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How would pressurized air act diferently?
Originally posted by himselfe
reply to post by gottago
True. Instead it disintegrated explosively barely a second after collapse began. Twenty-odd floors, dropping in a coherent mass--for it was still a coherent structure--simply exploded in mid-air. Big grey flower-bloom of destruction with structural members flying outward, some for hundreds of yards.
Disintegrated explosively? Hardly:
Originally posted by Griff
How would pressurized air act diferently?
Watch your own video please. What is going on in the last 2-3 seconds? How is that building mass disintegrating and flying out at the bottom right corner of the screen? Why is it roiling and moving outward?
Originally posted by himselfe
And you're a structural engineer? Worrying.
Note the differences between explosions and fluid dynamics.
Originally posted by himselfe
Perhaps since you are a structural engineer (and hopefully a competent one) you would care to back up what you state by showing us laymen, using scientific proof and data, how you arrive at such conclusions?
Given your expertise hopefully you can describe in unambiguous terms the process which you claim transfers the stress of gravity and mass from the bottom of the building (which has to bear 110 stories of weight) to the upper sections.
Granted beams and columns are used and designed to distribute loads, however like any scientific process, this distribution is not perfect, also the load must be transferred somewhere. While my understanding of structural design is basic, I always thought the desire with columns and vertical beams was to transfer the upper load downwards?
What benefit would there be to designing systems that transfer loads upwards?
The outer frame of the two towers was not designed to bear the full gravitational load of the tower, if you were to remove the load bearing capacity of the central support system that load would be transferred to the outer frame.
The outer frame on the lower part of the building could not transfer the load away and would collapse from the stress. Transferring the load upwards does nothing to reduce the effects of mass and gravity.
Well from what I've been reading you have been trying to assert that there was a conspiracy to place bombs in the basement, is that not right?
Since as yet there is no element of truth in the theory, what else could be the point?
I was talking about the lower parts that are attached to the ground holding the building up.
Care to cite?
Don't know where toilet seats come into it.
I'm sure as a structural engineer you understand the importance of the scientific process and proving your work or theory.
Are you claiming that the point of greatest stress would not fail when loaded beyond it's capacity?
What do either have to do with structural engineering?
I believe the core had to be taken out first for what we see yes.
One man's lie is another man's truth.
And the point of greatest stress would be the impact zones on the exterior.
Originally posted by himselfe
Nothing directly I guess, however I would hope that any structural engineer has a competent understanding of science.
In relation to fluid dynamics I believe they would be somewhat important to a structural engineer since you have to design structures with tolerance for various stresses including air, and in the case of bridges, water.
I am not disputing the fact that taking out the core structure at the base of the building could initiate the collapse sequence at the site of impact, what I am disputing is the idea that the outer structure at the base of the building could bear the entire gravitational load without failing prior to the upper collapse sequence reaching that point, especially considering the extra energy imposed on the support structure by the momentum of the upper collapsing sections.
But can you prove that the core had to be taken out first?
That does not answer the question of what other point there could be in asserting something for which there is no evidence other than alluding to conspiracy.
As far as I understand it when the entirety of a structure is concerned, the weakest point is not synonymous with the point of greatest stress, especially considering the comparably lesser forces the damaged upper section has to contend with as compared to the base of the structure.
what would be the flow rate of explosive gas as oppossed to gas being compressed and flowing out the core and out the exterior?
Only way IMO that it could have happened because a sagging floor truss still weighs the same as it did at start (so no added load just from sagging).
If you want, I can look up how much the moments would increase without the core. I believe they would be on the order of 8 times more without checking. That alone might be enough to fail the exterior without even considering the loss of interior load strength.
The only conspiracy I'm alluding to is the cover-up of the failure (incompetence) to stop the terrorists from placing a few bombs in the building. Either that or the cover-up that the buildings were built shoddy. IMO it's the former.
Originally posted by himselfe
To summarise what I understand of the official explanation is, it's not that the floors below the damage could not cope with the upper loads at rest, it's that they could not cope with the extra energy provided by the upper section collapsing, and the upper section collapsed due to the damage caused to vital support structure by the force of the impact and the resulting fire. While the fire was not intense enough to melt the steel, steel does soften under intense heat, and while under normal circumstances with all the support structure intact the steel might not have softened enough to fail even with a fire of that intensity, the impact of the plane would have taken out a proportion of the core support structure at the site of impact, thus transferring the load and increasing the stress beyond the capacity that the remaining structure could handle when exposed to intense heat.
What you fail to understand, is how fast the WTC collapsed. There is no possible way the weight of the upper section, and the "extra energy" from the upper section falling, could fall through the rest of the building WITH ZERO RESISTANCE.
The building was so strong, that all of the people who understand physics and reality are dumbfounded by the fact that the rest of the building did not slow down the upper section's collapse.
The upper section of building should have slowed down drastically while collapsing downward through the undamaged sections of the building.
The only plausible explanation for the upper section to fall so quickly through the undamaged part of the building is if the "undamaged" part of the building was actually being "damaged" by explosives before the upper section could crash into it.
Nobody, not even NIST or FEMA or ASCE or whoever, has given us a REAL reason why the upper section fell with no resistance.
Originally posted by IgnoranceIsntBlisss
Negative. Those "dark bands" are the mechanical (AC unit) floors. Instead of there being windows they were some sort of ventilation panels, that I still haven't seen any high resolutio images of to better understand what those window replacements were.
Um, so do you WANT! for it to have been explosives used?
Originally posted by himselfe
I think the extra load comes from the intense energy provided by the momentum of the upper collapsing section. Under normal conditions the floors below can bear the weight of the floors above because the forces are pretty static, when the upper sections collapsed on to the lower sections the force they had to bear was much greater because of the extra energy provided by the momentum.
Originally posted by himselfe
To summarise what I understand of the official explanation is, it's not that the floors below the damage could not cope with the upper loads at rest, it's that they could not cope with the extra energy provided by the upper section collapsing, and the upper section collapsed due to the damage caused to vital support structure by the force of the impact and the resulting fire. While the fire was not intense enough to melt the steel, steel does soften under intense heat, and while under normal circumstances with all the support structure intact the steel might not have softened enough to fail even with a fire of that intensity, the impact of the plane would have taken out a proportion of the core support structure at the site of impact, thus transferring the load and increasing the stress beyond the capacity that the remaining structure could handle when exposed to intense heat.
Originally posted by himselfe
To summarise what I understand of the official explanation is, it's not that the floors below the damage could not cope with the upper loads at rest, it's that they could not cope with the extra energy provided by the upper section collapsing, and the upper section collapsed due to the damage caused to vital support structure by the force of the impact and the resulting fire.
While the fire was not intense enough to melt the steel, steel does soften under intense heat
Steel beams in standard fire tests reach a state of deflections and runaway well below temperatures achieved in real fires. In a composite steel frame structure these beams are designed to support the composite deck slab. It is therefore quite understandable that they are fire protected to avoid runaway failures. The fire at Broadgate showed that this didn't actually happen in a real structure. Subsequently, six full-scale fire tests on a real composite frame structure at Cardington showed that despite large deflections of structural members affected by fire, runaway type failures did not occur in real frame structures when subjected to realistic fires in a variety of compartments.
This project was the first major effort to understand this behaviour using computational models of the Cardington fire tests. A full explanation of the mechanics that are responsible for the robust behaviour of unprotected composite frames in fire has been achieved and will be presented in detail in this report. Reaching this new understanding has been a laborious process, and numerous blind alleys had to be investigated along the way, however obvious the answer may now seem to the researchers involved in this project. It is possible that the conclusions will not seem obvious to others who have not been directly involved, however considerable effort has gone in to presenting the results of the project to provide as much detail as possible. Approximately 40 supplementary reports and over 10 technical papers have been written and appear as an appendix to this report. This amount of work has ensured that the conclusions presented have been verified by a number of independent approaches. Mutually reinforcing arguments were developed from the results of different computational models, application of fundamental mechanics and the analysis of test data. It is therefore with a great deal of confidence that these findings have been presented for close scrutiny by the profession. Once this new understanding of structural behaviour in fire is widely disseminated, discussed and understood, the way will be clear for completing all the other tasks which are necessary for full exploitation of the knowledge gained. This will lead to safer, more economic and rational design of steel frame structures for fire resistance.
The question may now be asked, what about the large deflections seen in real structures? Are those not a clear sign that ‘runaway’ was occuring? Figure 3.36 clearly shows that for temperatures below 300 °C, the deflections for the restrained beam are much larger than that for the simply supported beam, however they have nothing to do with ‘runaway’. These deflections are caused entirely by the increased length of the beam through thermal expansion and are not a sign of loss of ‘strength’ or ‘stiffness’ in the beam until much later. In fact approximately 90% of the defelection at 500°C and 75% at 600°C is explained by thermal expansion alone. Most of the rest is explained by increased strains due to reduced modulus of elasticity. However the behaviour remains stable until about 700°C when the first signs of runaway begin to appear.
I'm actually talking about the initial collapse. NIST says that the sagging floor trusses pulled the outer columns in. There was no momentum at this point.
BTW, I wasn't looking for a verbatum of the pancake theory. NIST has already thrown that out. But thanks anyway.
All I was trying to get at was that even if the core was taken out at the bottom we would still see the collapse at the impact zones from the outsides.
Originally posted by himselfe
To summarise what I understand of the official explanation is, it's not that the floors below the damage could not cope with the upper loads at rest, it's that they could not cope with the extra energy provided by the upper section collapsing, and the upper section collapsed due to the damage caused to vital support structure by the force of the impact and the resulting fire.
Not according to NIST, if you've read their report.
According to NIST, there was not significant heating to any support columns, as to cause softening and loss of yield strength.
Based on this comprehensive investigation, NIST concluded that the WTC towers collapsed because: (1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and (2) the subsequent unusually large jet-fuel ignited multi-floor fires (which reached temperatures as high as 1,000 degrees Celsius) significantly weakened the floors and columns with dislodged fireproofing to the point where floors sagged and pulled inward on the perimeter columns. This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.
In reality, and in real fires in steel-framed structures (I can post links to a particular study completed in 2000, after ~20 years of research, from the University of Edinburgh), the deformations caused during fires are caused by additional internal stresses induced between members from thermal expansion.
According to NIST, the exact opposite occurred, and instead of deflections from expanding beams/trusses, NIST says that the trusses were heated to the point of softening, sagged, and that somehow this sagging created a force pulling inwards on the exterior columns and causing them to buckle inward. Apparently, when a truss sags, it also becomes heavier?
Anyone familiar with the construction of the towers would also realize that the connection to the perimeter columns would fail long before the perimeter columns themselves would be jerked out of place.
NIST also did no particular lab tests or models to verify the above, which is their major hypothesis. They submitted a replica composite floor from the WTC to fire for about two hours, for both fireproofed and unfireproofed steel, but couldn't get any failures whatsoever, let alone anything resembling their hypothesis.
NIST’s findings do not support the “pancake theory” of collapse, which is premised on a progressive failure of the floor systems in the WTC towers (the composite floor system—that connected the core columns and the perimeter columns—consisted of a grid of steel “trusses” integrated with a concrete slab; see diagram below). Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon.