Econ41, I appreciate the effort you have made in your most recent post. I am not particularly interested in focusing on the free-fall acceleration issue to the exclusion of a discussion of models, as it is discussed elsewhere. I am interested in the question of modelling the Towers' collapse mechanism, whether in virtual space or physical reality (preferably the latter, of course). I agree that (iii) progression is the most interesting aspect of the collapse of the Towers, from my perspective at least. In other words, I am interested in how "Runaway Open Office Space Destruction" can be demonstrated, not discussed.
(My emphasis.)
Understood - and that is the specific narrow focus I am taking - despite the temptations to join in the several other discussions which are in progress.
The two topics we are joining together are:
1) Understanding the "ROOSD" process; BY
2) Demonstrating using physical models.
The gap between us is that:
a) You see models as both necessary and better than other methods; WHILST
b) I do not see any need for modelling of ROOSD (And don't see further modelling, esp physical modelling, as practical/possible for other main stages of the collapses but those not our current focus.)
I will need to press you further as to why you see modelling as either necessary or superior. And, continuing my intention to keep tight focus, here are what should be two points of final focussing:
First a minor one. "ROOSD" is one of the three mechanisms of the Twin Towers global progression collapse. The three are (1) "ROOSD" - runaway down the open office space WHICH (2) Left the perimeter unbraced causing/allowing "Perimeter Peel Off" WHILST (3) A similar strip down mechanism removed the bracing in the core leading to core columns falling. Discussion of these issues often is ambiguous as to whether ROOSD includes the total three mechanisms. My focus on the single mechanism.
Second as to the types of physical models. We have a "two by two" selection.
x) Do we model part or all; AND
y) Do we want it to "look like" the real thing OR do we want to demonstrate the engineering/physics. The two are not mutually exclusive BUT
some engineering models focused on a specific detail may not look like the real event.
I'm taking your need as being for something that "looks like" the real thing to suit a lay audience AND deals with the full mechanism.
However lets first look at some building blocks. The key reason why ROOSD happened was identified in this post by deirdre:
i think this pic says it all. the little clips holding Everything together are really all that matters, not sure how to model that.
For ROOSD to happen all that was needed was for a big enough impact load to land on that section of floor and it would easily shear the clips. Do you understand that and do you agree?
Now that is only one element and for each floor there were many such elements - but the same truth applies to each. I see no difficulty in concluding that - if one would shear THEN under the same circumstances - all the others would also shear. Do you agree?
Now let's take that part of the mechanism and put it into a physical model. This is one of the best efforts I have seen:
The core was not brought down by forces from above, although that played a part. It was brought down by a combination of lack of lateral support, and then being pushed and pulled from the side by a violent wave of thousands of tons of steel and concrete.
Look at the center column here:
Now try to imagine what the collapse (of this model) would look like if it were 30x as high. Even if I tape up the center column quite firmly, it's still going to collapse by itself at a certain height, and the vast accumulation of debris in the falling wave is going to make it collapse (in sections) much quicker.
IMO it looks very similar to the real thing. Near enough for me to see that it is relevant and a sound modelling of the
qualitative aspects of that single "vertically aligned stack of joists" part of the total mechanism. (Note "
qualitative" - we may need to discuss "
quantitative' at a future stage.)
We are now at a decision point:
Are you comfortable that you comprehend the mechanism of single floor joist element failure as the building block of "ROOSD"? AND
Do you agree that those single building blocks add up to the complete ROOSD mechanism?
No problem if you don't - it simply means I have more explaining to do. If so say which bits.
If you are in agreement so far we have all the foundation work completed to address the real questions including":
A) Would a model do it better?
B) For whom?
C) Why? / Why not?
...and some more that will no doubt emerge.
And I may wander off to join in a couple of the other mini-discussions - in case some old friends think I have totally lost the plot and some potential new friends doubt that I ever knew where the plot was.
EDIT PS Thanks to Mick West for help with the graphics.