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# Rect Water Storage Tank

## Rect Water Storage Tank

(OP)
I am looking at a design of a water storage tank.  The tank is not under pressure, so my design approach will be to accommodate only the hydrostatic pressure.  This tank is rectangular due to installation constraints.  The dimensions are 10'X4'-6"x18'tall.  I am having a hangup on the pressure at the bottom in relation to the design of the external stiffeners.  It seems to be too conservative for the external stiffeners (horizontal)to take the full bending load from the internal pressure at the base without taking advantage of the plate that makes up the shell.  How much of the shell contributes to the "top flange" of the stiffener for the bending stress.  Any help would be appreciated.

### RE: Rect Water Storage Tank

ag89:

To design the top angle of rectangular tanks I would use the following formula:

w=(0.036GH^2)/2  w=load per unit length; G=Spec. Grav. of fluid, and H=Tank height.

Once w is known the reaction at the top of the tank, R1, is taken as .3w (R2 at bottom is .7w).

Once R1 is known, the minimum moment of inertia, I, can be found by:  I=[R1(L^4)]/[192Et^2];  where L=Length of tank, E=Mod. of Elas.; and t=wall thickness;  be sure to watch your units.

Once the top angle is found, external vertical and horizontal stiffeners can be sized.  I would generally design the vertical stiffeners to carry the primary load, (or primary supports) and the horizontal stiffeners to carry the secondary load, (or secondary supports).  Design the vertical stiffeners to carry the load from 0 psi at the top to the increasing loading of 7.81 psi (for water at 18'), at the bottom.  Then the design of the horizontal stiffeners can be designed by utilizing the pressure seen by the stiffener at the required height on the tank.  If you are wanting to utilize a portion of the shell, I would use up to 4t, from the edge of the stiffener.

For max stress path into the foundation, be sure to weld the supports together at their intersections, as well as into the top angle and into the bottom chime of the tank.

You can utilize Roarks to design the shell by using the tables in the book.  From the 6th edition, I would use Table 26, Case Number 8d, 9d, or 10d.

Creating a spreadsheet for a rectangular tank would be an excellent idea, as many iterations are sometimes needed to find the right shell thickness, top angle, and the number and placement of vertical and horizontal stiffeners.  It has saved me many a number of hours.

### RE: Rect Water Storage Tank

(OP)
Just my initial thought about the stiffeners, I would have thought the stiffeners would be configured for the minimum length to support, i.e. the horizontal stiffeners will be supporting 10' of tank shell, where as the vertical stiffeners would be supporting 18' of tank shell.

### RE: Rect Water Storage Tank

I have a worked example of a open top rectangular tank with continuous horizontal wall stiffeners.  Itâ€™s similar to ASME 8 Appendix 13.  I can E-mail you a copy in .pdf format

Post@gowelding.com
www.gowelding.com

### RE: Rect Water Storage Tank

Concerning your specific question, BS5500 in App.G, when dealing with stiffening rings for supports, gives a total contributing length of shell equal to 10t+t2, where t is the shell thickness and t2 is the ring thickness (or width).

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

### RE: Rect Water Storage Tank

(OP)
welding1,

I have sent you an email requesting the example of the  open top rectangular tank with continuous horizontal wall stiffeners.  If you can still send it to me, I'd appreciate it.  Thanks

john.riley@encompasstx.com

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