Uplift due to Flooding

[Contraflexure74]

Hi,

I've got to design a foundation to support a boat sculpture beside a river that floods. My issue is that the river floods to an adjoining berm level of 4.5m above ground level. This will result in an uplift force on my foundation base. Am I correct in saying:

Force down due from 4m base is 4x4x1x24 = 384kN + a bit of soil over + a bit from self weight of sculpture.

Uplift force due to water at flood level = 9.81x6x4x4 = 941.76kN, so flotation is an issue.

Looks like I'll need a base of 6.5x6.5x1x24 = 1014kN just to deal with flotation alone.

My issue is the wider I make my base the more uplift I will get, i.e. my new uplift force is now 2487kN. Should I just use tension piles?

John.

 

[LittleInch]

Err, if this is a solid foundation made from concrete, there should not be any uplift force as concrete is heavier than water??

if your sculpture is hollow you might get some uplift force from that, but maybe I've not understood your structure.

Can you sketch it or provide a drawing?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Contraflexure74]

Hi Littlench

Sketch attached to original thread. It's a solid pad yes. You are correct there is no uplift. Apologies.

John.

 

[LittleInch]

That's better. Original answer still applies. Unless your sculpture is hollow or made of something with an SG<1 then you might have decreased weight, but no uplift force.

I guess the issue will be is there an additional force on the sculpture from flood water and is the soil resistance reduced on the concrete block. It's weight will effectively be reduced if the ground is now under water so might not have enough capacity o resist the overturning forces?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Contraflexure74]

Thanks for that.

How would one estimate the force from the river on the sculpture?

John.

 

[Teguci]

You are using 6 m for your buoyancy calculation, but you are only displacing 1 m for the footing and 11 m x 1m x 1m? for the sculpture. You'd probably make it easier on yourself if you figured out the net force on the sculpture (weight - buoyancy x SF) and the net force per unit volume on your footing (unit weight of footing - buoyancy x SF).

Some further things to investigate - the river can scour away the soil around your footing. The force of the river against your structure (what happens when a washed out tree crashes into the sculpture?)

 

[KootK]

Britain must have something akin to this: Link. The pile solution appeals to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[LittleInch]

Force on the sculpture is not easy, but you could give it a go by modeling it as a cylinder or flat plate or whatever shape is most similar when exposed to the river flow. An estimate of water velocity is required.

then force is CD x density x Area x V^2.

An irregular shape will be difficult to come up with a force but try a solid block as you should be able to find formula or drag coefficients.

Don't be surprised by a large number....

Do you have a picture or drawing of this "sculpture"??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Contraflexure74]

Will send on a sketch tomorrow. Thanks everyone. John.

 

[cvg]

recommend you include an anchor on your boat sculpture...