Here's a little more on the Midwest Express Center, its really dwarfed by McC Place, but the span is smaller than that of LSD.
Its actually technically 3 floors if you count the Mezzanine.
Check out the floor plans here:
http://www.midwestairlinescenter.com/floor.asp
Notice how even though the road only takes up so much space, about half of the main exhibit hall is elevated two stories off the ground for quite a ways.
The public parking lot in the diagrams is a surface lot and is slated for future expansion, if Milwaukee's convention business ever picks up that much...
^^^This is the part where it is suspended over the road (note the sweet blue lights), they hold the auto show here and all of the exhibit space directly above this picture (as well as the ballrooms on the first floor and the little rooms on the mezzanine) is full of cars, so I don't think it would be too much of a challenge to build like this over LSD. I suppose for a building that needs to support this much weight you basically just build a really big highway overpass...
I guess this is a mini version of what McCormick Place would look like if they added a second floor that went across LSD.