The chime provides a compression member to resist the 1 inch of water vacuum rating (minimum) required by the standard. This vacuum must be resisted even with the tank empty.
In the case of an open top tank, I would be comfortable allowing the chime to be reduced (forced in by incorrectly placed anchor bolts) to one-inch instead of trimming shell plate to maintain the two-inch chime. Is there anything else I should consider?
In the case where a reinforcing plate takes up a portion of the chime, the standard (API 650) recommends at least 1/2 of the bottom plate to project past the toe of the fillet weld at the reinforcing plate-bottom plate joint. His seems like a distortion limiting, or weld quality issue.?.?
Appendix (Annex) J tanks do have a smaller chime requirement. See J.3.2.3 and note that the bottom may be flat flanged (no chime) or a minimum of 1" beyond the toe of the shell-to-bottom weld. Appendix (Annex) A tanks can be quite large. For instance, it is possible to construct a 100 ft diameter x 32 ft high tank to Appendix (Annex) A so I do not see how the chime requirement should be relaxed for that portion of the standard.
In the AWWA water tank standard, the Section 14 design (ie, high-strength design comparable to API-650) requires the 2" chime, while the base design (more similar to the App. A design) requires 1" beyond the toe of the fillet. Meanwhile, API-12D for tanks up to 55' diameter doesn't require any specific chime. So it's hard to imagine that this is some critical number.
If I remember right, there is an issue in the shell analysis theory as to whether the bottom plate is assumed to have a plastic moment both inside and outside, or inside only, and perhaps the additional length was to help justify the assumptions there. If that's the case, then notching around a bolt should have minimal effect on it.
It's a "traditional" requirement. A small ammount of chime [chine] is required for welding the OD of the shell, and a further small ammount is required for the 1-Foot calculation method to be valid, i.e. using the floor and floor extension [chine] as a reinforcing member for the bottom of the shell. And it is a very good idea to have some more chine sticking out for 'sacrificial anode' to rust away first, before the corrosion gets to the weld.
When I add these up in my mind I get an inch or more; i presume that is how the 2-inch requirement arrived. When a tank is built in the field using 'classical' methods, the floor sketch plates are cut from straight plates. At that point, any reasonable floor extension - chine - can be established 1", 1.5", 2", 2.5" etc. For a new tank, I personally am perfectly happy with a nominal 2-inch chine, that never goes below 1.5" from the outer face of the shell.
OK what'a a chime (chine?)? Is this a non-USA term? I looked in api 650 and didn't see it. I googled the word. It took me to some web sites but I didn't see any definition. I looked in the dictionary and it said that it was a boat term where the bottom meets the side. This could be the shell meeting the bottom but where does the 2" come in? Is it just the bottom plate overlap?
