Where can I find the bolt shank diameter (bare shaft diameter) for container flanges?

I don’t understand what the original poster means; the bare diameter is simply the nominal diameter, for example, the bare diameter of M20 is 20.

There’s no such thing as a “bolt root diameter,” right? If it is used for calculating the bolt load-bearing cross-sectional area, the smaller value between the root diameter of the thread and the diameter of the shank is generally taken. The root diameter of the thread (i.e., the minor diameter of the thread) can be obtained from the basic dimensions for ordinary threads specified in GB/T 196-2003, or it can be calculated using the formula given in that standard; the result will be the same. The bare diameter of the bolt or stud (the minimum diameter of the unthreaded portion) should be obtained from the bolt standard selected. For example, according to JB/T 4707-2000, the bare shaft diameter of Type A studs for equal-length double-ended studs is equal to the nominal diameter, while the bare shaft diameter of Type B studs is equal to the root diameter of the thread.

Note 1 in Table 1 of NB/T47027-2012

NB/T 47020~47027-2012 covers pressure vessel flanges, gaskets, and fasteners; HG/T 20592~20635-2009 covers steel pipe flanges, gaskets, and fasteners. Which standard should be applied depends on the requirements specified in the drawings.

It refers to the minor diameter of the thread, that is, the most critical interface.

Generally, the smaller of the thread bottom diameter and the diameter of the middle section should be used as the basis for stress calculations