Sir,
Is it not possible to design a prismatic member using AISC 335-89 or AISC-360?
In the attached figure, the Grey color members are wire ropes and are specified as cable members. Staad does not design these members. More over, do the members connected to these will also fail as these are not designed?
Instead of assigning a prismatic circle, go to Tools -> Create User table and create a custom Pipe section with an inner diameter equal to 0. Such section should be designed.If the member is not designed, it does not have any impact on the design results of the members connected to it.