The NIST report, start to finish

Originally posted by Nutter
I can't believe we are going on with this. Jeez.

I'm fine with ending it, I have nothing to add to what I have already said.

So, this shreaded inch or two that has already put most of it's energy toward crashing through the outer columns still had enough energy to slice through core columns?

What makes you so sure that the plane lost most of its energy, or more importantly each subsequent component lost most of its energy when it hit the outer wall?

Which "real world physics" is this coming from again? Who performed the actual "real world physics" simulation that this is animated after anyway?

Here is a quote from the paper they published about the simulation and associated visual representation:

A model of the North Tower (WTC-I) was built by the
civil engineering members of the team. It included all
structural elements as well as the concrete floors (Figure
10). All stories were modeled, including those under-
ground. The simulation restricted to the upper 20 floors
of the building, however with increased detail meshing
near the impact region so as to achieve high accuracy of
the results.

The civil engineer named on the paper is Professor Ayhan Irfanoglu and you can find a paper he authored on the events here: www.cs.purdue.edu...

How did they get the correct information to do their FEA when everyone else is denied the information to perform a correct one?

They apparently used public domain information, which as I have said previously in this thread should be more than sufficient for accuracy within amateur error margins.

Also. I can't believe you guys are still claiming "real world physics" when we all know that in the "real world" the towers deflected when hit and that is definately not shown in the Purdue simulation.

I speculated the reasons for this in a previous post and now I have found the paper we can see it's confirmed by a quote from it. They limited the simulation to a small number of storeys in order to increase the simulation grid density without affecting runtime.

Originally posted by exponent

Originally posted by psikeyhackr
How can the core columns not move in the Purdue simulation of the north tower impact and yet the NIST provided empirical data in their 10,000 page report that the south tower moved 15 inches due to the impact? One of them has to be wrong. Since skyscrapers are designed to sway in the wind it is pretty certain the Purdue sim is wrong.

Well for a start, it's extremely unlikely that they included any columns above or below the impact zone in their simulation. That would massively slow it down and make working harder. It's common practise to focus only on the part that you care about, because that way you can increase grid density or use more complex elements.

For a second, Purdue didn't model office contents, so the momentum transfer was not as significant or as early as in reality.

All simulations are wrong to some extent, Purdue's sim shows that the plane would indeed penetrate walls and do significant internal damage, despite them not having the time or money to simulate office contents.

You are contradicting yourself with quotes you sourced.

A model of the North Tower (WTC-I) was built by the civil engineering members of the team. It included all structural elements as well as the concrete floors (Figure 10). All stories were modeled, including those under- ground. The simulation restricted to the upper 20 floors of the building, however with increased detail meshing near the impact region so as to achieve high accuracy of the results.

If they simulated the levels underground then they had to take into account the mass so how did their simulation not get the deflection due to impact? That makes them either more stupid than I thought or LIARS. So why haven't any other engineering schools caught this?

It looks like a buddy system playing CYA to me. Nobody rats on anyone else.

psik

Originally posted by psikeyhackr
You are contradicting yourself with quotes you sourced.

You didn't read properly:

A model of the North Tower (WTC-I) was built by the civil engineering members of the team. It included all structural elements as well as the concrete floors (Figure 10). All stories were modeled, including those under- ground. The simulation restricted to the upper 20 floors of the building, however with increased detail meshing near the impact region so as to achieve high accuracy of the results.

If they simulated the levels underground then they had to take into account the mass so how did their simulation not get the deflection due to impact?

They didn't simulate the levels underground, just as they said right in the quote that you quoted.

That makes them either more stupid than I thought or LIARS. So why haven't any other engineering schools caught this?

Because your false dichotomy failed to include the chance that you didn't bother to read the whole paragraph before trying to quote it and use it in your favour?

Originally posted by exponent

Originally posted by psikeyhackr
You are contradicting yourself with quotes you sourced.

You didn't read properly:

A model of the North Tower (WTC-I) was built by the civil engineering members of the team. It included all structural elements as well as the concrete floors (Figure 10). All stories were modeled, including those under- ground. The simulation restricted to the upper 20 floors of the building, however with increased detail meshing near the impact region so as to achieve high accuracy of the results.

If they simulated the levels underground then they had to take into account the mass so how did their simulation not get the deflection due to impact?

They didn't simulate the levels underground, just as they said right in the quote that you quoted.

That makes them either more stupid than I thought or LIARS. So why haven't any other engineering schools caught this?

Because your false dichotomy failed to include the chance that you didn't bother to read the whole paragraph before trying to quote it and use it in your favour?

At best they are being ambiguous. What do they mean by the difference between modeling and simulating? What would be the point of modeling down to underground levels if they were only rendering the vicinity of the impact if they were not computin the effects of that mass and stiffness?

Nitwits often deliberately use ambiguous language so they can then play semantic games claiming they meant whatever is convenient at the time.

By not have the core columns move their SCIENTIFIC simulation is WRONG.

psik

Originally posted by psikeyhackr
At best they are being ambiguous. What do they mean by the difference between modeling and simulating? What would be the point of modeling down to underground levels if they were only rendering the vicinity of the impact if they were not computin the effects of that mass and stiffness?

Because it was a visual depiction? It's possible they did some additional calculations I haven't seen that used the full mesh, but it's irrelevant to the point. You asked why the core columns did not move and this is why.

Nitwits often deliberately use ambiguous language so they can then play semantic games claiming they meant whatever is convenient at the time.

They didn't use ambiguous language, they specified it explicitly, don't blame them for your mistake.

By not have the core columns move their SCIENTIFIC simulation is WRONG.

All simulations are wrong to some extent, If you let a weight fall on earth for one second it will be doing 9.81m/s. That's just simple maths right? Well no, because there's air resistance, the local gravitational constant, any wind effects etc. You have to aim for an acceptable level of error in any experiment, and anyone other than a mathematician who tells you their results are perfect is probably an idiot.

Originally posted by exponent
A model of the North Tower (WTC-I) was built by the civil engineering members of the team. It included all structural elements as well as the concrete floors (Figure 10). All stories were modeled, including those under- ground. The simulation restricted to the upper 20 floors of the building, however with increased detail meshing near the impact region so as to achieve high accuracy of the results.

My question about the simulation of only 20 floors worth is how did they produce the correct amount of moment associated with the plane impact? Moment is force times distance. The moment of a 100 lb force acting 20 stories from the base is (100 x 20 (12.5)= 25,000 foot-lbs). The moment of a 100lb force acting 110 stories from the base is (100 x 110 (12.5) = 137,500 foot-lbs).

That is a huge difference in moment created.

But, Purdue forwent the whole fact that there was deflection, sway, and moment created.

Very "scientific".

Originally posted by Nutter
My question about the simulation of only 20 floors worth is how did they produce the correct amount of moment associated with the plane impact? Moment is force times distance to the centroid. The centroid of a 20 story building is 10 stories. The centroid of a 110 story building (+ 6 additional underground stories) is 58 stories.

It's even worse than you think, because the rotation was around the base of the towers, so it would be 20 storeys vs 110.

That is a huge difference in moment created.

Agreed, but how much of the impact energy went into deflecting the tower that much? That's something I am happy to discuss. Why do you think it's such a serious issue compared to modelling the actual destruction at the top? What are your predictions for the model changes if capturing the momentum transfer was added?

I like the fact you sarcastically call it "scientific" except they missed out all the office contents as I mentioned earlier. Where is your criticism of this? The momentum transfer caused by destroying desks and partitions and filing cabinets is a very important point in understanding structural damage. The swaying of the tower? It's useful for determining model accuracy and energy transfer, but not for structural damage, which is precisely what Purdue were looking at.

Originally posted by exponent

Originally posted by Nutter
My question about the simulation of only 20 floors worth is how did they produce the correct amount of moment associated with the plane impact? Moment is force times distance to the centroid. The centroid of a 20 story building is 10 stories. The centroid of a 110 story building (+ 6 additional underground stories) is 58 stories.

It's even worse than you think, because the rotation was around the base of the towers, so it would be 20 storeys vs 110.

We be thinking alike. I edited my post.

Agreed, but how much of the impact energy went into deflecting the tower that much? That's something I am happy to discuss. Why do you think it's such a serious issue compared to modelling the actual destruction at the top? What are your predictions for the model changes if capturing the momentum transfer was added?

My questions are if the towers were designed for 100 mph winds in a hurricane, that produces a significant amount of moment at the base. Was this moment superceeded by the plane impact? I'm not sure as I haven't calculated the force the plane would have hit, the distance it hit, etc. Not to mention that if the plane hit at an angle or not straight on (WTC 2) that would significantly change the deflection, sway, and moment.

The swaying of the tower? It's useful for determining model accuracy and energy transfer, but not for structural damage, which is precisely what Purdue were looking at.

That is a contradictory statement.

The swaying of the tower absorbed some of the energy. Which is precisely what Purdue was looking at.

Originally posted by exponent
They didn't use ambiguous language, they specified it explicitly, don't blame them for your mistake.

All simulations are wrong to some extent,

anyone other than a mathematician who tells you their results are perfect is probably an idiot.

You provided this quote:

A model of the North Tower (WTC-I) was built by the civil engineering members of the team. It included all structural elements as well as the concrete floors (Figure 10). All stories were modeled, including those under- ground. The simulation restricted to the upper 20 floors of the building, however with increased detail meshing near the impact region so as to achieve high accuracy of the results.

All stories were modeled, including those under- ground

The simulation restricted to the upper 20 floors of the building,

Those two statements are ambiguous. The underground stories were at least 80 stories away from the impact. How were they MODELED without being SIMULATED. If they had said RENDERED instead of simulated then I could understand it. Rendering is just producing the images. Modeling and Simulating are figuring out what happens.

ROFL

Purdue is a much of a joke as the NIST that can't specify the total amount of concrete in the towers.

psik

Originally posted by Nutter
We be thinking alike. I edited my post.

You should be aware that you can't idealise the WTC to a rigid body, so giving a value for the base moment is not really accurate, it's a minor nitpick but just so you know.

My questions are if the towers were designed for 100 mph winds in a hurricane, that produces a significant amount of moment at the base. Was this moment superceeded by the plane impact? I'm not sure as I haven't calculated the force the plane would have hit, the distance it hit, etc. Not to mention that if the plane hit at an angle or not straight on (WTC 2) that would significantly change the deflection, sway, and moment.

NIST is of course the source for this, and as you don't seem to be debating anything I will quote the answer for you.

Obviously WTC2 is the only candidate for impact analysis, as only two videos exist of the first impact, neither with enough stability, length or detail. For WTC2 the analysis indicated:

The peak amplitude of the movement in the north-south direction was determined to be 12 in. ± 1 in. at the 70th floor, which extrapolated to a value of 20 in. ± 3 in. at the roofline. Frequency analysis of the results revealed the fundamental north-south mode to have a period of 11.4 s, a torsional mode with a period of 5.3 s, and two higher frequency modes with periods of 3.9 s and 2.2 s. All periods were determined to an accuracy better than ± 0.1 s.

Here is an extract from the original wind deflection calculations:


As you can see, total deflection did not reach above 2 feet, and so wind deflection would have been greater in total. However, obviously the plane's region of impact was quite small, and so the forces are massively increased in that area, the two are not directly comparable, but the forces on the building significantly away from the impact zone probably are.

That is a contradictory statement.

The swaying of the tower absorbed some of the energy. Which is precisely what Purdue was looking at.

What makes you say they were looking at energy consumption? Like I said, surely you should criticise them much more harshly for not modelling the office contents?

Originally posted by psikeyhackr
Those two statements are ambiguous. The underground stories were at least 80 stories away from the impact. How were they MODELED without being SIMULATED. If they had said RENDERED instead of simulated then I could understand it. Rendering is just producing the images. Modeling and Simulating are figuring out what happens.

Modelling is the process of recreation, simulation is the process of figuring out what happened.

ROFL

Purdue is a much of a joke as the NIST that can't specify the total amount of concrete in the towers.

The only joke here is that you think disagreeing with their wording discredits them.

That is by no means a reasonable or logical step to take.

Originally posted by exponent
What makes you say they were looking at energy consumption?

If they were modeling damage, then they had to model energy. Damage just doesn't happen without energy.

Like I said, surely you should criticise them much more harshly for not modelling the office contents?

The addition of office contents and deflection would have produced a much closer to reality simulation IMO.

Originally posted by Nutter
If they were modeling damage, then they had to model energy. Damage just doesn't happen without energy.

That's true, but you implied they were aiming to study energy consumption, wheras they seem to make it clear they were trying to create a high quality visualisation of the impact and resultant damage.

The addition of office contents and deflection would have produced a much closer to reality simulation IMO.

Agreed. NIST did this, and so we do have the information available to us. Are you satisfied with the quotes I posted above?

Here are the next 4 topics, I will make an index of discussions so far later, but as we have such utterly beautiful weather here, I am going to go lie in the garden with a beer for a while

• Fireproofing failure meant some trusses were unprotected or severely under-protected
  
• Unprotected or semi-protected trusses in fire will heat quickly
  
• Heated trusses will sag due to a variety of reasons
  
• Sagging trusses will exert a greater horizontal force than those deflecting only at design requirements

I look forward to comments on these.

Originally posted by exponent

Originally posted by psikeyhackr
Those two statements are ambiguous. The underground stories were at least 80 stories away from the impact. How were they MODELED without being SIMULATED. If they had said RENDERED instead of simulated then I could understand it. Rendering is just producing the images. Modeling and Simulating are figuring out what happens.

Modelling is the process of recreation, simulation is the process of figuring out what happened.

ROFL

Purdue is a much of a joke as the NIST that can't specify the total amount of concrete in the towers.

The only joke here is that you think disagreeing with their wording discredits them.

That is by no means a reasonable or logical step to take.

A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system. Computer simulations have become a useful part of mathematical modeling of many natural systems in physics (computational physics),

en.wikipedia.org...

psik

Originally posted by exponent

Originally posted by Nutter
If they were modeling damage, then they had to model energy. Damage just doesn't happen without energy.

That's true, but you implied they were aiming to study energy consumption, wheras they seem to make it clear they were trying to create a high quality visualisation of the impact and resultant damage.

So you're saying the visualization was of first importance, and all the math and physics behind it was of secondary importance? I'm asking because the damage they're modeling is going to be just as inaccurate as whatever energies they were or weren't considering, and I doubt Nutter is alone in finding his priorities in the math and physics first.

Sagging trusses will exert a greater horizontal force than those deflecting only at design requirements

What deflection of trusses at "design requirements" are you talking about?


This sounds like some kind of convoluted excuse for why a phenomena that is known to produce smaller deflections than thermal expansion, is being favored over the phenomena that produces larger deflecting forces at lower temperatures. The perimeter columns are where the relevant deflection must take place, and you would have to get around to showing how the spandrel plates would have resisted any deflection, which was a no-go for NIST. That's the real-life design that NIST's hypothesis is required to overcome.

Originally posted by exponent

• Fireproofing failure meant some trusses were unprotected or severely under-protected
  
• Unprotected or semi-protected trusses in fire will heat quickly
  
• Heated trusses will sag due to a variety of reasons
  
• Sagging trusses will exert a greater horizontal force than those deflecting only at design requirements

I look forward to comments on these.

I already commented and you ignored it. If you are so intent on discussing these topics, why did you ignore my post on page 2 that addresses every point you wanted to discus? You're not even discussing them yourself.

I want to know why you and PLB, and probably others, think that sagging trusses put more force on the columns than non sagging trusses?

Where is your physics explanation for that?

Because here is the physics that proves you wrong...Steel when heated expands. IF the trusses heated up enough to expand (something else you need to prove before your hypothesis is excepted), then why did they not push the columns outwards? Instead they SAGGED, which means they could do neither push out, nor pull in the columns, otherwise they would not have sagged they would have pushed out first (which if you think they could pull in, then they would also as easily push out).

The sagging is not going to put more force on the columns, and even if they did the columns would have had no problem holding the extra force. Do you think building components are working on the edge of their ability?
No, building components are designed to do their work and have a large reserve, a safety factor, so that the component won't fail if their ability is exceeded.

Again your points of discussion are irrelevant, and ignore what is really important to the whole picture.

reply to post by ANOK

IF the trusses heated up enough to expand (something else you need to prove before your hypothesis is excepted),....

So you think that steel expanding when any heat is applied is only a hypothesis? Its not, it is a fact. And the expansion begins the moment the temperature of the steel exceeds its neutral temperature. A given piece of steel will be larger at 100 degrees than at 90 degrees. That is a fact. The difference may be minute, but there is a difference.

Originally posted by psikeyhackr
en.wikipedia.org...

psik

This article is about computer model within a scientific context. For artistic usage, see 3d modeling. For simulating a computer on a computer, see emulator.

Why are you so intent on trying to be right on this one minor point? It is really utterly irrelevant as we are speculating about their intentions and you are trying to discredit them based on wording you dislike.

The fact of the matter is that they modelled the collision with publicly available data and managed to get a reasonable reproduction. If they had included office contents it would likely have been a very strong match as this would reduce the core column damage to the same as NISTs estimates, and reduce the opposite side penetration as well.

Originally posted by bsbray11
So you're saying the visualization was of first importance, and all the math and physics behind it was of secondary importance? I'm asking because the damage they're modeling is going to be just as inaccurate as whatever energies they were or weren't considering, and I doubt Nutter is alone in finding his priorities in the math and physics first.

I'm not sure i'd rank them in importance terms, just that they were not looking to investigate the performance of anything but those few floors, so why waste a huge amount of expensive cpu time on modelling floors you're never going to use?

Once again, NIST did a much more thorough job of this, but Purdue should be considered more independent. Their results do not contradict each other, so you can pick and choose all you like really, whatever you trust or believe in more.

What deflection of trusses at "design requirements" are you talking about?

Trusses deflect under design load, they're limited by code and in the WTC I think mid span deflection maxed out at 2". I just wanted to make clear that I wasn't talking about an ideally flat truss as I heard talk of you drawing a free body diagram to debunk that particular point. No harm in adding a little more detail to my question.

The perimeter columns are where the relevant deflection must take place, and you would have to get around to showing how the spandrel plates would have resisted any deflection, which was a no-go for NIST. That's the real-life design that NIST's hypothesis is required to overcome.

I'm not sure why you think the spandrel plates would serve as a particular stiffener in this context. They were designed in a belt around the towers and worked as a stiffener in this axis, but in the truss axis they rely on trusses for support as much as any other perimeter column element.

Could you elaborate a bit further?

Originally posted by ANOK
I already commented and you ignored it. If you are so intent on discussing these topics, why did you ignore my post on page 2 that addresses every point you wanted to discus? You're not even discussing them yourself.

I'm sorry, I wanted to go in order and make sure there were no objections raised to the first points. If you want to quote your questions with regard to the 4 points I made above I would be happy to answer them.

I want to know why you and PLB, and probably others, think that sagging trusses put more force on the columns than non sagging trusses?

Where is your physics explanation for that?

They don't put more force on the columns, they exert a force in a different direction. As they sag, they become somewhat like a chain hanging between the core and the outer wall. Chains exert a significant portion of their weight horizontally inward, due to the geometry of their hanging: en.wikipedia.org...

Because here is the physics that proves you wrong...Steel when heated expands. IF the trusses heated up enough to expand (something else you need to prove before your hypothesis is excepted), then why did they not push the columns outwards?

They definitely did, many inches.

Instead they SAGGED, which means they could do neither push out, nor pull in the columns, otherwise they would not have sagged they would have pushed out first (which if you think they could pull in, then they would also as easily push out).

They definitely did push out, the sagging occurred at a later time after many truss seats had failed and the outer walls were less supported, the continued breaking of web / knuckle elements leads to sag.

No, building components are designed to do their work and have a large reserve, a safety factor, so that the component won't fail if their ability is exceeded.

That's certainly true, but in this case the safety factor was easily exceeded by the combination of extensive unfought fires and the initial damage to the structure.

Again your points of discussion are irrelevant, and ignore what is really important to the whole picture.

I don't think that's particularly fair. I'm trying to build up the whole picture here piece by piece, so instead of arguing about the whole thing at once and getting sidetracked, we pin down specific issues. That way we will both have information we know we can both agree on. That makes formulating any arguments in favour of your preferred hypothesis (nb: bsbray is going to explode at that word) a lot easier.

I've never looked at the trusses, and couldn't find what I was looking for in a quick google search, but could someone be so kind as to tell me if the trusses were coupled to the concrete slabs they were supporting? If so, at what distances (if not continuously)?

reply to post by exponent

They don't put more force on the columns, they exert a force in a different direction. As they sag, they become somewhat like a chain hanging between the core and the outer wall.

This would be the case if the trusses were simply supported at the columns. Correct me if I'm wrong, but I believe the column-truss connection was a fixed support.