Storage tank - Column Unsupporting length

[monopoly]

Hello,
One of my storage tanks has significant column deflection, approximately 12" from top to bottom. The rafters and columns are laid out in the attached photo.

Question: what is the K-value for checking the buckling or unsupporting length of the column?

I think the K-value should be 1.2, fixed at the bottom and rotation fixed and translation-free
K=2.0 is too conservative for the tank column, I guess.

Tank info: 110' dia. x 48' H

 

[JStephen]

I would use 1.0 for pinned-pinned.
Is the 12" deflection from a straight line or out-of-plumb?
Deflection from a straight line could be from eccentric loading, weld distortion (if a built-up column), differential movement under the base plate.
Out-of-plumb could be erection issues, racking of the whole structure, differential movement under the base plate.

 

[r6155]

Year of construction?
Corrosion allowance?
Original design calculation?
Colum type and dimension?
Materials?
Design drawings?.....etc, etc.

 

[HTURKAK]

Question: what is the K-value for checking the buckling or unsupporting length of the column?

I think the K-value should be 1.2, fixed at the bottom and rotation fixed and translation-free
K=2.0 is too conservative for the tank column, I guess.

K=1.0 . Rotation free, translation fixed at top and bottom .

Refer to API 6+50 Clause 5.10.3.2
Maximum Slenderness Ratios For columns, the value L / rc shall not exceed 180.

significant column deflection, approximately 12" from top to bottom

I would like to understand geometrical imperfection that is ; lack of verticality.

 

[monopoly]

I would use 1.0 for pinned-pinned.
Is the 12" deflection from a straight line or out-of-plumb?
Deflection from a straight line could be from eccentric loading, weld distortion (if a built-up column), differential movement under the base plate.
Out-of-plumb could be erection issues, racking of the whole structure, differential movement under the base plate.

The 12" deflection is from inspection out of plumb. The columns are made of W12x14 & W12x14 welded at 90oC. The column is deflected in the direction as shown of the arrow. It was built in 1968, so i assumed it is A36 materials.

 

[JStephen]

If the there is not any indication that the structure has moved, and if there is not any interior floating roof, I would say there's a good chance you are just seeing erection sloppiness.
If it uses a crow's foot that is not fixed to the tank bottom, it may be possible to plumb it up by shifting at the bottom, but I don't know there would be a motivation to do so if it's not causing any other problems.

 

[r6155]

When was the inspection before this one?
What is the actual thickness of the column?
Is this the only column with this defect?