The pre-collapse inward bowing of WTC2

Jeffrey Orling

Senior Member
right and the main contention is if the floor system is robust enough including the connections to substantially pull in the facade. It appears to me that without some sort of increased axial loads the amount of column deformation would not lead to the large IB and buckling. This is a guess... I don't know how out of "true" a column assembly like the twin towers could be and perform ...ie not buckle.

We do know that not very long after the IB the whole top descended and this is a clear sign that axial support was below service loads... whether from loss of strength, alignment or both.
 

econ41

Senior Member
It appears to me that without some sort of increased axial loads the amount of column deformation would not lead to the large IB and buckling...
You are conflating two issues there Jeffrey. Issue #1 - There is no necessity for increased axial load - provided p-delta is exceeded the column will buckle and the buckle will self-propagate until it folds/breaks or until it meets some externally imposed limitation. Issue #2 - within the specific context of IB at WTC the phenomenon was part of a cascade failure. That cascade may have re-distributed increased load to those columns. Separate aspect to and does not negate "Issue #1"

This is a guess... I don't know how out of "true" a column assembly like the twin towers could be and perform ...ie not buckle...
Near enough "p-delta" straight off the formulas. SO if you want to guarantee "perform ...ie not buckle" you need to keep it significantly less that p-delta - how much is the safety margin for you to decide. (And "safety" will not be linear with p-delta. Don't ask the next question - I would need to go back to the books. :rolleyes:).
We do know that not very long after the IB the whole top descended and this is a clear sign that axial support was below service loads... whether from loss of strength, alignment or both.
Must have been both - there was misalignment and no way could the columns have pretended they were still in line.
 
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jaydeehess

Senior Member.
Sure --- and that is an example of the point I was making.

Make sure you properly define your objectives......and that is a thread or three on its own

HOWEVER the topic here is "Inwards Bowing" ;)
oh, yeah, right.
It occurred, its indicative of a gradual progression to total failure as opposed to sudden failure due to the use of explosives.
Everything else is detail.
 

jaydeehess

Senior Member.
I'm not saying it caused failure. I'm saying it illustrates a relativly long and slow progression to failure.
There was a slow change in loading on those columns that eventually caused them to deform significantly over a period of minutes.

Did an inward pull cause it? Did a vastly increased vertical load cause it?
Personally I believe its both. A vastly increased vertical load was slowly caused to affect these columns
AND
due to a slight inward pull by sagging floor trusses/sections caused that deformation to preferentially be inwards.

If a floor connection had detached from perimeter column then that column would be free to bow outwards or inwards. Its possible that a pristine floor would also preclude outward bowing though.
Also might only need one column to be the first and bow inwards thus slightly pulling adjacent columns to follow suit, but that doesnt look like it was occurring. Looks more like several columns all began bowing at the same time.

But that's fire induced damage detail when it comes to whether or not, and yes this is not the OP, the collapse initiation was due to demolitions or not.
 
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econ41

Senior Member
I'm not saying it caused failure. I'm saying it illustrates a relativly long and slow progression to failure.
There was a slow change in loading on those columns that eventually caused them to deform significantly over a period of minutes.
Yes - time frame understood.
Did an inward pull cause it? Did a vastly increased vertical load cause it?
Personally I believe its both.
Has to be "inward pull" - may be "increased vertical load". See my comment to Jeffrey above. It could also be inwards pull and same vertical load. It is even possible inwards pull and less vertical but I doubt that one.
A vastly increased vertical load was slowly caused to affect these columns
AND
due to a slight inward pull by sagging floor trusses/sections caused that deformation to preferentially be inwards.
That is one of the possible options. But not the only viable one.
If a floor connection had detached from perimeter column then that column would be free to bow outwards or inwards. Its possible that a pristine floor would also preclude outward bowing though.
Also might only need one column to be the first and bow inwards thus slightly pulling adjacent columns to follow suit, but that doesn’t look like it was occurring. Looks more like several columns all began bowing at the same time.
Your reasoning understood and agreed. The "several columns" moving together scenario almost guaranteed due to the very strong spandrel connections unifying the columns into sheet.
But that's fire induced damage detail when it comes to whether or not, and yes this is not the OP, the collapse initiation was due to demolitions or not.
:)
 

jaydeehess

Senior Member.
Your reasoning understood and agreed. The "several columns" moving together scenario almost guaranteed due to the very strong spandrel connections unifying the columns into sheet.
:)
Understood. There's the situation of a line of columns, then there's that of a solid wall. The tower perimeter was something in between, a line of columns that were connected to each other.
Likely going to buckle similarly as a solid wall while visually it looks like a line of columns all bending inwards.
 

econ41

Senior Member
Understood. There's the situation of a line of columns, then there's that of a solid wall. The tower perimeter was something in between, a line of columns that were connected to each other.
Likely going to buckle similarly as a solid wall while visually it looks like a line of columns all bending inwards.
Exactly.

And that aspect BTW is where quite a few "discussions" have missed the point. The physics of "single sagging joist <> single perimeter column pull in" is text book simple. BUT tie all those columns together into a sheet - and remember that the floor joists were tied by the floor - and the whole problem becomes a couple of orders more complicated. And that is only part of the load redistribution problem of the cascade failure. Not surprising that so many people from "both sides" get .....confused.
 
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Jeffrey Orling

Senior Member
Exactly.

And that aspect BTW is where quite a few "discussions" have missed the point. The physics of "single sagging joist <> single perimeter column pull in" is text book simple. BUT tie all those columns together into a sheet - and remember that the floor joists were tied by the floor - and the whole problem becomes a couple of orders more complicated. And that is only part of the load redistribution problem of the cascade failure. Not surprising that so many people from "both sides" get .....confused.
Not to mention that the trusses were spaced at 80" oc and the columns at 40" oc so every other column had nothing actually acting laterally on it.
 

econ41

Senior Member
Not to mention that the trusses were spaced at 80" oc and the columns at 40" oc so every other column had nothing actually acting laterally on it.
Yes. Other than via the interconnections of the columns - spandrels etc.

Not a big issue in the overall context. Not a significant technical issue - it was neat 2 to 1. I cannot remember how Newtons Bit and enik handled that "two to one" aspect in their classic stand off. One of those rare instances where the truther engineer got it right over the "debunker". OR - to be more precise - the truther was "more right" - not as much wrong. And he acknowledged the error when I explained it to him. In fact it must be the only time I’ve had a truther engineer agree with me when I identified an error. Cannot remember another one.

The technical bottom line remains the same. You cannot quantify the physics of perimeter Inwards Bowing by using a 2 dimensional slice - whether one joist to one column or more correctly one joist to two columns. It takes full 3D multiple joists multiple columns to derive valid quantified data.
 
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Buildy

Member
Seems like a semantic argument. You could say the collapse of WTC2 started when the plane hit.

The inward bowing of the exterior was observed minutes (IIRC) before WTC2 fell. Are you suggesting the core was being slowly lowered for several minutes before the high speed portion of the "collapse", and this was slowly pulling in the exterior in the floors that were on fire?



The above image was six minutes prior to collapse.


Can you source this image? I can't seem to find it anywhere else.
 
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