@dylbie You asked several questions. I will respond to one - possibly more of them. But for reasons of overall coherent logic I will start with this one:
2. Does anyone have any theories as to why the core failed?
I have certainly presented comprehensive and in-depth explanations of all stages of the WTC Twin Towers collapses. Naturally such explanations include the aspects of core failure. Given this Forums "No Click" policy I won't provide links but can do so or provide quotations if appropriate.
(Others may have done similar work but I am not aware of any. If other members are aware of other comprehensive explanations they can comment separately.)
Both cores were still partially standing after the main collapse.
To be more specific some core columns were left standing and remained so for a few seconds. I suggest that the easiest way to understand why this is so is through understanding all the key features of the four distinct stages of Twin Towers collapses. I'll just outline them here - you can ask for more details if you need them. The "stages" are:
1) "initiation" stage - essentially a process of cascading failure of columns extending over about 1 hour (shorter for WTC 2 - longer for WTC 1) which reached a threshold point when the remaining columns were not strong enough to support their Top Block which then started to move bodily downwards. Causing near-instantaneous failure of the remaining columns which had survived to that point. The key feature of this stage was that of the three main methods of column failure ALL resulted in the "dropping" top part of the failing column moving past - missing or bypassing its lower part of the column. >> Remember "column ends missing" is ALL we need to know for this discussion at this time. Ask questions if you want more explanation.
2) "transition stage" > a chaotic mess. It resulted in a lot of horizontal translation movement associated with "tilting" and starting to "topple". Forget the details for now. All we need to know is that the horizontal motion made double sure that the column ends kept "missing" or "passing" AND - the main point - it imposed concentrated weight from perimeter columns on office space floor joists > starting "ROOSD" which was the driving process of the "progression stage". Proof of "starting ROOSD" is in this graphic which I can explain when/if necessary:
3) "progression stage" - which had two distinct "sub-stages":
3)(a) "early progression - the Top Block" and a similar-sized portion of the "lower tower" broke up in mutual destruction >> with falling debris continuing to "miss" or "bypass" the columns >> This substage is of no consequence to us at this stage of discussion.
THEN - the final stage which left those core column "spires" standing.
3)(b) "established progression"... involving "ROOSD" - a truther side inspired acronym meaning "Runaway Open Office Space Destruction". The concept AFAIK was first posted on a forum in Nov 2007 with this graphic:
The acronym "ROOSD" and the concept were put into mainstream discussion in 2009. It explains how debris fell down the "OOS" missing the columns. The concept is generally accepted these days (Since about 2010) as what really happened. Note It explains why the perimeter columns were left unbraced, free-standing and subsequently peeled off and toppled. It did not explain the core which
@dylbie is your current interest. For reasons of discussion history, nobody paid the issue much attention. In 2013 I extended the original "perimeter only" scope of ROOSD asserting that the behaviour in the core would be analogous. i.e. falling debris missed columns and sheared of floor beams in the same way as the debris falling in the OOS "outer tube" area sheared of the floor joists.
So that much should at least set an agreed context if you want any further discussion to explore more details. Your call.
In the simulations I've run, the debris at ground level completely overwhelms the base of the core and destabilises it. Just wondering if this is anywhere near accurate.
If your focus is on those few "spire" columns - your guess is probably in the right ball park. The core columns remained standing as spires because the progression of the collapse was so fast. And debris fell around the columns With no bias one way or the other to cause toppling. BUT the spires were too tall to remain - they were well beyond the effective length that could remain self-supporting. (The technical details involve a concept of "Euler Buckling" after the person who described it. Put simply a slender column can be too thin to stand up without horizontal bracing. So it will buckle. Buckle under load at some length. If it is even longer it will buckle under its own weight. And it will buckle easier/earlier if it has either imposed force or vibration. Resonant vibration was IMO possibly the trigger for those core column spires.
So does that answer or start to answer this part of your Question #3?
is that how on earth did the core survive this amount of damage and not collapse immediately. Seems to stay up for 10 seconds or so second before finally giving up.
If you need more or a fuller explanation just ask.