Jet fuel doesn’t melt steel, but it does dramatically weaken it December 17, 2015Posted by Tantumblogo in Admin, Basics, catachesis, foolishness, fun, huh?, non squitur, scandals, sickness, silliness, Society, technology.
A corollary to the below. Aircraft aluminum melts at about 1150-1200 Fahrenheit. But aluminum structure in aircraft is limited to speeds below Mach 2.3 continuous, or 2.5-6 for short bursts of time. At those speeds, aircraft skin reaches temperatures around 350-400 F. That is the maximum temperature that aluminum can be used without significant weakening. All metals weaken greatly at temps well below their melting point. That is why the SR-71, to travel at Mach 3.5 or so, had to be made almost entirely of titanium and exotic steels, to handle the 500-800 degree temps experienced. Likewise, the X-15 rocket plane, to travel at Mach 6 and endure temps up to about 12oo F (or about 650 C) had to be constructed from exotic steels like Inconel X. Regular steel would have softened too much even at those low temps to be used effectively in aircraft design.
Now, a building is not an aircraft, but the same basic principles do apply. Aircraft, to save weight, are typically designed with safety factors of about 1.5 (that is for commercial aircraft, military aircraft, to obtain more performance, generally have lower safety factors). This means the structural design is set to withstand 50% more load than the aircraft is ever expected to experience in even the most severe service. Buildings generally have safety factors of more like 2. This means there is more redundant structural strength, so that more structure can be lost, fail, or severely damaged before the overall design is compromised.
It is estimated that the jet fuel conflagration in the Twin Towers burnt at about 1500 F. While it is true that steel does not melt until much higher – 2500~2800 F – at 1500 F low carbon steel of the type used in building construction will be structurally useless. That’s the point of the video below. I, too, don’t want to get into the endless back and forth over 911 truthferism – I exhaust myself arguing one set of religious beliefs already, thank you – but it’s a quite valuable demonstration all the same.
Another factor not mentioned is creep. Creep is the (relatively) slow stretching out of a substance – typically member of some kind under load. Creep is directly related to operating temperature. The higher the temp, the larger the creep. A steel beam that doesn’t creep a micron at 200 F creeps like crazy at 1200 F. That can also cause structural collapse, as members creep in a fashion that causes things like rivets to pop as holes no longer align, or welded joints to give way. That can also precipitate structural collapse.
Creep also occurs when structural loading suddenly increases. A beam that normally operates at 50% design stress that suddenly goes to 110% – say, because other members were caused to fail by impact – will start to elongate. Over time, say, 12 hours or so, that could cause a structure to collapse even without the impact of heat.
I designed some plastic parts that worked like champs up to about 155 F. But go to 160, and over 24 hours they would creep into frankenstein shapes. And that was under no other load but gravity. I know plastic is not steel, but the same principle definitely applies.