Trusses that work like bi-metals

If you assume one of them is a conducting object then it will expand a little bit, a bi-metal consists of two metals that touch each other, both conducting sufficient, both with a different coefficient of expansion, that gives the effect. A truss is a truss and no bi-metal.

This page gives a picture about the effect of fire

http://guardian.150m.com/wtc/trusses.htm

einsteen

On June 15, 2003, a 39-year-old male career lieutenant (Victim 1) died, and another 39-year-old career fire fighter (Victim 2) was fatally injured while trying to exit from a commercial structure following a partial collapse of the building's flat metal roof, which was supported by lightweight metal trusses (bar joists). Victim 1, the lieutenant of the first responding engine company, reported light smoke on arrival at a discount store. The flat roof consisted of metal decking covered by wooden fiber insulation and asphalt. A suspended ceiling in the main store area approximately 12 feet high covered the metal roof trusses and obstructed their view from below. Victim 1 led his crew into the store to search for the fire. After proceeding approximately halfway to the rear of the store, he directed two fire fighters to go outside and bring in a preconnected hand line. Victim 1 and a lieutenant from the second en gine company proceeded to the rear of the store. This lieutenant found the fire behind the closed office door and exited to pull a second hand line. Soon afterward, Victim 1 requested a truck company to enter the store and pull ceiling tiles to search for fire extension; a third lieutenant and a fire fighter immediately responded. The truck company searching for fire extension observed fire in the truss void above the suspended ceiling. The accumulation of smoke and hot gases within the truss void mixed with fresh air as the ceiling was opened and violently ignited. The fire rapidly spread from the rear office area to the main store room through the hidden truss void. Victim 1 radioed that it was getting too hot and everyone needed to back out. At this point, the roof truss system at the rear of the store room began to fail, sending debris downward, with several bar joists coming to rest on metal merchandise shelving. Several fire fighters were able to escape by following the hose lines toward the front of the store. Victim 1 and Victim 2 (the nozzleman from the first responding engine company) became separated from the rest of the fire fighters and were unable to escape. As conditions worsened, the building was evacuated, and a rescue attempt for the two missing fire fighters was initiated. Fire fighters working at the rear of the building heard a personal alert safety system (PASS) device and quickly entered to investigate. Victim 2 was located in the rear storage room close to the office. A second PASS device was also heard further into the building. Soon after Victim 2 was brought outside, the rear of the building collapsed, preventing further rescue efforts until the fire was brought under control. Victim 1 was located approximately 1½ hours later. Victim 2 was hospitalized and died the next day [NIOSH 2004].

http://www.cdc.gov/niosh/docs/2005-132/#5

Truss systems are notorious in the firefighting community for their unpredictability, in fact "Never trust a truss" is a mantra for safe operations in structural fires. In a normally spread fire they may perform well. WTC was not a normally spread office fire. The "Molitov cocktail" of the aircraft set everything on fire almost instantly over multiple floors, greatly increasing the rate of temperature rise as opposed to a slow rise in the normally progression of a fire, so your examples are not really relivant to the WTC fires.

Grumpy

einsteen --- Those are interesting observations! Is there a way for you to post a vid-clip so that those of us (like me) who are unable to view videos can see the phenomenon?

By the way, the NIST steel collection contains a perimeter column which has folded down onto itself like an accordion. I had always assumed that this occurred somewhat further down the building.

shagster earlier posted a visual which appears to show that part of the northwest corner of the top block of WTC 1 failed to participate in the tilting.

Wedging: Ar 8 degrees of arc, the top of the south wall is about 0.6 meters inside the outer edge of the bottom portion of the south wall. Of course, the matter is not quite so simple at the moment of tilt initiation. Some serious inelastic behavior must occur right then.

However, the building was internally undergoing some deterioration earlier. The NYPD helicopter observer warned of collapse about 6 seconds before it did.