Hardfire: Physics of 9/11

[psikeyhackr]

Newton,

I really enjoy reading your posts.

You positively REEK of "knowing what you are talking about". This post is a great example.

Mackey & several others here suffer the same affliction.

Plus I get to learn a whole bunch about the ways that you guys look at the issues that are unique to your fields. It's fun.

[OK, that's all the sucking up I can stand for one day.]

I don't get this one, tho.

"A perimeter column at any floor must have the strength for both the axial component of the overturning moment above and the overturning moment delivered in shear by the diaphragm of that floor. This additional shear decreases at each floor as well as the height above base decreases."

By "this additional shear" in the 2nd sentence, I assume you mean "... the overturning moment delivered in shear ..." etc, in the first sentence.

For a first approximation, assume that each story's floor load per unit area is the same. And therefore, the total shear load is the same. And that it is shared approximately uniformly amongst all the external columns.

It seems to me that this would make the absolute value of the shear on the columns approximately the same on all stories (perhaps higher on the mech floors due to higher floor loads).

It's easy to see that the overturning moment decreases as you rise off the ground, but I'm not seeing this "moment due to membrane shear" doing the same.

thanks.

tom

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I can't tell if you are being sarcastic or if you actually buy that crap. I had to research "moment frame". I'll check if they ever used the term "moment frame" in the NCSTAR1 report when I get home tonight.

I figured that "moment" referred to "moment of inertia". People are always using that to make stuff seem complicated. Every one of the perimeter wall panels supported the edge of THREE floor slabs. So they were not about to rotate around the edge of any single slab. I bet the wall panels at the very top of the building never tilted as much as TWO DEGREES.

http://www.wisegeek.com/what-is-a-moment-frame.htm

Human beings have been building complex structures for thousands of years. I wonder when the term "moment frame" was invented".

I never heard the term CISC processor when I was at IBM. Maybe it wasn't invented until after I left when RISC processors were developed. But new jargon does not change how things work. It just impresses the people that don't know the jargon. People saying things I don't understand mostly just annoys me because after all of this time I have learned that I will have to spend 80% of the time researching the jargon and only 20% understanding the concept. And there is at least a 33% chance that I am just wading through some jargon I haven't heard before that just talks about something I already know.

It's a SNOW JOB!

paik

 

[Mr. Skinny]

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I can't tell if you are being sarcastic or if you actually buy that crap. I had to research "moment frame". I'll check if they ever used the term "moment frame" in the NCSTAR1 report when I get home tonight.

I figured that "moment" referred to "moment of inertia". People are always using that to make stuff seem complicated. Every one of the perimeter wall panels supported the edge of THREE floor slabs. So they were not about to rotate around the edge of any single slab. I bet the wall panels at the very top of the building never tilted as much as TWO DEGREES.

http://www.wisegeek.com/what-is-a-moment-frame.htm

Human beings have been building complex structures for thousands of years. I wonder when the term "moment frame" was invented".

I never heard the term CISC processor when I was at IBM. Maybe it wasn't invented until after I left when RISC processors were developed. But new jargon does not change how things work. It just impresses the people that don't know the jargon. People saying things I don't understand mostly just annoys me because after all of this time I have learned that I will have to spend 80% of the time researching the jargon and only 20% understanding the concept. And there is at least a 33% chance that I am just wading through some jargon I haven't heard before that just talks about something I already know.

It's a SNOW JOB!

paik

Translation: I have to spend time learning the definition of engineering terms I don't understand. I don't like that. Why do engineers feel the need to invent terms I don't understand? I don't want to have to go to engineering school, so they should only speak in terms I already understand. They just use their "special language" to confuseicate me!

 

[Newtons Bit]

.
I can't tell if you are being sarcastic or if you actually buy that crap. I had to research "moment frame". I'll check if they ever used the term "moment frame" in the NCSTAR1 report when I get home tonight.

I figured that "moment" referred to "moment of inertia". People are always using that to make stuff seem complicated. Every one of the perimeter wall panels supported the edge of THREE floor slabs. So they were not about to rotate around the edge of any single slab. I bet the wall panels at the very top of the building never tilted as much as TWO DEGREES.

http://www.wisegeek.com/what-is-a-moment-frame.htm

Human beings have been building complex structures for thousands of years. I wonder when the term "moment frame" was invented".

I never heard the term CISC processor when I was at IBM. Maybe it wasn't invented until after I left when RISC processors were developed. But new jargon does not change how things work. It just impresses the people that don't know the jargon. People saying things I don't understand mostly just annoys me because after all of this time I have learned that I will have to spend 80% of the time researching the jargon and only 20% understanding the concept. And there is at least a 33% chance that I am just wading through some jargon I haven't heard before that just talks about something I already know.

It's a SNOW JOB!

paik

I'm not about to apologize for your ignorance. Pick up a building code. It's one of three allowable systems for most structures. The other two being braced frames and shear walls or some combination of the three.

tfk: I'll explain what I'm referring to later. I'm using some terms that aren't going to be clear to most people.

 

[ElMondoHummus]

Translation: I have to spend time learning the definition of engineering terms I don't understand. I don't like that. Why do engineers feel the need to invent terms I don't understand? I don't want to have to go to engineering school, so they should only speak in terms I already understand. They just use their "special language" to confuseicate me!

You nailed it. You so nailed it. Dead on. Dead. On.

 

[tfk]

psik,

Excuse me for having missed it, but what is your professional background. I saw that you worked at IBM. What did you do there? And, beyond a job title, what is your expertise?

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I can't tell if you are being sarcastic or if you actually buy that crap. I had to research "moment frame". I'll check if they ever used the term "moment frame" in the NCSTAR1 report when I get home tonight.
paik

The reason that I asked you about your background is that I told you briefly about mine in this post: http://www.internationalskeptics.com/forums/showpost.php?p=4660613&postcount=1092

Newton is a professional. Clearly with a ton of experience. So am I.

I am wondering whether your question as to "whether or not I buy Newton's comments" is based upon your technical background. Or your political prejudices.

You seem to have a undisguised hostility towards engineers in general. Did one or more of them offend you in some particular way?

To answer your question, there were a half-dozen clearly true little revelations buried in that one post that had me sit back and say, "cool, so that's how they look at it". (Along with the one thing that I didn't get.) That's exactly why I posted back to him.

From Newton's description, a moment frame is a structure made out of a bunch of beams connected as rectangle. Connect them as triangles and it becomes a "braced frame". This isn't any attempt to fool you. Every specialty has its vernacular. In some fields (medicine, tax accounting & law), it's done to be exclusionary. Science & engineering tends to be far more democratic (well, meritocratic) and uses the vernacular as a short cut.

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I figured that "moment" referred to "moment of inertia". People are always using that to make stuff seem complicated.
paik

No, not in the slightest.

Newton can confirm this, but my guess as to the origin of "moment frame" is that it's a frame that is designed to resist a moment (i.e., a rotational or bending force).

Moment of inertia is not complicated. It is simply the rotational equivalent of mass. The ability of something to resist changes (i.e., inertia) in rotation depends not only on the AMOUNT of mass, but also on the DISTRIBUTION of mass. Specifically, the further the mass is spaced from the rotational axis, the tougher it is to change it rotational speed. Mass alone cannot capture the distribution part. Moment of inertia does.

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Every one of the perimeter wall panels supported the edge of THREE floor slabs. So they were not about to rotate around the edge of any single slab. I bet the wall panels at the very top of the building never tilted as much as TWO DEGREES.
paik

As I recall, just before collapse, the top tiled in the 10° range.

BTW, I answered several of your questions about the behavior of the buildings during collapse. Including the analog of landslides. No courtesy of response?

tom

 

[Newtons Bit]

tfk: it's called a moment frame because the connection between the beam and the column can resist moments without rotating. Thus the angle between the beam and the column is always 90 degrees (or whatever it was originally constructed at).

 

[GlennB]

.

I figured that "moment" referred to "moment of inertia". People are always using that to make stuff seem complicated. Every one of the perimeter wall panels supported the edge of THREE floor slabs. So they were not about to rotate around the edge of any single slab. I bet the wall panels at the very top of the building never tilted as much as TWO DEGREES.

Well, what with you demanding that NIST should have published exact masses/dimensions of WTC steel components for your convenience, and now complaining that experienced engineers are using proper technical terminology, I suppose this puts you fair-and-square in the category of 'endlessly whining loser'.

 

[psikeyhackr]

Translation: I have to spend time learning the definition of engineering terms I don't understand.

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Like I didn't know moment of inertia. ROFL High school physics that I knew in grade school. It is amazing how much science and engineering that people like Arthur C. Clarke put into their books. You just have to research the terminology in an encyclopedia and of course that tells you even more terminology.

I didn't know what "dead load" and "live load" were when I downloaded the NCSTAR1 report. LOAD means something different in electrical engineering. But just because someone doesn't know the jargon does not mean the concepts are in the least bit difficult. Most experts just make stuff complicated and use the jargon as a smoke screen.

Do you suppose the ancient Egyptians called them "dead loads" and "live loads"?

Is a Mummy a dead load or a live load?

psik

 

[Myriad]

I could see stepping down the column masses just above each mechanical floor instead of linear tapering; or a combination of linear tapering and additional step-downs above each mechanical floor. (As well as, naturally, using appropriately higher values for the floor masses and floor loads on the mechanical floors.) But just arbitrarily adding additional external column mass on the mechanical floors themselves doesn't seem to make any sense.

IIRC the spacing of the mechanical floors was greater than the others, wasn't it? In that case it might be necessary to increase the column thickness in that region because the unbraced column length is greater, resulting in a more slender column. I don't know enough about structural engineering to know whether that's reasonable.

I hadn't thought of that. Yes, if there were greater spans between bracings (floors) then a heavier column in that zone might be called for.

That might even explain why the exterior panel welds were aligned at (only) the mechanical floors: to allow for the stiffer columns to be substituted in on those floors.

In light of those possibilities, I retract my objection to modeling additional exterior column mass (even possibly exceeding the strength of the columns supporting them below) at the mechanical floors.

Respectfully,
Myriad

 

[aggle-rithm]

I never heard the term CISC processor when I was at IBM. Maybe it wasn't invented until after I left when RISC processors were developed. But new jargon does not change how things work. It just impresses the people that don't know the jargon.

Are you suggesting that the terms RISC and CISC are used simply to impress people?

There was no reason to differentiate types of processors before simplified instruction sets and pipelining were introduced. Afterwards, there had to be a way to distinguish the new technology from the old.

How would you suggest this be done?

And, how do you suggest that complex engineering concepts that didn't exist before skyscrapers were developed be discussed in nineteenth-century terms?

 

[Mr. Skinny]

.
Like I didn't know moment of inertia. ROFL High school physics that I knew in grade school. It is amazing how much science and engineering that people like Arthur C. Clarke put into their books. You just have to research the terminology in an encyclopedia and of course that tells you even more terminology.

I didn't know what "dead load" and "live load" were when I downloaded the NCSTAR1 report. LOAD means something different in electrical engineering. But just because someone doesn't know the jargon does not mean the concepts are in the least bit difficult. Most experts just make stuff complicated and use the jargon as a smoke screen.

Do you suppose the ancient Egyptians called them "dead loads" and "live loads"?

Is a Mummy a dead load or a live load?

psik

Again, you seem to be saying you want complicated things to be explained in terms designed to fit your level of understanding. I hate to use the term, but why must it be "dumbed down" to your level of education?

I believe you understand what moment of inertia is, but you complained that you had to look up "moment frame".

Seriously, you could have gone to school and achieved at least a masters degree in mechanical/structural engineering in the years since 2001. If it is so important to you that you understand the engineering aspects of the events of 9/11, you should both educate yourself, and politely ask others to help you understand.

Whining about others speaking above your level of understanding instead of trying to increase that understanding is just silly, and frankly rather boring.

Educate yourself.

 

[Newtons Bit]

Again, you seem to be saying you want complicated things to be explained in terms designed to fit your level of understanding. I hate to use the term, but why must it be "dumbed down" to your level of education?

I believe you understand what moment of inertia is, but you complained that you had to look up "moment frame".

Seriously, you could have gone to school and achieved at least a masters degree in mechanical/structural engineering in the years since 2001. If it is so important to you that you understand the engineering aspects of the events of 9/11, you should both educate yourself, and politely ask others to help you understand.

Whining about others speaking above your level of understanding instead of trying to increase that understanding is just silly, and frankly rather boring.

Educate yourself.

Especially when he's responding to posts that weren't directed at him.

 

[psikeyhackr]

Again, you seem to be saying you want complicated things to be explained in terms designed to fit your level of understanding. I hate to use the term, but why must it be "dumbed down" to your level of education?

Educate yourself.

.
You don't seem to understand the difference between words and ideas.

Creating complicated words for simple ideas is NOT intellectually impressive. It is just pseudo-intellectual bullcrap on the part of people PRETENDING to be intelligent. Memorizing complicated jargon is just a time wasting BORE.

Explain it so all of the laymen can understand it and move on to something sufficiently complicated to actually be interesting.

psik

 

[Mr. Skinny]

.
You don't seem to understand the difference between words and ideas.

Creating complicated words for simple ideas is NOT intellectually impressive. It is just pseudo-intellectual bullcrap on the part of people PRETENDING to be intelligent. Memorizing complicated jargon is just a time wasting BORE.

Explain it so all of the laymen can understand it and move on to something sufficiently complicated to actually be interesting.

psik

psik,

I'm not quite sure what to say, but I'll try.

In engineering, seemingly "complicated" words aren't so much created as they are agreed upon in the engineering community. Common engineering terms allow folks like poster Architect or NewtonsBit, to design something in their country, and have it built by architects/engineers in another county.

The language is not based on pretense whatsoever. It's based on having a common understanding among professionals in a particular speciality.

I'm sorry that having to learn new things bores you.

 

[Mr. Skinny]

Translation: I have to spend time learning the definition of engineering terms I don't understand. I don't like that. Why do engineers feel the need to invent terms I don't understand? I don't want to have to go to engineering school, so they should only speak in terms I already understand. They just use their "special language" to confuseicate me!

.
<snip>
Creating complicated words for simple ideas is NOT intellectually impressive. It is just pseudo-intellectual bullcrap on the part of people PRETENDING to be intelligent. Memorizing complicated jargon is just a time wasting BORE.

Explain it so all of the laymen can understand it and move on to something sufficiently complicated to actually be interesting.

psik

QED

 

[tfk]

Psik,

Like I didn't know moment of inertia. ROFL High school physics that I knew in grade school.

Let's see if I recall how this came up.

1. Newton mentioned a "Moment frame".
2. You said that you thought it was related to "moment of inertia".
3. You said that you thought that engineers use terms like "moment of inertia" to baffle the uninitiated.
4. I explained (perhaps needlessly) that "moment of inertia" is NOT in any sense difficult concept. Quite the opposite.
5. You feel slighted that that I "didn't think you knew what MOI meant." ..."ROFL."

Explain to me one thing, psik.

As we both now understand, MOI is a term that is significant only for a rotating mass.

When you originally interpreted the phrase as applying to the skeleton of a building, exactly what part of that skeleton did you think typically rotates?

But just because someone doesn't know the jargon does not mean the concepts are in the least bit difficult. Most experts just make stuff complicated and use the jargon as a smoke screen.

Most engineering is pretty straight forward. As long as you're willing to put a bunch of effort (usually in the midst of your "partying years"), be diligent & detail-oriented.

Real experts do not have to "baffle with the bull". Third rate guys do.

Now, I've known some ankle-biters who such annoying pests that I have cut short their wasting of my time in this way. But, in all truthiness, any alternative way of cutting short the interaction would have been far less kind. If you find that a bunch of engineers are treating you in this manner, perhaps the source of the problem is not their annoying habits.

tom

 

[tfk]

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Explain it so all of the laymen can understand it and move on to something sufficiently complicated to actually be interesting.

psik

Sorry Psik,

I tried to do exactly by constructing an analogy between vertical collisions and rock slides.

You see how well that worked out when one of the participants in the discussion is perversely argumentative. And a bit hypersensitive.

tom

 

[psikeyhackr]

Sorry Psik,

I tried to do exactly by constructing an analogy between vertical collisions and rock slides.

You see how well that worked out when one of the participants in the discussion is perversely argumentative. And a bit hypersensitive.

tom

It's not my fault that you can't figure out that the analogy doesn't apply.

Rocks and snow on the sides of mountains are not DELIBERATELY DESIGNED to hold themselves up like skyscrapers.

I provided a link to how the layers of snow are studied to understand why avalanches happen.

5. You feel slighted that that I "didn't think you knew what MOI meant." ..."ROFL."

I think you are wasting your time trying to figure out my emotional attitudes about this. I have a problem with people not having thought about the distribution of mass in skyscrapers within a few weeks after 9/11.

Physics ain't about ego games and that is what you people turn these DEBATES into.

psik

 

[Newtons Bit]

Newton,

I really enjoy reading your posts.

You positively REEK of "knowing what you are talking about". This post is a great example.

Mackey & several others here suffer the same affliction.

Plus I get to learn a whole bunch about the ways that you guys look at the issues that are unique to your fields. It's fun.

[OK, that's all the sucking up I can stand for one day.]

I don't get this one, tho.

"A perimeter column at any floor must have the strength for both the axial component of the overturning moment above and the overturning moment delivered in shear by the diaphragm of that floor. This additional shear decreases at each floor as well as the height above base decreases."

By "this additional shear" in the 2nd sentence, I assume you mean "... the overturning moment delivered in shear ..." etc, in the first sentence.

For a first approximation, assume that each story's floor load per unit area is the same. And therefore, the total shear load is the same. And that it is shared approximately uniformly amongst all the external columns.

It seems to me that this would make the absolute value of the shear on the columns approximately the same on all stories (perhaps higher on the mech floors due to higher floor loads).

It's easy to see that the overturning moment decreases as you rise off the ground, but I'm not seeing this "moment due to membrane shear" doing the same.

thanks.

tom

Okay, explanation time!

But first a bit of clarification:
When I refer to a column AT a floor, I'm actually referring to the column that is supporting that floor.
A "diaphragm" is the structural component that delivers horizontal forces from one part of the building to the lateral resisting system: the perimeter moment frames in the case of the WTC. The diaphragm in this case (and most cases) is the composite light weight concrete floor.

Imagine for a moment a wind event on the WTC building. And let's also for simplicity make this wind event perfectly normal to one face of the WTC. The wind pushes on one face of the building, and sucks on the opposite face. The exterior cladding of the building is loaded by this wind. The exterior cladding is attached to the floor at each level. This is one of many things that we in the industry call "shear". It is quite literally a shear force on the entire building as a whole. This shear force is then resisted by the floor diaphragm through bending. The floor diaphragm is supported by the lateral system. The force is then evenly distributed to the lateral system based on comparative stiffnesses. The moment frame that is parallel to the wind load will see the most (probably 99%).

The very large perimeter wall of the building acts like a beam. The columns in it are loaded elastically: compression at one extreme edge varying down to nothing near the neutral axis and then to tension on the other side. These forces counter-act the overturning moment (the effect of the wind that would push the building over). There is also shear in the columns, evenly distributed that is transfered to the base of the building. The foundation must resist this shear to keep the building from sliding.

Now then, this shear at each level from wind forces increases with height as the actual wind pressure increases with height.

Do you get it now?

 

[tfk]

Yeah, Newton, thanks.

I was thinking about the vertical shear between the floor slabs and the columns.

I'm familiar with the details of beam deflections. I designed electronic connectors & switches for years. And that's exactly how I think of the towers. Big ole boxy cantilever beams. Of course, my beams weren't as porous & discontinuous as this one.

We'd refer to the wind load as a distributed load, which results in a decreasing shear load as you move up from the ground.

Unfortunately, all of the really interesting stuff happens as it starts to starts to come apart. For the guys that did the analysis, it must have been like working on the shuttle disaster board. Fascinating and tragic.

Hard to imagine how frivolous some folks take it.

tom