Emre, hello Cd; It is given in cases where double reinforcement is required in slabs. But In your project, when converting Slabs to Hollow slabs, the reference beams are defined incorrectly, if you examine the slabs giving EA error, you can see when you pay attention to the spacing of the Rib beams (Rib intervals are not equal) I redefined the reference beams of the slabs. I saved the project at 6.0054. Good work."emrebagdu":3haxvp0o" said:I solved the project with hollow slabs, but it gives an error saying Cd in many slabs. I could not fix the error. I am attaching the project, I would be glad if you could help me. have a nice day