Other examples?
There's a list of buildings which have collapsed due to fire here: Historical Survey of Multi-Story Building Collapses Due to Fire.
Scanning this document, to include only building which are made of steel (and not e.g. reinforced concrete), shows only two:
Alexis Nihon Plaza Montreal, Canada
- Steel frame with composite steel beam and
deck floors; fire resistive without sprinklers
- 15 floors, Office
- Oct. 26, 1986, after 5 hour fire, which then
continued for 13 hours
- Partial 11th floor collapse
One New York Plaza New York, NY,
USA:
- Steel framing with reinforced concrete core,
fire resistive with no sprinklers.
- 50 floors, Office
- August 5, 1970
- Connection bolts sheared during
fire, causing several steel filler
beams on the 33-34th
floors to fall
and rest on the bottom flanges of
their supporting girders.
Can fire destroy steel?
Photographs of structural steel deformed by fire are shown on page 3 of The Reinstatement of Fire Damaged Steel and Iron Framed Structures.
Steel loses strength when it's heated, and eventually fails, which is why it needs protection.
Fuel-based fires get hot unusually quickly.
Normal/natural fires usually spread slowly, and are limited by available ventilation.
What about the WTC specifically?
There are two explanations of how/why fire caused the WTC collapse here:
The latter reference contains an explanation for the collapse (in the side-bar, on the right):
First, in the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001 likely would not have led to the collapse of a WTC tower. Second, the towers likely would not have collapsed under the combined effects of aircraft impact and the subsequent multi-floor fires encountered on September 11, 2001 if the insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.
It also says that they found no other causes:
Also, the investigation team neither found nor invoked any extraordinary events, beyond the terrorist attack that damaged the structure and removed the insulation, that led to the collapse of the towers.
The above (aircraft impact dislodging insulation) doesn't explain the collapse of Building 7. The analysis of the Building 7 collapse states that the failure of this building was more caused thermal expansion (steel changing size and shape): its fire-insulation wasn't affected by an airplane crash, but it failed in the 300°C-450°C temperature range, before it got hot enough (e.g. 650°C) for the steel to lose significant strength:
In the WTC 7 collapse, the loss of steel strength or stiffness was not as important as the thermal expansion of steel structures caused by heat.
There are various ways in which the WTC buildings and the WTC fire were famously unusual or unique.
The authorities learn from disasters and update the building codes and fire codes accordingly: perhaps that's another explanation for why we may not see an event quite this like one again.
