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API 650 Figure E-1 (Ci Coefficient)

API 650 Figure E-1 (Ci Coefficient)

API 650 Figure E-1 (Ci Coefficient)

I am working on a tank calculation for FRP vessels and using API 650 as a guide to develop the sloshing calculations. The graph provided in Figure E-1 (Ci coefficient) seems to limit out at 1.5 (H/D) ratio. Has anyone out there had experience in what to do when the magical 1.5 H/D ratio is exceeded?


RE: API 650 Figure E-1 (Ci Coefficient)

API 650 tanks typically do not exceed the H/D ration of 1.5 but there is no prohibition of greater ratios in the standard that I could find. I would be wary of exceeding the figure but the curve looks fairly linear in that region and you could probably extend the line a bit. I would however, dig up the documentation for this factor and make sure the equations that use it still work.

What is your H/D ratio? If it is fairly close to 1.5 you are probably on reasonably solid ground but if it is much higher I'd be sure to review the background papers and think you would be better off finding seismic parameters for a silo rather than a tank.

RE: API 650 Figure E-1 (Ci Coefficient)

First off- I'm going off memory here- but isn't that the factor for calculating the approximate impulsive period?
If so- you have two options. One is treat the tank as zero-impulsive-period, which simplifies the calculations and lets you avoid using that factor in the first place. The second is to go back to the Veletsos method, which is what that graph is based on, and work out the impulsive period for taller tanks from that.
The paper for that C1 graph is here, by the way: http://e-collection.library.ethz.ch/eserv/eth:1068...
The Veletsos method is sort of involved, and I was never 100% confident that I was doing it all correctly, due to some lack of clarity in the Veletsos paper itself.
With the Veletsos method, be aware that for a short wide cylinder, shear deflections need to be considered as well as bending deflections.
It's been a while, but if I remember right, Veletsos gave three different assumed deflection shapes to choose from. My method was to set up a spreadsheet to calculate it all three ways, then see which fitted better, and go with that one.

If your tanks are small enough, it may make more sense to treat the whole thing as rigid, including the sloshing component. This is one of the options permitted in ASCE 7. For tanks of typical API-650 size, the consideration of sloshing leads to a reduction in seismic forces.

RE: API 650 Figure E-1 (Ci Coefficient)

JStephen - a FAR better answer than mine and as usual right on point, with sources. Well done!

RE: API 650 Figure E-1 (Ci Coefficient)


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