ft. tall towers of 110 12.5 ft. floors
consisted of a perimeter of 244 structural steel columns forming a square
plan, with horizontal bracing (spandrels) and extremely rigid chamfered
peripheral steel columns from the 9th through the 106th floors spanned three
floors vertically, and were built into massive steel wall sections measuring
about 10 ft wide by 27.5 ft. (three floors) tall, consisting of three 14 inch
square hollow box columns spaced three ft. four inches apart, and welded to
thick steel plate spandrels.
building core, of dimensions 79X139 ft., was a veritable forest of 47
columns, horizontal and diagonal cross braced, which housed primarily
core columns varied in thickness with floor height. Individual core columns
in the lower core measured 52 x 22 in. (in plan), and were formed of 5 and 3
inch plate into almost solid steel shafts that weighed up to 56 tons. 
Other sources, including FEMA, note the average core box column cross section
to be 12" wide x 36" deep x 2" thick, having a cross sectional
area of 176 square inches. The box cross section construction changed to
relatively light I-beam cross section above the 85th floor.
Hufschmid indicates the perimeter columns also decreased in thickness with
increasing floor height.  Each tower weighed 500,000 short tons
or 454545 metric tons ( 1 metric ton=1.1 short ton). The intact 16 story
section above aircraft impact weighed 58,000 metric tons. Thus, the upper 16
of the 110 floor system, or 14.5 % of the floors, weighed only about 12.7 %
of the total building weight, so the majority of the mass per floor was in
the lower floors.