A code of practice for the structural analysis of manholes is being prepared by the DWA, and we are actively involved in it (committee work). We have been preparing static calculations for circular manholes since 1997 - consistently, for all materials including reinforced concrete.
The dimensioning is based on a vertically placed pipe (see trenching) with engineering additions. Circumferential and longitudinal directions are to be considered; often buoyancy control is required.