Design of RC Circular Water Tanks BS 8007

[engstructural]

Hi,

I an following a tank design from 'The Design of Water Retaining Structures - Batty + Westbrooke'

I have bee looking at the RC circular tank with a fixed base 15m Dia.

The calculations suggets checking the wall with a fixed base and calculating reinforcement etc. then when designingt he base as continuous the publication talks about calculation of the moments and ring tension in the wall caused by the forces in the base and the add them. As this point I am unclear the calculations would then produce the 'actual forces in the wall but there is no suggesting the wall steel should be re-checked / re-designed. - Would I be right to assume this should be done and therefore the sizing of the reinforcement in the 1st part of the example was not necessary?

Thanks for any help.

 

[Ussuri]

Its been a long time since I have seen the Batty and Westbrook publication, but I seem to remember doing the analysis for the wall and the analysis for the base followed by a moment distibution around the corner. Such that your moments at the wall/base connection are balanced.

 

[engstructural]

Thanks for the reply. This is what I was doing although the example had me design the reinforcing before moving on to the base design. (I thought this was odd)

My tank is 15m dia. internal x 6.0m ht. with 300mm thick base and wall.

The forces I got in the fixed base wall design were M (vert) = 18.2kNm.and ring tension force (267kN) service.

the moment redistribution from the base results in a ring tension 448kN total. this is significantly more - should this figure then be taken back and used in the wall reinforcement design?

If so it is a v. large tension force and may not be workable in mt reinforcement.

 

[Ussuri]

Your tank walls resist the internal water pressure through the horizontal ring tension in the walls, and because your base is fixed you have a vertical moment at the base.

If you had a sliding joint at the bottom all the water pressure is resisted by the walls in tension. The bending in the base is then caused by the weight of the walls and the weight of the water.

Now because you have a fixed joint between the wall and slab you have a radial edge moment at the base slab. Which will be different to the vertical moment at the base of the wall. The moments at the corner are then redistributed according to stiffness.

Also, because the corner is fixed there will be some additional tension transferred to the base because the base slab is restraining the bottom of wall.

What I am not following is how you get an additional horizontal tension in the walls due to redistributing a vertical moment at the corner between the base and wall. Am I missing something here?

 

[engstructural]

At the moment I have a small negative moment in the bottom section of wall. i.e. the inner face of the wall at the btm is in tension and at mid height the outer face of the wall is in tension.

When I calculate the fixed end moment in the slab and redistribute it at the wall the moment being transferred into the wall is causing the tension face of the wall at the btm of the wall to change sides resulting in the tension face being the outside face all the way down. This is then increasing the ring tension.

If i re-design the wall to have a hinged base what detail can i use to ensure this is achieved and ensure adequate waterproofing. (a small section drawing would be useful?

 

[Ussuri]

The moment at the base of the wall is vertical and carried by the vertical reinforcing bars. The ring tension in the wall is horizontal and carried by the horizontal reinforing bars.

There used to be some details in either Batty and Westbrook, Bob Anchor's book or the RC designers handbook. But i'm not sure.

Its not a detail I ever used because of the constructability issues and the risk of the joint leaking.