The complete collapse of the Twin Towers has more to do with the construction than anything else. Yes, it was the loss of fireproofing and subsequent fires that collapsed the building but if it were not for the construction, a total collapse may not have occurred. Ironically, the construction also saved many lives by letting the towers stand as long as they did. These pages explain the construction and provide evidence for collapse by fire.
"Tube in a Tube"
The designers coined the term "Tube in a Tube" to describe the buildings’ construction. The design was an innovation of its day. Thick steel columns ran up the middle of the building. This housed the elevators, stairwell, electrical conduits, water, sewer and other services which ran up and down the length of the buildings.
For you to understand the collapse, you will have to remember four main elements.
Core columns
Perimeter columns
Floor Trusses
Fire proofing
The columns of the building normally found evenly spaced out on a given floor became the outer wall of the building. This left large open areas for renting. A good explanation of this can be found on PBS.
Instead of encasing each column in heavy concrete, (normal fire proofing) the designers relied on 'sprayed on' fire proofing with a 2 hour rating to protect the load bearing columns and trusses from fire.
Each WTC tower was essentially bolted together like an erector set. Each average floor had 376 5/8" bolts, 188 Truss seats, 120 ViscoElastic pieces and 120 gusset plates.
Below is a diagram of the truss system with connections.
Trusses connected Perimeter columns to the core. Without this connection neither one could stand alone. Note how the columns are constructed in threes known as column trees. Each column is bolted onto the column beneath and to the tree beside it.
There was a 3 to 4 inch layer of concrete on the average floor. There were reinforced concrete floors in the core as well. Contrary to what some conspiracy theorists say, the core walls were NOT concrete reinforced. The columns in the core were also not incased in concrete. This was an error made by the BBC which grew a life of its own. Here is the article. Note the date it was created, Sept. 13, 2001, just 2 das after the collapse.
http://news.bbc.co.uk/1/hi/world/americas/1540044.stmMore evidence of no concrete protecting the columns.
http://www.lafire.com/famous_fires/2001-0911_WTC/2002-0500_LA-Firefighter_WTC-BuildingCollapse_Dunn.htmHere is a photo of the core sub levels during construction. You can see just how deep below ground level the buildings went. You can also see no concrete around the core or columns anywhere.
This type of construction makes a very open floor plan.
The fire proofing was sprayed on after the steel went up. This is called "Passive" fire proofing.
There is a big difference between this type of construction and others like the Landmark Towers in Fort Worth, Texas, which is often used as a comparison by conspiracy theorists.
There are large I-Beam columns encased in concrete. Large I-Beams also hold up the floors. The photo below was taken during preparations to demolish the building.
Note the vertical column above still has some concrete around it. If the towers had concrete around the columns, you would have seen columns with damaged concrete around them similar to this.
The Windsor Tower in Madrid is similarly built but with a concrete reinforced core added.
Now you have the general idea of the differences in construction between the towers and other high rise buildings. So why did it collapse the way it did?
A major factor was the fire proofing on the impact floors. The impact blew off the fire proofing as in the above PBS special.
