Load vs. Structure

A persistent mistake made by those who believe the WTC buildings to have falled due to controlled demolition is that the upper sections of the Towers could not have smashed their way through the comparatively larger lower sections. However, their mistake is based on the frame they have when viewing these collapses. Instead of understanding the collapses as Mass vs. Mass, they would better comprehend them by using the frame Load vs. Structure.

Table of contents

• Story Problems
• Load vs. Structure
  • How A Building Works
  • Static Loads and Dynamic Loads
  • The Collapses of the WTC Buildings
  • Mass Shedding and Piledriver Destruction

Story Problems

The AE911Truth slideshow presents a couple of "story problems" when discussing how the towers fell. They are accompanied by these illustrations:

The first has a hand holding the upper section of the WTC by the antenna, as if it were a olive on a toothpick. Beside it is a comparison by mass. The upper section is represented by 1 kilo, and there are five 1 kilogram blocks stacked up to represent the lower part of the building.

This misrepresents the mass distribution in the building, however. The higher the floor, the thinner the steel plates used to build the perimeter columns (allowing the columns to remain the same "size" all the way up). At impact level, most of the core columns had been swapped out with regular I-beams. So there was actually a great deal more mass below the impact zone than above. AE911Truth's case would appear to be stronger!

However, as you will see below, this doesn't matter at all.

The second shows the upper section supported by a crane over the lower section. Another crane holds a second upper section over thin air and the viewer is invited to guess which upper section will hit the ground first.

Clearly the one over thin air will hit the ground first. Again, this doesn't matter.

As you can see, the two pictures are illustrating similar points. The first is clearly comparing the mass of the top section to the mass of the bottom. The second attempts to show that the building below the upper section should have provided a great deal of resistance.

Both illustrations are highly prejudicial and seek to heighten personal incredulity? in the viewer. The hand holding the top section is ludicrous in its exaggeration, as are the markings of 1 kg. The second is less so, but I would still like to see the crane that could be built that much taller than a 110-story building and be able to support the mass of 15 floors of that buildings.

These mistakes of scale illustrate the true solution, however. AE911Truth wants you to consider this as Mass vs. Mass. Of course the greater mass should win, right?

Not at all.

Load vs. Structure

How A Building Works

High rise buildings are built to support their own mass (which does not change), the mass of the floor's occupation (which changes gradually but constantly), and to resist lateral forces (like the wind hitting the building or an earthquake). These (and other forces) are sometimes called loads.

It does this by using structure to distribute these various loads safely down to the ground. If there is a part of the structure that might experience bigger loads, the structure will use various time-tested systems to distribute that greater load throughout the structure so that no single part is stressed beyond its ability to do its work.

The building codes of various municipalities tell construction specialists a great many things about how they can build their buildings. Part of the code tells them how much their buildings must be able to accomodate when it comes to various stresses and loads. The WTC towers and Building 7 were no different. If a strong wind were to hit those buildings, they would have been expected to transmit that load safely through their structure and down to the ground. A building that could support its own mass, the gradual back-and-forth mass of its occupation, and the sudden loads from wind and earthquake is a building that is working well.

Static Loads and Dynamic Loads

I made a distinction above between loads that are assumed to be stationary, loads that are slow in building and slow in dissipating, and loads that exert their force on the building suddenly. This shows a basic division in loads that is important to understand when you study the collapses of these buildings: static loads and dynamic loads.

Static loads don't move and they don't change. They are constant stresses on the structure. Dynamic loads do change. Both the gradually changing load of the building's occupation and the sudden loads of wind and earthquake are dynamic loads.

And dynamic loads are the real problem for a building's structure. My favorite illustration of this is a brick and your head. If you were to place a brick carefully on your head, the structure of your body would easily distribute this gradual dynamic load through your skull and down through your skeleton to the ground. After that, the load of the brick becomes static, and your body's structure has no difficulty in continuing to transmit that static load to the ground.

But lift that brick just an inch from your head and then drop it. The dynamic load from the brick now will give you a painful start and maybe even raise a bump. Lift the brick even further — a foot, six feet — and the brick could seriously damage your head. It could easily kill you if it was high enough.

And yet it is the same brick! It is the same mass. Why is there such a difference in the load being transferred to your body's structure?

The brick is being accelerated by gravity. The further the brick falls, the more time gravity has to accelerate it, and gravity doesn't need long at all to give even this small amount of mass a great deal of energy.

The Collapses of the WTC Buildings

The problem with the story problems above is that they are comparing the mass of the section above to the section below the impact zones. What truly matters is the ability of the structure below to redistribute the dynamic force being delivered to it by the mass that's falling.

For example, if a brick falls from far enough, the structure of your skull won't be able to redistribute that dynamic load before the structure breaks. It won't matter if you just had a titanium hip replacement, say, or a new knee put in. The skull can't cope with the dynamic load, and it tears apart before it can transmit the load to the new hip or knee.

The same is true for the mass of the building section above and the structure of the section below the impact zone. If the mass of the section above delivers a dynamic load that's too large for the structure below to cope with, the structure below is going to tear apart before it can redistribute the dynamic load further down the building.

And the distance the upper section has to fall isn't very large at all. A paper released shortly after the attacks on 9/11 showed that once the sections above the impact zone had fallen a single story, the dynamic load they had to transmit exceeded the coping ability of the structure below by an order of magnitude. The upper sections, to turn the phrase around, had brought a gun to a knife fight.

So the first story problem misrepresents the situation at the towers tremendously. The mass of the section below doesn't matter. It is the structure that is going to distribute the dynamic load, and the structure below couldn't, not by a long shot. It doesn't matter that the mass below the impact is so much greater than the mass above. The force from the mass above is being increased by gravity to the point that the structure was unable to deal with it.

True, the structure got stronger as you went down. It would have to, since it has to deal with more and more of the load of the building above. But the falling mass was growing as well. The floors that were torn apart then contributed to the falling mass. The upper section continued to fall and so it continued to gain energy from gravity. The lower floors, as they fell, themselves gained energy from gravity. At a certain point, the energy required to tear the next floor apart would be a sliver of the energy possessed by the falling mass.

And so the second story problem also misrepresents the situation at Ground Zero. Yes, absolutely yes, the completely unimpeded section would hit the ground before the section falling through the building. But with the sizes of force we are dealing with (something conveniently masked by the improbable crane supporting the upper sections), that difference in time would be quite small.

Mass Shedding and Piledriver Destruction

Many times AE911Truth will point to how debris went everywhere in the collapses of the towers to show that the falling mass was not that great. The calculations published in actual peer-reviewed journals take this phenomenon into account. Richard Gage and his band of architects and engineers are invited to submit their calculations showing otherwise to a respected scholarly journal where they will receive their proper attention.

Another argument is that the piledriver appears to be destroyed in videos of the collapses. The "piledriver" is a convenient term for the top section above the impacts in both towers. Since they appear to be torn apart, Gage and AE911Truth wonder how it could then transmit these horrible forces to the structure below. This is the opposite mistake that they have made in the story problems above. Now they are comparing Structure against Structure. Just because the structure above is torn apart doesn't mean that the mass disappears. Except for the minority of beams that ricocheted away from the collapse, the mass was still right there, crashing down and destroying everything in its path.

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