The software calculates the required reinforcement for cone/cylinder junctions at both the large and the small ends. This calculation is performed whenever the internal pressure is greater than zero, and the reinforcing material is defined. If a knuckle is specified instead of a reinforcing ring, the knuckle calculation will be performed and the required area calculation will not.
When a knuckle calculation is performed, the software calculates both the required thickness and the maximum allowable working pressure for the toroidal portion of the knuckle, using the rules in Appendix 1-4(d).
When there is no knuckle, the software calculates the required area of reinforcement at the intersection of the cylinder and the two cones. Cones are required to have reinforcement at the large and small ends under internal pressure (Appendix 1-5) because of the tendency of the cone/cylinder junction to buckle under the radial load developed in the cone.
The Code calculates the maximum angle below which buckling will not occur as a function of the design pressure and allowable stress. This ratio is used because it is a pretty good indication of the diameter thickness ratio for the cylinder, and takes into account the strength of the material. This approach has the odd effect that when you increase the allowable stress you decrease the allowable cone angle. However, you will normally find that for a given thickness this effect is offset by the increase of area available in the cone for reinforcement.
Given that reinforcement is required, the area required is a function of the pressure and the square of the radius. Area available in the shell within one decay length may be included in the area available for stiffening.
CodeCalc will set the area required in the reinforcing ring to zero if either the allowed apex angle is higher than the actual apex angle or the area available in the shell is greater than the area required.