This is long, feel free to skip it.
The Popular Mechanics article is a great report. I hope some of these insensitive - ahem - 'ill-informed' people would take a look at it before spouting their political, biggot, conspiracy theories. I have something to add to that about the two building's demise, which i hope should give people comfort as it did me, to learn the how's and why's. It seems appropriate, to help dispell rumors.
In a conventional skyscraper, the iron structure is a grid of columns and beams. The higher the building, the more weight the structure needs to bear, and the more columns are used. If you look at the Empire State Building, its floors are a mass of columns. The design of the two towers was very innovative. In fact it was revolutionary. The designer saught to free up floor space by taking all those columns out of the floor space, and placed them in to the external wall. The vertical lines running up the side of the towers were not facade - those were the MANY iron columns doubled up on the out-side walls. The core of the building was linked t othe external wall by horizonal 'vvvv' shaped trusses. The weight was thus trasferred from many columns in the middle, to a reinforced outside wall, and ther result was office space free of the clutter of columns.
Sound weak? it's not.. the buildings structure was designed to be strong enough to hold the weight of the building, and also be able to take the impact of an airplane - the largest plane at the time, the B767.
When the planes hit the buildings, there was a massive explosion from the jet fuel, the building's shook and swayed, but the building's structure held. The designer's plan had worked. However there was something that was not taken in to account - fire.
Steel is sprayed with foam fireproofing. However, the impact of the planes simply blew the fireproofing off of the steel, like blowing dust off of a table top. The steel was now exposed to fire, and not just the fire from jet fuel. This was an office - there was paper, furniture - fabrics and chemicals which burn hot. It was this raging fire on open steal that did not melt but became weakend. The trusses linking the building's core to the external steel were fastened - not poorly or incorrectly but - relatively lightly. The trusses weakend and began to fall from btween the core and external wall. As these horizonal trusses began to fall, the exterior wall, bearing the weight of the floors above, began to pull away from the core, and the structure was compromised. The supporting steel, now disconected from the core, could not take the weight of the upper floors, and they pancaked - when a floor loses its support and falls to the floor below it, which can not take the extra weight, and itself collapses. Notice on replays that the building collapsing downward pours out at the sides.
The 2nd tower to be hit fell first. It wasn't burning as long. But it was hit far lower in the structure. The structure had to bear the weight of more floors, and was compromised sooner.
By the way - gravity pulls all things falling down at the same pace. Drop a bolling ball and a book from a plane and, excluding wind resistance on their round vs flat surfaces, they will hit at the same time.
The collapse of the structure was not the only excuse for loss of life. People had some time to get out. But there was a problem with the way the towers - and a lot of buildings - were designed. The core, the central block housing elevaters and emergency stair towers, was surrounded by fire retardant drywall - sheetrock. However the impact of the planes blew this drywall clear away - like the fireproofing - exposing the stairs to fire. Also, the planes sheared through the core. Anyone above the impact floors had no way to get down.
Buildings now are being designed in different ways.
For the last several years i've worked for a construction management company. I was in marketing, but i had access to a lot of interesting materials. In addition, my father is the engineer responsible for 1701 JFK Blouvevard ( Comcast Center ) in Philadelphia - which at 1007 ft will be the tallest building between new york and chicago. This tower was re-designed in direct response to the two towers.
The building has a concrete core. Not steal covered in drywall. It is a grid of 52 inch thick walls, and every day after it's been poured, it continues to get harder and harder. ( "Dad, if it's so strong, how will you ever take it down in 100 years? You don't. ) You can ram a plane in to it. You can blast it at the base. You can not punch through it. If a plane were to hit it and/or a terrible fire ensued, by the time the steel was weakened enough to collapse, everyone in the building could be inside the core safetly making their way down the stair towers. The building would collapse, and you were see this ominous concrete pillar towering over the city ( it already is ).
In addition to things like a concrete core, fireproofing has been enhanced since 1975 - with great improvements in the last couple years. It doesn't blow off from impact, and it resists greater temperatures over longer periods of time. Most buildings also do not use trusses like the two towers did.
I hope this helps some people understand what happened...
