Fires ignited by falling debris (from WTC 1) are the sole cause of the WTC 7 collapse.
- Debris from the collapse of WTC 1, about 350 ft. to the south,
impacted WTC 7, igniting fires on at least 10 floors.
- Other than starting the fires, the debris impacts were not a
- Fires on six of those floors grew from the time of the debris
impact (10:28:22 a.m.) and lasted until the building collapsed
- After several hours, the heat absorption ability of the
fire-proofing began to fail. The building's structural steel began to
soak up heat from the fires.
- A ''seated connection'' was used for some of the girders framed to
interior columns. The girder was held by four "erection
bolts" (''girders'' span between columns; ''beams'' span
- A 13.7 m (540 in.) long W33x130 beam heated uniformly to 600 °C will
expand along its length by 4.5 inches.
- In doing so it will produce a lateral force of 6.9 million pounds
(30.7 million newtons).
- The four erection bolts resisting this force had a total shear
capacity of 0.180 million pounds (i.e., 2.6 percent of the possible
- By the time the girders reached 164 °C the four bolts had failed
(i.e., torn in two, ripped apart, cleaved in twain, etc.).
- With the failure of the bolted connection, further thermal
expansion of the floor beams pushed a 13th floor girder,
between Columns 79 and 44, off its seat.
- The flooring system on Floor 13 subsequently failed, collapsing
onto the floors below, resulting in the collapse of floors 12 through 6.
- The floor collapse left more than 150 ft. of Column 79 without
- Without structure transferring load laterally, Column 79 buckled
- The transfer of load, once supported by Column 79, overwhelmed the
structural capacity of the other columns. They too failed.
- After burning for 6 hours and 52 minutes, WTC 7 suffered a
fire-induced progressive collapse. The building was completely
1. Erection bolts are not structural connections
In a seated connection, the beam or girder was supported by the seat,
which was welded to the column. Bolts were installed that fasten the
beam or girder to the seat for erection purposes These erection bolts
did not carry any gravity load; rather, they were installed to insure
that the beam or girder was held in place during erection. NIST found
no evidence that the girders or beams in WTC 7 were welded to the
seats. In a similar way. an angle or plate was bolted to the top
flange to prevent the beam or girder from twisting, but there was
little restraint to bending in the plane of the beam.
Consider the girder that spanned between Column 79 on the interior of
the building and Column 44 on the exterior. Thermal expansion of this
girder would have loaded the erection bolts in shear, since (1) there
were no shear studs anchoring the girder to the slab (and thereby
restraining elongation), and (2) the columns were prevented from
lateral movement because they were embedded in the floor slabs which
had considerable in-plane stiffness. Additionally, the expansion of
floor beams that framed into this girder, because the framing was
asymmetrical, tended to add additional shear load to the erection
bolts. The combination of these two shear loads could have failed the
bolts in shear. If the erection bolts were to fail, then there would
be no positive attachment preventing the girder from being pushed off
the seat. Source: (NCSTAR 1-9) VOLUMES 1 and 2, page 348.
2. Thermal expansion will break stuff
The first failures observed were of the shear studs, which were
produced by axial expansion of the floor beams, and which began to
occur at fairly low beam temperature of 103 °C. Axial expansion of the
girder then led to shear failure of the bolts at the connection to
Column 79; and, at a girder temperature of 164 °C, all four erection
bolts had failed, leaving that end of the girder essentially
unrestrained against rotation. Continued axial expansion of the floor
beams pushed the girder laterally at Column 79, as shown in Figure
8-26, in which failed shear studs and bolts were evident. When the
beam temperatures had reached 300 °C, all but three shear studs in the
model had failed due to axial expansion of the beams, leaving the top
flanges of the beams essentially unrestrained laterally. Continued
axial expansion of the girder caused it to bear against the face of
Column 79, generating large axial forces that led to failure of the
bolts connecting the girder to Column 44. When the girder temperature
had reached 398 °C, all four erection bolts at Column 44 had failed,
leaving the girder essentially unrestrained against rotation at both
ends. After failure of the erection bolts in the seat at Column 44,
continued axial expansion of the floor beams pushed the girder
laterally, where it came to bear against the inside of the column
flange. Axial compression then increased in the floor beams, and at a
beam temperature of 436 °C, the northmost beam began to buckle
laterally. Buckling of other floor beams followed as shown in Figure
8-27(a), leading to collapse of the floor system, and rocking of the
girder off its seat at Column 79 as shown in Figure 8-27(b). The
collapse process took time to occur in the LS-DYNA analysis, during
which the temperatures had ramped up to their maximum values in the
simulation. Source: (NCSTAR 1-9) VOLUMES 1 and 2, page 352.
3. Collapse Initiation
The simple shear connection between Column 79 and the girder that
spanned the distance to the north face (to Column 44) failed on Floor
13. The connection failed due to shearing of erection bolts, caused by lateral thermal expansion of floor beams supporting the northeast
floor system and, to a lesser extent, by the thermal expansion of the
girder connecting Columns 79 and 44. Further thermal expansion of the
floor beams pushed the girder off its seat, which led to the failure
of the floor system surrounding Column 79 on Floor 13. The collapse of
Floor 13 onto the floors below-some of which were already weakened by
fires-triggered a cascade of floor failures in the northeast region.
This, in turn, led to loss of lateral support to Column 79 in the
east-west direction over nine stories (between Floors 5 and 14). The
increase in unsupported length led to the buckling failure of Column
79, which was the collapse initiation event. Source: (NCSTAR 1-9)
VOLUMES 1 and 2, page 611.
4. "Free fall" is a myth
To further clarify the descent of the north face, NIST recorded the
downward displacement of a point near the center of the roofline from
first movement until the north face was no longer visible in the
video. Numerical analyses were conducted to calculate the velocity and
acceleration of the roofline point from the time-dependent
displacement data. The instant at which vertical motion of the
roofline first occurred was determined by tracking the numerical value
of the brightness of a pixel (a single element in the video image) at
the roofline. This pixel became brighter as the roofline began to
descend because the color of the pixel started to change from that of
the building façade to the lighter color of the sky.
The approach taken by NIST is summarized in NIST NCSTAR Report
1A, Section 3.6, and detailed in NIST NCSTAR Report 1-9,
The analyses of the video (both the estimation of the instant the
roofline began to descend and the calculated velocity and acceleration
of a point on the roofline) revealed three distinct stages
characterizing the 5.4 seconds of collapse:
- Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
- Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall).
- Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity.
:This analysis showed that the 40 percent longer descent time—compared
to the 3.9 second free fall time—was due primarily to Stage 1, which
corresponded to the buckling of the exterior columns in the lower
stories of the north face. During Stage 2, the north face descended
essentially in free fall, indicating negligible support from the
structure below. This is consistent with the structural analysis
model, which showed the exterior columns buckling and losing their
capacity to support the loads from the structure above. In Stage 3,
the acceleration decreased as the upper portion of the north face
encountered increased resistance from the collapsed structure and the
debris pile below. Source: Questions and Answers about the NIST WTC 7
Claims, assertions and questions :
Many of the claims and assertions by WTC 7 conspiracy theorists can be found at WTC 7 Draft Reports for public comment. The submission by Richard Gage of Architects & Engineers for 9/11 Truth is a good representation of how 911 conspiracy theorists rely on a lack of evidence to somehow draw conclusions (see: argument from ignorance).
Investigation by the National Institute of Standards and Technology*:
The National Institute of Standards and Technology (NIST) is responsible for conducting fact-finding investigations of building failures that resulted in substantial loss of life or that posed significant potential of substantial loss of life.
Final Report on the Collapse of World Trade Center Building 7
(The results of all NIST WTC investigations can be found at : NIST and the World Trade Center)
NIST: Questions and Answers about the NIST WTC 7 Investigation
How did the fires cause WTC 7 to
The heat from the uncontrolled fires
caused steel floor beams and girders
to thermally expand, leading to a
chain of events that caused a key
structural column to fail. The failure
of this structural column then
initiated a fire-induced progressive
collapse of the entire building.
According to the report's probable
collapse sequence, heat from the
uncontrolled fires caused thermal
expansion of the steel beams on the
lower floors of the east side of WTC
7, damaging the floor framing on
How hot did WTC 7's steel columns and floor beams get?
Due to the effectiveness of the
spray-applied fire-resistive material
(SFRM) or fireproofing, the highest
steel column temperatures in WTC 7
only reached an estimated 300 degrees
C (570 degrees F), and only on the
east side of the building did the
steel floor beams exceed 600 degrees C
(1,100 degrees F). However,
fire-induced buckling of floor beams
and damage to connections-that caused
buckling of a critical column
initiating collapse-occurred at
temperatures below approximately 400
degrees C where thermal expansion
dominates. Above 600 degrees C (1,100
degrees F), there is significant loss
of steel strength and stiffness. In
the WTC 7 collapse, the loss of steel
strength or stiffness was not as
important as the thermal expansion of
steel structures caused by heat.
Some people have said that a failure at one column should not have produced
a symmetrical fall like this one.
What's your answer to those
WTC 7's collapse, viewed from the
exterior (most videos were taken from
the north), did appear to fall almost
uniformly as a single unit. This
occurred because the interior failures
that took place did not cause the
exterior framing to fail until the
final stages of the building collapse.
The interior floor framing and columns
collapsed downward and pulled away
from the exterior frame. There were
clues that internal damage was taking
place, prior to the downward movement
of the exterior frame, such as when
the east penthouse fell downward into
the building and windows broke out on
the north face at the ends of the
building core. The symmetric
appearance of the downward fall of the
WTC 7 was primarily due to the greater
stiffness and strength of its exterior
frame relative to the interior
Did investigators consider the possibility that an explosion caused
or contributed to the collapse of WTC
Yes, this possibility was investigated
carefully. NIST concluded that blast
events inside the building did not
occur and found no evidence supporting
the existence of a blast event...
Is it possible that thermite or thermate contributed to the collapse
of WTC 7?
NIST has looked at the application and
use of thermite and has determined
that its use to sever columns in WTC 7
on 9/11/01 was unlikely.
Thermite is a combination of aluminum
powder and a metal oxide that releases
a tremendous amount of heat when
ignited. It is typically used to weld
railroad rails together by melting a
small quantity of steel and pouring
the melted steel into a form between
the two rails...
...Analysis of the WTC steel for the
elements in thermite/thermate would
not necessarily have been conclusive.
The metal compounds also would have
been present in the construction
materials making up the WTC buildings,
and sulfur is present in the gypsum
wallboard used for interior
An emergency responder caught in the building between the 6th and 8th
floors says he heard two loud booms.
Isn't that evidence that there was an
The sound levels reported by all
witnesses do not match the sound level
of an explosion that would have been
required to cause the collapse of the
building. If the two loud booms were
due to explosions that were
responsible for the collapse of WTC 7,
the emergency responder-located
somewhere between the 6th and 8th
floors in WTC 7-would not have been
able to survive the near immediate
collapse and provide this witness
*NIST is an agency of the U.S. Department of Commerce.