Collapse of the World Trade Center Towers, 4continued The Impacts Each tower was hit by a Boeing 767. The impact on the North tower was near the center of the North face at around the 95th story, with the plane hitting the side of the building square on and flying level. The impact on the South tower was some 15 stories lower, with the plane hitting the South face near the South East corner and impacting at an angle to the face of the tower. Fig 2 shows the impact directions and Fig 8 shows the plane about to hit the South Tower. 
Fig. 8 Plane About to Hit South Tower (Photo from CNN) The potential force of the impact from each plane can be approximately calculated and the figures are very large. The weight of each plane would have been approximately 150 tons, according to the media reports and Boeing data on this type of plane. The plane would have been traveling at around 800 kms/hour at impact. This gives a momentum of 150x800/3.6=33,333 tons.m/sec. If the plane was arrested by the building in effectively 0.6 seconds, which is a reasonable estimate based on a linear deceleration over the 63.5 m width of the building, then the force exerted on the building is the momentum/effective time to arrest, i.e. Force=33,333/0.6=55,555kN. To put that in perspective, the ultimate limit state design wind pressure over the entire height of the building is 220 kg/m 2 . This gives a ULS wind force on one face of the building of 58, 400 kN. Thus the potential force of impact from the plane is 95% of the design ultimate limit state wind load on the building! Especially in the case of the North Tower, not much of the plane was ejected from the building, so it is reasonable to assume the most of that potential force was absorbed by the building. Also the above calculation also does not take into account any additional force generated inside the building from blast loading due to, for example, exploding jet fuel. Having done this calculation it is more easy to understand what our eyes showed us – namely the planes slicing through the perimeter frames “like a knife through butter” as one reporter has stated. I contend that, having penetrated the perimeter frames the planes would have done much more that just stripping the fire protection off the columns as has been surmised by some commentators. The effect would have been to completely shatter and eliminate large areas of floor slab and many of the internal supporting columns, thereby immediately destroying much of the vertical load carrying system and leaving the rest vulnerable to any subsequent fire attack. This impact damage - not the severity of the fire – I contend is the principal cause of the ultimate collapse. However the nature and position of impact was different in each case and this led to different effects on each tower, with different collapse mechanisms. These effects are now discussed in as much detail as is possible based on the known information. continued... Related Articles | 
