THREE DISTINCT loading events triggered the collapse of the World Trade Center's twin towers on 11 September, according to last week's American Society of Civil Engineers/Federal Emergency Management Agency report World Trade Center Building Performance Study.
They were: aircraft impact, fire and structural failures.
The failure modes of WTC 1 and 2 varied because of the different location of the impacts and speeds of the hijacked Boeing 767 planes, says the report.
The first plane, travelling at 766km/h, hit WTC1, the north tower, at 8.46am between floors 94 and 98. The building collapsed one hour and 48 minutes later.
The south tower, WTC2, was hit at 9.03am between floors 78 and 84 by a plane flying at 944km/h. It collapsed after 56 minutes.
The first impact destroyed up to 36 perimeter columns in WTC1 over a four storey range, says the report, produced after extensive examination of video footage and interviews with survivors.
Parts of the plane which struck WTC1 travelled through the building, and were likely to have caused significant damage to core columns.
Interviews with survivors on the 91st floor 'suggest the possibility of immediate partial collapse of the framing in the central core, ' says the report.
While some localised floor collapses occurred near the impact, floor loads originally supported by the exterior columns were transferred to other load paths. Axial loads on some columns were increased by a factor of six, but because of their design for wind loading resistance, these columns still retained sufficient capacity for these extra gravity loads.
Loads were successfully redistributed after the impact, returning the building to a stable state.
'However, the structure's global strength was severely degraded. Although the structure may have been able to remain standing in this weakened condition for an indefinite period, it had limited ability to resist additional loading and could potentially have collapsed as a result of any severe loading event, such as that produced by high winds or earthquakes, ' the report notes.
Collapse of the adjacent WTC2 is dismissed as a cause of the WTC1 failure. Its final demise was caused by the intense fires which followed, says the report.
Both planes which struck the twin towers are estimated to have been carrying around 44,000 gallons of fuel, with around one third of this burning in a fireball rather than in a brief explosion lasting microseconds which would have caused greater structural damage.
Some blast type overpressures developed through liftshafts from burning vapours, adds the report.
Remaining fuel helped burn all combustible materials on the floors involved, bringing temperatures up to 1000 0C in a series of separate simultaneous fires, assisted by a ready supply of air from breaking windows.
The impact would have rendered fire protection systems useless, says the report. So many sprinkler heads were opened that pressure would have dropped dramatically, preventing water distribution while the rapid spread of fires prevented use of hoses by occupants.
The report cites the combination of loss of fire protection material and reduced strength of structural elements due to temperature increase as major factors.
It says that sagging of horizontal framing elements due to heat would have occurred, and, combined with loading from debris, would have hastened pancake failure of floors.
Loss of floors would have increased the laterally unsupported lengths of columns, causing buckling, it says. The dynamic effect of falling floors would also have hastened collapse.
Several factors caused WTC2 to collapse more quickly than its neighbour. The higher plane speed caused greater damage, while the lower impact height meant that columns taken out were carrying much greater loads than those hit in WTC1, so initial structural damage was more significant.