torstai 13. lokakuuta 2011

The following are comments on Chris Mohr's
"Part 3 Gage's Blueprint for Truth Rebuttal (Not Debunked): Tall Steel Frame Building Fire Collapses".

Contrary to what Mohr claims, steel is regarded as a good conductor of heat - for example, "A good conductor of heat, such as steel, will have a high thermal conductivity and a poor conductor of heat, such as air, will have a low thermal conductivity." (Hutchinson encyclopedia)

Mohr obfuscates issues in his discussion of partial collapses of buildings, such as Windsor Tower and Delft University. He claims that these steel-reinforced concrete buildings should have resisted fire even better than the three WTC skyscrapers, which were steel-framed buildings.

The problem with such a claim is that steel is a good heat conductor, while concrete is a poor heat conductor. I'll quote Jim Hoffman, whose work has been featured in Nature, Scientific American, Science Digest and Science News:
Steel is a good conductor and concrete is a poor conductor of heat. Thus in a fire, a steel frame will conduct heat away from the hotspots into the larger structure. As long as the fire does not consume the larger structure, this heat conductivity will keep the temperatures of the frame well below the fire temperatures. The same is not true of steel-reinforced-concrete structures, since concrete is not a good thermal conductor, and the thermal conductivity of the rebar inside the concrete is limited by its small mass and the embedding matrix of concrete.
Fires can cause spalling of concrete, but not of steel. This is because concrete has a small percentage of latent moisture, which is converted to steam by heat. Thus, a large fire can gradually erode a concrete structure to the point of collapse, whereas a fire can only threaten a steel-framed structure if it elevates steel temperatures to such an extent that it causes failures.
Hoffman's discussion of the Windsor Building fire is recommended reading and also relevant to the partial collapse of the Delft University.

The other examples Mohr provides have limited relevance.

The McCormic exhibition hall fire resulted in the collapse of the roof, and the single-story Charleston Sofa Super Store building fire resulted in the almost complete collapse of the roof.

In the Mumbai High North Platform fire, an explosion on the platform combined with an unlimited supply of fuel and oxygen.By contrast, the office furninshing in WTC 7 could only have sustained a ~20-minute fire at any given location, as acknowledged by NIST as late as the end of 2007: "At any given location the combustibles needed about 20 minutes to be consumed." Of course, the maximum of 200 °C temperature to which a 20-minute fire would subject fireproofed steel (manual by Finnish Steel Construction Association, p. 509) would have little effect on massive structural steel members. Accordingly, in the final report a year later, NIST claimed that the fires raged for hours at individual locations in WTC 7.

The Kader Toy Factory fire in Thailand is only marginally relevant, because the four-story factory was both poorly designed and built, and was reinforded with uninsulated steel girders to begin with.
The partial collapse of WTC 5 has been attributed to a combination of impact damage from falling debris and fires.

The Interstate 580 overpass failure resulted from a tanker truck explosion and fire, in so far as the case is structurally relevant at all.

Mohr argues that the precise 42-inch sagging figure (as against the 3 inches that the NIST fire tests produced) could somehow have been deduced from the claimed inward pulling of the buildings' exterior columns, based only on scant photographic evidence. As regards its fire experiment, NIST writes:
NIST contracted with Underwriters Laboratories, Inc. to conduct tests to obtain information on the fire endurance of trusses like those in the WTC towers[...] All four test specimens sustained the maximum design load for approximately 2 hours without collapsing[...] The Investigation Team was cautious about using these results directly in the formulation of collapse hypotheses. In addition to the scaling issues raised by the test results, the fires in the towers on September 11, and the resulting exposure of the floor systems, were substantially different from the conditions in the test furnaces. Nonetheless, the [empirical test] results established that this type of assembly was capable of sustaining a large gravity load, without collapsing, for a substantial period of time relative to the duration of the fires in any given location on September 11.
The authors of the article "Fourteen Points of Agreement with Official Government Reports on the World Trade Center Destruction" agree on all of the above, including the statement that the fires in the towers were "substantially different" from the conditions in the test furnaces: "the test furnaces were hotter and burned longer" as they did.

Summing up: some low, poorly planned and constructed steel buildings have indeed totally collapsed from fires; an oil rig mostly collapsed from an explosion and fires fed by an unlimited fuel and oxygen; and parts of buildings with a steel-reinforced concrete structure (much more prone to fire damage) have collapsed. However, a sudden total collapse of a skyscraper like WTC 7, in which 80 steel columns (or, in the case of its facade, 58 columns) had to snap without providing even a minimum of resistance throughout 8 floors NIST acknowledged in the fnal report that the skyscraper underwent "a freefall descent over approximately eight stories at gravitational acceleration for approximately 2.25 seconds" represents a completely different kind of case.

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