First let me say that I appreciate Mick's professionalism in this thread + I haven't actually read all the pages of this very long thread. But I just had to post the rather obvious reasons why we would need to be able to model better these catastrophal failures:
* In order to better understand how to avoid such events in the future.
- Is it eg. possible that rather minor damage or fire can cause whole steel structured skyscraper to collapse in short time (including the possibility of having eg. explosions in just one or couple of floors)?
- Or is it possible that even couple of steel columns' failure at some position of skycraper + floor failure can bring much of the structure down (if not whole building)?
* How many current buildings are in danger of same sort of event?
Good points Michael M and they have been addressed in the thread but let me present a brief (OK - I was an optimist.
) summary of my perspective. (Retired civil structural engineer with management experience including the level of policy which your questions raise.)
First I suggest we need to recognise two target audiences and two types of models.
The targets are either:
A)the professions and professionals involved in the building industry - high rise sector - and intended to solve problems those persons face; OR
B) persons including lay persons or members of other professions who may have an interest in some aspect of "why the buildings collapsed".
For brevity I will refer to those as "professional" and "lay" respectively in what follows.
The types of models are:
C) Professional research models intended to produce quantified data about some aspect of the events; OR
D) Demonstration of mechanism models.
The models needed for quantified reserch will tend to be partial models - models of specific sub-mechanisms. "Demonstration" models being more suited for visually showing what happened - often for a lay audience.
Now let me explain where those fit relative to the WTC 9/11 events and current state of understanding.
First recognise that all such buildings involve establishing of design parameters - usually with minimum requirements mandated by code. Which leads to a risk managed balance between safety levels and economic cost. All buildings designed to be safe - and safe by a good margin - within the envelope of design parameters.
All three WTC towers were taken well outside their design envelope.
The three "big factors" relevant to WTC collapse are:
1) All steel framed buildings are vulnerable to heat weakening effects from prolonged fire. All such buildings are designed with a view to two overlapping objectives in the event of fire. Those are:
(a) Ensure time for occupants to escape; AND
(b) Ensure time for active fire fighting measures to be implemented - taking over from any inbuilt passive or self powered fire inhibiting provisions.
2) WTC1 and WTC2 were hit by trauma way outside any design provision which was reasonable at the time they were built. That trauma overwhelmed the built in fire fighting strategy. (I wont derail but suggest it is not possible to design a commercially viable building with can structurally withstand any conceivable malicious attack.)
3) WTC7 was overcome by events so that it was taken outside the fire resistance aspect of its design. Note however that the primary fire objective of "get the occupants out" was achieved.
So with that setting of the scenario let me respond to each of your points:
* In order to better understand how to avoid such events in the future.
I suggest primary avoidance strategy would be to prevent such forms of attack. Obviously open to complex discussions.
- Is it eg. possible that rather minor damage or fire can cause whole steel structured skyscraper to collapse in short time (including the possibility of having eg. explosions in just one or couple of floors)?
Collapse due to "minor damage" was not what happened at WTC. So modelling WTC will not help prevent it. Design practice already includes assessing a wide range of "what if" situations including accident and malicious activity. I would suggest that there is no scope for modelling to assist.
- Or is it possible that even couple of steel columns' failure at some position of skycraper + floor failure can bring much of the structure down (if not whole building)?
Yes it is possible BUT the previous comments apply. We need to take care to follow the TWO paths of logic - i.e. "inside design parameters" OR "subject to deliberate malicious attack". My previous comments apply to the respective scenarios. In brief modelling doesn't help.
* How many current buildings are in danger of same sort of event?
Decide which scenario. I suggest non are in danger without deliberate malicious attack. And nothing structural can be done to reinforce exiting buildings in the event of deliberate massive attack. There may be some existing buildings where lessons from the WTC events could indicate modifications which may be prudent. BUT modelling is not the way to determine what is needed.
Now note that all your comments so far envisage the "professional" audience. None of them go to use of models for a lay audience.
So you concluding paragraph:
The events would indicate that these are valid points*. Either there were some other reasons for the total structural failure than fire** - and this is usually called conspiracy theory*** - or there might exist a grave danger that similar events (even from just fires and/or couple of rather small explosives) can bring whole skycrapers (or other tall buildings) down very rapidly and result in even thousands of lives lost.****
* I question the GLOBAL conclusion - reasons already outlined.
** There were - the "fire only" is a truth movement preferred "strawman".
*** Disagree - conspiracy theory is a theory that some conspiracy is involved when there is no evidence or argument to establish a prima facie case of conspiracy. Agreed however that "conspiracy theory" is an often misused term.
**** Hyperbole and conflation of aspects which do not lead to the conclusion. BUT this is the central point for a discussion.