In September 2001, the Pentagon was in the process of being retrofitted to mitigate the dangers presented by (external) bomb blasts. You can find an article on the retrofit here. Structurally, the Pentagon was reinforced concrete floors, supported by a reinforced concrete beam and column frame. To form the walls, the spaces between the concrete beams and columns were filled with ordinary bricks and mortar. The outer wall was decorated with a thin limestone facade. Thus the Pentagon was a typically constructed low rise office building. As such, it was dangerous in the event of external bomb blast.
Pictured on the left is a section of Pentagon wall. The concrete beams, columns and floor slabs are clearly visible, as is the brick infill between the concrete columns and beams. The outermost wall was decorated with a thin limestone facade (which can be seen in the photos below). The windows were two layers of ordinary glass (double glazed).
Such walls are dangerous in the event of an external bomb blast. The glass windows are particularly dangerous, as they shatter into a multitude of very sharp knives, which are then blown into the room at high speed. To eliminate this risk, the ordinary window glass was to be replaced with a tough laminated glass that is designed to absorb and resist the blast loads without shattering into small projectiles or leaving the frame as a single unit.
Merely replacing existing windows with blast-resistant ones was not enough as any powerful blast would blow the window together with its frame from its connection to the brick infill (this is due to the fact that brick-infill walls are inherently weak). The window would then become a dangerous missile, potentially killing those in the room. Thus, steel reinforcement (anchored at the top and bottom to the concrete floor slabs and not to the non-structural brick infill wall) was to be added around the windows. This steel reinforcement would also add some strength to the brick-infill walls.
Pictured on the right is a section of the wall after the retrofit. The reinforced concrete columns are shown in gray and the added steel reinforcement is in red.
Interestingly, a section of the wall (about 70 foot wide) where the aircraft hit the Pentagon had been retrofitted, but both sections immediately either side, had not.
To see that this is the case, examine the following photos of a section of the Pentagon wall immediately to the left of the impact zone. The limestone facade has been knocked off by the aircrafts impact, making it clear that this section of wall lacks the red steel reinforcement of the retrofit.
As this section was not retrofitted, the windows have two layers of ordinary glass. Notice that the glass in the window behind the burnt out car (that is, the ground floor window third from the left in the right photo) is unbroken. Now a Boeing 757 has a wingspan of 125 feet. Since we know that the aircraft hit the wall at roughly an angle of 50 degrees, it would impact some 125/sin(50) = 164 feet of wall, as pictured in the diagram below.
Hence, the above mentioned intact window, would have necessarily been struck by the aircrafts left wing. Since the window is unbroken, it clearly was not. Also, the tail of a Boeing 757 stands some 40 foot high (when its engines are resting on ground) and would have necessarily struck some of the unbroken windows on the third floor. Since many windows that would have been impacted by a Boeing 757 are still intact, it is impossible to believe that a Boeing 757 struck the Pentagon. For more on this, see this article.
The American Society of Civil Engineers Pentagon report estimates that the aircraft actually hit the wall at an angle of 48 degrees. This changes the 164 feet of wall, to 125/sin(48) = 168 feet of wall, which only makes matters worse.
By the way, an American Airlines Boeing 737 (the 100 or 200 series) fits all the known facts much better, although the fit is not perfect. Its wingspan of about 93 feet leads to a impact hole of width 93/sin(48) = 125 feet, much closer to the 120 feet reported in the American Society of Civil Engineers Pentagon report (and also found by measuring from photos). Another small adjustment, due to the fact that the aircraft was tilted at about 6 degrees to the horizontal (with the right wingtip highest), reduces the expected width of the impact hole by about another 6 inches.
The tail height of 35 feet (when its engines are resting on ground) also better matches the maximum hole height of about 25 feet. But why claim that the aircraft was a Boeing 757, if it was really a Boeing 737? For a guess at why, see this article.
Now, consider the above graphic taken from the American Society of Civil Engineers Pentagon report (local copy). This graphic really says it all. The impact hole is in gray. Red dots have been added to emphasize the wingtips and the top of the tail. Note, that the two leftmost, second floor, windows in the damaged area are not actually part of the impact hole, so the impact hole appears larger than it should. Note also, that if the left engine was pictured, it would have been partially below ground level. The graphic is on page 35. Incidently, their explanation for the lack of facade damage is:
..., the evidence suggests that the tips of both wings did not make direct contact with the facade of the building and that portions of the wings MIGHT have been separated from the fuselage before the aircraft struck the building.
Now examine the above sections of Pentagon wall. The first is to the left of the point of impact and just above where the left wing impacted. Still no evidence of the red steel reinforcement of the retrofit. The second photo is of a section to the right of the impact zone. The red steel reinforcement of the retrofit is also missing from this photo (if this section had been retrofitted then the steel reinforcement would have been visible across the top of the bottom window, where the limestone facade is missing).
In many windows you can see that the glass has broken, something the new toughened laminated glass was not supposed to do. Note also, that in places, only one of the two layers of the double glazing has broken. Note the impact damage to the floor slab where the right wingtip struck the building. For more detailed photos, see the article: The 9/11 "Missile Hits the Pentagon" Hoax.
Now have a look at the windows in the collapsed section of the wall.
Here the red steel reinforcement of the retrofit is plainly visible and the windows have not broken but have remained whole. The last three pictures have been taken from this photo (a large 1.6 MB version of it can be found here):
In conclusion: Doesn't it seem rather fortuitous, that the section of the Pentagon that was hit by the aircraft, was retrofitted, but sections either side of this were not?
And, on a different note, here are two short clips of the Pentagon collapse:
