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Discussion Starter · #1 ·
Note: This is not a conspiracy thread or anything of that sort so please refrain from turning it into that.

To my knowledge, the towers were made up of practically 100% steel when it came down to structural support and whatnot and yet the speed in which the buildings came down still defies any type of logic and it still makes me shiver to this day.:eek:hno:
So basically my question is simple: Were the towers actually safe when it came down to structural support and strength of the steel which supported the structure?
 

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The towers were not as safe as they could have been, but were structurally safe from what I've read. What do you mean by speed at which the towers came down? They stayed up long enough for everyone to escape below the points of impact. Are you referring to the "pancaking" or "progressive collapse?"
 

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Discussion Starter · #3 ·
The towers were not as safe as they could have been, but were structurally safe from what I've read. What do you mean by speed at which the towers came down? They stayed up long enough for everyone to escape below the points of impact. Are you referring to the "pancaking" or "progressive collapse?"
I'm referring to the collapse itself, yes.
Wouldn't the overall structure below the collapse wave resist the collapse to some extent? Seeing how it was a support within a support?
 

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I'm referring to the collapse itself, yes.
Wouldn't the overall structure below the collapse wave resist the collapse to some extent? Seeing how it was a support within a support?
First of all, The World Trade Center was designed with more of an exoskeletal like an insect or a crustacean. The walls themselves were load-bearing walls designed to carry much of the weight of the building rather than using more steel in and around the core in order to create more rentable office space. Now when the buildings became weak at the point of impact from the explosion and from the heat of the fire, the floors above that point came crashing down on the floors below. No building is designed to take that much weight crashing down on the top of the rest of it. Thus, you had a cascade effect -"pancaking" or a "progressive collapse."

That is why the south tower collapsed first even though it was hit after the north tower. There was more weight bearing down on the weakened part of the building. I was on my way to work when the towers collapsed and thought there must have been bombs planted inside to make that happen, but when I got home and actually watched footage of the collapse on the news, I saw the cascade effect, the progressive collapse.
 

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It's kinda difficult to comment on this subject without at least entertaining or referencing a few of these so-called "conspiracy" theories. It's pretty much becomes an 'elephant in the room' scenario if you want to purposefully ignore some of those aspects or viewpoints.
On saying that, I am new here so I'm not entirely sure as to what extent you're allowed to discuss these things anyway. Just my two-cents worth FYI, take from it what you will:

RE: the relative safety of the towers themselves. Just remember, the towers were designed to take the impact of a Boeing 707-320 (a plane very similar in overall size, weight and fuel capacity to the 767-200's used in the attacks).

I'm assuming they (the original architects and engineers) didn't neglect the fact that planes carry a lot of fuel, and therefore the likelihood of large-scale fires (coupled with some rather large holes in the walls) was not something that would compromise the structural integrity of the buildings. Certainly not enough to initiate a total collapse.

To quote Frank A. Demartini (on-site construction manager for the World Trade Center): "The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door - this intense grid - and the jet plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting."

To quote John Skilling (Head structural engineer for the World Trade Center):
"Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed, ... The building structure would still be there."

So, to answer the original question: Yes the original design was structurally safe.
 

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One Brickell CityCentre
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So, to answer the original question: Yes the original design was structurally safe.
Yes, I do agree! And because the World Trade was of an exoskeleton design with the curtain walls designed to carry much of the weight of the building, when the planes destroyed the core of the buildings, the weight of the upper floors shifted to the load-bearing curtain walls, and those walls supported enough of the weight of the floors above long enough for the people below the point of impact to get out of the building. The design of the buildings actually bought time for a lot of people.
 

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I would say (in relation to the outcome) the structures started out strong (over-engineered to a degree), and then became progressively weaker. Like anything else operating out of specification, the chances of failure are then increased dramatically.

I guess that's why we have so many theories regarding foul-play. How could these building's that on paper should've survived such an attack, fail so badly? The plot thickens...
 

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Is anybody reading what I said?
 

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Discussion Starter · #10 ·
Is anybody reading what I said?
You covered why the towers collapse, but you didn't really answer my initial question.

Why didn't the unaffected 92 and 76 floors worth of structural support below the impact points resist the collapse? Why 11 seconds if we were told these buildings were as strong as they can get?
 

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How could these building's that on paper should've survived such an attack, fail so badly? The plot thickens...
They were not designed to survive such an attack. Boeing 767s did not yet exist when the World Trade Center was built.
 

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You covered why the towers collapse, but you didn't really answer my initial question.

Why didn't the unaffected 92 and 76 floors worth of structural support below the impact points resist the collapse? Why 11 seconds if we were told these buildings were as strong as they can get?
I did answer your initial question right here where I'm quoting myself above. When the buildings became weak enough at the point of impact from the explosion and from the ensuing fire, the floors above slammed down onto the floors below. No building is designed to take that much weight crashing down on top of it at one time and the top of the building became heavier and heavier as it collapsed, thus "progressive collapse." How can you say I didn't really answer your initial question? What is it you don't understand about this at this point? The buildings were not as strong as they could get. They were as strong as they needed to be. If they were as strong as they could get, they would have been built with a lot redundant steel the way the Empire State Building was built, but would have smaller offices. Nobody builds like that anymore because it's not really necessary wth computer technology, let alone not cost effective.

Now when the buildings became weak at the point of impact from the explosion and from the heat of the fire, the floors above that point came crashing down on the floors below. No building is designed to take that much weight crashing down on the top of the rest of it. Thus, you had a cascade effect -"pancaking" or a "progressive collapse."

That is why the south tower collapsed first even though it was hit after the north tower. There was more weight bearing down on the weakened part of the building. I was on my way to work when the towers collapsed and thought there must have been bombs planted inside to make that happen, but when I got home and actually watched footage of the collapse on the news, I saw the cascade effect, the progressive collapse.
 

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Note: This is not a conspiracy thread or anything of that sort so please refrain from turning it into that.

To my knowledge, the towers were made up of practically 100% steel when it came down to structural support and whatnot and yet the speed in which the buildings came down still defies any type of logic and it still makes me shiver to this day.:eek:hno:
So basically my question is simple: Were the towers actually safe when it came down to structural support and strength of the steel which supported the structure?
I suppose the answer to that question has to be 'no'. Clearly, you can't say that a building is safe if over 2,000 of its occupants lost their lives due to its collapse.

It really depends on how you define 'safe'. No building is totally safe - all that we can ask is that it be as safe as reasonably practicable. That means that we have to think about whether the designers back in the late 60s should have foreseen the possibility that their building would have airplanes deliberately crashed into them by terrorists who were prepared to lose their own lives in pursuit of their goals.

Since the disaster eleven years ago, a huge amount of investigation has been carried out by structural engineers into just how the building behaved following the crashes and how the towers eventually collapsed. This has involved frame by frame analysis of thousands of images of the towers backed up by computer analysis. Much of this analysis is in the public domain for anyone who cares to look and so your statement that the collapse 'defies any type of logic' has been well and truly refuted.

Whilst a lot of the analysis is pretty complicated, the basic reasons for the collapse are fairly simple and are just elementary physics.

As Quantum X described, the towers had a 'shell and core' type construction with the central core, composed of steel columns, taking the vertical load of the structure and the external skin, composed of fabricated steel sections also taking its share of the vertical load but also the bending load due to wind load on the towers.

This design allowed a large area of column free office space to be provided on each floor, important in a modern building, and it is also believed that the heavyweight cladding of the exterior, with its narrow windows, appealed to the Japanese architect who suffered from vertigo and hated the huge expanses of glass with their vertiginous views common in many skyscrapers.

The floors also played an important role in the structure as, apart from the obvious function of supporting the loads from people and office furniture, they also acted as large diaphragms effectively bracing both the external skin and the central core.

Structural engineers are largely of the opinion that the floors were the reason for the buildings eventual collapse. Whilst adequate to support all ordinary loading, they were of relatively flimsy (lightweight steel truss with concrete topping) construction and with weak connections to the core and external skin.

Another problem with the shell and core design, which was especially notable in the case of Tower No. 2 was that the concentration of building services in the centre of the building, meant that the escape stairs were very close to each other and the plane impact effectively closed off all means of escape for the people above the crash zone. Had stairs been located at each corner of the building, many of the people above may have survived.

When the planes hit the towers, they caused massive damage to the external skin, which is clear from all the photos and videos of the event. About two thirds of the columns on one side of the tower were knocked out by the impact. What is less clear, but which has been proved by computer analysis is that several of the internal floors and some of the columns of the central core would have been knocked out.

The reason that the towers didn't collapse right away following the destruction of so many load bearing members is down to the fact that the designers had incorporated a huge structural steel truss into the roofs. So, following the impact, the external wall of the building and the floors that it supported, effectively hung from this truss and the load was distributed into the damaged central core.

The towers remained stable for some time after the crashes but the heat of the fire caused by the airliners fuel (backed up by that due to the paper, timber and plastic etc in the building) caused the exposed steel members to lose their strength (steel does this at temperatures well below that at which it melts). This effect was made worse by the fact that the brittle sprayed-on fireproofing of the building had been blown off by the impact explosion.

In addition, the failure of the floors meant that the steel columns of the building, overloaded by the load transfer, damaged by the impact and weakened by the intense heat also lost the propping effect of the floors (steel members subject to compressive loads need to be held in position at regular intervals to stop them from buckling). With that combination, collapse became inevitable.

Although the causes were the same, the two towers collapsed in different ways. Tower 2, the first to fall, did so due to the failure of its external skin. Tower 1 failed due to the failure of its central core.

As soon as the collapse was initiated, the speed of that collapse is not remarkable. The towers, I believe, weighed about half a million tonnes each, which means that the weight of the sections above the collapse zone would have been something like 100,000 to 200,000 tonnes - heavier than a fully laden supertanker. Once that started moving, only a massively strong structure could have stopped it or slowed it down. Effectively, the towers 'unzipped', the huge mass of the top section forced its way through the bottom section with the relatively flimsy floors offering little resistance and the external skin (just bolted together) being pushed out of the way. The central core probably lasted longer but the removal of its lateral restraint as the floors surrounding it collapsed meant that it could no longer support its own weight.

The idea that the towers were designed to withstand plane crashes is true but has been distorted by the conspiracy theorists.

In carrying out the design of structures such as this, you have to take into account the possibility that a plane could be flown into the building and, at the time of the design, that was not that extraordinary as a plane did fly into the Empire State Building during World War II. That was an accident caused by the pilot losing his direction in low cloud and it resulted in some structural damage to the building and the deaths of some people inside it.

By the 60s, planes were a lot bigger and faster and so the worst scenario considered by the designers was that a Boeing 707 would get lost and accidentally hit one of the towers. The most realistic scenario imagined at the time would be that a plane, coming into land at one of New York's airports would be flying low and accidentally collide with a tower.

The difference between that scenario and what happened on 911 was that the planes were deliberately flown at 400mph into the towers, as opposed to the maximum 200 mph of a plane on its landing approach (planes never normally fly at such speeds so close to the ground).

That makes a huge difference because, as anyone who is familiar with dynamics knows, the energy contained by a moving object is proportional to the square of its speed and so, the planes may have hit the towers at twice the envisaged speed but will have imparted four times the energy.

There is another factor as well. Both planes were at the beginning of cross-continental journeys and were, therefore, carrying their maximum fuel load as opposed to a landing plane that would normally have much less fuel on board. Not only did that add to the volume of fuel and hence the intensity of the fire but it also meant that the wings, where the fuel is stored, acted like battering rams and, instead of being shredded by the steel columns were heavy enough to smash them out of the way.

Leslie Robertson, the structural engineer responsible for the World Trade Center, stated that he was not aware just how a building could be protected against a massive fuel fire and that had probably not been taken into account in the design of the fire-proofing.

The investigation into the WTC collapse did result in a large number of recommendations, not all of which have yet been implemented.

Structurally, it was realised that more structural continuity was required in skyscrapers to guard against progressive collapse - highlighting the failure of the floors within the WTC.

Means of escape were also investigated and, apart from the need to have widely spaced escape routes, as I mentioned above, the need to have protected lifts to prevent people being faced with the need to climb down 100 storeys to get to a place of safety was recognised.

The inadequacies of fire-proofing in the WTC has led to the requirement either to embed structural steel members in concrete or use some blast-resistant material such as intumescent paint (which swells to form a protective coating when subject to) heat.

So, probably the towers were as safe as they probably could be given the level of knowledge back in the 60s but modern towers should be a lot safer.
 

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Discussion Starter · #14 ·
Thanks for that in depth analysis Mister S, there's no doubt in my mind that a collapse was inevitable and i throughly understand the cause of the collapses as through out the years, i've done my own research about the subject.
What escapes my grip is the speed in which these buildings folded onto themselves. I totally grasp the idea of the buildings being extremely heavy(as it is obvious) and that no matter what, the structure below would have been completely destroyed but these buildings should have resisted the collapse somewhat, even if we're talking seconds, seconds that could've made the difference between the area of free fall and not.
 

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Thanks for that in depth analysis Mister S, there's no doubt in my mind that a collapse was inevitable and i throughly understand the cause of the collapses as through out the years, i've done my own research about the subject.
What escapes my grip is the speed in which these buildings folded onto themselves. I totally grasp the idea of the buildings being extremely heavy(as it is obvious) and that no matter what, the structure below would have been completely destroyed but these buildings should have resisted the collapse somewhat, even if we're talking seconds, seconds that could've made the difference between the area of free fall and not.
I don't think there is any great mystery about the speed of the collapse. The fact was that the only thing that could have slowed it would have been the floors. I'm not familiar with US design codes but if they are anything like the ones we use in Britain then the floors would only have been designed to carry the weight of furniture and occupants plus an additional 60% to act as a safety factor. That figure would have been massively exceeded by the load from the collapsing floors above and, of course, as the collapse progressed, more and more floors would be added to the total moving load and the speed would have been increasing so the forces that the floors were subjected to would have been astronomical. Even so, the collapse would have been slower than freefall, but not that much.

I don't know if a more traditional design such as that for the ESB would have made much difference. I guess that the collapse might have been slower but not avoided.

One way of looking at it is from the point of view of conservation of energy. Each tower took about two to three years to construct - every day of which would have been spent with cranes lifting heavy sections of steelwork and hoppers of concrete through hundreds of feet and expending loads of energy in the process. Then, with the building occupied, the lifts would be used to bring heavy items of furniture to every floor of the building.

All of that energy would have been embedded in the building as potential energy and then, in a matter of seconds released as the tower collapsed. That explains the total devastation of the site and the fact that items within the towers, such as desks, computers and, of course, people were pulverised in the collapse.
 

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Discussion Starter · #16 ·
I don't think there is any great mystery about the speed of the collapse. The fact was that the only thing that could have slowed it would have been the floors. I'm not familiar with US design codes but if they are anything like the ones we use in Britain then the floors would only have been designed to carry the weight of furniture and occupants plus an additional 60% to act as a safety factor. That figure would have been massively exceeded by the load from the collapsing floors above and, of course, as the collapse progressed, more and more floors would be added to the total moving load and the speed would have been increasing so the forces that the floors were subjected to would have been astronomical. Even so, the collapse would have been slower than freefall, but not that much.

I don't know if a more traditional design such as that for the ESB would have made much difference. I guess that the collapse might have been slower but not avoided.

One way of looking at it is from the point of view of conservation of energy. Each tower took about two to three years to construct - every day of which would have been spent with cranes lifting heavy sections of steelwork and hoppers of concrete through hundreds of feet and expending loads of energy in the process. Then, with the building occupied, the lifts would be used to bring heavy items of furniture to every floor of the building.

All of that energy would have been embedded in the building as potential energy and then, in a matter of seconds released as the tower collapsed. That explains the total devastation of the site and the fact that items within the towers, such as desks, computers and, of course, people were pulverised in the collapse.
The speed would've made sense had it been only the floors that collapsed but it was the entire structure and we're talking about the 47 column core and the outer steel grid along with the floor trusses.
I think all of this put together would have been enough to slow down the collapse to some extent and it obviously didn't.
Which leads me to believe the overall structure wasn't very well assembled or strong enough.:eek:hno:
 

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The speed would've made sense had it been only the floors that collapsed but it was the entire structure and we're talking about the 47 column core and the outer steel grid along with the floor trusses.
I think all of this put together would have been enough to slow down the collapse to some extent and it obviously didn't.
Which leads me to believe the overall structure wasn't very well assembled or strong enough.:eek:hno:
The collapse of the outer steel skin is quite simple to explain. As the mass of the structure above the crash site forced its way downward, the skin was simply pushed out of the way, which is why some of the elements of the outer skin were found embedded in neighbouring buildings.

The central core is not so easy as we never saw what happened to it due to the massive plume of dust and smoke that the collapse generated. However, if you look at some of the pictures of the collapse of WTC1, you can clearly see some of the structural steel columns towering hundreds of feet above what was left of the tower. Those columns would be completely unbraced so would have collapsed within seconds.

What seems to have happened is that the building lost its structural integrity. Each element depended on its neighbour to hold it in place. As far as I am aware, there was no heavy cross-bracing on the core structural columns and so, with the restraint of the perimeter floors gone, they would simply have collapsed (probably aided by the collapsing floors pulling them down by what was left of their structural connections).

Evidently, the building wasn't strong enough to stand up to an attack using airliners but, on the other hand, it stood for thirty years and took all the strong winds that battered NYC in that time as well as a large truck bomb placed in its basement and so it performed well within its design parameters.

The fact is that the towers collapsed and the speed of collapse does not necessarily indicate a badly designed or built building. Maybe another building would have taken a few seconds more to collapse, maybe not - but nobody can say that for sure.
 

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people often confuse weight and mass, unfortunately the collapse was a lessen on how when you have enough mass and you start to move it, it is very difficult to stop it. I didn't explain that very well but hopefully someone will understand what I meant.
 

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people often confuse weight and mass, unfortunately the collapse was a lessen on how when you have enough mass and you start to move it, it is very difficult to stop it. I didn't explain that very well but hopefully someone will understand what I meant.
It is very easy to confuse weight and mass, especially in Imperial units where the same units are used for both (unless you buy your potatos in pounds force).

What you are describing is momentum (mass x velocity). The structures of the towers were designed to cope with the weight of the structure above but not the momentum of the collapsing structure - it would be like trying to stop a supertanker in a matter of feet.
 

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They were designed to take the impact of "multiple" 707 airliners, which, are slightly smaller than the 767(?) that hit each. They had a strong steel core in the center and they essentially doubled the load bearing weight of the tower itself by creating a steel exterior. The steel exterior (a skeleton was essentially what it was) acted as a support for the center core of each building. It's what made their height possible at the time.
 
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