WTC 7

What was WTC 7?

WTC 7 was a 47 story steel skyscraper that collapsed at 5.20 pm on September 11th 2001.

WTC 7 had the following government agencies working in it:

United States Secret Service
The Department of Defense
The Immigration and Naturalization Service
The U.S. Securities and Exchange Commission (SEC)
The Mayor's Office of Emergency Management
The Internal Revenue Service Regional Council (IRS)
The Central Intelligence Agency (CIA).

Various companies rented office space, notably:

Salomon Smith Barney (SSB), floors 28-45
Standard Chartered Bank
Federal Home Loan Bank of New York
First State Management Group
ITT Hartford Insurance Group
NAIC
EEOC
Providen Financial Management
American Express Bank International

Why did it collapse?

Cause of collapse at time of writing (August 2006) is unknown. FEMA did not manage to explain it, and NIST state that their report on WTC 7 "will appear in a separate report."

What are the main theories?

FEMA has various possible theories, but none of them are likely according to their own analysis. All of their theories attempt to explain how fires (evident in various photographs and videos) caused the weakening of the structure. They say "The specifics of the fires and how they caused the building to collapse remain unknown at this time."

Doesn't FEMA think the collapse is a bit strange?

There are hints that they suspect something out of the ordinary has happened. They introduce WTC 7 in the report with: "Prior to September 11, 2001, there was little, if any, record of fire-induced collapse of large fire-protected steel buildings."

What were the features of the collapse?

Almost free fall collapse in 6.5 seconds (approximate)
Squibs and flashes seen in some videos
Part of the penthouse on the roof drops a few seconds before the main collapse starts, shortly followed by the remainder of the penthouse and then the building itself
Debris mostly contained within the building footprint, corroborates the video evidence of the core columns failing first.

WTC 7 fell in approximately 6.5 seconds according to many sources including st911.org. This has been shown to be very close to free fall speed, which is 6.0 seconds. The main difference between WTC 7 and the twin towers collapses, is that WTC 7 lost structural support at the base of the building (whereas WTC 1&2 collapsed from the top).

There were also squibs and flashes seen on some videos[2].

WTC 7 Squibs

The floors are virtually indistinguishable due to dust along two vertical debris clouds, even though the collapse mechanism could not have occurred at the higher floors, as the entire structure is moving downwards. FEMA also thinks that the structure collapsed from the lower floors, highlighting the inconsistencies with the upper floor anomalies.

What evidence exists apart from video, audio and witness testimony?

According to NIST, they did not recover any steel from the collapse. However, FEMA (who did a short study before NIST), did some limited analysis of steel from both the twin towers and WTC 7.

Other evidence comes in the form of seismic recordings.

What do the seismic recordings show?

They record the time and the signal duration of the collapse. The signal duration was 18 seconds[11], well over double estimates for the collapse speed. Videos show part of the penthouse dropping first a few seconds before the main part of the collapse started. The seismic recordings show that there must have been some large scale collapses occuring inside the structure before the building visibly collapsed.

What's the most unexplainable hard evidence?

The melting of some parts of steel due to a combination of oxidation and sulfidation[3]. According to FEMA's Appendix C, the minimum temperature required for this process is approximately 1000 degrees centigrade.

Where else would the melting of steel occur?

It is commonly formed in small quantities during demolitions of steel framed buildings. Linear cutter charges are used to cut steel for decommissioning of structures[4],[5]. The charges create a high velocity plasma which melts through metals at high speed.

How did the sulfidation of steel happen?

FEMA has no explanation and recommends further study to determine the cause[1]. No such study has been attempted by the government.

Could the sulfidation come from linear cutter charges?

There is only limited information available on the internet regarding linear cutter charges. However, they require an explosive such as RDX[6] which does not contain sulfur. The explosive would need to contain sulfur for the sulfidation of steel to occur.

Where else has sulfidation occurred?

FEMA states that sulfidation of steel occurred in debris from one of the twin towers, WTC 1 or WTC 2. Of course it is likely that sulfidation occurred in the debris from both towers, as they underwent very similar collapses.

There is no evidence of sulfidation of steel occurring in other building collapses.

Are there any theories for the origin of the sulfur?

According to st911.org, the patent filed here: http://www.dodtechmatch.com/DOD/Patent/PatentDetail.aspx?type=description&id=6766744&HL=ON describes a thermate reaction which could be used as a cutter charge. The design is patented by the US Government.

Importantly, thermite is not an explosive reaction; it is an incendiary. However, it is not known by the author if a thermite derivative could be an explosive.

If you wanted to cut a steel beam, surely the linear shaped charge is a better idea than a thermite reaction?

Certainly a linear shaped charge with RDX would be more controllable than thermite. Of course linear charges make a noticable bang, whereas thermite is relatively quiet.

So if someone wanted to take down a building with explosives how would they do it?

Controlled demolitions often use linear shaped charges, most notably in the oil industry due to their large scale steel structures, using conventional explosives such as RDX[4].

The structural columns are weakened using linear shaped charges, with delays set to progress the collapse vertically up the building[7]. As most building cores are continuous from the base to the top, an explosion on the core at a low level causes a noticable dip of any structures on the roof before the main collapse begins. This is observable on many videos of controlled demolition, for example the Underwood Typewriter Facility here http://www.implosionworld.com/cinema.htm. The reason to weaken the core first is to cause the building to collapse inwards as it is falling. This helps to prevent asymmetric collapse, and was wrongly coined "implosion"[9].

Is it possible to use thermate/thermite for demolition purposes?

It's unlikely that thermite and its derivatives would be used to demolish a building on their own. My speculation is that thermite could be placed inside a drilled hole in a steel beam, either from above or from the side of the target beam. Thermite's products are a mix of molten iron and aluminium oxide, and high temperatures over 1000 C, as is stated in the patent[10].

However the logical inference is that thermite could be used on some core columns, with the remainder broken at some point with RDX linear cutter charges. The thermate could be used to weaken the structure, with relatively few linear cutter charges needed to initiate the building collapse.

The only real advantage of using thermate over RDX for cutting beams, is that thermate burns rather than explodes (and therefore does not create a loud explosion).

What was the extent of the fires?

It's not currently known exactly, due to the lack of hard evidence. Even when viewing WTC 7 in a limited number of video clips, they show smoke only on the south face. The smoke also appears to come out from most floors, although it's hard to know for certain as smoke is mixing in from other nearby fires. There is very little evidence of typical flames coming from the building, but they can be seen in some photos, notably this one:

It's not known at this time how fires could have started so low in the building, but it is assumed that falling debris smashed through the south face, igniting the offices. A few video clips show a fairly large amount of smoke coming out of the south wall[12]. Some people argue that WTC 7 either did or didn't have "raging fires". I think the true answer is somewhere in between. If the fires were truly raging, there would probably be large numbers of visible flames emitted. However, the smoke is evidence of fire, meaning there must have been some fairly widespread fires. It's important to remember that in the case of the Twin Towers, the combustibles in any area were thought to last only a maximum of 20 minutes (see the NIST report). Therefore the fire in WTC 7 would have had similar movement (because it is a similar type of office fire), with the fire leaving any particular area after around 20 minutes.

We know the building burned for around 7 hours. Fires generally work their way up large buildings, and so it's likely that after 7 hours many floors would have been burnt out. It's hard to explain why the collapse began at the bottom of the building. FEMA has various ideas, but none have a high probability of occurring. For example, there is a theory that stored diesel was ignited, causing massive heating and damage to the interior. FEMA think that this is extremely unlikely to happen and therefore there is a no official explanation for the collapse. There is also no explanation for the sudden symmetric failing of many columns spread out across the lower floors. Even if fuel was leaking out (and was hot enough to cause severe structural damage), it's not known how the damage caused managed to fail many parts of the building simultaneously. WTC 3, 4, 5 and 6 all had far more falling debris and fires, but they did not collapse entirely. Even WTC 6 which had most of it's interior collapse, managed to resist a complete failing of the structure.