Bottom Annular Plate of an Atmospheric Tank?

The bottom annular plate is the outer ring of plates to which the tank shell is welded. It is usually of thicker material than the rest of the floor plates. In a tank the size that you mention I would hazard a guess at around 8 mm, the floor could well be thinner, say 6 mm (which is about a minimum) but you need to be guided by the codes and local regulations and to consider you corrosion allowances.

From API 650:

3.5 ANNULAR BOTTOM PLATES

3.5.1 When the bottom shell course is designed using the allowable stress for materials in Group IV, IVA, V, or VI, buttwelded annular bottom plates shall be used (see 3.1.5.6). When the bottom shell course is of a material in Group IV, IVA, V, or VI and the maximum product stress (see 3.6.2.1) for the first shell course is less than or equal to 160 MPa (23,200 lbf/in.2) or the maximum hydrostatic test stress (see 3.6.2.2) for the first shell course is less than or equal to 172 MPa (24,900 lbf/in.2), lap-welded bottom plates (see 3.1.5.4) may be used in lieu of butt-welded annular bottom plates.

3.5.2 Annular bottom plates shall have a radial width that provides at least 600 mm (24 in.) between the inside of the shell and any lap-welded joint in the remainder of the bottom and at least a 50 mm (2 in.) projection outside the shell. A greater radial width of annular plate is required when calculated as follows:

In SI units: 215 t_b / HG)^0.5

where t_b = thickness of the annular plate (see 3.5.3), in mm,

H = maximum design liquid level (see 3.6.3.2), in m,

G = design specific gravity of the liquid to be stored.

In US Customary units: 390 t_b / (HG)^0.5

where t_b = thickness of the annular plate (see 3.5.3), (in.),

H = maximum design liquid level (see 3.6.3.2), (ft),

G = design specific gravity of the liquid to be stored.

3.5.3 The thickness of the annular bottom plates shall not be less than the thicknesses listed in Table 3-1 plus any specified corrosion allowance.

3.5.4 The ring of annular plates shall have a circular outside circumference but may have a regular polygonal shape inside the tank shell, with the number of sides equal to the number of annular plates. These pieces shall be welded in accordance with 3.1.5.6 and 3.1.5.7, item b.

3.5.5 In lieu of annular plates, the entire bottom may be butt-welded provided that the requirements for annular plate thickness, welding, materials, and inspection are met for the annular distance specified in 3.5.2.