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RE: Circular Concrete Tanks without Prestressing

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David,

The main reason for the change was the move from Working Stress Design to
Limit State Design (originally called Ultimate Strength Design).  The 1960's
version predates my oldest which is 1972, but the concept was basically the
same.  

The old 1960's version kept you on the very low end of the allowables for
rebar in a round about way to control cracking.  Once the cracking
calculations were included (Z calcs) and the load factors were modified, the
ACI 350 and the PCA, "Circular Concrete Tanks without Prestressing" came up
with basically the same leak proof tank design.

Be diligent with your seismic loads.  You should be using the AWWA D100 to
develop the impulsive and convective fluid forces.  I would advise you to
look at the appendix of Chapter 14 of the 1997 NEHRP to see how the new
NEHRP / IBC mapped values effect the impulsive and convective fluid forces.
The difference can be significant.  It would be a shame to design a tank
today that would require a rehabilitation when the new code comes out later
this year.

Regards,
Harold Sprague

> -----Original Message-----
> From:	David Hall [SMTP:Dahl(--nospam--at)deainc.com]
> Sent:	Tuesday, January 25, 2000 6:12 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Circular Concrete Tanks without Prestressing
> 
> Our firm has designed a circular concrete tank to be constructed in
> northern Washington.  The design was done under the above listed
> publication.  However, this design was done from what I am now told a
> 1960's version of this publication.  I have a copy of the 1993 version.
> 
> Anyway, the base slab was detailed as being placed in segments (8 total)
> opposite of each other within the tank with expansion joints separating
> these pours and an 18 " wide by 12" reinforced continuous footing below
> this joint to prevent differential sentiment which would cause leaks.
> This is the recommended method prescribed in the 1960's version.
> 
> The 1993 version as eliminated this method and recommends using ACI 350R
> with consideration given for the "z" factor to reduce cracking.
> 
> Can anyone tell me why the 1960's method was removed from the code.  I am
> providing the final QM review and the engineer who designed the tank
> insists his design is the way to go.
> 
> Also, that tank is above grade and seismic considerations were accounted
> for.  Plans are going out to bid shortly.
> 
> Thank you for your assistance.
> 
> David A. Hall, S.E., P.E.
>