Unclear picture, other angle please.
Try and remodel the blackened floor or just copy the floor from upper level that renders well. Or are they not the same profile?
I'm a visualizer in an architect firm. I always get sketchup models from designers. When I import it in 3DsMax I will always make sure that I attach all objects which share same material, and detach the whole bunch of polygons again, I don't know why, but for some reason by doing this, 3DsMax will read the detached polygons better. By doing this it eliminates the wrong surface normal, easy for you to apply and change material, also eliminates blackened area, and remove GI blotchiness when using Vray render (I dont know about other renderer though). As though the polygons have defects when you first import it from other apps, but after detach it resets to Max's own native polygons. Correct me if I'm wrong, but it works for me all the time.
Edit: Maybe not all the time, but most of the time.
