Calculating Pressure of Moving Water on Foundation 1

[sjhgeotech]

Does anyone have a tip on how to calculate the force exerted on a stationary object by a moving mass of water (e.g. pylon foundation in a river)?

Thank you.

 

[rconner]

Not professing to know much about that specific field, but I believe you are perhaps talking about calculations in the field of fluid dynamics for a sort of "drag" force on the object you are talking about [from the flow(ing fluid)]. I believe plots of "drag coefficients" are available for various shapes and Reynold's numbers e.g. an "infinitely long cylinder" if applicable, to assist in determining the forces. Some keywords like this may allow you to find much information regarding your specific cases in e.g. fluid dynamics references or the web. While certainly not insignificant, I suspect these forces might be sometimes quite small compared to getting hit by a big ship, barge, or even floating tree trunk log etc.!!

 

[JedClampett]

You need to get the AASHTO Standard Specification for Highway Bridges and refer to Section 3.18 for a complete approach to this issue.

 

[BigInch]

Figure V^2/2/g if you like to err on the conservative side for water hitting dead on and stagnating. If its flow past a shape, you'll have to get the drag factor for the particular shape and angle of attack.

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[looksatstars]

i don't know that field either but wouldnt it be Force=mass of water * acceleration of the water

 

[BigInch]

Does this help?

Stagnation pressure = (Depth + (water_velocity ^ 2 / (2 * g)) * 62.4 lbs/ft3

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[Ingy]

sjhgeotech,


F = (rho)*(A)*(Cd)*(V^2/2g)

where
rho = water density
A = projected area of the obstruction normal to the current
Cd = Coefficent of drag (=2 for sheet pile cofferdams)
V = velocity of the current
g = acceleration due to gravity

in english units rho =~ 2g and using Cd = 2

F = 2AV^2

Good luck!

 

[sjhgeotech]

Ingy,

Section 13.18.1.1 of the AASHTO gives equation (3-4) as Pavg = K*(Vavg^2), where the Pavg equals pressure in pounds per square foot.

Where did you get your equation F = (rho)*(A)*(Cd)*(V^2/2g)
Also, I have round concrete piers. What friction factor Cd do you recommend and is there a reference for them?


Thanks.

 

[BigInch]

Drag factor for round shapes is 0.8 x width. Just about any fluids text will show Cd = 0.8 for round shapes as well as the formulas we all gave you for calculating either the pressure or force. All the formulas yield the same answer in one unit or another.

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