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Re: Sanitary Structures

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My company works almost exclusivly in the field of water and wastewater plant design.  I understand your frustration.  The approach that I use is to just use working stress design.  I go one step farther and adjust my allowable steel stress, fs, based on the reinforcing spacing and max Z factor.  For example, if I'm designing a wall for a wastewater basin and I think that #7 @ 12" will work, I use an allowable design stress of 17.88 KSI.

On several occassions I've tried to design using the Load Factor provisions of ACI 350 but almost without fail, I need to redesign the steel after checking service load stresses for crack control.

I do use the LFD provisions for any load case with seismic loads.

Kipp Martin, P. E.
Portland, Oregon

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 >=3D 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 <=3D 115 k/in.
>
>Step 4:  If that spacing is less than I found in Step 1, I shorten my =
>spacing and perhaps changr 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.