Large Concrete Tank Design 2

[ARLORD]

I am looking for advice from those of you who have experience designing large concrete water retaining structure. I am familiar and have used the procedures presented by PCA Circular and Rectangular Concrete Retaining Structures. However, they only provide design guidelines for walls with length-to-height ratios of 4 and below.

I have a concrete tank that has a L/h ratio of 9. There are 4 walls, with the longest being more than 250 feet long. Do you treat this as a retaining wall. The wall will be about 30 feet high. Preliminarily I will need a 3ft slab, 3ft wide wall at base. My two wall loading conditions are (1) Tank full, no soil outside, and (2) Tank empty, soli full ht outside.

 

[jike]

For a long, 30 foot high wall, I would look for a way to reduce the 3 foot wall thickness. I might try something like pilasters on the backside of each wall with horizontal tie beams across the top of the structure spanning between pilasters.

 

[jike]

You could even add horizontal beams on top of the wall spanning between pilasters. The wall could then span 2 ways (1)vertical: base to horizontal beam and (2)horizontal: pilaster to pilaster.

 

[ARLORD]

I can taper the wall so that it is not 3ft for the full height. I just wanted insite on the overall wall-mat design. Is it common practice to treat as a "retaining" wall type problem assuming an arbtrary heel length, maybe 25feet in this case. Or is there another method or procedure that I am not aware of.

 

[JedClampett]

First, 30 ft. is a honking tall tank wall.
If you have sufficient slab support on the top, you can design the wall as a propped cantilever. Otherwise, you design it as a retaining wall. And you will need a 3 ft. thick wall at the base. Be careful at the corners, as the wall will be additionally supported there and shears and moments will be high there. You'll also need a very thick slab at the wall, probably 39 inches thick.
For 250 ft., you'll need expansion joints too.
As far as jikes' suggestions, I've tried them all and usually just end up with a thick wall. Contractors find it easier to build and understand.

 

[ARLORD]

JC,

Yes I agree, it's a big wall. There is no cover at the top to act as a support/brace. It is a full height cantilever.

Since the wall is loaded from both sides, is it wise to taper the wall, or is it common to do so in this application, I guess I can design for the smallest d within the height of the taper.

 

[lexim]

Designing the structure with pilaster columns within the tank will be a very Safe option, I will rather add tie beams probably with gratings and walkway to connect the columns.
On the wall design, the wall will have hoop stresses (compression and Tesnsion for load cases) better to modelthe structure in structural analysis software to determine the resultant stresses.

This will be by iteration with thickness of wall being a variable and the rebars quantities in the horizontal direction too. (It is what is prescribed in API 650 for steel tanks for petroleum products.)

 

[JedClampett]

Tapering the wall saves some concrete, but makes the wall harder to form and pour. Don't forget, they need to get their stingers in a narrower opening at the top of the wall than the place where the vibration is needed. And if the wall is done in a single lift, that's going to be a challenge regardless.
Some contractors hate tapered walls, some don't. I've done it before and not on other walls, so I'm pretty ambivalent. I'd only taper one side to make the reinforcing just a tad easier.
Make sure your footings are designed for sliding and overturning both ways (just soil and just water) just like your foundation textbook tells you.

 

[jike]

Another option is to use butresses/counterforts.

 

[blackmaddog]

From left field, outside the box ... Can you connect the apposing walls with struts or partial height walls? The pressure on the opposing walls is equal and opposite so a strut will transfer the loads in compression or tension and greatly reduce your bending moments.

 

[hokie66]

I agree with everything Jed said. Keep it simple.

 

[rapt]

Round is much more efficient!!

 

[VikingEngineer]

It sounds like this wall may be used in something like a sewerage works or water reservoir. Have you considered using an earth bund over part of the height, if this is well engineered and compacted, it may be able to support 45 degree sloping ground slabs. The balance of the wall could be a vertical cantilever but it may need to rest on piles. The earlier thread about expansion / contraction joints is important.

Careful design and the use of good soils is vital.

 

[RMunoz1361]

ARLORD,
I recently designed an aeration basin matching those 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 with #9 dowels at 6-inches on center with vertical steel at 12-inches on center. 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 horizontal 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.

 

[hokie66]

RMunoz,
Don't you mean vertical cracking near the base? The base slab has already shrunk, now the wall wants to shrink but is restrained, so it cracks vertically.

 

[ARLORD]

RMunoz1361,

It sounds like you designed the 146' long wall with a support at the top. What did you do with the 50' wall. If the 50' wall was designed as a cantilever retaining wall how thick was the wall full height, 3' thick the full height?

 

[RMunoz1361]

hokie66,
You got what I was trying to saying about doubling the horizontal reinforcement near the base to reduce the vertical cracking due to the wall wanting to shrink while the vertical dowels are being restrained by the already cured foundation/mat.


ARLORD,
I placed a 6-feet wide walkway around the entire structure to pin the top of walls to reduce the load requirement near the base.
Are you also the ARLORD on

http://www.engineerboards.com/?

RMunoz1361 or RMunozIII

 

[ARLORD]

RMunoz,

Thanks, and yes I am a member of both forums.