The multiple floors of a high-rise building create the cumulative effect of requiring great numbers of persons to travel great vertical distances on stairs in order to evacuate the building. In the evacuation of the World Trade Center high-rise office towers following the terrorist bombing in 1993, the tens of thousands of building occupants successfully and safely traversed some five million person-flights of stairs.
Evacuation and planning information
NFPA report
"High-Rise Building Fires"