How long did it take the Twin Towers to collapse?
The South Tower collapsed 56 minutes after being hit, and the North Tower 102 minutes after being hit. Once the buildings began to fall, each took less than 30 seconds to fully collapse. The South Tower, though hit second, collapsed first.
Parts of the South Tower reached the ground about 9 seconds after its collapse began at 9:59 AM EDT, approximately 56 minutes after being hit. The central core of the lower 40 floors caved in within 25 seconds thereafter. Debris flattened or filled the seven basement levels within a few seconds.
The TV mast and roof from the North Tower struck the ground about 11 seconds after the descent began, at 10:28 AM EDT, 102 minutes after being hit. The central core of the lower 60 floors completely collapsed within 10 to 15 seconds after being bypassed by the falling upper floors.
Most of the interior collapse was hidden by dust and debris, which also spread over the surrounding buildings and streets.
If a tornado had struck the twin towers before their destruction on 9/11 they would likely have suffered broken windows but would probably not have collapsed. EF5 damage to a high rise building is described as "significant structural deformation," which could result in collapse. However, there has never been an F5 or EF5 tornado recorded in the state of New York.