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Re: Min. Reinforcement in Pedestals

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Mr. A. Karimzadegan,

	Having read most of the other correspondence related to your question I
am given to understand that you have a large diameter pedestal on a
foundation.  Presumably this is to support a tank or a large diameter
process vessel which requires a footing larger than the pedestal.  If it
were some rectangular equipment I would expect the pedestal to conform
to the shape of the equipment; however, this is not important.

	The pedestal should contain enough reinforcing to resist any tensile or
bending loading; in addition, the surface should have sufficient
reinforcement to resist cracking due to sudden changes in surface
temperature (actually, sudden reductions in temperature which would put
the surface in tension).

	I haven't calculated the surface reinforcing requirement for years. 
Once upon a time, the first time I was faced with the problem, I did do
some calculations.  As I recall we eventually decided to use the code
requirement for minimum (temperature) reinforcing in slabs, 0.002*Ag,
and assuming a 12" (300 mm) "slab" for the surface of the pedestal.  A
quick run through the arithmetic indicates about 0.3 square inches per
foot (600 square millimeters per meter); which  equates to #5 (15 mm)
bars at 12" (300 mm).  This is, admittedly, a bit arbitrary; but every
engineer I know does it this way and none of us has had any problems
using this as a minimum for surface reinforcing.

	If you really want to do some calculations you might estimate the
largest temperature drop (for the surface) that can occur in a one hour
period and calculate the tensile stresses using the formula

	Stress = E*Alpha*Delta(T)

and assume all of this temperature drop reduces to zero 12 inches below
the surface.  Doing that (using english units) you get something like

	E = 3,000,000
	Alpha = 0.000,006,5 / degree Fahrenheit
	Delta(T) = 20 degrees Fahrenheit in the air (assumed) 
(note that the air-to-concrete boundary layer will reduce Delta(T) in
the concrete to about 20% or 4 degrees Fahrenheit.

This results in 78 p.s.i. Tension at the surface or 5616 pounds per foot
(in each direction, of course) for the surface 12".  This would amount
to about 15,000 p.s.i. in a #5 bar if the bars resisted ALL of the
tensile stress.  Although I would normally use #5 bars at 12" I did
suggest that you use #6 bars because I didn't know (and I still don't)
all the details of your problem.

	The above example is, admittedly, also a bit arbitrary; if you really
want to get exotic you might do a full blown heat flow analysis (even
using finite element analysis if you want to but that is getting to be
absurd) to determine the actual concrete temperatures.  I doubt,
however, that you will want to do this more than once in your lifetime
for foundations.  I actually did this as part of my Master's thesis on
"Hoop Stress Due to Ice in a Circular Tank"  One of the difficulties I
found was in modeling the air boundary layer; I found this boundary
layer had approximately the same insulating properties as a 16
centimeter thick layer of ice.

	You also have other reinforcing requirements.  If you have anchor bolts
with significant tensile loads you should ensure that the load path from
the anchor bolt to the concrete to the reinforcing to the footing is
adequate.  You must also provide between the anchor bolts and the edge
to prevent a "break out" of the anchor bolt at the edge.

	I hope this is sufficient for your needs.

				Regards,

				H. Daryl Richardson

Karimzadegan wrote:
> 
> Dear Richardson
> 
> Your suggested reinforcement seems reasonable to me also, but if there is no
> reference or mathematical logic support the idea, then any other
> reinforcement could also be used. So please let me know if you have such
> supports for your suggestion.
> 
> Best Regards,
> A. Karimzadegan
> 
> ----- Original Message -----
> From: "Daryl Richardson" <h.d.richardson(--nospam--at)shaw.ca>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Wednesday, December 26, 2001 10:01 PM
> Subject: Re: Min. Reinforcement in Pedestals
> 
> > Karimzadegan,
> >
> > Unless you can find some code requirement to the contrary or some
> > loading that requires greater reinforcing I would be inclined to use #6
> > (20 mm diameter) bars at 12" (300 mm) each way on all exposed surfaces.
> >
> > Regards,
> >
> > H. Daryl Richardson
> >
> > > Karimzadegan wrote:
> > >
> > > I have a question regarding the Min. reinforcement which we shall use
> > > for the large pedestals. The question is the for the pedestals under a
> > > large equipments (e.g. 6~7m in Dia. ) we need to construct a pedestal
> > > to reach to the required elevation ( e.g. from -1.000m below ground to
> > > 0.300 above ground with 6m dia. pedestal ). It seems that application
> > > of about 1% or even 0.5% reinforcement is very large and unnecessary
> > > and in these cases the pedestal action is more close to a foundation (
> > > for which the main reinforcements shall be top and bot. bars instead
> > > of vertical bars ) instead of column action. If anyone have any useful
> > > suggestion in this regard, please let me know.
> > >
> > >
> > >
> > > Best Regards,
> > > A. Karimzadegan
> > > PIDEC Co.
> > > Shiraz
> > > Iran
> >
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