well, this should be the end of this little debate. not that anyone actually thought there was a concrete core except one person.....
but here are pictures of the steel core during construction.
taken from
The WTC Cores As Seen In Construction Photos - Evidence based research
In the above picture we get to see only the tops of the core columns sections with the floor built up around them.
The builders would then place another section of core columns on top of the ones seen in this picture.
They would weld and cross-brace the next 3 story part of the core.
Then another 3 floors would be built and the whole process would repeat itself over again.
If we remember that the cranes are located inside the corners of the core, we can identify each column.
The row of columns closest to the viewer is either row 500 or row 1000. These are the biggest and strongest columns in the building.
The pair in the foreground to our right are columns 504 and 505 (or
1004, 1005, depending which way is north).
This is a very informative picture of the base of the core taken during construction. This photo was taken at about the same point of construction as the other black-an-white picture shown before.
Some concrete footings are visible.
Once again please notice that the tops of all columns are at the same elevation.
In the picture above, you can see a long lateral truss connected along the outside edges of the 500 columns. This lateral brace has small rectangular protrusions of steel spaced every 4 feet or so along it's side. The floor joists that support the flooring in the entire open office space will be bolted to these metal protrusions, so this lateral braces support all the floor joists to be suspended to make the next higher floor.
This gives us further evidence that the inside connections of all floor joists in the open office space were supported directly only by the strongest, outermost core columns.