Phil one needs to understand that the splice connections as you noted in the previous comment were indeed the weakest point in the 34 sections of 36' long columns adding up to 1,362 feet tall columns. The connections were basically to hold the columns in alignment and did not contribute to their ability to provide axial support. So a large force such as a plane could easily sever those connections. But the plane couldn't and didn't sever all 236 facade and 47 core column connections.
The top sections did come down when there was failure of all the columns at the plane strike zones. This failure process began with the mechanical destruction of several columns by the plane impact. But we can see that even losing those columns did not cause the tops to drop and break apart.. descend and cause the cascading floor failure. Additional loss of strength was required. The columns which survived the plane impact were now carrying additional load. Whatever reserve strength they had (FOS) was now decreased. But they still had some left.
The next process of strength erosion was weakening those columns... We know that heat DOES weaken steel. The hotter the steel, the less yield strength it has. You don't have to melt a material for it to lose yield strength. So we need to assess how the fires which ensued from the plane strike affected the steel. We can't see in the core and so we can only know by telltale evidence.. such as early movement or examination of the steel post collapse. We can't measure the temps of the steel from video inside the core.... at least with sufficient precision for it to be meaningful. We do know something about how hot various materials burn. But we don't know precisely where they were burning etc... and of course how much flammable material was present.
Of course, there could be explosives or other devices to weaken the steel.
We do know from examining the core steel from strike zone that the surviving columns eventually buckled. There is no dispute because a buckled column show specific signs... distortion of the flanges and webs. These signs are seen ONLY in the columns which came from the plane strike zone. No columns above or below were buckled. All other columns show clean breaks at the weak connections as you have noted.
The buckled columns means that they were unable to support the load of the upper stories. That load had not increased... and so the conclusion is the columns LOST strength.. that is the aggregate strength of the surviving columns. And this as mentioned was from heat weakening and or other causes such as *mischief*. There doesn't seem to be tell tale signs of explosions... but I can't say each column was examined. Bolt removal does not reduce yield strength.. only facilitate taking the columns apart.
The mystery ... one among many... is what supplied the HEAT to weaken the remaining columns pushing their FOS below 1... and eroding their yield strength? Many will claim that office materials burning could not weaken the steel... and we know it can't melt nor does it have to melt the steel, nor do we see any signs of melted steel core columns from the strike zone (they could have been spirited away of course.. but that's a cop out claim).
So we need to know what the reserve strength was for starters .. and that is something we CAN determine. If the reserve strength was so high it would take the destruction of many columns before the tops would collapse buckling the few remaining. For the sake of argument lets suppose there were 25 columns each was equal and they had an FOS of 5. This means that 1/5 of the columns could support the load and one would have to destroy 20 of the 25 or reduce the yield strength of those so much from heat that the couldn't carry the load upon them let alone any extra (reserve strength). Such a structure with FOS of 5 would have huge columns... and this would be not only uneconomical but a sort of over kill which is unjustified by experience in the real world. Accordingly typical high rise structures have an average FOS or 1.42 or 42% reserve strength. My calculations indicate that the twins core steel was in fact above average and had an FOS between 1.65 and 1.85. This is important to know to determine how many columns or how much aggregate strength they had to have to support the top section. And of course, each column was a bit different and had different loads and different FOS. Then one needs to consider the load paths for redistribution as the reserve strength is destroyed. If a massively strong column is surrounded by much weaker ones and the massive one is severed or destroyed its load moves to the already weaker/smaller adjacent columns. This conceivable could fail then instantly... while the reverse would not.. sever or fail the weaker one which redistributes its load to adjacent stronger ones stronger one will likely have enough reserve strength to hold.
We just don't know what was going on there and we can only model so much. We can't rule out placed devices but we can't assert that they had to be present either. However without the signs of the steel being exploded up there... we probably should look for incendiary type... high heat creating devices. And these might be masked by the natural fires. I don't know. We do know that there were several thousand gallons of jet fuel which burned up there. Anyone who claims that no jet fuel made it into the towers does not understand physics and mechanics. Physicists CAN calculate the amount of fuel which produced the fire balls seen on impact..and they have and it does not account for all the fuel alleged or assumed to be on the plane. Lots of assumptions here. For example maybe the planes were fakes and tankers loaded with fuel??? We can't make any claims that they were or weren't. Or even that they were fully fuel loaded commercial jets. We don't know. or maybe they carried and delivered some incendiary materials to the strike zone... We don't know.
We do know the core buckled... the tops were unsupported and came crashing down and breaking apart and delivered in the range of 30,000 tons of additional load to the floors they fell upon and crashed into and that this was the beginning of the collapse phase of the flimsy floors.... gravity driven.
Read this:
"Illuminist14 wrote:Thanks, I think its easier for me to say dynamic load although you know what I mean, I gather its not really the correct terminology in physics. And I think its unlikely that the collapse would have full alignment of columns, therefore its difficult to see that x10 figure as reality.
Greetings Illuminist14.
I am a retired civil/structural engineer and also trained as a military engineer which included demolitions.
I have been following your discussion in the other thread and I think I can put forward a perspective on these issues which may assist you. Here it is. If it is of no value to you simply ignore it.
I want to focus on how the falling top section of either twin tower met the lower section. You have seen the "solid block" explanations and several people here have given you assistance but mainly in the abstract physics.
You also say "And I think its unlikely that the collapse would have full alignment of columns,..." which is a very useful insight. Let me take it a step further. A critical stage of the collapse of either of the Twin Towers was that brief period of time when the top section of the tower first started falling. At that stage we can know something for certainty. That is that all the columns had failed. Some of them the broken ends were already passing. Others the column may have been bending, not yet broken but for all those columns there was no way that the column ends would ever go back into a load bearing end for end alignment.
Now if you can accept that it makes explanation easier. If you have doubts we can either set them aside for now or I can explain further. Your choice.
So the top section of tower is falling onto the bottom section. The layout of the bottom looks like this:
The top section is the same layout but upside down. And there would be a lot of bent and broken bits between the two. Again can you accept for the moment that the bent and broken bits are of no significance. We can deal with any concerns later if necessary.
So what we have is a bit of a wire basket comprising a central core with a grid of columns joined by cross beams; an open office space surrounding that core and a perimeter rectangular tube of columns at the outside.
Point 1 there is clearly no "solid block" to land on any other "solid block" - it is sort of "wire basket" on "wire basket" - with some bloody big wires but, in scale, still wirebasketty.
So what does land on what? I am going to explain the "progressive collapse" (or "global collapse" which is an alternative name) as three distinct identifiable and provable mechanisms.
Here goes the first one - Mechanism #1 - Call it "ROOSD" - I'll come to why shortly.
The key to understanding the "global collapse" is that the open floor area of the tower was weakly fastened to the outer perimeter and the core. The floor to column connections had only to be strong enough to hold up one floor (plus a bit but don't worry about that bit) Anything more than about one other floor landing on that open floor would shear the floor off the core and perimeter columns.
So if something hits the top floor of the lower tower and it is heavy enough it will shear that floor which will fall on the next floor and shear it and fall on the next....all the way rapidly down to the ground or as close to ground as it can get with the accumulated bits of broken building making a raised heap.
That process has been identified by many people. Major_Tom posting on this forum calls it "ROOSD" for runaway open office space destruction. M_T's work goes into a lot more detail and is the definitive work in my opinion. And I have described it "top down" as something landing on floor below it which is a bit of a simplification - bear with me till I get the overall scheme clear then we can deal with some extra details.
So the first of our "...three distinct identifiable and provable mechanisms" is that the floors of the open office space collapsed downwards in a "pancaking" fashion". Doing that they left the outer perimeter columns standing without bracing. And that set the scene for mechanism two:
Mechanism #2 - perimeter column peel off. The unbraced outer perimeter columns simple fell over as complete sheets of varying sizes. Some medium size as per this picture:
![[Image: outercolsheets2.jpg]](http://conleys.com.au/webstuff/outercolsheets2.jpg)
...some very large ones being about 1/3rd the building height. Some big ones shown here on the ground:
![[Image: SheetsOfCols400.jpg]](http://conleys.com.au/webstuff/SheetsOfCols400.jpg)
...again M_T has done what I judge to be the definitive work and posted here on this forum.
So that is mechanism 2 - the collapsed floors left the outer perimeter unbraced and it fell over.
Notice that there is no end for end loading of columns and the columns as they fell were quite straight - they had not buckled. Most failures seem to be where I would expect them at the bolted joints.
So that leaves Mechanism #3 - Core Strip down
Remember that as the top bit of the tower started to fall all the columns had failed and had either broken or were bending just before breaking. And there was no way that top bit of broken column could land and stay landed on bottom bit of same column. (Explanation of that available later if you need it.)
So as the top bit of core lands on the bottom bit of core what hits what?
It must be cross beam on cross beam. And two points follow:
1) the cross beam is a hell of a lot weaker than the column against vertical loads; AND
2) It had most of the weight of multiple stories of core landing on the cross beams for one floor level.
So overwhelmingly large force hits relatively easily broken object with inevitable conclusions.
For the purists it is not as many times overwhelming as the overloading of the OOS floor to column connectors. Bur orders of magnitude overload doesn't care if it is one or two or three orders - it is overwhelming overload.
So I have set out the three mechanisms which happened at close to the same times. Mechanism #1 and Mechanism #2 as close to provable as we can claim whilst using scientific language and supported by visual records of evidence. Mechanism #3 slightly less provable due to limited visual evidence but using the Mk1 Grey Cell Head Filler says "good enough for me".
Where next? Does that explanation start to fill in a picture for you. If it does we can flesh our some of the details which I have glossed over in this first overview.
I'm not the expert on analysing visual evidence but two of the best are Major_Tom and femr2 both longer term members here than Me the newcomer. And they, principally M_T I think, have posted a lot of evidence to support the claims and descriptions I have now given.
Questions?
..and if it is no value we can halt there,
thanks
Eric
ozeco41 Posts: 247
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Re: Deceleration
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SanderO » Sun Jun 10, 2012 9:31 pm
Very well stated!... I am going to post this to a listserve...
One minor point... the wire basketry was really a good analogy because the cross sectional area of the 47 core columns at that level was a tiny fraction of the 10,000 square feet of the core... just about 40 square feet... and the perimeter at that level (tower 1) was about 45 square feet... so we're talking 85 SF of steel column are compared with the 40,000 square feet of the foot print ... a whopping .2% spread over 236 perimeter and 47 core columns... so you can see how wire baskety the structure was. When the tops DID come down as a result of columns losing the aggregate strength to hold up what was above them... they *buckled* and when columns buckle they *go out* of column.. that is to say they are no longer nice and straight and erect.. but all kittwampus and bent and the downward forces make / allow the weak and unstable column to translate laterally... likely they all find the weakest direction and like a flock of birds or a school of fish all move in the same direction. The tops are NOT under and stress they are now *falling* and the fish lead them the chosen way (the path of least resistance).
Note this COULD as Ozzie notes lead right onto the bracing which was connected to the four faces of the columns in the core (three on the perimeter of the core and 2 at the corners of the core)... but it could also lead diagonally so the columns above... now came down on.... NOTHING as in shafts... or on a slab within the core. We can't know what they impaled or hit... it's one of 3 possibilities or perhaps a little of each... air, slabs or bracing. For it to be bracing the buckling would be in one of the two axes of the core.. east-west or north south. If it decided to not go in one of these directions it would miss the bracing. And of course if it was a shift on axis it would be off axis for about half the columns anyway.
This is a minor point because as Ozzie says neither the slab nor the bracing and certainly not the air in shafts would slow the collapse of the top wire basket... the array of 283 columns. Once the sucker was headed down with no axial support... the bottom was a goner... The tops HAD to be supported by the columns below... the ONLY thing in the structure which COULD support that load... which is why the suckers are so large in cross section.
SanderO Posts: 1012
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Re: Deceleration
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SanderO » Mon Jun 11, 2012 5:55 am
The real key to understanding ROOSD in my opinion is that the floor system in the twins was not only not very strong... it was in fact weaker that the NYC Code requirement of 100 psf deign load.. to 58 psf. This essentially means as Ozzie notes that the floors simply cannot support significantly more than in the case of the twins 58 psf... static or dynamically applied. And when you have multiple floors.. or the materials making up those floors.. dropping onto an undamaged floor.. that undamaged floor shatters like dropping a brick on a thin pane of glass. Once the floor collapse began the floors get crushed. By the way this is the fate of the floors in CDs... but CDs haven't been done to buildings taller than 20 something stories and the increased number of stories may account for the amount of (pulverization) and communition of the concrete. One might think of a tumbler machine. You place an object inside the tumble with the hard grinding material and turn it on. The longer the tumbler turns and agitates the mix the more rounded and ground up the target object becomes. If you leave it in there long enough it is ground to dust.
http://www.youtube.com/watch?v=UBGVPAluutY
"Tumbling grit is sand-sized particles of silicon carbide. That is where the name "grit" comes from. It is an abrasive material that is placed in the tumbler barrel with your rocks. Silicon carbide is much harder than the rocks that you will tumble. When it is caught between tumbling rock particles it creates tiny scratches on them. This continuous scratching wears down the rocks and rounds them. Sharp edges on the rocks are worn down more rapidly than flat surfaces because the grit is rubbing against them on two or more sides. The result of this grinding is smaller, rounded rocks."
The *tumbling* process in the ROOSD was scaled up enormously... and the time frame condensed to a few seconds. But the concrete was very *weak* to begin with and had no stone aggregate.. almost like gypsum wall board.
While ROOSD requires various parameters be met... a smaller tower could go ROOSD but likely produce larger chunks and accordingly less concrete dust. SCALE matters.