WTC Collapse Simulation using Unity/Besiege

Jeffrey Orling

Senior Member
Damage Caused by Heat

Let's discuss

Where was the acting
in the core - %​
outside the core - %​
both inside and outside the core % & %​
Did the fire spread...
From where to where​
Why?
Where was the main impact:​
Beams in the core​
did the expand and sag​
did they expand and push
did the concrete they supported break up and drop?​
Bar trusses supported the outside the core slabs​
did the expand and sag​
did they expand and push​
over the entire footprint​
in some areas only​
Columns in the core​
did they grow weaker and carry loads​
did they grow weaker and buckle​
did the end connections hold or did they shear
were they displaced by the beam expansion
which were the most and least effected by heat​
Connections​
did they shear and fail
beam stubs to columns
beam stubs to beams​
column to column​
 

Mendel

Senior Member.
A typical office fire starts on a single floor (and stays on a single floor), then the sprinkler system comes on, and a short while later the fire department attends to it and saves the building.
You don't design a building against someone spilling several tons of jet fuel over several floors and setting them alight.

Fire safety is always compromised against cost. What you want in case of a really big fire is for everyone to make it out safely, and for most people in thw WTC below the impact zone with the demolished stairwells, this worked (though analysis identified necessary improvements).
 
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econ41

Senior Member
Why would you design a building so that an unattended fire on a small group of floors can demolish the entire building in under an hour?
You don't. You design for an anticipated level of risk.

Steel framed high rise towers are a compromise. Certainly in the era when WTC Towers were built, they had to be lightweight steel frames. And steel frames are vulnerable to fires causing heat weakening.

So they're a compromise designed with "fire rating" as a major design consideration. "Fire Rating" is expressed in hours. Three hours for most of the WTC Complex.

The purpose of fire rating is to allow time. Time for:
(1) Occupants to escape >>which is the over-riding#1 priority; AND
(2) Time for active firefighting to be implemented. Protect the building - minimise damage - which is the second priority.

The fire rating is based on the normal (expected) progression of an office fire i.e. a fire involving normal office contents within the normal office partitions fit out.
The goal is to prevent steel frames from heating to temperatures that would compromise the structural stability. It is achieved by techniques that include insulation and fire sprinklers.

None of the WTC Towers fires were the fires designed for. The Twin Towers fires were much larger in several dimensions. Instantaneous start of fires on several floors. Fuel concentrated to one side of the building due to "BoeingDozing" (The aircraft pushing office contents and fitout material across the building) and the instant start of fires aided by aircraft fuel acting as an accelerant. PLUS fire sprinkles were disabled.

AND there was no active firefighting which, at minimum, would (a) put firefighters at the fire front with hoses AND would ensure the boosting and replenishment of fire sprinkler supply water.

Bottom line the Twin Towers fires were well outside the designed fire parameters and collapsed well before the fire rating time. WTC 7 was also outside design but not as far. So WTC1 & WTC2 designed for 3-hour fire rating failed well before the three hours. WTC 7 - not taken as far outside parameters - lasted nearly twice its fire rating.
 
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Jeffrey Orling

Senior Member
The answer is that steel's vulnerability to failure are fire protection, suppression and fighting.
Unfortunately in a multi story building... the mass of only several floors "released" by a failure at one level is enough to destroy the floor it falls on and that repeats all the way to the ground. No single floor can be built economically or practically to support these sorts of loads.

It should be noted that the runaway collapse was essentially the overloaded slabs, not the columns. Columns failed from loss of bracing when the floors had collapsed... which provided lateral support/bracing for the columns.

Fire protection and fighting systems need to be robust and redundant. WTC buildings' crippled fire protection and fighting systems all failed to prevent fires from fatally weakening the structure.

Steel typically is protected by "insulation" to prevent temps from rising and lowering strength. This alone is not enough. Active fire fighting/suppression is required. Sprinklers failed and fire fighters were unable to fight the fires.

++++

So design engineers of the WTC buildings used column free floor space outside the cores. Cores were for vertical conveyance and mech shafts and contained bathrooms. These designs has floors spanning from core to perimeter. Traditional high rise steel frames use a grid system of "bays". The floors are "structurally" isolated from one bay to the next. A local floor failure within a bay usually will not spread laterally to other bays. The construction is more expensive... so it was not used.
The wide open office floors facilitated entire floor collapse... and essentially left the core and the perimeter standing. Floor collapses can destroy essential column bracing... the beams that support the floors.

In 7WTC it appears that there was a "local" floor collapse in the NE quadrant which undermined structure at the lowest floors and that involved load transfer structures which rapid spread and undermined the entire core leading to a floor collapse of the entire footprint. This left the facade which fell soon afterward as the floor collapse undermined its support.

The twins fate was sealed when there was a runaway floor collapse... leaving unbraced columns which could not stand without bracing.

Floors are typically designed to carry only about 2-3 times the anticipated superimposed loads. More than that... a runaway collapse is the outcome.
 
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dylbie

New Member
Hi all, sorry not been here for a while.

I've been running some more "simulations" with Besiege recently and think I might have a makeshift solution to simulate the floor collapses.

One problem I had with the previous versions was that the floors to column connections were much too strong (almost indestructible in fact!) so as soon as the mass of the upper moving section hit the intact section below, it 'stalled', ejected a lot of material sideways, and in lots of cases stopped the whole collapse. This obviously isn't consistent with what happened.

The solution I have reduces the strength of the floors so the impact of the collapse, and a lot of the material falling from above, now continues down inside the tube of exterior columns in a much more fluid motion as was seen in real life.

The only problems I have left are the fact the top 'block' does not remain intact. I honestly don't think that this can be overcome with this software, so not much I can do here. Also, the core isn't putting up much of a fight once the floors and exterior columns have fallen down to ground level. This leads me on to some questions...

1. Does anyone have any estimates of total collapse duration times, including the core? All I've seen so far is either 12-13 seconds for 2WTC and 15-16 seconds for 1WTC. The video taken from the boat on the Hudson of 1WTC would suggest the time taken from initiation to complete core failure were much longer than this.

2. Does anyone have any theories as to why the core failed? Both cores were still partially standing after the main collapse. In the simulations I've run, the debris at ground level completely overwhelms the base of the core and destabilises it. Just wondering if this is anywhere near accurate.

3. I've asked this before, but has anything done to calculate the build up of debris at lower levels and any lateral damage this may have caused? Once the debris/material starts building up inside the 'tube' it starts damaging the exterior walls. I notice in some of the photos of ground zero the lower sections which remained intact are bowed outwards. I also noticed a photo where part of the exterior wall is intact, but there is a hole section of steel which appears as if its been punched out. One odd thing which relates to question 2, is that how on earth did the core survive this amount of damage and not collapse immediately. Seems to stay up for 10 seconds or so second before finally giving up.
 
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Landru

Moderator
Staff member
Hi all, sorry not been here for a while.

I've been running some more "simulations" with Besiege recently and think I might have a makeshift solution to simulate the floor collapses.

One problem I had with the previous versions was that the floors to column connections were much too strong (almost indestructible in fact!) so as soon as the mass of the upper moving section hit the intact section below, it 'stalled', ejected a lot of material sideways, and in lots of cases stopped the whole collapse. This obviously isn't consistent with what happened.

The solution I have reduces the strength of the floors so the impact of the collapse, and a lot of the material falling from above, now continues down inside the tube of exterior columns in a much more fluid motion as was seen in real life.

The only problems I have left are the fact the top 'block' does not remain intact. I honestly don't think that this can be overcome with this software, so not much I can do here. Also, the core isn't putting up much of a fight once the floors and exterior columns have fallen down to ground level. This leads me on to some questions...

1. Does anyone have any estimates of total collapse duration times, including the core? All I've seen so far is either 12-13 seconds for 2WTC and 15-16 seconds for 1WTC. The video taken from the boat on the Hudson of 1WTC would suggest the time taken from initiation to complete core failure were much longer than this.

2. Does anyone have any theories as to why the core failed? Both cores were still partially standing after the main collapse. In the simulations I've run, the debris at ground level completely overwhelms the base of the core and destabilises it. Just wondering if this is anywhere near accurate.

3. I've asked this before, but has anything done to calculate the build up of debris at lower levels and any lateral damage this may have caused. Once the debris/material starts building up inside the 'tube' it starts damaging the exterior walls. I notice in some of the photos of ground zero the lower sections which remained intact are bowed outwards. I also noticed a photo where part of the exterior wall is intact, but there is a hole section of steel which appears as if its been punched out. One odd thing which relates to question 2, is that how on earth did the core survive this amount of damage and not collapse immediately. Seems to stay up for 10 seconds or so second before finally giving up.
When did Besiege (a $15 game engine) become good enough to model the collapse? I though you admited it wasn't very good.
 

dylbie

New Member
We've established that it isn't good enough to model the collapse in detail... but for a £15 game, on a consumer grade PC, it doesn't do a bad job. It's also led me to ask some questions that I wouldn't have thought about, and which I haven't seen discussed before.
 

Jeffrey Orling

Senior Member
When did Besiege (a $15 game engine) become good enough to model the collapse? I though you admited it wasn't very good.

Hi all, sorry not been here for a while.

I've been running some more "simulations" with Besiege recently and think I might have a makeshift solution to simulate the floor collapses.

One problem I had with the previous versions was that the floors to column connections were much too strong (almost indestructible in fact!) so as soon as the mass of the upper moving section hit the intact section below, it 'stalled', ejected a lot of material sideways, and in lots of cases stopped the whole collapse. This obviously isn't consistent with what happened.

The solution I have reduces the strength of the floors so the impact of the collapse, and a lot of the material falling from above, now continues down inside the tube of exterior columns in a much more fluid motion as was seen in real life.

The only problems I have left are the fact the top 'block' does not remain intact. I honestly don't think that this can be overcome with this software, so not much I can do here. Also, the core isn't putting up much of a fight once the floors and exterior columns have fallen down to ground level. This leads me on to some questions...

1. Does anyone have any estimates of total collapse duration times, including the core? All I've seen so far is either 12-13 seconds for 2WTC and 15-16 seconds for 1WTC. The video taken from the boat on the Hudson of 1WTC would suggest the time taken from initiation to complete core failure were much longer than this.
I don't have anything for 2WTC... but 1 WTC was I believe: (hard to determine the precise end of the collapse as its shrouded in a huge dust cloud. The floor collapse appears to have reached a "terminal velocity" around 100' / second.
Einstein Puffs_page1.jpg

2. Does anyone have any theories as to why the core failed? Both cores were still partially standing after the main collapse. In the simulations I've run, the debris at ground level completely overwhelms the base of the core and destabilises it. Just wondering if this is anywhere near accurate.

Parts of both cores survived because the ROOSD phase the collapsing floor mass BYPASSED the columns... striped off most bracing beams with the exception of some elevator shaft framing.... which can be seen as "ladder looking" surviving core columns. All columns collapsed post floor collapse from Euler buckling and instability. Tallest surviver was col 501 which was 72 stories tall before it toppled.
3. I've asked this before, but has anything done to calculate the build up of debris at lower levels and any lateral damage this may have caused? Once the debris/material starts building up inside the 'tube' it starts damaging the exterior walls. I notice in some of the photos of ground zero the lower sections which remained intact are bowed outwards. I also noticed a photo where part of the exterior wall is intact, but there is a hole section of steel which appears as if its been punched out. One odd thing which relates to question 2, is that how on earth did the core survive this amount of damage and not collapse immediately. Seems to stay up for 10 seconds or so second before finally giving up.
The core columns were incredibly thick at the bottom of the towers... made from plates up to 2.5" thick. The 5 story"trees" were made from 6" thick plate. I can only guess that the debris mass had largely vertical impulse and the debris mass perhaps braced the columns as it filled in around them. There was a lateral component to the debris pile which would push the facade outward.... which would behave like pouring gravel in one spot.
 
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econ41

Senior Member
When did Besiege (a $15 game engine) become good enough to model the collapse? I though you admited it wasn't very good.
The fundamental problem remains - failing to define the goal. There are two main motivations that seem to be driving the project viz:
(1) Having fun forcing a gaming engine to mimic a structural collapse; AND
(2) Producing something that can help persuade some target class of person to understand the physics of the collapse.

Those two logically "AND"ed not "OR"ed. Both are valid in some balance.

The problem with all the "is it good enough" questions depends on who is the defined target. @dylbie can decide for himself if it provides him with enough fun. BUT the range of targets for persuasion is potentially wide. From raw novice, CT committed truther lay-person across to highly experienced and qualified professional persons. And it won't persuade those at either extreme whether the committed CT OR the experienced professional. The committed CT is not available for persuasion other than indirectly. Whilst, of the experienced professionals, I and several other members here comprehend the WTC collapses sufficiently well that we can advise @dylbie as to the accuracy of the model. Which by definition says it is not of value to persuade us -- we already know.

Bottom line I suggest is we cannot know whether "a $15 game engine" can produce results of persuasive value until we decide the level of "good enough" that is needed for the target audience. Hence the need to define "Who is the target for persuasion"? i.e. "What is the objective"?
 

econ41

Senior Member
@dylbie You asked several questions. I will respond to one - possibly more of them. But for reasons of overall coherent logic I will start with this one:
2. Does anyone have any theories as to why the core failed?
I have certainly presented comprehensive and in-depth explanations of all stages of the WTC Twin Towers collapses. Naturally such explanations include the aspects of core failure. Given this Forums "No Click" policy I won't provide links but can do so or provide quotations if appropriate.

(Others may have done similar work but I am not aware of any. If other members are aware of other comprehensive explanations they can comment separately.)
Both cores were still partially standing after the main collapse.
To be more specific some core columns were left standing and remained so for a few seconds. I suggest that the easiest way to understand why this is so is through understanding all the key features of the four distinct stages of Twin Towers collapses. I'll just outline them here - you can ask for more details if you need them. The "stages" are:
1) "initiation" stage - essentially a process of cascading failure of columns extending over about 1 hour (shorter for WTC 2 - longer for WTC 1) which reached a threshold point when the remaining columns were not strong enough to support their Top Block which then started to move bodily downwards. Causing near-instantaneous failure of the remaining columns which had survived to that point. The key feature of this stage was that of the three main methods of column failure ALL resulted in the "dropping" top part of the failing column moving past - missing or bypassing its lower part of the column. >> Remember "column ends missing" is ALL we need to know for this discussion at this time. Ask questions if you want more explanation.

2) "transition stage" > a chaotic mess. It resulted in a lot of horizontal translation movement associated with "tilting" and starting to "topple". Forget the details for now. All we need to know is that the horizontal motion made double sure that the column ends kept "missing" or "passing" AND - the main point - it imposed concentrated weight from perimeter columns on office space floor joists > starting "ROOSD" which was the driving process of the "progression stage". Proof of "starting ROOSD" is in this graphic which I can explain when/if necessary:
ArrowedROOSD.jpg

3) "progression stage" - which had two distinct "sub-stages":
3)(a) "early progression - the Top Block" and a similar-sized portion of the "lower tower" broke up in mutual destruction >> with falling debris continuing to "miss" or "bypass" the columns >> This substage is of no consequence to us at this stage of discussion.

THEN - the final stage which left those core column "spires" standing.
3)(b) "established progression"... involving "ROOSD" - a truther side inspired acronym meaning "Runaway Open Office Space Destruction". The concept AFAIK was first posted on a forum in Nov 2007 with this graphic:

003c350.jpg

The acronym "ROOSD" and the concept were put into mainstream discussion in 2009. It explains how debris fell down the "OOS" missing the columns. The concept is generally accepted these days (Since about 2010) as what really happened. Note It explains why the perimeter columns were left unbraced, free-standing and subsequently peeled off and toppled. It did not explain the core which @dylbie is your current interest. For reasons of discussion history, nobody paid the issue much attention. In 2013 I extended the original "perimeter only" scope of ROOSD asserting that the behaviour in the core would be analogous. i.e. falling debris missed columns and sheared of floor beams in the same way as the debris falling in the OOS "outer tube" area sheared of the floor joists.

So that much should at least set an agreed context if you want any further discussion to explore more details. Your call.
In the simulations I've run, the debris at ground level completely overwhelms the base of the core and destabilises it. Just wondering if this is anywhere near accurate.
If your focus is on those few "spire" columns - your guess is probably in the right ball park. The core columns remained standing as spires because the progression of the collapse was so fast. And debris fell around the columns With no bias one way or the other to cause toppling. BUT the spires were too tall to remain - they were well beyond the effective length that could remain self-supporting. (The technical details involve a concept of "Euler Buckling" after the person who described it. Put simply a slender column can be too thin to stand up without horizontal bracing. So it will buckle. Buckle under load at some length. If it is even longer it will buckle under its own weight. And it will buckle easier/earlier if it has either imposed force or vibration. Resonant vibration was IMO possibly the trigger for those core column spires.

So does that answer or start to answer this part of your Question #3?
is that how on earth did the core survive this amount of damage and not collapse immediately. Seems to stay up for 10 seconds or so second before finally giving up.
If you need more or a fuller explanation just ask.
 
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econ41

Senior Member
Parts of both cores survived because the ROOSD phase the collapsing floor mass BYPASSED the columns... striped off most bracing beams with the exception of some elevator shaft framing.... which can be seen as "ladder looking" surviving core columns. All columns collapsed post floor collapse from Euler buckling and instability. Tallest surviver was col 501 which was 72 stories tall before it toppled.
Concisely and accurately stated.
 
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