First it was the “new phenomenon”  of “thermal expansion”.
Now, Sergei Dudarev, of the UK Atomic Energy Agency, says the Twin Towers collapsed due to “unusual magnetic forces “.
Specifically, as described by the BBC, Mr. Dudarev argues that:
“The peak in this pliability is at 911.5C, but begins at much lower temperatures, at around 500C (932F) - a temperature often reached during building fires.
The steel backbone of the Twin Towers was probably exposed to temperatures close to this, when insulating panels – meant to protect the buildings’ structural frame – were dislodged by the impacts of the hijacked planes.
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The roaring fire mid-way up the building heated the steel struts, and once temperatures rose above 500C the structure became elastic, and collapsed under the force of the floors above.”
Is he right?
Well, as noted in Appendix A of The World Trade Center Building Performance Study :
In the mid-1990s British Steel and the Building Research Establishment performed a series of six experiments at Cardington to investigate the behavior of steel-framed buildings. These experiments were conducted in a simulated, eight-story building. Secondary steel beams were not protected. Despite the temperature of the steel beams reaching 800-900 C (1,500-1,700 F) in three of the tests (well above the traditionally assumed critical temperature of 600 C (1,100 F), no collapse was observed in any of the six experiments.
And Underwriters Laboratories tested the steel components at the Twin Towers and found they could withstand fires for hours without failure:
“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.” (NIST, 2005, p. 140).
Other fire tests have also failed to cause failures, collapses or “unusual magnetic forces” at high temperatures.
What about actual office fires?
The 2005 Madrid skyscraper fire “reached 800 degrees Celsius (1,472 F), said Javier Sanz, head of Madrid firefighter”  (see pictures here ), and lasted some 20 hours. Indeed, the fires in the Twin Towers were much cooler than many office fires, as indicated by the color of the flames and the black smoke pouring out of the windows.
As Steve Watson notes :
We have previously pointed to the innumerable number of buildings  that have suffered roaring fires across the majority of their floors for hours and remained standing. Seemingly the steel beams in these buildings were not subjected to the same “unusual magnetic forces”.
Furthermore, a far more extensive fire occurred in WTC 1 itself, prior to enhanced fireproofing of the building, on February 13, 1975 . The fire burned at much higher temperatures for three hours and spread over six floors , including 65% of the 11th floor and the building core, yet it caused no significant damage to the steel structure and no trusses had to be replaced. There were no “unusual magnetic forces” present on that day.
Furthermore, referring to the collapses, the original NIST report concluded  that ‘the existing condition of the fireproofing prior to aircraft impact and the fireproofing thickness on the WTC floor system did not play a significant role’”.
Any “thermal expansion” at the World Trade Center was not a new phenomenon, but something that building designers and fire safety engineers have taken into account for decades if not hundreds of years.
Likewise, any “magnetic forces” at the WTC should have been less severe than those present in fire safety tests and actual office building fires, which have never before led to complete collapses. Indeed, despite the apparently advanced science which Mr. Dudarev hints at, he actually admits this is nothing new:
“He said blacksmiths had exploited this property for hundreds of years”.