RC Water Tank (square) - base/wall moment transfer

[Sense Check Me]

Buried RC Tank - 9m x 9m (square) x 6m deep, plus 1.5m nib. Simplified the geometry for discussion purposes.

I've considered two loads cases; 1) tank full and no backfill, and 2) tank empty with backfill in place.

The walls are designed as two-way spanning 'panels' (using boundary conditions of top = free, other 3 edges = fixed) using published tables.

The base is designed as two-way spanning (with 4 continuous edges, accounting for the benefit of the external nib). The bending moments determined for the hydrostatic pressure acting on it's underside.

However!!!!....My question is.....how is the wall/base detail treated? Due I need to design the whole base slab for the vertical moments applied from the wall panels note: fixed at base)? The vertical moments from the walls are significantly higher than those in the base slab due to hydrostatic pressure.

Please could people enlighten me?

 

[KootK]

Due I need to design the whole base slab for the vertical moments applied from the wall panels note: fixed at base)?

I would think so yes. Stuff like serviceability and crack control aside, once wants to be telling a story of wall equilibrium that is consistent with base slab loading.

 

[Sense Check Me]

Let's take the case for the tank full. For equilibrium, would the downward component of the internal contents cause a relieving effect to the base slabs bending moments?

Similarly, for the backfill case - would the weight of the soil acting on the nib cause a reduction in the base slab bending moments?

Due to the size of the structure (it's actually several of them) it would be advantageous to optimise the reinforcement (for both commercial and sustainability reasons).

 

[KootK]

Let's take the case for the tank full. For equilibrium, would the downward component of the internal contents cause a relieving effect to the base slabs bending moments?

Probably.

Similarly, for the backfill case - would the weight of the soil acting on the nib cause a reduction in the base slab bending moments?

Also probably although the benefit would seem negligible unless it's a very large nib.

In both cases, I'd recommend drawing free body diagrams of the situations. Put 'em in equilibrium, pat yourself on the back for a job well done, and move on.

Due to the size of the structure (it's actually several of them) it would be advantageous to optimise the reinforcement (for both commercial and sustainability reasons).

It always is. That doesn't much change the nature of the task before you however. Design efficiently, deign safe, pay attention to crack control in a liquid retaining structure.

 

[Sense Check Me]

It's a substantial nib (more than 1.5m) needed for a variety of reasons (flotation, base prop to shoring and for access during construction).

I've drawn a free body diagram, but I've become a little unstuck due to the base and walls both acting two-way spanning.

I think I may need to simplify it, use a free body diagram assuming a linear wall (so ignoring the box effects and two-way slabs) and seeing what happens wrt equilibrium which may inform what relieving effects could be expected.

This is really insightful, thankyou.

 

[KootK]

That is a nib of some significance. I see what you mean about the two way action complicating it. Feel free to post FBD's hear if you'd like some feedback on them. We love that kind of thing.

In a lot of structures, we do that business where we'll assume boundary conditions that aren't quite true and then just put in some "detailing reinforcement" for crack control and reflecting the likely "true" stress distribution. I'm reluctant to do that in water retaining structures where crack control may be paramount however.

 

[Sense Check Me]

Excellent points raised.

I agree with the caution for careful control and detailing wrt to crack widths (flexural and thermal), indeed its what's governing the design.

I'll post put a free body diagram tomorrow, look forward to more discussion.

 

[HTURKAK]

If you need the soil wt on the nibs against flotation , GWL is high enough ...


Probably you may need 3)tank empty with backfill in place with seasonal highest GW ..


If the tank is open , the use of boundary conditions of top = free is OK .. If there is a roof slab, top = simple support , other 3 edges = fixed is better..



Designing the whole base slab for the vertical wall moments is reasonable..But modelling the whole tank 3D with FEM will be better approach..
The base slab should be designed for three loading cases,

- Tank empty , GWL at minimum, active soil thrust at min.
- Tank empty, GWL at max. ,active soil thrust at max..
- Tank full ..

 

[Sense Check Me]

The backfill is taken to be earth, groundwater and surface surcharge.

As the groundwater level was unknown at start of design, I took GWL at ground level which also made the analysis using published tables easier for triangular and UDL pressures.

Tank is open (no roof).

As the base slab is built on a firm base, the loads are assumed to act uniformly across the base (I think this is termed plastic analysis).

I agree with 3D FEM model would help resolve these issues. However, I don't have the capability to do this myself and for various reasons (lack of resources, skills, budget, etc.) it appears this can't be undertaken yet there is a real push for a highly efficient design.

 

[HTURKAK]

If you are looking for a highly efficient design, the starting point could be to get the actual geotech data and GWL info.

The assumption of the GWL at ground level overkills the efficient design..

If the tank is open and the dimensions are 9m x 9m (square) x 6m deep, i would prefer to model unit width rather than moment coefficient tables.