Which is right for modelling steel stiffener, beam element or shell element???

[Khaleelo]

Hi
I modeled a steel load-bearing diaphragm in abaqus. I used for modeling the stiffeners shell element and after that I used beam element. there was difference between the failure load and failure mode in the two models. I think there is a wrong in modeling the stiffeners. which is the right element or the right way to model the stiffeners.

Thanks in advance.