Wind Load on an Open Top Rectangular Tank

[waytsh]

I am designing an open top rectangular SBR and am wondering the best way to consider wind load on the structure. I am designing per ASCE 7-02 and the tank is only going to be out of the ground about 11'. The remaining 14' is buried. I am not so much concerned with the wind pressure as with the wind suction being combined with the internal fluid pressure which is controlling. Normally I would have used Eqn. 6-20 for wind loads on other structures, but then I got to thinking about the potential for suction forces. Am I picking gnat crap out of pepper or would the components and cladding equations be better suited for this situation? How are you guys handling this? Thanks in advance.

 

[JedClampett]

If the SBR is going to be made of concrete, I seriously doubt the wind loads are going to affect the design. You are already designing for an fluid load of 63 pcf. Your wind load will only be about 20 psf and the suction is a percentage of that. If you're nervous, you can run the numbers and prove it to yourself.
Wind loads do have a bigger effect on steel tanks, especially when empty.

 

[Lutfi]

Use a solid sign, model it as it has a single large pipe column (your tank) and see what ASCE 7-02 would give you as far as wind force.

I am like Jedclampt, if the tank is full; wind is not going to govern. If it is empty, I still don’t think it will govern because the earth pressure will be your worst load case.

Good luck

 

[waytsh]

Thanks for the quick responses. I agree the wind pressure will not have a significant impact when the tank is full. What I was wondering is if I should consider leeward suction as an additive force to the internal pressure. If so should this suction be calculated per the building components and cladding equations? Actually as I thumb through ASCE I don't see a load combination for Wind and Fluid pressure. I'll have to check IBC. I suppose you're right and I'm overanalyzing the situation. My tank walls won't be designed that close anyway. I just wanted to be thorough since the design will have to go out for review.

Thanks again to both of you for the quick response!

 

[JStephen]

FYI, wind pressure/ suction is never added to the hydrostatic pressures in the design of tanks. If you did add it, most codes allow a 1/3 increase in allowable stress (or reduction in load factors as applicable), and the combined loading with the change in allowables would be less than hydrostatic alone.

 

[Ron]

The effect of the wind on the leaward (inside wall of windward face) face would only have an effect for about 2 or 3 feet down from the top. As for the "negative pressure" effect of wind blowing across the open top, the greatest effect will be the rise in liquid level as a result and I doubt that it will be much.

 

[MotorCity]

If the tank is open, the internal pressure is zero (atmospheric pressure).

 

[connect2]

There's no combination of F and W in ACI-350M-01 either, just a statement that L,W, or E can not be used to reduce the effect of F (9.2.5) in the case of concrete.