Try asking the Port Authority for the Building Code that the WTC architects abd engineers were supposed to follow.
Apparently as a NY/NJ bi-state agency, they are not required to the follow the New York City Building Code.
Ok, so then they would need to follow the NYS State Code one would figure, but apparently (according to newspaper articles on the subject), the Port Authority determined they were only required to follows their own Port Authority building code.
Apparently, (again as stated in newspaper articles on the subject) they can't find that document also.
While this didn't impact structural engineering since all codes reference AISC and ACI codes, apparently this "no code" code allowed the Port Authority to omit sprinklers and other fire protection measures.
Some of these deficiencies (sprinklers for example) were apparently rectified after the 1991 events.