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Well,... from the evidence, the answer is: Not that much happens.

We consider two examples from fires that were known to be very hot.

The June 1990 Broadgate Phase 8 fire.

The above photo shows a number of trusses and a buckled steel column after the Broadgate Phase 8 fire.

Note that the fire was hot enough to buckle the steel column.
Note that even though the trusses were obviously in the same fire, they show no sign of buckling

Consider the following quote from here (section 1.1) concerning the fire.

"On the 23rd June 1990 a fire developed in the partly completed fourteen storey building in the Broadgate development. The fire began in a large contractors hut on the first floor and smoke spread undetected throughout the building. The fire detection and sprinkler system were not yet operational out of working hours.

The fire lasted 4.5 hours including 2 hours where the fire exceeded 1000°C (1832°F). The direct fire loss was in excess of £25 million however, only a fraction of the cost (£2 million) represented structural frame and floor damage. The major damage was to the building fabric as a result of smoke. Moreover, the structural repairs after the fire took only 30 days. The structure of the building was a steel frame with composite steel deck concrete floors and was only partially (fire) protected at this stage of construction. During and after the fire, despite large deflections in the elements exposed to fire, the structure behaved well and there was no collapse of any of the columns, beams or floors.

The Broadgate phase 8 fire was the first opportunity to examine the influence of fire on the structural behaviour of a modern fast track steel framed building with composite construction."

The trusses used in the Broadgate phase 8 construction had a 45 feet (13.5m) span.
The World Trade Center Tower construction used trusses with both 35 and 60 foot spans.

And note that, the sprinkler system and other active measures were NOT operational at the time of the fire and most of the steel was not fire protected.

For more on this fire see this report (local copy).

The February 1975 World Trade Center North Tower Fire.

This 110-story steel-framed office building suffered a fire on the 11th floor on February 23, 1975. The loss was estimated at over $2,000,000. The building is one of a pair of towers, 412 m in height. The fire started at approximately 11:45 P.M. in a furnished office on the 11th floor and spread through the corridors toward the main open office area. A porter saw flames under the door and sounded the alarm. It was later that the smoke detector in the air-conditioning plenum on the 11th floor was activated. The delay was probably because the air-conditioning system was turned off at night. The building engineers placed the ventilation system in the purge mode, to blow fresh air into the core area and to draw air from all the offices on the 11th floor so as to prevent further smoke spread. The fire department on arrival found a very intense fire. It was not immediately known that the fire was spreading vertically from floor to floor through openings in the floor slab. These 300-mm x 450-mm (12-in. x 18-in.) openings in the slab provided access for telephone cables. Subsidiary fires on the 9th to the 19th floors were discovered and readily extinguished. The only occupants of the building at the time of fire were cleaning and service personnel. They were evacuated without any fatalities. However, there were 125 firemen involved in fighting this fire and 28 sustained injuries from the intense heat and smoke. The cause of the fire is unknown.

The WTC North Tower suffered no serious structural damage in this fire. In particular, none of the trusses needed to be replaced.

So, here is a very serious fire (which spread over eleven floors) in World Trade Center North Tower. The very same building that "collapsed" on 9/11. Although the 1975 fire lasted about 4 hours, it caused no serious structural damage. However, according to the government/media fairy tale on 9/11, the 2001 fire, which lasted only 1.75 hours, caused not only serious structural damage, but the entire building to collapse.

There should be a law against telling such fairy tales as these government/media fables.

Anyway, in both of the above mentioned pre-9/11 fires, the trusses survived the fires without replacement and supported the buildings for many, many years after the fires were put out.

So did truss failure cause the "collapses" of the World Trade Center buildings? Of course not.