On the 11th September 2001, 19 Hijackers flew planes into New Yorks Twin Towers, and The Pentagon, with a fourth plane crashing in Shanksville Pennsylvania.

After the airliners crashed into the world trade center, they caused massive structural damage, however the towers had been designed to withstand the collision of an airliner, and they continued to stand. But one thing its designer Leslie Robertson says was never anticipated was the effect the fuel the aircraft was carrying would have.

It is alleged that the jet fuel could not have melted steel in the twin towers and therefore there must be another reason for the collapse. This is to a point correct. However steel does not need to melt to lose tensile strength. It is a maleable and ductile metal & the burning fuel set fire to the contents of the world trade center, spilling across a large area, spilling down lift shafts, and starting a large amount of fires.

During the collisions approximately 60,000 square feet of spray on fireproofing was blown off, this fireproofing is very fragile at the best of times, and this exposed trusses which knitted the buildings core's to the outer skeleton to the intense heat of the fires raging inside the buildings.

For structures like the world trade center, A36 grade carbon steel is used to create load bearing structure of immense strength, this steel has a high carbon content; its tensile strength is 250 mega pascals, with an ultimate load strength of 400 mega pascals. But as this steel gets hotter, the carbon inside this steel vibrates, the ferrite and carbon particles spread out, and the metal becomes softer and more maleable, its ability to withstand bending moments drops significantly.
Steel begins losing tensile strength at just 300 degrees C, and steadily loses strength up to 800 degrees C, and continues losing tensile strength at a lower rate up untill its melting point of 1540 degrees C

Sources: Corus Steel

University of Manchester school of structural fire engineering

Despite the hat truss structure being designed to support twice the weight of the floors above, it was fatally weakened by fire in a way it was never designed. Under such intense stress the metal succumbed to fatigue and failed, resulting in the floors above weighing 100's of thousands of tonnes falling onto the floors below and shearing the links that bound them together.

Skyscrapers are 90% air, and as the collapsing floors fell down, the air inside was also pushed down and compressed, and floors below the wave of destruction, the compressed air took the path of the least resistance, and this meant some of it went out of the windows.

Those floors then fell with the floors above onto the floors below as the building progressively collapsed and the floors were crushed together. The speed of the collapsing floors of the north tower reached 125mph, and the force of the impacts on the floors below showered debris away from the tower. Some of it tearing a 90 foot gash into the lower floors of WTC7 and starting fires inside that building. Diesel pipes for emergency power systems inside WTC 7 were severed, releasing diesel fuel onto the fires which had broken out. The collapse of the twin towers damaged the water mains in the area, cutting off the sprinkler systems for WTC 7.

WTC 7 was so badly damaged, that the decision was made to pull fire teams away from WTC 7.

Some allege that RDX based plastic explosive charges were used to bring down the twin towers, to me this seems absurd, RDX is flammable (thats how it works) it melts at 206 degrees C and has an ignition temperature of 234 degrees C. If detonated RDX explodes with a velocity of 26,000 feet per second. There was an intense fire, and in the initial 1000 degree's C blaze it is highly likely that any explosive charges would either immediately detonate (immediately bringing down the towers) OR burn off harmlessly.

Some instead allege that thermite was used and that molten pools of steel were found in the building. Thermite is based on Aluminium, and the skins of the aeroplanes which crashed into the towers were made of aluminium (as was some metal in the world trade center), it is quite possible that upon oxidising in the fire, the aluminium of the aircraft burned producing a small thermite reaction. It must be pointed out though, that the 'pools of metal' conspiracy theorists refer to were actually the crushed wreckage of several floors mangled together as the building pancacked down.



More to come...