Perhaps this problem exists because buildings should not collapse "easily". And they don't. We also know that CD of tall building place charges on columns low in the structure and time the sequence of destruction to control where / how it falls... into a relatively compact pile of the property it is built on. Engineers design these destructions to conform to their objective of a "neat" collapse.
The impact of a large plane hitting a tower which is 95% air and less than 1% structural steel would destroy the plane and elements of structure where the plane impacted. There was not enough energy to "push" the tower over... and the tower was not stiff enough to act as a unit and break its connections at the base to allow it to tip over.
The plane strikes caused massive local structural damage which was not fatal with the fires which ensued. The towers survived the plane impacts... and one of the reasons is that they were mostly AIR.
If the entire mass of a twin tower were in one "column" standing as high as the towers were.... assuming that the column could self support and was not too slender.... and the plane hit it... what would happen? The plane would be sliced and the column likely would sway but remain standing. Engineer could calculate the forces needed to be applied at a dynamic point load at the equivalent height of the 92nd floor to break the column free at the foundation and cause it to topple over. This is a somewhat useless thought experiment.
The simplified explanation that is sought ... popular "every-man science"... would acknowledge that the collapse was a PROGRESSIVE process. And in a very real sense it was a runaway, ie unstoppable progression. Mick's models show a runaway progressive collapse in a highly simplified way.
Every-man wants to know how plane damage + fires lead to the building completely collapsing. And in the case of 7wtc... how fires alone can produce this result.
So Mick's model and the discussion of ROOSD is basic settled trivial engineering. A beam can support X pounds and then it fails... Once the sufficient threshold mass was dropping inside the tower... it would destroy each floor consecutively and uninterrupted. That accounts for the slabs. The columns and the facade fell for other basic structural reasons... and it is described in the Euler formulas for the limits of slender columns to self support. There is a limit of how tall a flag pole of X diameter can be. Try to extend it taller and it would buckle under its own weight... and topple over.
So....
Every-man needs to learn and understand what fire and heat do to building materials aside from combustion. The fires were fueled by available combustible materials... jet fuel at first and then office contents immediately thereafter.
Steel and concrete do not burn at the temps of the fires in the WTC.
"The behavior of concrete at high temperatures is influenced by several factors, including the rate of temperature rise and the aggregate type and stability. Abrupt temperature changes can cause cracking and spalling due to thermal shock, and aggregate expansion can also produce distress within the concrete.
High temperatures also affect the compressive strength of concrete. Above 212º F, the cement paste begins to dehydrate (loses chemically combined water of hydration), which gradually weakens the paste and paste-aggregate bond.
The temperature that concrete has reached often can be determined by observing color changes in the aggregate. For example, limestone aggregates turn pink when they reach about 570º F, which can result in substantial loss of compressive strength."
Spalling -
"There are two drivers for spalling of concrete: thermal strain caused by rapid heating and internal pressures due to the removal of water. Being able to predict the outcome of different heating rates on thermal stresses and internal pressure during water removal is particularly important to industry and other concrete structures.
Explosive spalling events of
refractory concrete can result in serious problems. If an explosive spalling occurs, projectiles of reasonable mass (1–10 kg) can be thrust violently over many metres, which will have safety implications and render the refractory structure unfit for service. Repairs will then be required resulting in significant costs to industry.
[2]"
Heat weakens and eventually destroys concrete.
Heat weakens steel
Heat causes steel to expand.
Excessive heat caused havoc in the WTC steel engulfed in flames. This included:
lateral beams / bracing
columns
bolted knife connections
splice plates
bar trusses
metal Q decking
All of the above are "settled engineering". What is NOT KNOWN is.... where the heat was causing the problems.... which beams and columns and so on were losing their ability to perform to design specification. We don't know the precise fuel load.... it can only be "guessed at" with some reasonable certainty. We can't know the precise sequence of failures without SEEING those failures which were taking place behind/inside the facade.
All that can be done are theoretical models of the sequence of failures which occurred to the compromised plane damaged structure. This is one "guess" of a possible "sequence of inside the core failures" for 1wtc.
It attempts to show the failure of core columns progressing southward.
It does not show what was happening to the bracing between those columns which was expanding and exerting lateral forces and perhaps causing the misalignment of columns, destroying bearing area and leading to buckling.
Loads do not disappear... so they are redistributed when a column "fails" and this causes other columns to assume increased loads which can lead to their being overloaded. THERE IS excess capacity in the design of the steel... beams and columns. But when the excess capacity is insufficient the steel member will fail and then the loads are redistributed and the these steel failures go "runaway".
When aggregate axial capacity is no longer sufficient to support what is above... "above" drops down.
All of this is settled engineering. There is nothing mysterious to discover about the WTC collapses... other than:
THE PROGRESSION OF FIRE/HEAT THROUGH THE STRUCTURE
THE PROGRESSION AND LOCATION OF STRUCTURAL STEEL AND CONCRETE FAILURES
This is unknowable without "being there". We can only make educated guesses based upon what WE CAN SEE... building movements, smoke, etc.
++++
So the discussion of interest may be.... can skyscrapers be designed to isolate and arrest structural failures to prevent them from going "runaway"?
Are some structural systems less PRONE to going runaway?
and the corollary...
Are some structural systems MORE prone to going runaway?
The above includes the construction DETAILS.
As building construction/design decisions are economically driven (as well as code and technology)... are designs less prone to going runaway more or less expensive or the same cost to build... including the time to erect the structure?
I leave it there for now.