Interesting, if you don't know much about construction, like me you never would have thought about things like this, and it all sounds so logical when you read it. It's a very interesting looking building that's also a very interesting enginering project.About the gap in the building: they're going to close that when the two towers are connected at the top. The building is very complex with all the angles and they're not sure yet how the building will 'settle' itself. The building is erected in a slightly more upright angle than its final position due to its enormous weight. When everything is connected on top, the building is fixed at groundlevel.
As you can see on this last picture they are pouring concrete around the steel collumns of the construction. In this way, the collumns can take higher pressures. The entire construction works as a hull to create stability. The forces on the construction are not the same everywhere, so in some places the stability beams are closer together (there are more crosses in some places, view the renderings too).