Wind loading 2

[Mech2325]

Hi,I did calculations for two vertical vessels on skirt support...one had a tan tan length of 5.2m and other had a tan tan lenght of 23m...in the compress report, for wind, the operating shear and bending moment values shown are different for operating and empty case....while they are same for operating and empty case in case of vessel with 5.2m vertical tan tan lenght... Why is this so??

 

[racookpe1978]

Well, repeat the calc's by hand.

NEVER blindly accept ANY unusual results from your own spreadsheet, your own program, somebody's else's program, or somebody else's spreadsheet.
But, given that, the 23 meter high tower is substantially taller - further up, wind loads are higher - for the same wind speed - because the ground effect goes away. Thus, your program could be "logicing" itself into reducing the wind force on the shorter PV. There should still be some effect.
Are they the same diameter?
Are they containing the same fluids at the same weights at the same levels, and comparable densities and heights?

 

[Mech2325]

Both have fluids with same specific gravity...however in the case of 23m vessel the gust factor being calculated is different for its operating and empty case but of 5.2m vessel the gust factor is calculated is similar for both operating and empty case

 

[Mech2325]

My question actually is why, for the same vessel, i.e, for 23m height is the wind shear different and moment different for operating and empty conditions??

 

[JStephen]

My guess: The program is calculating the gust effect factor per ASCE 7, and the gust effect factor depends on the natural frequency of the structure, which changes with the weight of the structure.

For tall slender structures, you can also get a P-delta effect influencing the moment, but it shouldn't influence the base shear, unless, as above, it affects the natural frequency.

 

[Mech2325]

JStephen : Thanks for your valuable feedback.