water tank wall/base slab moment transfer theory

[bazflexure]

what if I design wall for fixed support at bottom.
I think moment from wall will not be transfered to base slab decause the joint between slab and wall is restrained from rotation. As it will tend to rotate soil pressure below slab and behind wall (passive)will increase which will prevent joint from rotation.As long as soil pressure is below bearing capacity no or very little curvature will develope in base slab.So external moment from wall will resisted by forces developed in soil.

please comment on this explanation.

Secondly i have not read in any book talking about moments in base slab when tank is fully filled .

 

[JedClampett]

I'm not sure where you're going with this, but I can assure you that wall moments are transferred into the slab at the base. The moment is eventually resisted by the bearing of the slab on the soil, but that doesn't eliminate the transfer of moment at the joint.

 

[kslee1000]

Point 1 - Revolutionize finding on soil-structure interaction. Where you leaned from - rigid joint rotation is prevented by sourrounding soil?

Point 2 - True, because it is not the most critical stage for underground tank design that most engineers worry about.

Point 3 - Are you practicing structural engineering under any supervision? Talk to your superior to get bonus on these. Good luck.

 

[minorchord2000]

Jed Clampett is exactly, 100% correct.

 

[bazflexure]

I know there will very little rotation of joint but we cannot determine the slab share of moment by convention moment distribution method

 

[oneintheeye]

there will be moment transfer dependent on the stiffness of the elements. you need to determine or use charts.

 

[kslee1000]

Why not?

Without consider the cover, you have a continuous beam (2 side walls and the slab) that is supported by pins/hinges at the lower corners. Make proper assumption on supports on top, moment distribution sure works for this type of arrangement.

 

[kslee1000]

Correction:

The lower (wall-slab) corners are right joints using moment distribution. They are assumed (pre-smart spring modelling tool made available) pinned externally for computer analysis to solve global stability catches.

 

[asixth]

bazflexure,

This is what I think is happening. The cantilevered water tank section is wanting the base slab to hog upwards, and therefore the soil distribution is not uniform but is concentrated at either end of the base slab.

Refer to my free body diagram I sketched. The bending moment in the wall is still required to be transferred to the base slab. There is no mechanism for the soil to take out this moment at the wall-slab joint.

My apologies for the bending moment diagram. In Australia we draw the bending moment diagram on the tension face. I think it is because of the amount of concrete design we do here so we need a visual on where to provide reinforcing steel.

Like all good engineers, my CAD abilities a limited for now, so I had to use paint to do my sketch.

 

[apsix]

I believe the OP's structure is a buried tank where the water pressure can be resisted by passive pressure from the soil.
Depending on the level of backfill compaction, type of backfill, stiffness of wall and slab etc, the wall to base moment transfer will vary.

 

[oneintheeye]

apsix I wouldnt rely on backfill to resist forces. but on moment transfer;

if you design the wall to take the full moment (no distribution), make the base at least as thick or slightly thicker it will therefore be stiffer and attract moment. If your wall is x thickness with y area of steel. The base will be stronger with the same steel, moments on each element will be less than the initial wall moment after redistribution has been done therefore if your wall works for non ditributed moment then it will be fine after distribution.

The only problem is how this effects ring tension values maybe.

 

[pba]

For passive resistance to develop may require significant movement of the ground. - This would crack the concrete and allow the water to escape...

Even if the soil is very stiff it only contributes the the stiffness at the wall/slab interface - you still have the stiffnesses of the slab abd wall to consider so it could only ever provide some help - never totally replace the slab stiffness.

 

[miecz]

You'll want to test for leakage prior to backfilling. For this load condition, there is no passive pressure.

IF you don't have access to more sophisticated tools, treat the joint at the base mat as rigid and do a 2D analysis of the tank cross section for 2 load conditions.

1. Filled, not backfilled
2. Empty, backfilled

For the filled condition, not only does the base mat take the bending moment from the wall, it also is in tension from the shear at the bottom of the wall.