Another option that keys off of Paul’s suggestion is to make separate scenes, one for the floor plan line work and a second for the beams and structural elements. Create separate viewports for them and set the line weight for the floor plan to be fairly thin and the beams fairly heavy.
Or, you could create a different style for the scene with the floor plan set so the lines are gray. I did a sort of combination in this quickie example. I set the style for the plan to have red edges so the difference would be obvious in the screen shot although you could set the lines to be gray or whatever color you want and I made the edges heavier for the viewport showing the I-beams. Both viewports are rendered as Vector in LO.
Since this is two different viewports and two different scenes from SketchUp, it would be easy enough make modifications to the structural elements without impacting the view of the plan.