Answer:
This check box for details of the serviceability limit state design is not available if the minimum reinforcement As,min has not been selected.
In order to find the most economical reinforcement, it is necessary to activate several "design types," such as:
- Direct calculation of crack width wk
- As,min and limit diameter lim ds
- As,min and limit spacing lim sl
As soon as several "design formats" are available, the check box can also be activated (see the image).