What would be the goal of modelling structural beams?
- if the purpose is to properly factor in the additional thermal mass of e.g. concrete (+ rebar) columns and beams, then you can either adjust the thickness of e.g. the concrete slab above, or else consider adding InternalMass objects.
- if the purpose is to instead model e.g. shading effects or point thermal bridges of beams, then there are specific solutions to address these.
So what is it specifically you're aiming at?
EDIT : From your follow-up comment, you're looking at beams as shading surfaces. Helix Energy has a nice video on adding shading surfaces in OpenStudio. Here, I added a window to the initial model below, then 6 shading surfaces to represent 3 cantilevered beams extending from the facade.

I find it's best to first delineate the outline of any new shading surface using SketchUp's tape measure (as a drawing tool). I'd also ensure a small gap (> 25mm) between a beam (as shading surface) and a nearby wall. Then use the OpenStudio Plugin's Add New Shading Surface Group : pick one of the outline intersections, then click ENTER. This allows you to re-draw over the outline of each shading surface using lines. Once all shading surfaces are re-drawn, click somewhere outside the shading surface group. Save your model. These new shading surfaces will appear in the OpenStudio Application Facility tab, Shading sub-tab.
Finally, I suggest you familiarize yourself with EnergyPlus options when it comes to solar and shading (see here and here).
Below is a basic, 3-space OSM model, the geometry of which is built up in 4 steps (clockwise) using the OpenStudio SketchUp Plugin:

- First, outline floor diagrams.
- Select all, use the "create spaces from diagram" Plugin feature to generate 3D spaces (all same height), then double-click on a specific space (to adjust its height).
- Mouse-click its roof, and use SketchUp's "Move" tool to increase its height.
- Draw a line to split its roof surface, select the line, and again "Move" it upwards.
This is as basic as I can describe it using SketchUp, when it comes to variable height spaces with sloped roofs. I suggest experimenting a bit ...
Hope this helps.