Your drawing looks pretty good - in the drawing at right, you can see the crane runway girder sitting atop the step in the roof support column.
The roof trusses are built-up from structural shapes, and might be welded, bolted, or riveted together. The truss shown below is built-up from basswood angles, with styrene gusset plates. The bottom and top chords are 6"x6" angles, back-to-back, with the gusset plates sandwiched between them, while the intermediate members are 4"x4" angle, single pieces only (although I don't recall if they were supposed to be single or if they should've been double, but I just got lazy)

The span across the bottom member is 75', and the distance from the bottom to the peak is 27'.
For the casthouse floor, I'm not sure what the spacing of the floor support columns would be, but judging from photos of my model, it looks like 20' to 25', and this would be in a square pattern under the entire floor. These were connected by a grid of I-beams. The floor itself would've been reinforced concrete, with an overlay of hearth refractory brick. Concrete, when molten iron or steel hits it, tends to explode.

The slag and iron runners were set into the floor and were also lined with refractory. For my model, the support columns for the crane runways and roof were separate from the floor support columns, so there was a double row of columns around the perimeter. The columns at the ends of the casthouse (beneath the gable ends) were not as large as those supporting the roof and cranes - probably 1'6"x1'6" at most, since they supported only the end wall.
The crane runway is usually composed of heavy I-beams, although longer spans would use a built-up girder similar to a railroad girder bridge, while exceptionally long spans would be a truss, again like a bridge.
Hope this info helps. I never worked in the blast furnace or the open hearth, but I do have some photos, plus the book from U.S.Steel, and of course a vague memory of those blueprints. I see that Dean Freytag's book on modelling the steel industry is now available - about $70.00 at the LHS.
Wayne