Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Min. Reinforcement in Pedestals

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dear Richardson

Thank you for your response. It helps me a lot.

Best Regards,
Ali Karimzadegan

----- Original Message -----
From: "Daryl Richardson" <h.d.richardson(--nospam--at)shaw.ca>
To: <seaint(--nospam--at)seaint.org>
Sent: Sunday, December 30, 2001 12:15 AM
Subject: Re: Min. Reinforcement in Pedestals


> 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
> > >
> > > ******* ****** ******* ******** ******* ******* ******* ***
> > > *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> > > *
> > > *   This email was sent to you via Structural Engineers
> > > *   Association of Southern California (SEAOSC) server. To
> > > *   subscribe (no fee) or UnSubscribe, please go to:
> > > *
> > > *   http://www.seaint.org/sealist1.asp
> > > *
> > > *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> > > *   send to the list is public domain and may be re-posted
> > > *   without your permission. Make sure you visit our web
> > > *   site at: http://www.seaint.org
> > > ******* ****** ****** ****** ******* ****** ****** ********
> >
> > ******* ****** ******* ******** ******* ******* ******* ***
> > *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> > *
> > *   This email was sent to you via Structural Engineers
> > *   Association of Southern California (SEAOSC) server. To
> > *   subscribe (no fee) or UnSubscribe, please go to:
> > *
> > *   http://www.seaint.org/sealist1.asp
> > *
> > *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> > *   send to the list is public domain and may be re-posted
> > *   without your permission. Make sure you visit our web
> > *   site at: http://www.seaint.org
> > ******* ****** ****** ****** ******* ****** ****** ********
>
> ******* ****** ******* ******** ******* ******* ******* ***
> *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> *
> *   This email was sent to you via Structural Engineers
> *   Association of Southern California (SEAOSC) server. To
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> *   send to the list is public domain and may be re-posted
> *   without your permission. Make sure you visit our web
> *   site at: http://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********