I can't use non linear material property for my rib beam and when I want to model a rib beam in a slab that has non linear material property. Rfem5 will send an error message. Can someone help me out
Hi Chukwudalu,
Avoid Using Rib Member in This Context. It's recommended not to use rib member in this case.
You can find detailed instructions in the following FAQ: Design of Ribbed Plate in Cracked State with RF-CONCRETE NL and Export of Stiffnesses
Feel free to reach out if you have any further questions! 
Best regards
Stefan Hoffmann
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