Well, I didn't particularly wish to do this but....... there are a few questions which need to be answered in the context of what actually happened.
Here are the basics - in overview and applicable to WTC 1 and WTC 2.
First there was a sequence of events which should be agreed common ground to any reasonable proponent from "either side" if I list them this way:
- (Someone may have pre-positioned demolition devices - multiple options have been suggested); There may have been pre-cutting to assist collapse;
- Plane strikes tower and does damage, starts fire, disables fire fighting etc BUT tower remains standing;
- Fires rage and do other damage. (Pro demolition supporters would want the option to include some demolition here depending on their specific hypothesis);
- Some trigger point is reached and the Top Block starts to move downwards;
- The "Global collapse phase is started;
- ...and rapidly runs to completion;
- ...aftermath - not of interest for this post.
Now I would like to address the several references to the "Global Collapse" phase. It starts at the trigger point of "4" and how we got to four is now irrelevant - whether by crash plus fire plus consequences plus a uniquely vulnerable building OR all that plus a little demolition.
Save the debate of what caused the initial collapse for later - let's look at the global collapse.
Now in the global collapse phase there were three parts of structure potentially available to resist and possibly arrest the fall of the "Top Block". Those three are:
- The columns of the outer wall tube;
- The core; AND
- The floors between the core and outer wall (accepting that those floors depended on the columns of the core and the outer wall tube).
Now taking the outer wall columns first the evidence is clear that they peeled off and fell away and were not crushed. Whatever the proportion of load they were designed for they were not involved in resisting the fall of the top block with their full strength. Two options we need to consider
- The no demolition option - that the descending mass sheared the joist to column connectors; AND
- The demolition option - they were cut (and the option supported by most pro demolition claimants id explosive cutting of "something" and it wasn't the outer columns so it must be some part of the joist and the connection is obvious preference). (But "their call" - it is their hypothesis.)
Next come the outer tube floors which I would posit failed at the inner end by the same mechanism as the failure at the outer end. And I am still leaving the option open for the pro demolition folk to insert explosives.
Then the core. There are many claims for the core. I will take only the first and obvious steps at this stage. The core consists of the falling bit of the top block and the standing bit of the lower tower. Despite all the rhetoric about it being strong (it was in its original and designed function) the collapse situation at the start saw the core as if it was the two cut halves of a bird cage with very thick bars and beams. BUT a hell of a lot of space.
Many folk from both sides seem to presume that the falling core would land on the standing core and meet a full strength resistance. That is clearly not so. I will explain qualitatively at this stage leaving quantitative till later.
(And this is the point where I move from what should be agreed position towards my own interpretation.)
And I remind the non-engineers that the original usage of a column properly braced is to resist axial loads where compression is the critical and most likely mode. If deflected or bent and if not loaded centrally the axial strength is severely reduced. Similarly cross beams in the core are not designed for the massive loads of multiple floors.
So the "thick wire bird cage" of the top block core falls onto the thick wire birdcage of the lower core and a lot of the columns do not land on columns properly centred to take full original design axial load. Some columns miss altogether at the first fall others "hit and glance off," possible bending one or the other. And some horizontal beams land on other horizontal beams and bend them because they impact with way over design load. And to add insult to injury those struck beams when bending pull their end columns out of line and weaken those columns.
And the chance of some columns missing altogether must be high. (Remember I am not quantifying at this stage.)
Now there are many permutations and combinations BUT the total effect is that the falling core meets only the resistance from a fraction of the strength that the core originally had. AND it is a dynamically applied load.
So simply put and leaving all the interaction and second order effects till a later post this is what happened. (Now fully my opinion - and still leaving justification till later)
The falling Top Block wedged itself inside the outer columns ("proof" relatively trivial but deferred) It took no significant part other than resisting the fall to the shear strength failure of the joist connections (Or taking no part if we accept demolition in the global collapse)
The inner end of the joist to core column fails the same way and for the same reason. Either the falling weight of floors plus debris was overwhelming Or demolition.
The core contributes some resistance but could not be enough to stop the top block - several aspects here and all answerable later if needed.
Now in that scenario it is clear that no explosives were needed. Nor are any detailed calculations. 10 floors of total structure in the lesser case lands
dynamically on the floor designed to hold one full floor load
statically and it has a safety factor. No support from outer tube very reduced support of core by core. And the core would not add significant support if any to the outer floors. Again another bold assertion backable by reasons if needed.
So there is the basic outline of what I suggest happened. Other posters in recent days/weeks have supported parts of it - I have not seen the full overview posted in the couple of weeks I have been her.
So that is the background against which recent questions about the "Global collapse" can be answered. And I look forward to other comments or answering some of those questions myself.
