
if the soil is poor and/or the overturning loads are high, the footing may need to be significantly larger than the tank. in that case, a pedestal (the size of the tank) resting on the footing would be a more efficient design. if the footing is much wider than the tank's diameter, footing thickness may need to be increased to ensure that foundation is stiff enough for loads to be uniformly distributed to the soil/piles.
perhaps other reasons, but i have used the above criteria often.
a couple of reasons to keep the anchor bolts out of the footing in the case you describe are to simplify the placement and to allow shorter anchor bolts.  you need to hold a fairly tight tolerance on bolt placement at the top, and that gets harder if the bolts are sticking 10' up in the air.  it might also help avoid interference at the bottom.
a lot of foundations for tanks of those dimensions will just be single slabs of whatever diameter required.  a pedestal would be used where there is a significant savings in concrete over a single slab of the same total thickness.  but it complicates design, detailing, and forming.
usually, if the tank is supported on the mat or pile cap, i place the anchor rods directly in the concrete.  i use sleeved anchors to allow a bit of flexibility in placement and call for the sleeves to be grouted after placement.  
if the tank is supported by legs, i put the rods in a pedestal to keep the anchor rods out of muck.
one more thing you need to consider is containment.  if the vessel ruptures, your walls have to be high enough to contain the full volume of the tanks contents.
as far as being preferable to place the rods in a pedestal instead of the footing, think about the accuracy required for placing the rods in a large mat with little tolerance for error versus placing the rods in a much smaller pedestal with the same tolerance for error.
thank you for the input.  this answers my questions and will be very helpful for my design.  
a pedestal isn't inherrently required.  even when present, it is not uncommon for the anchor rods to extend down into the foundation slab.
the main reasons why i can see to avoid the placement of the anchor rods in the slab are related to constructability.  i agree with most of what chipb says regarding the use of sleeved anchor and the tolerance issues with placement of the rods.
one other thing about the present of a pedestal.... it increases the punching shear perimeter of the vessel.  probably more of an issue with a tall vessel rather than a short / squat tank.  but, worth mentioning.  
josh