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Large Concrete Tank Design

Large Concrete Tank Design

Large Concrete Tank Design

(OP)
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.  

RE: Large Concrete Tank Design

I would take unit plan x-section at a few feet interval, and analyze it as hollow rectangle structural frame with load intensity varies with depth. You can place uniform dummy (unit) load on it, then multiply depth to the results to obtain internal reactions at locations/elevations of interest.

RE: Large Concrete Tank Design

First of all, some editions of the PCA Tables have major errors (corrected with an innocent looking errata), that might affect your results.  Personally, I'd throw them away and get the Bureau of Reclamation Publication on the Design of Rectangular Plates.  Then get the largest aspect ratio case in the publication, a/b=1.5, and design the corners for that.  If the moments are large, they're large.

RE: Large Concrete Tank Design

(OP)
JD,

I have those tables also.  You say use the largest ratio a/b=1.5.  My a/b = 5.  How can you justify using 1.5.

RE: Large Concrete Tank Design

By the way, since the wall length is much greater than the height, if you have designed the wall as cantilever, then it would be excessive to place full load on the unit box frame again. I would use unit diplacement method (assume cantilever subjects to unit load) to estimate equivalent spring support at each frame level, then provide the spring supports along the box walls but at the corners. Any analysis program will be handy, however, the process can be lengthy and tedious. I think a 3D plate FEM maybe the best fit to take on this task.  

RE: Large Concrete Tank Design

If your tank is above ground, it will have tension in addition to bending at the corners.  That tension will likely be highest near the top of the tank, as well.  

In any case, the bending and shear are usually quite high at the corners.  Consider keeping the walls 3 feet thick at the corners, and stepping the thickness as you did from the base.

RE: Large Concrete Tank Design

Chamfering the corners combined with angled bars is a possible way of handling the large moment.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Large Concrete Tank Design

(OP)
miecz,

Yes, I think I will try going with 2ft thick wall at the corners.  I initiailly thought of doing this but was not sure what others have done.  I'm curious what has been done in the past to deal the the corner forces for tanks this large.

RE: Large Concrete Tank Design

You know, one of the assumptions of the tables from the  Bureau of Reclamation Publication is isotropy (See Method of analysis on sheet 3.)  I think the PCA tables make the same assumption.  I think that stepping the thickness of the walls violates that assumption.  I would add an extra factor of safety if using the tables.  

RE: Large Concrete Tank Design

Back to your question about six posts ago; The reason that using the corner forces from a/b=1.5 is a good approximation is that with that aspect ratio, the interior of the wall is not affected very much by the corners (and vice versa), similar to your case.  We commonly do this in our place.  
I did a tank a while back that was sized 400 ft. by 88 ft. by 20 ft. deep. The walls were tapered from 32 inches to 21 inches.  I used a cantilever approach plus analyzing the corners with the Bureau of Reclamation tables.  I needed #9's at 6 inches horizontals at the corners, with reinforcing actually reducing toward the base.
I wouldn't be shy about trusting the numbers and providing the thicknesses and reinforcing as above.  You'll only get one shot at this, and you don't want to lose sleep over it.

RE: Large Concrete Tank Design

(OP)
JD,

I see that you tapered your wall.  I am stepping the wall because I think it is easier to construct.  Do you see any disadvantages to stepping over tapering.   

RE: Large Concrete Tank Design

Technically neither one really meets the assumptions of the standard tools (PCA or Bureau of Reclamation), but I think if you're conservative with your approach you'll be fine. I've used stepped walls. If you want to be more precise, you can model the walls in STAAD or some other finite element program.  But that's a lot of work.
One other point to consider;  You're imposing construction joints at every step.  You'll have to dowel in reinforcing to meet the new wall thickness and put in waterstop.  With a tapered wall, the contactor can build gang forms, pour the wall full height and re-use the forms.  And you have a lot of wall. Even though they complain about tapered walls, they do have advantages.

RE: Large Concrete Tank Design

Is this an enclosed tank (with roof cover), or open top reservoir?

RE: Large Concrete Tank Design

(OP)
Open tank, no roof.

RE: Large Concrete Tank Design

If that is possible, you may provide a ring beam on top of the tank to increase out of plan stifness of the wall (preferably the beam width is wider than the top width of wall), thus, reduce the upper corner forces. I am still thinking a 3D plate model is best choice for your situation.  

RE: Large Concrete Tank Design

Any consideration to a round tank? Much more efficient design without the stress concentrations. We have been building round tanks from 75' - 200' in diameter for a number of years. Only to heights of 12'. Contractors have also been using their conventional forms with the additon of a spacer on the outside every so often.

RE: Large Concrete Tank Design

ARLORD,

You probably remember reading my post on a previous thread that you started, if not here it is with somemore info in it:

I designed an aeration basin matching these dimensions (146-ft long x 50-ft width x 27-ft height).  Since the L/H was greater the 2, for simplicity the wall will perform as a cantilever member, unless I restrict the top of wall.  The basin had an immediate wall running parallel with the 146-ft, so placed a 6-ft wide walkway (strut) cross all three walls, at the center of the structure (T'ing across the 50-ft dimension) to brace the wall at top.  I also used a 6-ft wide (acting as a beam framing into it strut) walkway around all perimeter walls.  If did not do this the required flexure strength would be like #10 at 6-inches on center, to much to close at the base.  The end result was a 30-inch thick wall at the bass with #9 dowels at 6-inches on center with vertical steel at 12-inches on center.  I stepped the wall down to 20-inches just past mid height.  I sized the wall base (thickness) for the shear load without the additional environmental load factor (concrete only, I did not use any steel for shear capacity) and the flexure steel as required by ACI 530 with the environmental load factor.  Be careful with the expansion and construction joints locations, I think I used 1-EJ and 3-CJs.  

Make sure you follow the requirements in ACI 530 for temperature reinforcement.  And double up on the temperature reinforcement as the base of the wall for at least 4D to 6D (height) to prevent vertical cracking as the wall cures.  The foundation for the basin was a 3-ft thick mat with piles at 10-ft on center each way.  
    
Just a reminder: The design of the crack control reinforcement in both directions is based on ACI 530 with unfactored loads.

What I worked on is not the same scenario as yours, but you could always create (3) walks crossing the 100-ft dimension, support them at (3) locations to provide yourself a modification of what I did and you must use STAAD to simplify analysis with the walkways.  This should reduce you moment requirement at the base, but you will still probably be at a 32 to 34-inch wall.

Hope this helps.

RMunoz III

RE: Large Concrete Tank Design

(OP)
RMunoz,

I don't have control to add walkways.  I have to live with the free top.  I have #11@6 on the interior and #10@6 on the exterior, vert bars at the base.

I am planning to use 1 Expansion Joint and Construction Joints at 40ft oc.  Did you continue the expansion Joint from the wall thru the mat foundation. Did you stop the reinforcing at your Expansion Joint.  I am assuming CJ stand for Control Joint.  Di you stop the reinforcing at that joint as well.   

RE: Large Concrete Tank Design

(OP)
RMunoz,

I see CJ is construction Joint.  I think you wanted to type ACI 350 and not 530 for the crack control design.

RE: Large Concrete Tank Design

(OP)
RMunoz,

Also, you mentioned crack control in both directions.  Did you have bending in both directions.  Did you use the actual calculated fs (aci 350 10.6.4.6), or did you use fs,max(aci 350 equation 10-4, section 10.6.4.1.  aci 350-06.

RE: Large Concrete Tank Design

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
 

RE: Large Concrete Tank Design

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.

 

RE: Large Concrete Tank Design

(OP)
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.

RE: Large Concrete Tank Design

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

RE: Large Concrete Tank Design

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
 

RE: Large Concrete Tank Design

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.

 

RE: Large Concrete Tank Design

JAE,
   Like always every contractor has there preference.
RMunoz
 

RE: Large Concrete Tank Design

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.

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