Analysis of Skyscraper Structural Design

The perimeter tube design comes to life when you rearrange evenly distributed

columns in a building to a tube-in-tube arrangement.

These office spaces were incredibly elegant, convenient, and

spacious, with no irritating columns inside the

office areas.

This was the sole reason why the engineers chose this unique perimeter tube

design for both skyscrapers. To truly appreciate this

choice, let's quickly compare the design of these

offices with those found in the

iconic Empire State Building. As you can see, the gigantic

columns here eat a lot of useful office floor area.

Connecting the external columns and core at a high span of sixty feet

was only possible by a truss arrangement. The beams would have sagged.

To facilitate the construction process, they prefabricated parts of the

floor. They then clubbed some of the trusses

and secured them together

by placing a steel plate over them.

The trusses are connected with the columns in a rather unique fashion.

These truss connections were made possible via these tiny

and crucial steel

seats, the main culprit of the collapse

and least mentioned in the reports

offered by the government. Let's take a closer look at it.

The truss' upper cord is rested on this inverse L-shaped steel

piece.

This seat is then welded to the spandrel plate via these tiny standoff

plates. Keep these connection details in mind,

as they played a

crucial role during the collapse of the towers, as we'll learn later.

As the hijacked jetliner crashed into the North Tower, it ruptured the maze of

perimeter columns for up to six to eight floors.

This crash also damaged about 60% of the core columns.

The elevators and stairwells were damaged too, trapping around

1,600 people above the tower.

This impactful hit of the Boeing jetliners could have led to a tower

collapse like this, but it didn't.

Thanks to the strong foundation of this building and also due to thousands of

these

tiny viscoelastic dampers in the building.

These rubber-based dampers absorbed some impact force.

In fact, these dampers provided the towers with

incredible flexibility,

such that on a windy day, they could sway up to 12

inches. The catalyst of the collapse was the huge increase in

temperature following the crash.

A large amount of jet fuel burned vigorously on those six to eight floors,

causing a spike in temperature. If the trusses lacked the protective

thermal

coating we discussed earlier in the video, the tower would likely have collapsed in

as little as ten to 15 minutes. However, thanks to the

thermal insulation spray, this was delayed for over an hour.

Burning fuel caused the heat to reach a whopping 1,000 degrees Celsius.

Although steel does not melt at 1,000 degrees Celsius, it does surely

soften and would have caused the truss to sag, as shown.

This means that the weight of the sagging concrete floor would dent the perimeter

columns inside. This is due to the force which

is acting inward.

Remember, the perimeter tubes are also weakened due to the temperature

increase.

When an axial force passes through a perimeter tube design with a dent,

it just collapses at the region of the dent.

This is a clear case of buckling. You've probably all experienced or at

least seen buckling. Buckling occurs when a compressive load acts

axially on a slender beam. Because of the dent, buckling happened at

a much lower load. Can the strong central core save the

top floors from falling down? Unfortunately, no, it

can't. We've already seen how the floors

are weakly connected to the core.

Moreover, due to the high temperature, these connections were further weakened.

During the collapse of the floors due to the dent, the core's support would simply

break. With almost no connection to the core,

the result is clear:

the collapse of the entire top floors downwards.

The mass of the top 15 floors was equivalent to the Titanic,

coming in at a mind-blowing 38,000 tons.

In addition, the hat truss and the Boeing were also falling down.

The sudden impact force of this huge weight over the floor right below it

trashed the lower floors. The perimeter tubes also almost

immediately buckled, since the impact force was huge

and compressive in

nature. This caused an unfortunate chain reaction,

and one by

one, the floors kept crashing down.

The weight also kept increasing as the building collapsed.

When viewing footage of the tragedy, you must have surely seen the shredding of

the

perimeter columns during the collapse. This was due to buckling.

The North Tower came down in 13 seconds, and the South Tower

took just 11 seconds.