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Professional engineer Jon Cole cuts steel columns with thermate, debunks Nat Geo & unexpectedly repr

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posted on Jan, 14 2011 @ 06:09 AM
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Originally posted by -PLB-
So since you also seem to reject the sagging trusses explanation, what do you think caused the bowing?


If the truss connections are failing, then wouldn't the columns experience a large section of unbraced length? When a column is unbraced, Euler buckling comes in to play. Why was Euler buckling dismissed?




posted on Jan, 14 2011 @ 07:16 AM
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reply to post by Nutter
 


It may have been a combination. It seems that NIST only tested the scenarios individually:



Column buckling did not occur when lateral support was lost at three floors under the
expected gravity load that included dead plus service live loads.

Column buckling did occur when lateral support was lost at three floors and the gravity load
was increased to 150 percent of the expected gravity load simulating redistribution of load to
the exterior wall.

Column buckling was found to occur when an inward lateral load (pull-in) of approximately
12 kips was applied to three adjacent floor levels. The inward deflection of the exterior wall
when it could no longer support the gravity load (i.e., at the buckling load) was
approximately 10 in.

edit on 14-1-2011 by -PLB- because: (no reason given)



posted on Jan, 14 2011 @ 08:24 AM
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reply to post by -PLB-
 


You see just one more reason not to believe the NIST report as it did exactly that....tested in a singularity to meet the models conditions.

Inward bowing of an exterior wall was a necessary but not sufficient condition to initiate collapse.
(NCSTAR 1-6 p322 para2)

The primary role of the floors in the collapse of the towers was to provide inward pull forces that
induced inward bowing of exterior columns (south face of WTC 1; east face of WTC 2). (NCSTAR
1-6 p lxxix para1)

Sagging floors continued to support floor loads as they pulled inward on the exterior columns.
There would have been no inward pull forces if many of the floor truss seats had failed and
disconnected. (NCSTAR 1-6 p lxxix para2)

As the exterior wall buckled (south face for WTC 1 and east face for WTC 2), the column
instability propagated to adjacent faces and caused the initiation of the building collapse.
(NCSTAR 1-6 p lxxix para3)

Floor sagging induced inward pull forces on the south wall columns. (NCSTAR 1-6 p lxvii
para12)


now none of this was observed these are simulated senarios in the report.

Also when nist is using as a senario only one 5/8" A325 bolt but there were two...
just another ommision on NIST"S part.

now also if the amount of expansion due to thermal expansion stated by NIST the bolts should have sheared off well before any sagging of the trusses happened once again removing the lateral forces for which bowing would be needed to occur.

Now you say what could have caused the bowing...well the one thing that NIST never ever considered and never ever looked at.....removing the inner core from bottom up....and i do not mean the entire core at once...segments of the core...remove the resistance....the core drops and causes buckling....NIST tried to fit the collapse to the model not fit the model to the collapse.

The model tells us what did not happen so NIST had to fudge data to fit the models.

As i showed you before how the NIST report on BLDG 7 had a model that showed extreme buckling and twisting...but was that the observe senario in the actual collapse...no.



now there is a damper in tact...two A325 bolts...not sheared off.

lets look at some seats on the exterior columns...the seats are are torn apart...the bolts are not sheared off.

you see for me these are all things that should have been looked at in detail....and a so much better analysis could have been done...the crime scene was cleaned up and not well investigated.

these are concerns that people have....it is not the abilities of the people doing the NIST report...it is the methodology and failure of NIST to look at all POSSIBLE senarios...that would fit the actual collapse.

now as i showed previously the resistance from the lower structure was removed as that is the ONLY way the top section could continue accelerating all the way down...IF it fell the way Bazant has stated in the progressive collapse theory the upper section would have inevitably encountered resistance...and would therefore not be under continued acceleration.



posted on Jan, 14 2011 @ 09:07 AM
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Originally posted by plube
Now you say what could have caused the bowing...well the one thing that NIST never ever considered and never ever looked at.....removing the inner core from bottom up....and i do not mean the entire core at once...segments of the core...remove the resistance....the core drops and causes buckling....NIST tried to fit the collapse to the model not fit the model to the collapse.


Removing the core from the bottom up? Do you mean removal of the connection between the floors and core? Or the core columns themselves? How would either have been accomplished? Shouldn't there be visible evidence of this? On what evidence do you base this theory?


now as i showed previously the resistance from the lower structure was removed as that is the ONLY way the top section could continue accelerating all the way down...IF it fell the way Bazant has stated in the progressive collapse theory the upper section would have inevitably encountered resistance...and would therefore not be under continued acceleration.


Bazants model proves that when this resistance would be most optimal, meaning the top section fell exactly on the support columns, there would still be acceleration. So the condition for acceleration is satisfied for the least likely case.



posted on Jan, 14 2011 @ 11:06 AM
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Once again logic seems so elusive to you....The upper section needing to have remained INTACT for the Bazant model to work, It requires that it IMPACTS the lower section in order for the collapse to proceed...as it has been shown it just keeps on accelerating...as the video also showed....it just keeps accelerating....The reports Fail in so many ways because they both make huge assumptions...they Assume a global failure throughout and entire floor...they assume a straight down collapse...but as observed there was tilting in both instances....they assume the fires became hot enough to cause failure across entire floors.
The NIST report uses terms such as the rigid blocks decent or tilting and doward movement...do you realize why they use these terms...because if there was buckling and slow tilting or a creeping movement downward(as one would expect) then the whole point of the upper section falling onto the lower section would not cause the progressive collapse.
the only way it works is to have the upper section remain INTACT and FALL...therefore IMPACTING the lowersection....that is why these papers are worded the way they are...they are worded this way to MAKE the models work...now ask yourself logically....did the say 93rd floor or say the 94th floor just magically get swept aside so the upper section could impact the lower section therefore causing the collapse negating the conservation of momentum and energy....you see you can't increase the mass to overcome this conundrum you would have to increase velocity.
Now Bazant states it would have to fall at least one story for this upper section to Impact the lower section to initiate this collapse....so therefore...the impact would show in the acceleration graph as a point way out of place to show the deceleration of the upper section at the point of impact....does this show from observational date from videos....no it does't....and the rate of fall is easily calculated...as has been shown.
now NIST reports the sections as RB+12 rigid block then we have DS-6 the six floors of the damaged section where the planes impact and damage due to fires floors 93-98...and of course RB-92 the lower section.
Now keep in mind the lower section is actually built stronger than the rb-12+ as in the skyscraper the columns taper as you get higher...(normal construction).
So are are led to belive the weaker RB-12 FELL and impacted the stronger lower section and brought down the building...We are only talking North tower right now because in order for the reports to function it has to work in the tower with the least mass.
Now as for this supposed evidence the evidence of fires and sagging trusses of core damage...are ALL hypothetical....they are only done through modeling...and even a lot of the observed data was ignored by NIST...now as for your question of wheres the evidence for explosives....did NIST do a model based on explosives..No.... did they look for evdence of Explosives....No. ....would models of explosives in building show controlled demolition may have brought the buildings down in the fashion they fell I would say most likely.
the evidence for a complete forensic investigation into the towers fate has been....removed and destroyed....the company that ove saw this operation was none other than CDI....but no suprise there huh.
so you see it is a sorry state of affairs...you would tend to believe the papers bought and paid for by the same people suspected in the demolitions....rather than use your own logical engineering brain to make fair and reasonable deuctions....am i saying it was definately explosives...no...i am saying it is a definate POSSIBILITY which should have been looked at from the onset unless ther was something that was not meant to be seen by Joe public.




edit on 113131p://f09Friday by plube because: (no reason given)



posted on Jan, 14 2011 @ 01:25 PM
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reply to post by plube
 

That's how I feel about it, the south tower's upper section leaned 22 degrees estimate, and disintegrated. There are closeup pictures of a crack opening up on the SE corner seconds before collapse. It then leaned and twisted away from the building as it disintegrated. The north tower's lean upper section, after seeing "new" video from a better angle, is actually much more severe, since it was nearly top down by the time it landed in the street. There is also close-up video at the point of collapse at the north tower, that show the central columns on that face slipping sideways, while those on either side collapse with no lateral movement. I don't yet know why that is, but it is there. The link is to a clip of the south side I think, of WTC1. when it collapses, it is leaning extremely away from your view, but in the closeups you still see that face falling and twisting downward, even though the rest of the top section has gone down in the opposite direction,

www.youtube.com...



edit on 14-1-2011 by smurfy because: Add link.



posted on Jan, 14 2011 @ 01:43 PM
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reply to post by bsbray11
 


You didnt read my response fully.

I said, in the beginning, when the trusses were being heated first, the trusses began to expand. But since the exterior columns were still not affected by the heating they were unable to move or bulge out. This allowed for the trusses to start sagging since they cant expand outwards. However, once the exterior columns were heated to the point where they began to creep and soften, this allowed for the now sagging trusses to pull in.



posted on Jan, 14 2011 @ 02:10 PM
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Originally posted by GenRadek
reply to post by bsbray11
 


You didnt read my response fully.

I said, in the beginning, when the trusses were being heated first, the trusses began to expand. But since the exterior columns were still not affected by the heating they were unable to move or bulge out. This allowed for the trusses to start sagging since they cant expand outwards. However, once the exterior columns were heated to the point where they began to creep and soften, this allowed for the now sagging trusses to pull in.


That sounds reasonable enough, but only if that event happened that way. The point I'm making is that okay, everyone knows that the towers fell just inside and outside an hour, but NIST themselves did not labour on how long such an event would take, just as you describe. It seems to me that NIST are saying, Well buildings fell, so this must be what happened. That can't be the right way to do it. Anok pointed out that he didn't think there was enough time for that effect. I'm saying that NIST didn't bother to make a time marker.
edit on 14-1-2011 by smurfy because: Text.



posted on Jan, 14 2011 @ 02:57 PM
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Originally posted by GenRadek
I said, in the beginning, when the trusses were being heated first, the trusses began to expand. But since the exterior columns were still not affected by the heating they were unable to move or bulge out. This allowed for the trusses to start sagging since they cant expand outwards. However, once the exterior columns were heated to the point where they began to creep and soften, this allowed for the now sagging trusses to pull in.


Remember we are talking about ONE hours worth of fire.

Not the best example but this should make my point...


What I found was that a 10" rod will lengthen about 1/8" when heated to a bright orange (around 1000C or 1800F). That's a simple enough number to remember and adjust for.

www.craftkb.com...

So how much do you think realistically the trusses could have expanded after one hour of heat? They would have been nowhere near bright orange in colour. Even the fires themselves would not have reached 1000C in an hour, let alone all the steel.


Of interest is the maximum value which is fairly regularly found. This value turns out to be around 1200°C, although a typical post-flashover room fire will more commonly be 900~1000°C. The time-temperature curve for the standard fire endurance test, ASTM E 119 [13] goes up to 1260°C, but this is reached only in 8 hr. In actual fact, no jurisdiction demands fire endurance periods for over 4 hr, at which point the curve only reaches 1093°C.

www.doctorfire.com...

Sorry but the sagging trusses hypothesis fails when you look at it in detail.
edit on 1/14/2011 by ANOK because: typo



posted on Jan, 14 2011 @ 04:24 PM
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Originally posted by smurfy
reply to post by plube
 

That's how I feel about it, the south tower's upper section leaned 22 degrees estimate, and disintegrated. There are closeup pictures of a crack opening up on the SE corner seconds before collapse. It then leaned and twisted away from the building as it disintegrated. The north tower's lean upper section, after seeing "new" video from a better angle, is actually much more severe, since it was nearly top down by the time it landed in the street. There is also close-up video at the point of collapse at the north tower, that show the central columns on that face slipping sideways, while those on either side collapse with no lateral movement. I don't yet know why that is, but it is there. The link is to a clip of the south side I think, of WTC1. when it collapses, it is leaning extremely away from your view, but in the closeups you still see that face falling and twisting downward, even though the rest of the top section has gone down in the opposite direction,

www.youtube.com...



edit on 14-1-2011 by smurfy because: Add link.

Sorry meant to say second last post "view from North" it might actually be more northwest.
edit on 14-1-2011 by smurfy because: Text.



posted on Jan, 14 2011 @ 04:39 PM
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Originally posted by -PLB-
Removing the core from the bottom up? Do you mean removal of the connection between the floors and core? Or the core columns themselves? How would either have been accomplished? Shouldn't there be visible evidence of this? On what evidence do you base this theory?


If the core structure's load-bearing capacity were compromised in any way then one result would be the buckling of perimeter columns via the hat truss. That doesn't mean those columns buckling were the cause of the collapse, it just means they would be a symptom of the core being compromised. If the core was compromised, that was the main load-bearing part of the structure. It carried at least half of the total building loads and I've seen it stated from various sources as carrying anywhere from 50-60% of the total loads, all within a confined core area inside the building, and all of the columns being accessible from elevator shafts and other service shafts

That's not a theory, or hypothesis, those are just facts. The core structure carried a critical amount of the building loads, and if it failed, it would try to transfer its loads onto the perimeter columns via the hat truss. That's just how it was. So when it comes to buckling perimeter columns, that's when the hypotheses come into play. What caused that? So far I am still not seeing any way the trusses could have caused it. There are too many problems, too little evidence, and no reproduction of that alleged mechanism whatsoever. Not to mention the number of buckled perimeter columns by themselves aren't enough to cause a global collapse, and when WTC1 started falling, we know its core structure was already compromised at the same time the perimeter was. So that's one more "clue" as to what was really happening, for anyone smart enough to realize that.


Bazants model proves that when this resistance would be most optimal, meaning the top section fell exactly on the support columns, there would still be acceleration. So the condition for acceleration is satisfied for the least likely case.


Not hardly because we've already shown he has to change parameters to make them unrealistic to get these results at all. When he accounts for the ejected mass for example, all those other correlations go right out the window. And that's just one thing he has to assume to get his model to work. You've already admitted yourself that it was a simplistic model.



posted on Jan, 14 2011 @ 04:43 PM
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Originally posted by GenRadek
I said, in the beginning, when the trusses were being heated first, the trusses began to expand. But since the exterior columns were still not affected by the heating they were unable to move or bulge out. This allowed for the trusses to start sagging since they cant expand outwards. However, once the exterior columns were heated to the point where they began to creep and soften, this allowed for the now sagging trusses to pull in.


The trusses would still be trying to expand, not contract. If the perimeter columns lost that much integrity then if anything those columns would still be trying to bow outwards. Not to mention I haven't seen any data to indicate the perimeter columns lost that much strength from being heated anyway.



posted on Jan, 14 2011 @ 05:22 PM
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reply to post by ANOK
 


ANOK, I'm glad you asked.

Take a look through these fire fighting magazines about light steel trusses and behavior in fire conditions:

www.firenuggets.com...

www.postandcourier.com...

www.firerescue1.com...

commandsafety.com...

www.fireengineering.com... lapse-under-fire-conditions.html

www.chesapeake.va.us...

www.fireengineering.com... and-variations.html

www.fireengineering.com... rame-construction.html

www.fireengineering.com... of-service.html

www.fireengineering.com... rame-construction.html


Lightweight steel truss bar joists-a parallel array of light steel beams used to support a floor deck or low slope roof. Steel girders located in the front, center, and rear of this building support the lightweight trusses. The joists are welded to the steel girders. During a fire, the lightweight trusses absorb heat rapidly and can fail in five to 10 minutes. If the roof is heavily loaded with air-conditioning and other units, collapse may occur sooner


Now ANOK, I dont know where you are getting the idea that light steel trusses take an hour to reach failure point, when according to fire dept. safety sites and publications, light steel trusses can reach failure point in a fire within 10 minutes. 10 MINUTES! Some with fireproofing may be rated for an hour or so, IF the fireproofing is intact and adequat, two things that were not at the WTCs impacted floors. Comparing a 10" rod to a light steel truss is meaningless and irrelevant.

And why should it take an hour for the fire to reach 1000C? Isnt 1000F enough? Heck, you average office fire can reach those temps easily. Now throw in a burning airplane, and tons of paper and debris, plastics, chemicals, and you have a whole new set of fuel for the fires. Also, its reported that light steelt russes can fail in temps of 800F. 5-10 minutes of exposure and they fail:

tkolb.net/tra_sch/TrussSys/TrussSystemFailures3.pps

Look through this powerpoint on Truss System Failures. They clearly state that light steel trusses exposed to fire will fail RAPIDLY, within 10 minutes. Now, the WTC fires burned for an hour, with floors that were damaged by impact, which dislodged the fireproofing. Also it was known that there was possible substandard fireproofing on the trusses, which on some was missing, too thin, or lost chunks.

It does not take long to reach 1000F. Ask any firefighter. Heck, ask thedman. He would be more than happy to explain to you fire dangers, fire temps, and truss failure.

The below is taken from the pdf:
www.aspiringfirefighters.com...


Examples of steel truss failures
 Cold drawn steel cables can totally fail at 800 F
 At temperatures above 1,000 F the expanding steel in bar joist trusses can
exert lateral thrust forces on surrounding masonry walls sufficient to cause
their collapse.
 Expansion within metal trusses may also cause the bottom chord of buckle
and fail, resulting in downward thrust and collapse of the roof or floor.



Any one or all three of these may have been present in the WTCs prior to collapse.

It seems to me, you missed all of these important facts. If memory serves me well, I'm pretty sure you will just ignore all of this as irrelevant anyway, which goes back to my reluctance of trying to add any actual factual information that may help you in understanding the complex mechanisms of the onset of collapse of the WTC towers. By the way, the above posted information is the CORRECT way to do research, and looking into relevant information. I'm surprised you never bothered to do any research into light steel truss and their fire danger.

Also, it can be said that because of the tube-in-tube design of the WTC, coupled with light steel truss flooring, that maybe the reason why they collapsed the way they did, but no one wants to admit that the design itself was severely flawed. And before you go and run off to post the 1975 fire in the WTC, remember this: The fire was actively fought with water, the floors affected had sufficient and "fresh" asbestos fireproofing installed on the affected floors. Also the steel was thicker and more resistant to the fires.

www.junkscience.com...

But the fireproofing installed on later floors of the WTC were blown on fireproofing. It was also suspected the the new fireproofing was crappy in comparison to the original asbestos. But thanks to the bad publicity of asbestos, they switched. This may very well have been the fatal mistake that would not be noticed or mentioned until 9/11/2001.

Unfortunately the fireproofing did not survive the collapses, since the floors were all smashed together, it would have been difficult to check the extent of fire-proofing loss during the clean up and investigation.



posted on Jan, 14 2011 @ 05:28 PM
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reply to post by bsbray11
 


Well they would be expanding, however, would it be even? What if one part expanded more than another? What if one was damaged? The trusses were sagging, now as to whether the sagging was a result of heating or cooling, I cannot say for certain. Some say it was during the fire and the heating, others say it was during the cooling. But what if it was from the heat causing the sagging, pushing out the columns slightly, then as they sagged down low, started to cool causing the steel to contract? We may never know 100% exactly what was going on. But we can make educated guesses based on what we know. And we know more than we realize. All we have to do is combine the different knowledges of steel, fires, trusses, etc etc etc and combine it all. We need to use all available facts and information, and not shun or throw out things that dont jive with the pre-determind conspiracy flavor of the day. That is why I bring up fireproofing and light steel trusses and the dangers of them in fires. For some reason, the TM believers all ignore this angle. Why? But they will jump on any mention of thermite/ate/nano/paint on, high explosive whatever, no matter how ridiculis.



posted on Jan, 14 2011 @ 05:38 PM
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Originally posted by GenRadek
reply to post by bsbray11
 


Well they would be expanding, however, would it be even? What if one part expanded more than another? What if one was damaged?


Better yet, how would an expansion result in a contraction force at all?


The trusses were sagging, now as to whether the sagging was a result of heating or cooling, I cannot say for certain.


"Sagging" happens when a beam/truss is trying to expand but is constrained on the ends. You already said as much yourself. "Cooling" doesn't cause sagging. When the sagged beams in the Cardington tests cooled they just retained the same shape, since they had been heated until they permanently deformed. In less extreme heating an elastic deformation takes place and the beam would just revert to its normal shape afterward.


We may never know 100% exactly what was going on.


In other words you may never know that the hypothesis you're repeating is valid at all, and basically it's unprovable and thus not actually scientific at all. All real science is subject to the scientific method and is thus provable, demonstrable, and repeatable. Where did NIST reproduce this mechanism by causing trusses to sag and pull perimeter columns inward? Or anyone else? That would be actual science.


That is why I bring up fireproofing and light steel trusses and the dangers of them in fires. For some reason, the TM believers all ignore this angle. Why? But they will jump on any mention of thermite/ate/nano/paint on, high explosive whatever, no matter how ridiculis.


Maybe because they're not actually the danger you have convinced yourself they are. And neither is thermate as ridiculous as you think it is, and yes, it's spelled ridiculous. Did you even watch the video in the OP? Let me guess, you just totally ignored it and only decided to look at the thread at all after 50 pages of buffering from that inconvenient video.



posted on Jan, 14 2011 @ 06:15 PM
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I can't see how those those trusses acted in one way in the simple models some have described, like just bridging across for the core to the perimeter. They also had tranverse supports attaching one truss to the other, like a spiders web and in the literal long run also attached to the perimeter walls for a number of columns off each four corners, if there was bowing inward and the trusses were the cause, then there should have been bowing in each of the four corners as well to some degree or other. It's probably safe enough to say there would be bowing around the impact areas, though more like from impact. I'm sure however that the core in WTC1 did not progressively collapse, but it could have "stepped down" at ground level or somwhere near.That would cause mayhem above, and all according to how much "step down" there was. The link below shows that the core below the top section is still standing in WTC1 for a few moments after total collapse,

www.youtube.com...

It could also explain the peculiar tendency of the mast to stay vertical while sitting on the hat truss, which should be adjusting itself, according to the OS. What would cause the "Step down" is open, but the subject of this thread is as good as and better than anything I have yet heard because it is a demonstrated application of collapse. Ironically the "Step down" is usually a failure effect of usual demolition in buildings with a concrete reinforced core.
edit on 14-1-2011 by smurfy because: Text.



posted on Jan, 14 2011 @ 06:29 PM
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reply to post by GenRadek
 
Not very good examples for comparison are they? useful as they are.



posted on Jan, 14 2011 @ 06:31 PM
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reply to post by GenRadek
 


Again you're missing the point.

The argument is 'can sagging trusses create a pulling force on the columns they are attached to'.

Your firefighter links do not cover that.

You are also ignoring again the KNOWN temperatures room fires reach in KNOWN times. There is no reason the fires at the WTC were any hotter than would normally be expected. The link I gave you about room fire temps takes into account furniture, plastic etc. Jet fuel would not make things burn hotter, only quicker.

Also fire temps do not equate to steel temps. The room might reach 1000c very quickly but it takes a long time for that heat to transfer to the steel enough for it to fail.

Yes the 10" rod is relevant. You seem to think 'lightweight' means smaller than a 10" rod? That rod was hot enough to be orange, a bigger steel truss that could not have got that hot is not going to expand more than a rod made from, a probably, lower grade steel.

And nowhere have you shown that a 'lightweight' steel truss can exert a pulling force on larger 'heavyweight' (lol) steel columns if it sags from heat.

(I might add more to this post)
edit on 1/14/2011 by ANOK because: 'cause



posted on Jan, 14 2011 @ 07:35 PM
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reply to post by ANOK
 



Also fire temps do not equate to steel temps. The room might reach 1000c very quickly but it takes a long time for that heat to transfer to the steel enough for it to fail.


So then why is it that firefighters are so worried about light steel trusses failing from fires, if they take a long time to heat up, according to you?

Or wait, did you read the articles or not? Did you ignore the rapid onset of failure of the trusses when exposed to fire? 5-10 minutes, not hours.

As to can the trusses cause a pulling force? Probably, if you include the entire floor decking that is resting on it. Are you including that as well? What about if the exterior columns are also heated to plasticity, where creep can take place? Remember creep? Steel can also creep when heated and put under a load. Your bar comparison is irrelevant unless its a 30ft long bar with a load on it.


And nowhere have you shown that a 'lightweight' steel truss can exert a pulling force on larger 'heavyweight' (lol) steel columns if it sags from heat.


Just one? What about the whole floor segment with a whole bunch of them doing the same thing? It wasnt just ONE truss, but many acting in unison.


You are also ignoring again the KNOWN temperatures room fires reach in KNOWN times. There is no reason the fires at the WTC were any hotter than would normally be expected. The link I gave you about room fire temps takes into account furniture, plastic etc. Jet fuel would not make things burn hotter, only quicker.


But you are forgetting the extra weight of the aircraft inside, the debris inside, etc etc etc. And yes, plasitcs burning will burn hotter than say, paper, or wood. What about the flammable fumes given off by the burning material? The flammable gases can also add to the heat, and what about when they catch fire? You are confusing the temperature scales F and C. Most office fires burn at about 1200F, not celsius. That is what is constantly tripping you up. You were saying temps of 1200C. Big difference between 1200F and 1200C. Gotta be careful. But as was mentioned before, temperatures in a fire of 800F can cause a truss to sag and fail in a very short period of time.

Here is another page on just how fast a light steel truss can be affected by fire:
www.cdc.gov...


Findings reported by the National Engineered Lightweight Construction Fire Research Project indicate that unprotected lightweight steel C-joists fail within 4 to 6 minutes of exposure to fire [Grundahl 1992]. Testing conducted by the U.S. Bureau of Standards (now known as the National Institute of Standards and Technology, or NIST) showed that unprotected steel open-web bar joists reached 1,200º F in 6 to 8 minutes [Brannigan 1999]. Table D-1 illustrates that steel retains only 25% of its original strength at 1,200º F and retains only half its original strength at approximately 900 ºF. Building design calculations are based on original strength at normal temperatures. At elevated temperatures, steel may retain no excess strength.

Steel is noncombustible and does not contribute fuel to a fire. This property may cause a false sense of security and overshadow the fact that steel loses strength when exposed to temperatures commonly found in structural fires. Steel has a high thermal conductivity, which means it can transfer heat away from a localized source and act as a heat sink. As long as the flame impingement is localized, the steel can transfer heat to other regions of the member-and thus the time to reach the critical temperature is delayed. If an intense fire is evenly distributed along the steel member, the critical temperature may be reached very quickly. Steel also has a high coefficient of expansion that results in the expansion of steel members as they are heated. As an example, a 50-foot-long steel beam heated uniformly over its length from 72° to 972° F will expand in length by 3.9 inches. The same beam uniformly heated to 800° F would expand by 3.2 inches; if heated to 1,200° F, the beam would expand by 4.9 inches [Grundahl 1991; Cotes 1997].

•Expansion within metal trusses may also cause the bottom chord to buckle and fail, resulting in downward thrust and collapse of the roof or floor.

bolding mine.

What if the bottom chords failed first from the fires? the downward thrust would be a good reason for the inward bowing, as each truss section failed like this. Having the floor truss sag, with the weight of the floor above it pushing down, is going to create a pulling force. The floor truss is also softening up and creeping down towards failure. And say the bottom cord of it snaps, well there you go, failure.



posted on Jan, 14 2011 @ 10:28 PM
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Originally posted by GenRadek
As to can the trusses cause a pulling force? Probably, if you include the entire floor decking that is resting on it. Are you including that as well?


How is that supposed to negate the thermal expansion and turn it into a pulling force?


What about if the exterior columns are also heated to plasticity, where creep can take place?


That's well over 600 C. What evidence do you have the perimeter columns were heated that much?



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