The Heiwa Challenge

[Gamolon]

These guys use the term rigid body in this article:
http://www.luxinzheng.net/publication3/FEM_DEM_CSE09.htm

In this model, every floor is a rigid body without rotation displacement and the adjacent stories are connected with an axial spring and a shear spring.

I suppose they are wrong also?

I must be confused Heiwa. I see the term "rigid body" used all the time when people are talking about structural analysis, but I fail to see you explain how and when the term should be used. You say Bazant is wrong for using it to describe the upper part of the WTC tower.

How should the term be used in a structural sense? What structural designs can the term be used to describe when performing calculations?

 

[Heiwa]

AA. Are you are saying that in the world of structural engineering, there is no such thing as a "rigid body" be it in real life OR when doing calculations?

BB. If you AREN'T saying that there is no such thing, please provide an example of either a real life example or when/how a "rigid body" should be used in calculations.

CC. You are claiming that Bazant is applying the term "rigid body" in error. So please explain how one should use/apply the term "rigid body" in a structural sense.

I'm all ears.

I know most of you understand the term "rigid body", but I want Heiwa's explanation for it.

AA. In structural analysis/design/calculations no element is rigid, i.e. all elements can deform, when a force is applied. The whole purpose of structural analysis is to calculate these deformations (and associated stresses).

BB. Therefore I never use rigid elements in structural analysis! Each element has its material properties, like e.g. steel. Rigid steel does not exist!

CC. In Bazant's simple 1-D model of one line (C - the 'WTC 1 upper block') colliding with another line (A - the 'WTC 1 weak lower flexible block') he assumes that line C is rigid and cannot deform. Therefore C deforms A in a collision but A cannot deform C; A becomes a 'rubble' line B that is A compressed 4 times!! Complete and utter nonsense. Why not do a proper 3-D analysis using correct sub-elements? It is not difficult!

In real structural analysis there is no need to assume that parts or elements are 'rigid'! Just give them the proper material properties.

It seems Bazant has designed a bridge when he was young and I assume that no part of that bridge was rigid.

In very simple beam static analysis done long-hand, where you are not really interested in local deformations, but only where forces and moments are going, to get a feel for the structure for detailed design later, you can assume that the complete 'beam' is rigid. With a PC, you evidently give the beam its proper dimensions and material non-rigid properties from start so that it can deform.

Happy?

 

[Heiwa]

These guys use the term rigid body in this article:
http://www.luxinzheng.net/publication3/FEM_DEM_CSE09.htm

I suppose they are wrong also?

I must be confused Heiwa. I see the term "rigid body" used all the time when people are talking about structural analysis, but I fail to see you explain how and when the term should be used. You say Bazant is wrong for using it to describe the upper part of the WTC tower.

How should the term be used in a structural sense? What structural designs can the term be used to describe when performing calculations?

Well, if you put in 'rigid body' mass elements in a structure and connect them with various springs (non-rigid elements) that can break and then drop one of these 'rigid body' mass elements, like these Chinese do, on another 'rigid body' mass element in the structure, then evidently only the springs will deform (and maybe break).

Look at the figures in the paper. The 'rigid body' mass elements are not damaged - they just pile up on ground - undamaged! This has nothing to do with reality.

 

[tsig]

<snip>This has nothing to do with reality.

True

 

[Gamolon]

AA. In structural analysis/design/calculations no element is rigid, i.e....

Then can you please tell me why I am finding references to "rigid bodies" from engineers? If no element can be considered "rigid" in your eyes, then why do engineers use them when performing calculations and consider certain sturctures as being rigid?

For example, from this site:
http://www.woodheadpublishing.com/en/book.aspx?bookID=1538

- Free vibration of rigid bodies without damping

- Generalized SDOF system: rigid bodies

- Program 14.4: MATLAB program to find the natural frequency of beams or rigid frames

Here is a quite from this site which explicitly says "every floor is a rigid body":http://www.luxinzheng.net/publication3/FEM_DEM_CSE09.htm

In this model, every floor is a rigid body without rotation displacement and the adjacent stories are connected with an axial spring and a shear spring

So please explain. How can you say that rigid bodies do not exist yet I can find MANY references to them when dealing with structural calculations?

BB. Therefore I never use rigid elements in structural analysis! Each element has its material properties, like e.g. steel. Rigid steel does not exist!

Again. How is it that YOU say rigid steel does not exist, yet I find references about rigid bodies all over the place?

Are you, a NAVAL ARCHITECT, trying to tell me that STRUCTURAL ENGINEERS are completely wrong because YOU say rigid bodies don't exist per your definition and that they shouldn't be used in calculations which is why YOU don't use them?

 

[twinstead]

Hey. If Heiwa says rigid bodies don't exist, it doesn't make a difference HOW many engineers mention them; since he's smarter than all the other engineers on Earth, we can safely assume that rigid bodies do not exist.

Get with the program people!

 

[Newtons Bit]

Err, rigid bodies DONT exist. They're an approximation that makes math a whole heck of alot easier in engineering analysis.

Before modern Finite Element Analysis and hi-end computers it was absolutely necessary to assume certain elements of structures as rigid.

 

[Grizzly Bear]

Hey. If Heiwa says rigid bodies don't exist, it doesn't make a difference HOW many engineers mention them; since he's smarter than all the other engineers on Earth, we can safely assume that rigid bodies do not exist.

Get with the program people!

What I find ironic about Heiwa's approach is that he uses precedents [wrongly] to state that a particular failure mechanism is impossible but then states as fact that progressive collapse is impossible under any circumstances without looking back at any precedents.

Da twoof works in many fascinating paradoxes.

 

[twinstead]

Err, rigid bodies DONT exist. They're an approximation that makes math a whole heck of alot easier in engineering analysis.

Before modern Finite Element Analysis and hi-end computers it was absolutely necessary to assume certain elements of structures as rigid.

Oh, sure. Bring expertise into the discussion. I HATE when you do that

So what the hell is Heiwa talking about anyway?

 

[Gamolon]

Err, rigid bodies DONT exist. They're an approximation that makes math a whole heck of alot easier in engineering analysis.

Before modern Finite Element Analysis and hi-end computers it was absolutely necessary to assume certain elements of structures as rigid.

I understand that.

My whole point is that Heiwa is saying that a "rigid body" element (whatever it is being used to represent in a calculation) has no place in structural calculations. Which is complete and utter crap based on the fact that I have found references to "rigid bodies" in all sorts of structural engineering discussions and calculations. Here is a quote from Heiwa from another thread:

In structural analysis there are no rigid elements

I would like him to explain how he can make this kind of statement when structural engineers are quoted as referencing them. Obviously he has no clue, being only a naval architect, and has not discussed any of this with any structural engineer at all. The proof is right there in front of him and everywhere else.

Rigid bodies ARE used when doing structural calculations and he is completely off his rocker to say otherwise as the proof is in my post above showing REFERENCES to rigid bodies.

What I want to know is if structural engineers do use the "rigid body" element to perform calculations, how are they determined? For example, in my post above, there is mention of a "floor" being considered a rigid body. What is the criteria used, if any, that will define when one "element" (such as the floor mentioned above) can be considered a rigid body and another element (Bazant's upper tower part) cannot? Is there any?

 

[Newtons Bit]

Oh, sure. Bring expertise into the discussion. I HATE when you do that

So what the hell is Heiwa talking about anyway?

He has this fantasy that rigid objects automagically destroy any non-rigid object they touch. It's quite ludicrous.

He later one changed his opinion such that if the floors weren't rigid, then the building would no longer self collapse. I assume this has to do with his rubbish theory on friction stopping the collapse.

 

[Heiwa]

I understand that.

My whole point is that Heiwa is saying that a "rigid body" element (whatever it is being used to represent in a calculation) has no place in structural calculations. Which is complete and utter crap based on the fact that I have found references to "rigid bodies" in all sorts of structural engineering discussions and calculations. Here is a quote from Heiwa from another thread:



I would like him to explain how he can make this kind of statement when structural engineers are quoted as referencing them. Obviously he has no clue, being only a naval architect, and has not discussed any of this with any structural engineer at all. The proof is right there in front of him and everywhere else.

Rigid bodies ARE used when doing structural calculations and he is completely off his rocker to say otherwise as the proof is in my post above showing REFERENCES to rigid bodies.

What I want to know is if structural engineers do use the "rigid body" element to perform calculations, how are they determined? For example, in my post above, there is mention of a "floor" being considered a rigid body. What is the criteria used, if any, that will define when one "element" (such as the floor mentioned above) can be considered a rigid body and another element (Bazant's upper tower part) cannot? Is there any?

Thanks for your kind words about naval architects. That some young 'scientists' still use 'rigid bodies' in their calculations of whatever is just lack of proper education. A rigid body or element does not exist!

You wonder if a "floor" can be considered rigid. Are you sitting on one or not? If yes, look down! Does it look rigid? Tell me how thick it is, etc, etc, and I will tell you it is not rigid at all. But maybe you can enlighten me and all those naval architects you refer to.

But that was off topic! In this thread you shall design a structure where a small part can one-way crush the remainder by gravity only! You are only allowed to drop this part on the other part and you must demonstrate the famous 911 POUFF!!!! Upper part one-way crushes lower part!

 

[nicepants]

To answer your last question any smart engineer knows that a structure consists of elements connected to one another. So there are no TWO parts; one mass above and individual or a collection of floors (one part??) below in a structure like WTC 1.

The mass above happens to be a collection of floors held apart by columns. Only a fool considers that one mass, one part, or worse, one rigid block!

If you were to place the "mass above" onto a large scale it would register the weight of the entire mass.

ETA: It doesn't really make any difference though. 100 1g masses traveling at 10ft/sec have the same energy as one 100g mass traveling the same speed and both weigh the same.

Breaking a large mass into smaller pieces doesn't change its weight.

If you get that - the upper part (C) is just an assembly of elements - you'll soon find out that it cannot produce a one-way crush of anything similar, e.g. a part (A) below. Reason is that A will damage C at contact.

Won't C also damage A? Suppose the bottom floor of C is crushed and teh top floor of A is crushed. Now the mass that is falling is even larger as it continues to fall.

Heiwa, if you're not a troll and you truly don't understand this simple high-school physics, perhaps you should take a stroll over to your nearest university, stop at the physics department and have a discussion with a qualified physics professor who can explain this to you in detail.

 

[FineWine]

Thanks for your kind words about naval architects. That some young 'scientists' still use 'rigid bodies' in their calculations of whatever is just lack of proper education. A rigid body or element does not exist!

You wonder if a "floor" can be considered rigid. Are you sitting on one or not? If yes, look down! Does it look rigid? Tell me how thick it is, etc, etc, and I will tell you it is not rigid at all. But maybe you can enlighten me and all those naval architects you refer to.

But that was off topic! In this thread you shall design a structure where a small part can one-way crush the remainder by gravity only! You are only allowed to drop this part on the other part and you must demonstrate the famous 911 POUFF!!!! Upper part one-way crushes lower part!

Okay, Heiwa, you've been asked this question many times and you've always babbled incoherent gibberish. You're sitting in a chair on the 97th floor. Floors 98-110 collapse and fall on top of your floor. What magic protects you from being crushed?

 

[BigAl]

But that was off topic! In this thread you shall design a structure where a small part can one-way crush the remainder by gravity only! You are only allowed to drop this part on the other part and you must demonstrate the famous 911 POUFF!!!! Upper part one-way crushes lower part!

Can I hammer it to cause structural damage, add weight that exceeds local loading limits and set it afire and let it burn to cause more damage before I drop part A on it?

 

[Heiwa]

If you were to place the "mass above" onto a large scale it would register the weight of the entire mass.

ETA: It doesn't really make any difference though. 100 1g masses traveling at 10ft/sec have the same energy as one 100g mass traveling the same speed and both weigh the same.

Breaking a large mass into smaller pieces doesn't change its weight.

Mass is always mass. But 100 small masses m dropped one after the other on the floor, ping, ping ...., is different from dropping one big, solid M = 100 m on the floor, BANG.

What do you want to say? This thread is about something else! See post #1.

 

[Gamolon]

That some young 'scientists' still use 'rigid bodies' in their calculations of whatever is just lack of proper education.

So you're saying that any structural engineer who uses "rigid bodies" in a structural calculation is wrong and that they should never be used?

 

[Heiwa]

Can I hammer it to cause structural damage, add weight that exceeds local loading limits and set it afire and let it burn to cause more damage before I drop part A on it?

See post #1. Just design/build a structure of any type, size and scale, then disconnect the upper 1/10 part (C) of the structure, lift it a little and drop it on the 9/10 structure below (part A).

Now GRAVITY is supposed to assist you to win The Heiwa Challenge! If C one-way crushes A due to GRAVITY, you are a WINNER. So far - 1800+ posts here - nobody has managed, incl. NIST, NASA, FEMA, ASCE, Bazant, Seffen, Mackey, MIT, Harvard & Co. They all say it will happen ... but they cannot show it in a simple test!
I asked GWB to have a try and you know what happened?

Curious?

GWB suggested that .... ha, ha, ha ... GWB said .... ha, ha, ha ... lol, lol ... . Sorry, I am laughing so I can't ...

 

[Heiwa]

So you're saying that any structural engineer who uses "rigid bodies" in a structural calculation is wrong and that they should never be used?

Yes! How could you miss that. I have said it seeal times. But, sorry, this GWB ... he said ... unbelievable .... sorry. I have to lay down and rest !!!

But, pls, continue ask off topic questions.

 

[BigAl]

See post #1. Just design/build a structure of any type, size and scale, then disconnect the upper 1/10 part (C) of the structure, lift it a little and drop it on the 9/10 structure below (part A).

Now GRAVITY is supposed to assist you to win The Heiwa Challenge! If C one-way crushes A due to GRAVITY, you are a WINNER. So far - 1800+ posts here - nobody has managed, incl. NIST, NASA, FEMA, ASCE, Bazant, Seffen, Mackey, MIT, Harvard & Co. They all say it will happen ... but they cannot show it in a simple test!

Then you are dealing with some hypothetical structure that bears no resemblance to the WTC towers and what happened to them on 9/11. Because of that, I find the HEIWA Challenge(tm) exceedingly uninteresting.