The reason for the WTC collapse is because of something known as "Creep of Metals."
Creep is divided into primary, secondary and tertiary. This is illustrated on a strain-time creep curve for a constant stress and constant temperature.
the key here is constant temperature
stress on metals in a building structure being consistent depending on its design
Creep is not to be confused with fatigue.
Fatigue is a process of crack initiation and crack growth under cyclic loading.
Tertiary creep occured and this resulted in the catestrophic failure of the structures.
Strain increases with increasing temperature and higher stress results in higher creep
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Creep is defined as the continued slow straining of a material under the influence of a constant applied loading action.
Strain e = f(s, t, T)
Under short term loading
Strain e = f(s, T)
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The initial strain at t=0 corresponds to the initial elastic strain. Primary creep strain occurs over a short time, then there is secondary creep (or steady state creep), finally there is tertiary creep which occurs over a short period of time and is associated with catastrophic failure.
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During steady state creep (secondary)
de/dt = K exp (-B/T) where de/dt increases with T
and
de/dt = Cs^n where de/dt increase with increased stress s
B, K and T and n are constants for a particular metal or alloy
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so whats the big deal, and why the Towers imploded?
Creep strain becomes important when the operating temperature exceeds 0.3 to 0.4 Tm (Tm is the melting point in degrees Kelvin).
At room temperature Creep in Metals is not significant for construction metals (however it is a problem for aluminum and lead)
Typically,
The melting point of iron is 1807K and 0.33 Tm is 602K (one third of melting point). This is about 329 degrees centigrade.
it is belived that the length of time that the jet fuel was burning superheated the iron I beams to above 329 degrees centigrade.
That is what caused the implosion.
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Creep in Concrete
Concrete, believe it or not, also exhibits Creep.
Because of the complex constitution, creep in concrete is influnced by a lot of things such as:
1. cement content
2. water to cement ratio
3. cement type
4. environmental conditions such as humitity
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As an aside
The Great Ship Titanic also suffered a similar fate where the metalic propeties of the rivets used to bolt the ships haul were not known.
The ice waters of the Atlantic caused the rivets to become brittle, because of the nature of the Iron Carbon Steel alloy used.
The theory here is that tough materials absorb a lot of energy, and low toughneess (brittle) materials absorb little energy.
At sufficiently low temperatures, steel exhibits brittle failure (low toughness), and behaves like glass.
conclusion
Education is the key.
And as the saying goes, "Empty heads are the root of all Evil," which is what all the conspiracy theories are all about (basically a load of hog wash).
