The collapse of the south tower and the North tower are both because of the same reason; The heating of the key internal supports, Though we will focus on the south tower, as it collapsed first, and received more serious damage from the impact of the plane. The Boeing 767-200ER, united airlines flight 175 weighed 125.9 metric tonnes when it collided with the southern side of the WTC2. It was travelling at 870km/h it had a force of 110 000 kN The weight of the plane consisted of 98656kg of the airplane, cargo, passengers and other miscellaneous items, and 30000kg of jet fuel. The plane hit the southern side of the WTC2 at an angle of 6° vertically, 15° laterally, with a roll angle of 38° clockwise.
These factors all increased the damage to the WTC2 through a number of ways, Firstly, that the damage was spread over more of floors, meaning more chances to damage something critical, secondly, because of this the fuel and ensuing fires were spread over more floors, meaning the floors in the centre were surrounded by two or three floors of fire, both above and below it. And thirdly, as the plane entered diagonally across one corner of the tower, it acted much as a three legged table would, unbalancing one corner of the tower. Instead of the WTC1 where the plane collided at nearly 90° to the face of the building, in the centre, mainly damaging the internal supports on one side compared to the WTC2s damage to the internal supports on one corner.
As you can see, the planes collided on different sides of the separate towers; the WTC1 was hit on its northern side, while the WTC2 was hit on its southern side.
The Structure of the WTCs was common of the time they were built. They had a strong core surrounded by a latticework of steel supports supporting the outer wall. This latticework was aimed at stopping the collision of a plane, although a smaller one than the 767.
(generic design for each level of the WTC 1&2)
Concerning the actual steel involved in the construction, it would have been impossible to have exactly the same steel used for all supports. In fact, 2 0f the 24 samples of the perimeter columns had unsatisfactory yield strengths to NIST standards, as well as 2 of the 8 core column samples. There were several different types of steel involved in the collapse of the WTC but to summarise, we will focus on two types of steel used; the steel with a yield force of 4.1x108 N/m2 for the perimeter columns and 2.5x108 N/m2 for the core columns. Whenever steel is mentioned from now on, it will be pertaining to steel with yield strength of 4.1x108 N/m2 when pertaining to the perimeter columns and 2.5x108 N/m2 when pertaining to the core columns, that is, not concerning broad statements. The perimeter columns had a static stress of 4.2x108 N/m2 and a longitudinal strain of 0.0160m at room temperature. And the core columns had a static stress of 2.6x108 N/m2 and a longitudinal strain of 0.0336m at room temperature. Meaning that,
at room temperature.