In RM Bridge inside OpenBridge Designer, the best practice for Diaphragms & Stiffeners is creating a separate Link segment for these elements. The Links can be accessed at the Main Girder menu, at the top right, as shown in the screenshot below. The most significant advantage is that the Links option for modeling allows the users to set these elements' Status as active or inactive, as shown below. This is a powerful technique because it simplifies the process as we can still use the same cross-section definition for segments with and without diaphragms and stiffeners. 

communities.bentley.com/.../276371    

Another possibility for modeling the diaphragms is more simple and used when the local results are not the main interest, but the longitudinal main behavior. Then, users can just create different Cross-Sections to be applied in this region, using, for example, open, closed, partially, or fully filled Cross-Sections, as shown in the image below with the screenshot from 4 different CS of Australian T4-2500. The use of Variables, Tables, and Formulas also can help in creating different CS.

One example that can be loaded for reference and use is the one explained at the link below:

communities.bentley.com/.../9-rm-bridge-steel-design---training-example

Link Segments detailed technical information and examples can be found at the link below:

communities.bentley.com/.../18-modeler-link-segments-truss-modeling