Shear reinforcement in underground tank wall 1

[Aakalim103]

I am designing a underground tank where the depth of the tank is 9m. I am getting have high shear at the vertical joint of the walls and it seems i would need some kind of shear reinforcement at the wall corner. But i am not sure if these results are realistic since i have never seen shear reinforcement in the walls which led me to think maybe there is something wrong with my model or calculations.

The tank is 9m deep with 400mm thick outer walls. It will be back-filled with soil and we have to consider the water table at the ground surface. The base slab is 600mm thick and resting on rock. I have made a FEM model, considering the wall to be fixed at base and top. (Top slab is 300mm thick). I have not applied any stiffness modifiers since we have to keep cracking at minimum for underground tanks. I know that the shear force will depend on the loading but is it reasonable to give shear reinforcement in such walls?

 

[HTURKAK]

If shear reinf. is necessary, the wall thickness and loading should be questioned. Apparently, the wall thick . 400 mm is not enough for 9.0 m ht. What about the loading and combinations? i feel ,the hydrostatic load is multiplied with a factor 1.6 ..If the GWL at ground surface, the tank will fill float when empty.. What is the permanent GWL?

Pls share more info. (size of the tank, GWL, soil cond's, loading and combinations)to get more valuable comments...

 

[strucbells]

I design a number of these structures and I have a few concerns based on your descriptions. As HTURKAK noted, the particulars really matter for these types of tanks.

I'm not sure where you're located (other than not the US given your units), so some of these may not apply (ie seismic considerations). We generally use ACI 350 and ACI 350.3 in the US for design of these types of structures.

Generally for my projects we consider a number of different loading scenarios for every underground tanks.

1. Full hydrostatic load with no soil backfill (this will be the condition for any leak testing that occurs during initial construction to prove the tank is watertight).
2. No hydrostatic load with full soil backfill (during construction or any maintenance work that will occur).
3. Hydrostatic, static & seismic soil and hydrodynamic suction on trailing wall with hydrostatic & hydrodynamic pressure and no static soil on leading wall.
4. No hydrostatic load with full soil load & seismic soil effects.

What are your overall dimensions of the tank? Is this a conventional reinforced concrete structure? Any intermediate walls or framing or just a 6 sided box?

Some quick napkin calcs for you to consider, assuming conventional construction and typical US approach.

1.4*2/3*(1/2*62.4 pcf *(27ft)^2) = 21.3 k/ft ultimate shear at the wall/slab joint due to hydrostatic load alone, assuming the tank is full. Maybe you have a couple of feet of freeboard and this is overly conservative.

For a ~15" wall..0.75*2*sqrt(4500psi)*12in*12.5in = 15.1 k/ft shear capacity at the wall/slab joint.

For hydrostatic loads alone, you're looking at ~1.40 DCR for shear through the wall near the wall/slab joint. Expect seismic load effects & soil loads (with water table at grade) to increase this further.

Typical approach is to design the concrete alone to resist the shear for walls loaded out of plane.

 

[r13]

Question:

1. Is this a circular tank or rectangular?
2. Did you try model the wall as two way plate element?
3. Don't you have net uplift problem?

 

[Aakalim103]

I know its very late now, but I talked to the project manager and we ended up increasing the thickness of the walls to avoid shear reinforcement. Thank you all for you answers.

I usually get a email notification when there is a reply to my thread but I guess for these replies I did not get any email notification, so I almost forgot about this thread. Besides, shortly after I designed that tank, I was laid off, apparently due to the worsening financial situation caused by covid19 pandemic. Got a new job recently and came to this forum for some advice and that's when I realized I have unread replies.

Thanks again for the help.

 

[r13]

At least you come back and acknowledge the email notices. Good luck on your new job.