So what did happen on 9/11? How did two airplane crashes bring down two 110-story skyscrapers? How did the collapse of WTC2 and WTC1 also cause the collapse of WTC7? What exactly happened?
If the answers to those questions interest you, keep reading.
In the sibling post I wrote about the conspiracy crazies who believe the Towers and WTC7 were brought down deliberately by our government using explosives.
A somewhat less utterly preposterous theory suggests it was American and/or Israeli agents the flew the planes. In some versions the passengers were off-loaded and vanished somehow, in others they were captives. (Which ignores the air-to-ground phone calls made by some of those passengers. Melissa Etheridge has a heart-breaking song about one such case.)
But here’s what the NIST scientific analysis of the events found.
Let’s start with a breakdown of the events of that day:
- 08:46:30 AM American Airlines flight 11 (a Boeing 767-200ER with 11 crew & 76 passengers), flies into WTC1, floor 96, at 440 MPH.
- 09:02:59 AM United Airlines flight 175 (a Boeing 767-200ER with 9 crew & 51 passengers), flies into WTC2, floor 81, at 540 MPH.
- 09:58:59 AM WTC2 collapses (00:56:00 after impact)
- 10:28:22 AM WTC1 collapses (01:41:52 after impact). The collapse of WTC1 causes notable damage to WTC7, including starting fires on multiple floors.
- 11:30 AM No water to fight fires in WTC7
- 02:30 PM Official order to cease efforts with WTC7
- 04:33 PM C on Edison shuts off power to sub-station under WTC7
- 05:20:52 PM WTC7 collapses
The first plane’s impact centered on floor 96 and damaged floors 93-99. That left ten floors intact above. The second plane’s impact centered on floor 81, damaging floors 77-85. That left 24 floors intact above. The second impact leaving over twice as many intact floors above it is a key reason the second Tower fell first, only 56 minutes after impact. It took the first Tower nearly twice as long, 101 minutes, to fall.
When WTC1 fell (around 10:30 AM), its collapse caused considerable damage to WTC7. About a half hour later, fires were reported on multiple floors. Four hours after the collapse (at 2:30 PM) efforts to save WTC7 were abandoned. Emergency workers had their hands full, and many had already died in the collapse of the Towers. Fires in WTC7 continued to burn unchecked.
The fires in WTC7, as well as those in the Towers, had plenty to feed on. Offices are filled with office furniture made from wood and plastic, lots of paper, and considerable cloth. Most fire protection measures are designed to prevent fires from spreading to other offices, but do little to stop a fire already burning in an office.
In the case of the Towers, jet fuel from the fully loaded planes played a big role in getting massive fires started on multiple floors. This is one reason the Towers fell so quickly, but WTC7 took almost seven hours for to fall.
And it was, indeed, the fires that caused the collapse in all three cases.
Conspiracy crazies claim this supports their theories, because fires feeding off wood, paper and plastic are not hot enough to melt steel. They’re not even hot enough to cause significant softening, even factoring in the jet fuel.
They are correct about the temperature of the fires, but it was not softening of the steel beams, let alone melting, that caused the collapse. The short version of what happened is that the fires ended up damaging the integrity of the steel frame, and that is what caused the collapses.
In order to fully understand this, you need to understand a bit about the design and construction of the WTC buildings. You also need to appreciate one simple fact about metal: it contracts when it’s cold and expands when it’s hot. (Anyone who lives in an area that experiences both winter and summer has likely encountered this directly). To see why that is at fault, we need to consider the design of the buildings.
The World Trade Towers used a skyscraper design that was unique at the time. Most skyscrapers consist of a metal framework that supports the building. This framework looks essentially like a big set of cubes stacked together.
The structure provided by the framework is rigid and supports its own weight as well as the weight of the concrete floors and everything else in the building. A consequence of this design is that it requires vertical beams at regular intervals. Large offices necessarily have vertical columns throughout.
The World Trade Towers used a different design. To provide an entirely open inner space (a big selling point at the time), the building was constructed around a central core containing elevators, stairs, heating/cooling vents, water/sewage pipes and all electrical and phone lines. Unlike most skyscrapers—where the outer wall is just a facade hung on the frame—the WTC outer walls were designed to be load bearing. The outer walls in this case were an important part of the construction.
Each floor consisted of a set of horizontal beams. Heavy long beams (60 or 35 feet) hung between the inner core and the outer wall. Shorter bridging trusses hung between the long beams to provide support and rigidity. As in all such buildings, corrugated sheet steel lay atop the beams with a 4-inch concrete floor poured over. This design allowed the entire floor to be open, with no vertical beams. The only obstruction was the central core.
This design is exactly what caused the collapse, and it is why the buildings fell straight down (another thing conspiracy crazies cite as “proof” of deliberate demolition).
Structural steel has a thermal coefficient of about 0.0000063 inch/inch per °F. The NIST study estimates the fires reached about 1100 °F. (Candle flames burn as hot as 2,600 °F, but structural fires are “cooler” due to incomplete combustion.) Note also that, while it certainly doesn’t melt or bend, steel does lose its strength at these temperatures.
The formula for thermal expansion is:
Plugging in a 60-foot (720-inch) beam and a Temp-Diff of 1,030 °F (1100 – 70):
So the long beams, the primary floor supports, expanded by nearly 5 inches!
And since these beams were mounted only at their end points, the expansion plus the weakening of the steel caused the beams to buckle and bust off their supports. This caused the floors to collapse. As I’ll touch on in a moment, this was the primary source of the collapse of WTC7.
Remember that the outer wall was structural. It carried part of the building weight. When the planes blasted through the outer walls of the Towers, that support was lost. The plane moving through the floor also stripped the fire-proofing from the beams, exposing them to the fire.
The fires also affected the vertical beams in the core. In their case, thermal expansion was counteracted by the weight of the upper floors. This caused them to compress which reduced the load on them. The roof trusses (designed to spread the load between the core and outer wall) transferred the load imbalance to the outer walls. Except one of those walls was badly damaged by the plane. Even without that damage, the building design was not intended for such an imbalance. With the load balance severely compromised, the building could no longer bear the weight of the upper floors.
A close examination of the available video shows the upper part of WTC2 tilting towards the damaged wall just as it begins to collapse. Each floor of the Towers contained 43,000 square feet of concrete and sheet metal, 60 long iron beams, 24 short iron beams, considerable other metal supports and all the office contents. Multiply that by 10 for WTC1 and by 24 for WTC2. That’s a huge amount of weight that, once it starts downwards, has incredible inertia.
All that fell straight down into what was essentially a square tube. Once the collapse began, the strong outer walls below tended to somewhat contain the collapse leading the building to “pancake.” The crazies claim the buildings fell too fast; some claim they “free fell.” This is clearly false, as the videos show the expended debris falling faster. The Towers fell as fast as they did due to the sheer amount of weight in motion. As that mass hit each floor it smashed through them easily. And each floor added to the mass, amplifying the effect on the lower, undamaged, floors.
In the case of WTC7, there was a somewhat similar scenario. The fires caused horizontal beams to expand, buckle and come off their mounts. This left a key vertical beam unsupported. The building’s design used the horizontal beams to hold the vertical beams in place. Without that support, the vertical beam buckled from the building load. This caused WTC7 to collapse inwards.
And that is the horrifying story of what happened that day eleven years ago. Various careful computer models and physical simulations have demonstrated the behaviors described above. The physics, the logic and the facts all fit.
Given that the conspiracy theories aren’t supported by logic, reason or facts, the conclusion is crystal clear.
(Click on the illustrations above for a bigger image. All are from the NIST reports.)