I am trying to do modal analysis of spar. I have modeled flanges using CBEAM and web using CSHEAR elements . I got modal frequencies but I am not sure if they are correct. I found a technical article which tells to use CROD/ extensional stiffness on all 4 sides of CSHEAR as CSHEAR is unstable element. I didn't use CROD/ extensional stiffness in my model. Can anyone guide me on use of CSHEAR elements (should I use CROD)?