**Ask a monkey a physics question thread**

[Renton]

afaik the beams didn't melt, they were just weakened by heat, which happens at well under the melting point. And the molten metal that conspiratards were talking about wasn't steel, but aluminum, which has a lower melting point than steel, i believe.

[OngBonga]

afaik the beams didn't melt, they were just weakened by heat, which happens at well under the melting point. And the molten metal that conspiratards were talking about wasn't steel, but aluminum, which has a lower melting point than steel, i believe.

The beams must have melted, otherwise it remains solid and therefore retains at least some of its ability to offer support to the building.

And are we talking about the molten metal seen pouring out of the corner of the tower? Who knows what that was? I saw a docu that claimed it was tested as iron. I treat both that claim and yours as speculative. Not that it matters. If it's just aluminium, it's not from the supporting beams.

It's clear that the fire got hot enough to melt steel. If it didn't, then the implication is that such a skyscraper is critically vulnerable to a standard office fire and we should evacuate every single steel supported skyscraper in the world as a matter of urgency until scientists understand completely why the steel failed.

[MadMojoMonkey]

The beams must have melted, otherwise it remains solid and therefore retains at least some of its ability to offer support to the building.

I hope I have given you pause to reconsider the strength of this line of reasoning.

"At least some" becomes "not enough" at some point.

And are we talking about the molten metal seen pouring out of the corner of the tower? Who knows what that was? I saw a docu that claimed it was tested as iron. I treat both that claim and yours as speculative. Not that it matters. If it's just aluminium, it's not from the supporting beams.

I'm not talking about that, no.

It's clear that the fire got hot enough to melt steel. If it didn't, then the implication is that such a skyscraper is critically vulnerable to a standard office fire and we should evacuate every single steel supported skyscraper in the world as a matter of urgency until scientists understand completely why the steel failed.

I think you're letting your emotions guide you, here.

This wasn't "a standard office fire." This was a fire that had a shit load of accelerant (jet fuel) poured into many floors of the building at once. This severely limited the ability of standard emergency response procedures to control and contain the blaze.

[OngBonga]

"At least some" becomes "not enough" at some point.

Sure, but the structural integrity of the building went from 100% to 0% in a very short amount of time. That does not support the idea that there was a gradual weakening, which one would expect under the circumstances. It implies that once the stress reached critical point, that once collapse was inevitable, it was almost instantaneous. We also have the problem of what should be an uncompromised steel structure below the impact level and the range of the fire completely failing to support the collapsing mass, not even to the point of tipping it.

I find it intruiging that an educated physician can accept such a uniform and rapid collapse of a reinforced steel stucture due to impact and fire at the top of the structure. It just seems so implausible. Especially given we're supposed to believe a passport managed to emerge from the rubble intact.

I mean come on.

Is it fear that stops people from seriously questioning what we're told? Or do you guys just have too much faith in humanity? The older I get, the less shocked I am by what they did. In terms of lives lost, 9/11 was a drop in the ocean compared to the price paid so far by humanity for this war on "terrorism", which is basically WWIII in all but name. This has been going on for a century now, the global superpowers being actively engaged in some form of war practically non stop since WWI. It's still going on. They'll do whatever it takes to try and get an advantage.

Do you think those in power could not put a price on the heads of 3k Americans? Well, they did.

Does anyone remember that thread about pushing a button for a million quid and killing someone random by doing so? Something like that, anyway. How many people press the button? How many people press the button when it's 3k people and maybe $100 billion?

[MadMojoMonkey]

Sure, but the structural integrity of the building went from 100% to 0% in a very short amount of time. That does not support the idea that there was a gradual weakening, which one would expect under the circumstances. It implies that once the stress reached critical point, that once collapse was inevitable, it was almost instantaneous. We also have the problem of what should be an uncompromised steel structure below the impact level and the range of the fire completely failing to support the collapsing mass, not even to the point of tipping it.

What do you mean by "very short amount of time?"
Note: the collapse of each building took tens of minutes from impact to critical failure.

***
F = ma = dp/dt

dp/dt is change in momentum with respect to time.

When a single floor collapses, it slams down into the floor below. It delivers significant momentum in a short time, causing a spike in the reaction forces which support the floor it slams into. The delivers a wave of stress through the entire structure of the building. The wave moves at the speed of sound through the various materials. If any parts of the structure are pushed beyond their yield stress, either due to prior weakening or simply the magnitude of the stress, then they will permanently deform. This displaces the stress from those supports to other supports, which are now bearing more load. If those supports were previously weakened, by whatever means, then they are notably more likely to yield, themselves. Rinse and repeat.

If that lower floor later falls, then the effect is repeated, but multiplied by the added weight of 2 falling floors worth of debris. This usually leads to complete collapse of the entire building in a process called - I'm not making this up - pancaking. It is extremely likely that the process will destroy the structural core of the building where the floors are collapsing, which drops the whole top of the building, at which point... it's all over.

It is very true that once the collapse is begun, it happens quite rapidly. The collapse of the structure due to overloading travels through the structure at the speed of sound in steel. This will look "instantaneous" on a seconds-based time scale. Slow it down enough, and you'd see a wave of failures pass through the structure, each failure making the next failure more severe.

I find it intruiging that an educated physician can accept such a uniform and rapid collapse of a reinforced steel stucture due to impact and fire at the top of the structure. It just seems so implausible. Especially given we're supposed to believe a passport managed to emerge from the rubble intact.

I mean come on.

[...] removed 'cause not physics-related

Physicist - not physician. Unless you're talking about someone else besides me.
I only speak for myself, not any club or group or otherwise.

It's not like the entire building went up in flames. Much of the structures never burned, just fell.

It would be implausible if there wasn't a fair amount of debris that was mostly unscathed by the fall due to the randomness of it all and some bits would just have "gotten lucky" in how they came down. It's perfectly reasonable that a bit of paper could have been sandwiched in between 2 things which prevented Oxygen from getting to it, and therefore, it couldn't burn.


What's intriguing to me is that I can offer you a step by step reason for why every step of the process is plausible. I can do so in ways that you can personally test to determine whether or not I'm fooling you. Yet, you do not perform the tests, and you say I'm fooling you, and that it is not plausible.

But then I can talk GR where I'm hard pressed to present you any way you could test it yourself, and you accept what I tell you and do not accuse me of fooling you.

*shrug*
People are strange.

[OngBonga]

I think you're letting your emotions guide you, here.

No doubt. Not very scientific of me. Sorry, I forgot it was the physics thread.

[MadMojoMonkey]

No doubt. Not very scientific of me. Sorry, I forgot it was the physics thread.

-.-
Not sure if snarky.

[MadMojoMonkey]

afaik the beams didn't melt, they were just weakened by heat, which happens at well under the melting point.

I doubt that any steel I-beams melted clean through. The question as to whether ANY steel could be melted is what I tried to answer.

I tried to make the point about the reduction in structural integrity due to simply heating the steel. How's this for a practical example:

Consider sword making. The blacksmith heats the steel so that it is glowing a reddish-orange color due solely to its own temperature. This puts the Steel in a temperature range close to 1,500 - 2,000 C (based on the observed color, so wide error bars). When the steel is this temperature, it is weak enough to be shaped by the blacksmith.

I bring this up to help you understand in an intuitive way that the steel doesn't need to be heated to melting in order to have significantly lower structural rigidity.


Another factor is the loading. As one structural element weakens or is deformed, it displaces that stress to nearby structural members. These members which take on the new load are now more likely to fail, since they are now closer to their max loading. This can cause a chain reaction through a rigid structure.

And the molten metal that conspiratards were talking about wasn't steel, but aluminum, which has a lower melting point than steel, i believe.

Disclaimer: I'm not researching conspiracy theories on this and the wackos who latch onto the path of "IDK -> it must be" with no steps in-between.

Confirmed that Aluminum has a lower melting point than Steel. Aluminum cans can be melted in most camp fires, as any beer-swilling camper can affirm. Steel cans don't usually melt in a normal campfire, but you can certainly do it with a little knowledge about air-flow. Something like a Dakota fire hole can be used to allow controlled oxygen feeding to the fuel, and a rudimentary blast furnace can be implemented in this way.

A Dakota fire hole is basically a U-shaped hole in the ground, with fuel in one hole, and nothing in the other. The empty hole provides Oxygen feeding to the base of the fire, keeping all the air flow through the combustion chamber moving upward. This 2nd hole can have a fan or bellows placed over it to make a simple blast furnace, increasing the combustion rate.