Large Concrete Tank Design 1

[ARLORD]

I am designing a concrete tank, 280ft x 100ft x 30ft deep, containing water. The b/a and c/a ratios are larger than tabulated PCA design guides. Therefore, I designed the walls as a retaining wall structure, no problem. 3ft thick at base for 12ft first of height, 2ft thick for next 12ft, 15" thick at top for 6ft.

My question is what to do at the corners. How should I design the horizontal reinforcing at the corners. Using the PCA tables for horzontal moments, the maximum moment is near the top, and require way too much reinforcing.

What is the best approach for the corner horizontal wall reinforcing design.

 

[RMunoz1361]

Arlord,
Yes, you would have to stop the reinforcing at the expansion joint through the slab plus wall and use some smooth bars to transfer the shear, etc... As for the crack control you would check for bending in both directions. Check the Mx-bending near the base wall at the center of the structure and long the corners of the structure for My. I have ACI 350-06 but decided to use ACI 350-01 with my calculated fs since I thought it would be easier that way.

RMunoz

 

[JAE]

RMunoz1361 -
What shear were you transferring across your expansion joints? Not sure there is any unless you had differential settlements.

Also - In speaking with contractors - stepping the wall form is more expensive and difficult. Most that I've spoken with prefer the tapered gang form method.

 

[ARLORD]

Is designing a tapered wall any different from a straight wall. If the tensile reinforcement is on an angle do you make any adjustments or design as usual.

 

[JAE]

The bending moment on section of wall is simply the same - b, d, f'c, etc. I design as usual.

 

[RMunoz1361]

JAE,
I have heard opposite of what you have heard from contractors and being a former one myself. Installing tapered forms requires more labor such as special detailing at corners, since these corners bars will vary in length (unless you make some standard sizes). The only downside with stepped construction will be an increase on project cost for the extra concrete you will use.

Shear transfer at an expansion joint would depend on its location to a point of fixity in the perpendicular direction. For example, I had an elevated walkway that linked (3) walls together and was within 6-ft of an expansion joint and had to consider some shear transfer to its relative stiffness at that given location.

These tapered forms you used, were they for a circular to rectangle/square structure?

RMunoz

 

[JAE]

Rmunoz,

A good friend of mine (a superintendent for a large contractor) was the one who told me to use tapered over stepped (for a rectangular tank). With stepped, they have to use separate sets of wall forms, set at the exact height of the step. Also, separate sets of bar mats - interrupted at the step vs. a simple sloping single mat of steel.

 

[RMunoz1361]

JAE,
Like always every contractor has there preference.
RMunoz

 

[haceng]

Hi - I'm from the UK and I've been designing using the Moody Charts for over 20 years and have also designed many of these tanks uaing FEA and Grillage. With a very large open tank without ties,the corner moments can be up to 2 /3 of the vertical free cantilever moments. This can be proved quite conviningly by analysing a simple panel(I use Staad Pro). The previous suggestion of using the max ratio in the Moody charts (a / h) = 1.5 i.e. Length / Height of 3 is a reasonable starting point (as it is the limit of the charts). The horizontal moments do increase as L/H increases beyond that but to a lesser and lesser degree. The point of contraflexure does not shift significantly either so the problem is quite local. You will have the problem with combined moment, tension and shear. (See my topic posted today on this - I only joined tody)

It is very unwise to design these tanks as cantilevers with nominal horizontal bars at the corner because it will crack there seriously and leak.

A good way of dealing with the large requirements for reinforcement is to bundle bars at the corner. If your tank is very large and the walls are pure cantilevers it is worthwhile tapering the walls and putting splays in the corner (as suggested eleswhere) the contractor won't like it but it is a small sacrifice in the context of the size of the tank and deals with a very local problem efficiently. It will also help you with shear as the critical point will be the end of the splay.