API-650 Intermediate Wind Girders to Prevent Buckling - Considerations for Full Tank? 1

[UofIAdam]

Hello,

I'm currently reviewing an existing above ground API-650 steel water storage tank to determine the level of protection that will need to be provided to account for tornado winds.

API-650 provides equations for the determination of the maximum height of the unstiffened shell. Pretty simple. However, am I correct to assume that the equation considers the tank to be empty?

If I'm trying to protect a tank that's full, and keep it full, is there any way to account for the internal water pressure as counteracting the wind pressure?

My thought is to do the following: Referring to the 2007 edition of the standard, Section 5.9.7.1 provides an equation for the maximum height that contains a design wind speed (V) factor. Halfway down the same page is the equation for velocity pressure which also contains the wind speed (V) factor. Could I adjust the velocity pressure by subtracting the internal water pressure, recalculate the "effective" wind speed, and use the effective wind speed to calculate the allowed tank height?

Sorry, this isn't a very well phrased question. Not sure how else to ask, though.

 

[JStephen]

I think the proper approach there would be to simply say "Full tanks don't blow in. Period." There's really not any reason to do calculations in that case. And I don't know how you'd come up with calculations that were real meaningful.

If the tank has two or three feet of freeboard above the liquid, you could check that span of shell for stability.

For the "full" condition, you'd want to check foundation bearing. Typically, overturning of the tank itself is based on an empty tank. You could use the equations from Appendix E for the liquid acting with the shell to calculate increased stability due to the water.

The tank roof may not be adequate for the wind uplift loads. If wind speeds are high enough, design of ladders, stairways, and similar items may be inadequate.

 

[UofIAdam]

Thanks for the reply JStephen.

I agree that it won't blow in. I'll need to do some more searching to find studies that prove this, though. I'd appreciate it if you could point me to a reference or two that may have data, if you have any.

I'll review the tank roof and the 1 foot span of freeboard at the top of the wall as well.

 

[fegenbush]

UofIAdam,

Additional studies should not be necessary, as it can be shown with a free body diagram:

1) Calculate the design wind pressure, say 18 lb/ft^2
2) Take a 1 ft x 1 ft square section of shell. Assume the shell acts as a flat plate.
3) Apply fluid pressure (say, water) to one side of the shell with one edge of the shell even with the water level. Call this side the top.
4) Apply the wind pressure to the opposing side of the plate.
5) If the fluid pressure is greater than the wind pressure, the tank cannot blow in. This process is conservative since it neglects the curvature of the plate and the bending of the plate, both of which resist the wind pressure also.

Example:

Unit Weight of Water: 62.4 lb/ft^3
Pressure at Bottom of 1 ft column: (1 ft)(62.4 lb/ft^3)=62.4 lb/ft^2
Average Pressure over plate: (0.5)(0 + 62.4 lb/ft^2)=32.2 lb/ft^2
Wind Pressure: 18 lb/ft^2
32.2 lb/ft^2 > 18 lb/ft^2 therefore the tank cannot blow in for the top 1 ft of shell. Lower sections of shell have more hydrostatic pressure and are thus more resistant to blow in.

 

[mariog123]

"If the fluid pressure is greater than the wind pressure, the tank cannot blow in."

Yes, the theory behind API 650 is "the modified U.S. Model Basin formula for the critical uniform external pressure on thin-wall tubes free from end loadings", subject to the total pressure of 1.72 kPa (36 lbf/ft2), corrected with specific velocity V.
The phenomenon considered is cylinder buckling under external pressure.
No net external pressure, no buckling!

 

[UofIAdam]

Thanks everyone.