Ground slab to water tank

[sidnum]

Hi,
I am trying to design the base slab of a water retaining tank. The tank is above ground and the only loading on it is from the retained height of water. The tank measures 5m by 6m by 4m deep, with a freeboard of 500mm. The moments and shears in the walls have been worked out using two way slab theory (table 2.78 of Reynolds reinf. conc. handbook, if anybody is familiar with it)

The wall and base slab are to be designed for full fixity.
Can I carry the moment at the base of the wall into the edge of the slab, like it is a portal frame?
How do I work out the moment in the centre of the tank, if any?

I tried looking at this like an inverted T cantilever retaining wall and work out the moments at the base due to bearing pressure. However as there is no over-turning moment (O/T moments from water cancel each other) there is an average uniform bearing pressure of Total load over area. This seems too simple?

Any help or references to other sources would be great.
I should add, I work for a small office and we don't have any FEM software, so this needs to be hand checked.

 

[JedClampett]

You can FEM it to your heart's content, but if you design the edges for the wall moment and the horizontal reaction, the interior will take care of itself. As a matter of fact, it's common to thicken the edges for a distance and use a nominal interior thickness, like 8 inches. It wouldn't be worth it for your layout.
The soil pressure is basically the water plus wall weight divided over the whole slab area. It might vary a bit, but that usually isn't a large amount, unless you use the thickened edge design I mentioned above.
Be careful with the construction joint details and you should be in good shape. Don't optimize. 12 inches is a good minimum wall thickness, unless the shear loads require more.
Go to your antique store and get a copy of "Moments and Reactions for Rectangular Plates" from the Department of Interior. It's better than any computer program.

 

[sidnum]

Hi Jed,

Found that resource on line! Thanks for that.

Will the base slab have tension due to bending on the water face throught or will some develope on the soil face at mid-span?

 

[JedClampett]

I'm not sure I understand your question, but the top bars in the slab will be in tension due to bending, plus there is a tension component from the outward wall reaction (sometimes ignored, but if you're doing this the first time, you should resolve all the forces).
The bending moments go away as the water weight resists them as you move inward on the slab. But it's pretty traditional to use the same top reinforcing all the way across. It's unlikely the bottom bars will go into tension, as the water tends to transfer loads in midspan as direct bearing (load through slab to soil), unless you have bad soil. Once again, we tend to use the same reinforcing top and bottom. You can't hardly be too conservative in water holding structure design.

 

[sidnum]

Nope you answered my question thanks

 

[msquared48]

Don't forget that... You also need to consider the overturning effects here, filled and empty, for wind, seismic, and sloshing effects...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com