NIST's New Mechanical and Metallurgical Analysis of Structural Steel (WTC)

[stickdog]

Highlights:

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 temperatures 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 one of these three forensic evidence indicated that the high temperature exposure occurred 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.

...

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!

...

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.

...

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. ... 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. ... 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.

...

All floor truss 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 ... 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.

...

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.