Reply #3: Some problems with his arguments...

[Carefulplease]

(I've just posted this in another thread)

You are right. One can no longer say that no structural engineer has ever challenged the impact+fire explanation of the collapse.

I've listened to the recent interview of Charles N. Pegelow by Jim Fetzer. He seems to base its assessment on the premises that (1) no perimeter column and no core column experienced a temperature exceeding 250 degree and 500-525 degree respectively. (Fetzer suggested that these were Fahrenheit figures, he concurred. I though this was a mere slip but he later reiterated that these were Fahrenheit figures he was talking about.)

He also assumes that (2) the whole structure is 9 times stronger than what is required to support the dead loads, in order for it to be able to resist the worst case wind loads.

He assumes that (3) the "reinforced concrete" floors would not fail because the slabs are under compression.

He claimed that the fires were not hot enough to (4) break the windows or (5) melt the aluminum claddings on the perimeter columns.

He concurs with Fetzer that (6) the towers were designed to withstand the impact of a Boeing 707.

His main argument is that because of the enormous reserve capacity of the towers, "some aircraft impacts" and "some fires" would not bring down the structure. This is not a compelling argument.

All the premises listed above are false or ungrounded.

(1) The NIST figures he cites are Centigrade, not Farenheit. They concern small, non representative physical samples of a fraction (1 to 3%) of the 287 column segments on the impact floors of WTC1 and WTC2.

(2) There is no ground for this figure. The numbers I've seen in the literature for the reserve capacity of cold columns are closer to ~500% for the perimeter columns and ~33% for the core columns, not an overall ~800%.

(3) The floors would still sag when the bottom cords or the supporting trusses expands. The top cord and the slabs resisting in compression is irrelevant.

(4) The windows were observed to brake as the fires progressed.

(5) The claddings weren't in contact with the steel. They were insulated from it. And a large temperature differential is expected between the inner and outer webs of the columns. This phenomenon in itself promotes buckling.

(6) The towers were not designed to withstand the impact of an aircraft travelling at cruise speed. The engineers considered the possibility of an impact from an aircraft lost in fog attempting to land on an approach course. There is a 6 to 10 fold difference in kinetic energy between the envisaged scenario and the 9/11 impacts.