Tank dome roof rafters

[Tony P]

Hi Guys, Good Morning!
I need help regarding Roof rafter beam under and over-Rolled radius tolerance for dome roof API-650 Tank.
Can anyone provide such information. thank you in advance for your time and assistance.

 

[IFRs]

This is not a question we can answer. It can only be answered by the structural engineer who designed and analyzed the dome. Under rolled (too flat) is probably worse from a structural perspective but both will become fit-up problems in the field. If it is 1 percent different from design it might be OK, or not. Too many unknowns. 12 foot diameter or 200 foot diameter, location, loads, factor of safety, etc.

 

[JStephen]

Tank tolerances in general are presumably based on a number of factors:
-What is required for structural stability
-What is required for proper functioning (ie, so a floating roof will operate, etc.)
-Cosmetic appearance of the finished product
-What is typically achievable using "good practice"
-What can be measured conveniently
For the "what is typically achievable" on a rolled rafter, you might check either with the companies that do such forming, or the companies that make the machines used for this. If you're looking to specify tolerances for work yet to be done, this would likely be the main consideration.
In some cases, modest changes in radius will make almost imperceptible changes in actual profile. (Example: Rolling a 50' long beam to 100' radius or to 99' radius will make about 3/8" in the depth of the rolled beam, which may be insignificant in the construction).
With straight beams, there are straightness tolerances in ASTM A6 or A20. It doesn't seem reasonable to me that the tolerances from theoretical shape for a rolled beam would be any smaller than what the mills furnish for a straight beam. (Not to say those tolerances are necessarily achievable in a rolled beam, though).
Variation from beam to beam might be a little different than having all beams rolled to the wrong radius.
It may be possible to make corrections in rolled profile with flame straightening.
If beams are welded to the roof plates, weld distortion may cause considerable straightening or additional curvature.