IFR Pontoon Moment of Inertia - How to figure?

[jcoots]

I have come across some calculations for a floating roof recently and noticed that when analyzing/designing the pontoons the engineer used the Moment of Inertia of the Pontoon cross section when looking at the Pontoon bending. I found that the moment of inertia was calculated normally for a 4-sided cross section except that the corners were treated differently. I don't know exactly what was done differently but I know it was something. Can anybody shed some light on why this was done. Do large thin hollow cross sections require something different or is this pontoon specific, etc?

Thanks for any enlightenment.

 

[JStephen]

Hard to tell from your description. Perhaps the fillet weld area was included. Perhaps the extension of plate beyond the joint was included. Perhaps a reduced effective width for wide unstiffened sections was used.

 

[jcoots]

I know it the weld area wasn't being considered, at least for this part. The number I am refereing to seemed to includ a few inches of plate in both directions from the corner, but I couldn't tell exactly how the number was derived. I'll see if I can find those calcs again and provide what info I can. Can you elaborate on the "effective width for unstiffened sections".

Thanks,

 

[TankDude]

I believe that a common way of determining these contributing portions of the pontoon is by using 16t at each corner and for a portion of the deck plate at the inner rim connection. I believe that these are considered as the only portions of the pontoon contributing to the overall stiffness and resistance to bending.

 

[JStephen]

Look in the appendices to AISC-ASD 9th Ed. It has a method for calculating a reduced effective width of a wide thin section stiffened on each side. Using it will normally give a lot more effective width than the 16t. It can be awkward to work with as the stress is based on the effective section and the effective section is based on the stress, and the 16t is much simpler to use.

 

[JoeTank]

Each of the major tank manuafacturer's use a slightly differnt approach to the design of pontoon floating roofs. Each company's procedure is "tempered" by the experience that what's on paper is not the complete story. As floaters have become larger, the design procedures have evolved at address new criteria or phenomenom not previously observed. The design expertise truly resides with the tank fabrication firms and they are not going to share or publish what they have individually paid for over several decades of development and refinement. Please don't rely heavily on duplicting a set of calcs based on what has been given to you. I am pretty sure that the calcs don't tell the whole story.

Joe Tank

 

[jcoots]

Thanks for all the input. Your answers are about what I expected.