The center structure consists of a whole lot of I-beams and posts.  There are also walls containing windows and doors.  Within these walls is a computer room that is walled off from the engines and pumps.  I have glued in everything except the top two I-beam pieces.  These will be glued in later when the interior parts, engines and pumps are installed.  Once the top pieces are glued in, I can true up some of the misaligned posts. The entire structure is 3 dimensional except for where the truck arms will attach. At that point you can see sheets of styrene.  I wanted to get a really good attachment point for these arms.  Now that I think about it, if you mount the LUT and Saturn V on top of this model, all the weight is placed on top of the truck arms and goes straight through to the trucks.