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Re: Sanitary Structures

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Chris:

I assume that you are doing this by ACI 350. To get about the same resteel that we used to get by WSD, they introduced the addtional 1.3 factor that gets you pretty close on the first time around. I developed some tables years ago that I still use today which give me the capacity and the Z factor  at full stress level for different slab / wall thicknesses, bar sizes, covers and spacings. All I have to do is proportion the req'd Z-factor (95 or 115)  by 1.3 and the ratio of req'd capacity to the actual capacity. You then just pick the bar and spacing from the chart. This method is very fast.

If I have something unusal that is too difficult to analyze by hand, I use Visual Analysis / Design to model it by finite element and let the computer design everything including checking working stress levels and Z-factors.

I hope this answer is helpful to you.
 

Jim Kestner, P.E.
Green Bay, Wi.

Chris Towne wrote:

 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.
 

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