"When the shell is a portion of the sphere, it tends to spread outward at the discontinuous edge. To counteract this a ring is added, but the ring and the shell distort by different amounts, which results in stresses in the shell. These incompatible strains must be reconciled analytically, which is not too difficult a task for simple spherical shells. However, when the shell has isolated supports and few (if any) planes of symmetry, it is a severe problem..."