Another recycled echo in the echo chamber claims new evidence of explosives in the towers.

This comment was posted to reddit on May 08, 2015 at 5:05 am and was deleted within 15 hour(s) and 12 minutes.

Another recycled echo in the echo chamber claims new evidence of explosives in the towers.

I don't expect you to read, let alone understand this, but here is a post from /u/ in /r/Askengineers on the topic of collapse.

Explain to me how a falling mass can crush the structure below it, without decelerating, whilst continuing to accelerate and still crushing the progressively stronger structure below.

Hopefully this will answer your question:

Basic structural engineering

To understand this, you first have to understand basic structural design, as well as the WTC's specific design. Generally, structures are built to withstand two main types of loads:

Static loading to support its own weight, also called gravity load. Static simply means that there are no moving parts.
Wind loads. These loads are applied laterally (sideways) to the structure.

WTC was designed in a manner where the main structural beams were built in the center, called the inner core, like this. The inner core supported the majority of the structure's static loads.

The outer beams were connected to the inner core using truss beams like this. In addition to sustaining the wind loads, the 244X perimeter beams added redundant support for up to 50% of the static loads of the building. This is why the building did not collapse upon jet impact, because only redundant columns were removed, leaving the inner core columns mostly intact.

Static vs. Dynamic

Even though it's true that there was still ~80% of undamaged structure below the impact, this does not mean it was "progressively stronger". Nearly all structures are NOT designed to handle dynamic loads.

Dynamic, as opposed to static, means there are moving parts transferring load. A good example is aircraft landing gear, which retracts when it hits the ground in order to transfer the loads into the rest of the landing gear mechanism.

In a landing gear, you do not design it to handle only the weight of the aircraft, but some multiplication of the aircraft's weight. A typical landing gear design will handle 4-5X the aircraft's weight. Structures are not designed to the same limits as landing gears because they aren't expected to handle dynamic loads.

For the WTC, a floor collapsing onto the floor below was the dynamic load, and the sign that progressive collapse had begun.

Failure modes

If the static and/or dynamic loads exceed the design limit of the structure, it will fail and will propagate through the structural members in a few ways:

Material yielding. This simply means the loads exceed the strength of the material, and the structural member will literally shear, tear, bend, or deform to give way to the load.
Beam buckling. A beam buckles if it loses material strength, or loads beyond the design limit are transferred into the member.
Catenary failure. This is when lateral structural members "sag" under all the weight from above it has to support, due to the loss or failure of a column underneath. This has the double action of pulling in adjacent columns inward while also pulling downward.

The critical buckling force is determined using the equation F = (pi² E) / (KL/r)²

where E is Young's Modulus, and KL is the effective length of the column, and KL/r is the slenderness ratio (with r being the radius of gyration). A thin column has small radius of gyration and a stocky column has large radius of gyration. The slenderness ratio determines elastic or inelastic mode of buckling failure. Columns with small slenderness ratios are called short columns.

Short beams tend to fail at the material yield limit, while long slender beams tend to buckle before bending to the material yield limit before failing. Either case will result in a structural member failing completely because it cannot return back to its elastic state.

Sequence of events leading to failure

Now knowing these concepts, here is the sequence of events leading up to structural failure:

As an act of terrorism, a plane crashed into the building substantially changing the structural integrity of the overall frame.
The plane was loaded with jet fuel that in turn ignited office furnishings that had been concentrated in a corner by the force of impact.
The collision disrupted/loosened/knocked off fireproofing of structural members, especially the main span floor trusses that ran between the exterior wall and the interior core.
Over time the floor trusses began heating and losing their structural integrity, including sagging into a catenary mode. This eventually failed one end of their gravity/lateral mode connections. The failure to either end would be problematic for the unsupported length of the building's columns, and lowering its vertical load carrying capacity. The failure at the exterior wall is most likely due to heat and bolt sizing. Perhaps, those floor trusses then collapsed onto the lower level spread heat/fire to the adjacent stories and added to the heat exposure of the exterior columns, increasing their KL/r ratio beyond their design limit for steel at a given temperature. Or possibly, that failure mode might have initiated at a mid-story column splice.
The lack of support to the exterior columns continued until failure was initiated. A seam-like failure indicated a failure may have been imminent and only needed one to fail for all to fail.
Progressive collapse is initiated which is unstoppable once initiated.

TL;DR: Why can't progressive collapses be stopped?

Even though the lower undamaged structure was designed to be able to handle the upper damaged 18 stories, this is only true in a static sense. Once the loads became dynamic (due to gravity), the dynamic loads had already far exceeded the structural design limit and the failure had to propagate until all the energy is dissipated.

This is true for nearly every structure in existence, and certainly true for every skyscraper: once progressive collapse begins, it can't be stopped because the supporting structure below cannot handle the many multiplications of the load it was designed to support.

It's erroneous to think of a skyscraper as a solid column of material that's able to stop falling objects. In reality a skyscraper is more like a skeletal structure with thousands of interconnected supporting members. The loss of too many members/supports will cause failure. A giant mass falling through the center of the skeleton will only knock more of the structure away, further propagating the collapse.

Another comment I wrote regarding the role of fire damage in initiating progressive collapse: https://www.reddit.com/r/AskEngineers/comments/33tl74/how_did_the_wtc_north_tower_top_section_fall_and/cqr683e

Finally I will conclude with a quote from the 2008 NIST report on WTC 7:

The observed descent time of the upper 18 stories of the north face of WTC 7 (the floors clearly visible in the video evidence) was 40 percent greater than the computed free fall time. A more detailed analysis of the descent of the north face found three stages: (1) a slow descent with acceleration less than that of gravity that corresponded to the buckling of the exterior columns at the lower floors, (2) a freefall descent over approximately eight stories at gravitational acceleration for approximately 2.25 s, and (3) a decreasing acceleration as the north face encountered resistance from the structure below.

References:

Edit: typos and add more references.

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