The large open floor space of the WTC

Each floor of the towers possessed an open unobstructed area of roughly 200 by 200 feet. The pursuit of adaptable and open floor space by the client, and satisfied by the engineer, resulted in a problem for the fire service which was two-fold: the potential incidence of fire on each floor approached an acre in area, virtually impossible to extinguish under normal conditions (typical fire loading and serviceable standpipes and elevators), and the absence of intermediate walls and partitions which would have served as redundancies in the structural system of the frame. Interior walls would have both compartmentalized the floor area to resist the spread of fire and simultaneously provided an alternate load path in the event of floor collapse from above. [Firehouse]

Typical Floor Plan of the World Trade Center

See also: The 9/11 WTC Collapses: An Audio-Video Analysis

What Really Happened