Why there is the full-perimeter reinforcement preset and I cannot change it?
For selection of any other reinforcement layout, no circular cross-section is available in the reinforcement group. If you remove members with circular cross-section from the reinforcement group, it is possible to select optimized distibution again.
Linear and nonlinear analysis of reinforced concrete members with reinforcement concept
Design of reinforced concrete members and surfaces (plates, walls, planar structures, shells)