The software calculates the required reinforcement and moment of inertia for the cone/cylinder junctions at both the large and the small ends. This calculation is performed whenever the external pressure is greater than zero, the cone is taken as a line of support and the reinforcing material is defined. If a knuckle is specified instead of a reinforcing ring, the knuckle calculation will be performed and the area of reinforcement calculation will not. If the user specifies that the cone/cylinder junctions are not to be taken as a line of support, then the area of reinforcement and moment of inertia calculations will not be performed.
Cones are required to have reinforcement at the large and small ends under external pressure (Appendix 1-7) because of the tendency to buckle under axial external loads.
At both the large and small ends there are requirements for the area of reinforcement and moment of inertia of the reinforcement. The area of reinforcement is based on considerations similar to those described for internal pressure. The required moment of inertia of the reinforcement is a function of the strain in the ring at the cone/shell junction, which is in turn calculated using the Code materials chart from the stress in the ring. See the comments on stiffening rings in the external pressure section for further insight.
The maximum apex angle is taken from Tables 1-8.1 in Appendix 1 of the ASME Code. The software calculates the ratio P/SE. Note that this angle applies only to the large end of the cone - the small end always requires at least a little reinforcement.
The area required in the reinforcing ring will be set to zero if either the cone angle is less than the maximum angle (large end only), or the area of reinforcement available in the shell is greater than the area required.