Highlights of NIST's new WTC Mechanical and Metallurgical Report

[stickdog]

Summary:

1) No WTC-7 steel was recovered or analyzed.

2) No unprocessed, intact floor trusses were recovered or analyzed.

3) No testing for explosives (or sulfidation or other residue of any kind) was performed.

4) Only 12 total core columns were recovered from WTC-1 & WTC-2 combined).

5) Of the recovered core pieces, none showed exposure to tempertures in excess of 250 C.

6) Of 170 examined areas on the perimeter column panels, only THREE showed exposure to temperatures in excess of 250 C and for at least one of these three other forensic evidence indicated that the high temperature exposure happened AFTER the collapse.

7) No recovered steel showed any evidence of exposure to temperatures above 600 C for any significant time.


http://wtc.nist.gov/pubs/NISTNCSTAR1-3Draft.pdf

A total of 236 recovered pieces of WTC steel were cataloged; the great majority belonging to the towers,
WTC 1 and WTC 2. These samples represented a quarter to half a percent of the 200,000 tons of
structural steel used in the construction of the two towers. The NIST inventory included pieces from the
impact and fire regions, perimeter columns, core columns, floor trusses, and other pieces such as truss
seats and wind dampers.

The collection of steel from the WTC towers was sufficient for determining the quality of the steel and, in
combination with published literature, for determining mechanical properties as input to models of
building performance.

...

The failure mode of spandrel connections on perimeter panels differed above and below the impact zone.
At or above the impact zone, bolt hole tear-out was more common. Below the impact zone, it was more
common for the spandrels to be ripped off from the panels. The change in mode may be due to shear
failures as the weight of the building during collapse came down on these lower panels. There was no
evidence that fire exposure changed the failure mode for the spandrel connections.

...

Failure of the limited number of recovered core columns was a result of both splice connection failures
and fracture of the columns themselves. One recovered core column (WTC 2, column line 801,
floors 77–80) may have sustained damage as a direct result of the airplane impact; however, the welded
splice to the column above survived intact.

...

In both towers, most of the perimeter panel floor truss connectors (perimeter truss seats) below the impact
floors were either missing or bent downward. Above this level, the failure modes were more randomly
distributed. This behavior apparently resulted from the building collapse sequence.

Of the 31 core floor truss connectors (core seats) recovered, about 90 percent were still intact, although
many were extensively damaged. Only two were completely torn from the channel.

...

A coating on the SFRM prevented the loss of the SFRM in some locations on the perimeter columns.
This coating appeared as a band of white features on the SFRM wherever two aluminum panels met on
the exterior columns of the buildings, becoming visible when the panels were dislodged. This may be a
coating applied to protect the SFRM from moisture infiltration at the aluminum panel joints, acting to
preserve the SFRM even when the SFRM was knocked off both above and below those locations.

...

The pre-collapse photographic analysis showed that 16 recovered exterior panels were exposed to fire
prior to collapse of WTC 1. None of the nine recovered panels from within the fire floors of WTC 2 were
observed to have been directly exposed.

NIST developed a method to characterize maximum temperatures experienced by steel members using
observations of paint cracking due to thermal expansion. The method can only probe the temperature
reached; it cannot distinguish between pre- and post-collapse exposure. More than 170 areas were
examined on the perimeter column panels ...
Only three locations had evidence that the steel reached temperatures above 250 °C.

These areas were:
• WTC 1, east face, floor 98, column 210, inner web,
• WTC 1, east face, floor 92, column 236, inner web,
• WTC 1, north face, floor 98, column 143, floor truss connector

Other forensic evidence indicates that the last example probably occurred in the debris pile after collapse.
Annealing studies on recovered steels established the set of time and temperature conditions necessary to
alter the steel microstructure. Based on the pre-collapse photographic evidence, the microstructures of
steels known to have been exposed to fire were characterized. These microstructures show no evidence
of exposure to temperatures above 600 °C for any significant time.

Similar results, i.e., limited exposure if any above 250 °C, were found for two core columns from the
fire-affected floors of the towers.

...

No steel was recovered from WTC 7!

...

The availability of images with details of the impact hole in WTC 2 was much more limited than for
WTC 1 (refer to NIST NCSTAR 1-5A). This was primarily due to geography, where the land area south
of the WTC complex was much less extensive than to the north, and tall buildings to the south made it
difficult to get clear views. In addition, most of the professional and amateur photographers who moved
toward the area came from the north and did not try to go south of the WTC complex. Fig. 6–2 (top)
shows one of the few images where smoke shrouded only a portion of the damage to WTC 2 caused by
the airplane impact. In Fig. 6–2 (bottom), markers were again added to identify observable failure modes
in the exterior columns. The damaged region encompassing the starboard wing tip area (outlined roughly
in yellow) was unobservable in all available photographs and video footage due to the large and persistent
volume of smoke emitted from those floors. For visualization purposes, the black lines indicate the
location of the bolted column joints in the exterior wall. Detailed post-collapse analysis of the failures
was not possible around the impact hole in WTC 2 because no identified panels in NIST’s possession
came from this area.

...

Several images taken from different angles and with different smoke and lighting conditions were
examined to characterize the damage. Fig. 6–3 is a view of the northeast corner of WTC 2 centered on
approximately the 82nd floor. The 81st floor, where the engine exited, was piled full of burning debris.
The corners of the buildings were designed such that every other floor had a column in this space, an
example of which can be seen above the hole, indicated by the arrow. The plans of the building show that
the 81st floor was one without a column, consistent with the image, so that the exiting engine did not
remove one as it exited the hole. Also, examination of this and two other images showed that the
columns that border the hole to the left and right appear undamaged, so that the engine appears to have
not damaged the building structurally as it exited.

On the right side of the image in Fig. 6–3 is the north face of WTC 2. It was believed that a large
component of a landing gear exited the building on this face near the northeast corner in the region of the
80th floor. On Fig. 6–4, the arrows indicate where the columns have been cut (regions of missing column
denoted by blue arrows) or where a bolted connection has broken and bent the column ends outward
slightly (green arrows). The bolted connections were broken at the 81st floor in columns 257 and 258.
The entire column was cut through at columns number 254 and 255, also at the 81st floor. (Column 254
retained an intact outer web, however . The web had fractured free of the rest of the column
over a distance of slightly more than two floors. Since the flange tips can be seen on either side of the
outer web, the fractures occurred within the flange plates.) These exterior columns were no longer load
bearing.

...

The five panels located in the airplane impact zone had greater damage than those outside of this region.
There did not appear to be any difference in failure modes or damage characteristics whether the panels
were exposed to fire or not, with the exception of the buckling phenomenon observed for the inner web
plates.

...

From the observations made of the spandrel connections on recovered panels, there was no difference in
failure mode whether or not the panels were exposed to pre-collapse fires.

...

The most significant observations were made when the panels were categorized by their as-built elevation
within the building, Fig. 6–28. Of the 28 floor truss connectors at or below the impact floors for WTC 1,
93 percent were either missing or bent downwards. Only 37 percent of the 38 floor truss connectors
above the impact floors had similar characteristics. Comparable results were found for WTC 2 where
88 percent of the floor truss connectors below the impact floors were bent down or missing, while only
55 percent of the floor truss connectors above this region had such damage characteristics.

...

Table 6–2 lists the 12 identified core columns, including as-built locations and possible conditions to
which they may have been exposed prior to the collapse of the buildings. Due to the small number of
samples, statistical data analysis of the damage features and failure modes would be of little use.
Therefore, in-depth descriptions of the four significant pieces (C-80, C-88a, C-88b, and HH) that were
located within the fire zone floors are provided below. For reference, Fig. 6–29 displays the position of
the columns within the core and with respect to the perimeter panel damage.

...

Core columns C-88a and C-88b, from WTC 2, were unique among the recovered core elements in that the
columns were still connected at the welded column splice, Fig. 6–31. Both columns were 42 ksi built-up
box columns with their shared splice in the 80th floor level. The lower 16 ft of C-88a was recovered
(floors 80 through 82), whereas the upper 8 ft of C-88b was still attached (primarily within the
80th floor). The location of the column C-88b suggested that impact damage may have been sustained.
Both webs and the “south” flange were severely bent to the east and had flame cut ends, Fig. 6–32. The
bends occurred just below the connector for the channel associated with the 80th floor. The “north”
flange had a less significant bend toward the north, with approximately 80 percent of the failed end of the
plate as a fracture. There was no visible necking of the plate in the area of failure, with the remaining
portion of the plate having been flame cut during the recovery effort. This flange was fractured roughly
2 ft below the splice in the 80th floor level. The fracture surfaces were too corroded for analysis.

Aside from the three columns discussed above, only one other core column was recovered from within
the fire region.

...

Samples received at NIST were either small sections (less than 3 ft to 4 ft in length) of chord and rod
material (Fig. 6–35a) or large, entangled masses in the form seen in Fig. 6–35b. According to Structural
Engineers Association of New York (SEAoNY) volunteer members involved in the recovery effort, it was
necessary to “ball up” any truss pieces of significant size to facilitate removal from the WTC site and
subsequent handling in the recovery yards. This was necessary since the primary and bridging trusses for
a given floor were welded together into huge grids. During the collapse, these lightweight floor sections
were severely damaged and could not be easily removed. Thus, it is unknown when specific damage
features occurred. Further, it is likely that sections of multiple trusses were “balled” together and labeled
as a single unit rather than that NIST labeled samples consisting of just one pre-fabricated truss unit.

Attempts were made to identify the as-built location of the truss material, but unlike the other major
structural elements, identifying marks could not be found on the trusses. Difficulty was also encountered
in trying to measure the length of the truss members to determine if they were of the 35 ft or 60 ft variety.
It appears that no whole length sections were recovered. A large majority of the compressive resistance
welds between the chords and rods were observed to have failed. This was evident from the large amount
of chord material recovered without rods attached and vice versa. It is unknown when these failures
occurred or what caused them.

...

NIST has developed a novel approach to evaluating the primer paint on the structural components for
evidence of exposure to high-temperature excursions (see Appendix D of NIST NCSTAR 1-3C). This
method was found to be relatively easy to implement and robust enough to examine an entire component
in the field. Calibration tests in the laboratory showed that, although there was little or no change in
color, the primer paint used on the WTC steels that reached temperatures over 250 °C cracked (similar to
a “mud cracking” pattern) from the difference in thermal expansion between the paint and the steel. Since
deformation and environmental effects can also cause mud-cracking, the absence of mud-cracking
indicates the steel has not exceeded 250 °C, but the presence of mud cracks cannot be assumed to be
caused by high temperature.

Visual inspection for the fire effects on recovered steel was conducted solely on the perimeter panels and
core columns, as these were the only structural elements that had known as-built locations. Twenty-one
panels were selected with numerous locations on the inner webs, flanges, spandrels, and floor truss
connectors for each floor level analyzed, providing that sufficient paint was available for the analysis.
Core columns C-80 and HH from WTC 1 were examined while C-88a and C-88b were tested from
WTC 2; these columns resided within the fire floors for their respective buildings. The entire length of
each core column was examined, and evaluations of the primer paint were made when sufficient paint
was available for inspection.

Over 170 areas associated with the 21 exterior panels were analyzed, and the results may be found in
Appendix E of NIST NCSTAR 1-3C. These 21 panels represent only 3 percent of the panels on floors
involved with fire and cannot be considered representative of other columns on these floors. Only three
locations (less than 2%) showed evidence of paint mud cracking!

Four of the core columns with known as-built locations were examined for mud cracking of the paint.
For columns C-88a and C-88b, sufficient paint for analysis was not available. For columns HH and C-80,
few areas of paint were observed (three to five spots per column) with no indication of temperatures over
250 °C.

...

The spandrel steels identified as having been exposed to fire prior to the collapse of the building showed
no microstructural evidence of change. Similar results indicated that three of the four seats observed to be
exposed to severe pre-collapse fire conditions did not experience significant microstructural changes as a
result of the exposure. However, the seat with the melted binder (Fig. 6–38) did show signs of
microstructural alteration as a result of elevated temperature exposure, though it was unknown when this
exposure occurred. Finally, in the several columns with known pre-collapse fire exposure, metallographic
analysis provided no conclusive evidence that the steel exceeded 625 °C, based on calibrations in furnace
exposure studies of WTC steel reported in NIST NCSTAR 1-3E.

...

Based on microstructural analysis of the recovered structural steel, there was no evidence indicating that
the pre-collapse fires were severe enough to affect the steel microstructure of these pieces. Based upon
this evidence, it is believed that no steel was recovered which experienced temperature excursions above
600 °C for any significant length of time as a result of the pre-collapse fires.

...

Because NIST recovered no steel from WTC 7, it is not possible to make any statements about its quality.
The recommended values for the stress-strain behavior were estimated using the same methodology that
was used for the WTC 1 and WTC 2 steels (NIST NCSTAR 1-3D). The static yield strengths were
estimated from historical averages and corrected for testing rate effects.

Because, prior to collapse, WTC 7 did not suffer any high-strain rate events, NIST made no effort to
estimate high-strain-rate or impact properties of the steel.

No metallography could be carried out because no steel was recovered from WTC 7.

...

Discernible changes to the microstructure of furnace exposed WTC steel were observed when exposed to
a temperature of 625 °C for as little as 0.25 h. At or below 500 °C, no microstructural change was
apparent using light optical microscopy. The hardness of these samples varied for a given temperature
and material type, but correlated well with microstructural observations.

...

Of the 31 core floor truss connectors (core seats) recovered, about 90 percent were still intact though
extensive damage may have occurred. Only two were observed to have been completely torn from the
channel.

...

The pre-collapse photographic analysis showed that 16 of the 33 exterior panels recovered from WTC 1
were exposed to fire prior to building collapse. None of the nine recovered panels from within the fire
floors of WTC 2 were observed to have been directly exposed.

It is difficult or impossible to determine if high-temperature exposure occurred prior to or after the
collapse. Of the more than 170 areas examined on 21 exterior panels, only three locations had mudcracking
of the paint, indicating that the steel may have reached temperatures in excess of 250 °C.

Annealing studies on recovered steels established the set of time and temperature conditions necessary to
alter the steel microstructure. Based on the pre-collapse photographic evidence, the microstructures of
steels known to have been exposed to fire were characterized. These microstructures show no evidence
of exposure to temperatures above 600 °C for any significant time for the recovered pieces.

Two of the core columns with as-built locations in the fire-affected floors were examined for paint
cracking. The few areas with sufficient paint for analysis did not show mud cracking patterns, indicating
the columns did not exceed 250 °C.

[StrafingMoose]

1. Hey, nice find!

So basically, you've got a governemental agency (NIST) contradicting the official story on some of the data concerning the crux of the crux of this story... Good find! But this document is due out in september, let's see what happens!