Deep Concrete Tank Wall Design for Shear 1

[mhijazi]

I am working a 40'-deep underground rectangular concrete tank. Due to soil and Hydrostatic pressure loading, the wall thickness is becoming over 5 feet near the bottom. Any ideas to reduce the wall thickness..?
Can I use additional vertical reinforcement to help withstand the shear...?
Thank You.

 

[waytsh]

You can consider the effect of your vertical reinforcement by using equation 11-5 in ACI 318-05. Just remember that if you are an environmental structure you will then need to multiply your shear load by the durability factor as defined in ACI 350 since you are now using the steel to resist the shear force. You can also increase the shear capacity by considering any axial compression load you have in the wall by using Eq. 11-7. Hope this helps.

 

[mhijazi]

Thank you, I will check if this helps. Some people suggested using Shear Friction reinforcement, but I was not sure about that..!

 

[waytsh]

I have never used shear friction but I have used "U" or "Z" stirrups typically at the same spacing horizontally as the vertical reinforcing and then spaced vertically per Eq. 11-15 or the minimum spacing requirements. Whichever controls. I suppose you have considered increasing the compressive strength of your concrete as well?

 

[mhijazi]

Yes I did (5,000 psi). Thank you again.

 

[NS4U]

you could taper the wall to reduce the amount of concrete

 

[NS4U]

I would recommend analyzing your wall using MCFT.... with a wall that thick the crack with will be so large you will lose aggregate interlock and will likely not get the 2*sqrt(f'c) aci allows.

Adding shear reinforcement is a very good idea... it's not ideal for construction, but it keeps your cracks small, and will allow you to get a thinner wall... coupled with the fact that your shear will drop off rapidly the higher up the wall you go... I strongly suggesting adding some amount of reinforcement.

 

[JedClampett]

I'd just make the walls 5'-0" thick, if that's what your calculations show. I've used shear reinforcing and tapering walls, but in the end once you have a certain thickness wall, the increases are pretty cheap, just the concrete cost.
For a 40'-0" deep tank, I'm not surprised that the walls are thick. By the way are you using two way action?

 

[PanamaStrEng]

Would the tank configuration and use allow you to use mid-height struts between walls? (Like what you would do to brace a deep excavation, but permanently).....

 

[RMunoz1361]

Some of underground concrete structures are controled in the design by uplift (floating) if ground water is an issue.
And thick walls and slabs may be needed.

 

[miecz]

Are you taking yur critical section for shear at the bottom of the wall or at a distance "d" from the bottom?

 

[jike]

Are you taking advantage of the compressive load in the walls in determining the Vc (Equations 11-5,6 & 7)? In this case Nu = positive for compressive load.

 

[NS4U]

jike, I don't think that will add anything significant, for NWC you'll get a compressive stress of about 42 psi at the base of wall... which is next to nothing

looking at Eq 11-4, you need to be closer to a 1,000 psi compressive load to even get your Vc to 3*sqrt(f'c)

 

[mhijazi]

Thanks to all of you for the great comments and tips. Below are some answers to the questions came up:
- The walls will be stepped up may be three times (Tapering may cost more)
- Internal Bracings are included the design
- We are looking at the shear at the bottom of the Wall not at a Distance d from it. I would like to leave this in the back pocket (last resort) if you will
- The structure is modeled in Risa -3d as a plate Element Model.
- Tieback system is not feasible for Easement issues.

Adding reinforcement at the bottom beyond what is required for flexural steel may change the mode of Failure of the section (i.e. we tend to design for Rho max to insure steel yielding first).

 

[miecz]

Have you calculated the difference in wall thickness by checking the shear at "d"? I'm all for being conservative (see the 3% overstress thread), but this can make for a significant cost savings.

Also, there are some hidden costs in stepping your walls; custom formwork, rebar bends, handling, shop drawing prep and review, etc. I once did a cost comparison for 50`x120'x24' tank, at the suggestion of the esteemed value engineer, and decided to stick with a constant wall thickness. As JedClampett pointed out, your comparing stepping costs to the cost of concrete material, not your unit wall cost.

I've modeled with RISA, but I've never had 5 foot thick plates. I'm curious, what size mesh does one use for a 5 foot thick element?

 

[mhijazi]

I will check shear at a distance d.
As far as Risa, our model has 2'x2' mesh.

 

[civilperson]

Try modeling the walls as fixed at the bottom and at the two sides with triangular loading. The corners of wall junctions can easily be fixed ends with corresponding maximum moments. Shear maximum for design is at "d" above the bottom support. If the horizontal dimensions of the walls are greater than 20 feet, try adding either counterforts or buttresses at midspan of the wall. 5 feet thick is a bit strong but still buildable if needed for neutral buoyancy.