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RE: Sanitary Structures

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Chris,
 
You are on the right track.  Stay in strength design and make the checks. 
The load factors for tanks are higher, and it helps to be consistent.  You
did touch on why working stress stayed around so long with the environmental
engineers.
 
Give yourself a break and create a MathCad template.  It lets you play
around with the steel spacing and sizes to zero in on the optimal design
very quickly.  And your calculations can be checked by non-computer guys.
 
I designed in working stress and "ultimate strength" design for several
years, and I did it with hand calculations & slide rule (in the real old
days).  I developed a pile of eraser crumbs on my lap as I optimized. 
MathCad keeps the eraser crumbs in the CRT.  
 
Once the loads are identified it only takes about 10 minutes to design and
optimize the steel even checking stresses, the Z factor, and deflections
using the MathCad (about 4 pages of calculations).
 
If you do a couple of tanks, you have paid for the software.
 
Regards,
Harold Sprague
The Neenan Company 
 

-----Original Message-----
From: Chris Towne [mailto:ctowne(--nospam--at)newwave.net]
Sent: Wednesday, February 10, 1999 2:28 PM
To: SEAINT List Server
Subject: Sanitary Structures


I do a lot of concrete tank design and I was wondering how other engineers
tackle the problem of cracking.  The process I go through is vigorous and
time consuming, but I feel it is necessary.
 
Step 1:  Find Steel that satisfies phiMn >= Mu
 
Step 2:  Find the actual steel stress (unfactored)
 
Step 3:  Find the maximum spacing for that steel stress and bar size that
satisfies Z <= 115 k/in.
 
Step 4:  If that spacing is less than I found in Step 1, I shorten my
spacing and perhaps change
            bar size and go to Step 3.  I repeat steps 3 and 4 until I have
the most economical section
            that satisfies both strength and cracking.
 
I've found out that most of the time if the steel stress is under about 24
ksi, cracking is ok.  I was wondering if others just use working stress
design and make the whole process simpler.  I was taught using ultimate
strength and I feel the need to check it even though it never governs.
 
Chris Towne, E.I.T.