September 11, 2001, changed our lives.
A lot.
Political differences seemed to disappear, at least briefly.
New security measures filled the world’s airports.
Thousands of families mourned the loss of loved ones.
And, of course, the New York City skyline lacked its two tallest buildings.
But there’s one change that you probably didn’t notice, because it took place inside
new high-rise buildings.
And it’s probably not what you'd expect.
Before we dive in, we want to acknowledge that this is perhaps the most analyzed event
in modern American history. It was a tragedy.
But it occurred nearly 20 years ago, meaning a significant portion of the American
population doesn’t have clear memories, if any, of what happened.
In order to understand what changed we first need to outline how the towers were originally designed.
The plans for a World Trade Center date back to 1946, but those were for a single 70-story
building - pretty different from the Twin Towers.
The twins were designed by Japanese-American architect Minoru Yamasaki after his firm won
the commission in 1962 - a renowned job that landed him the cover of TIME Magazine.
On April 4, 1973, the completed towers debuted as the tallest in the world.
But their height wasn’t their only remarkable feature.
The design- structural design of the towers, and for that matter, the architectural design
was quite unique.
This is Ronald Hamburger.
He was one of a few structural engineers recruited to do a postmortem on the Twin Towers after
they collapsed.
He explained that earlier skyscrapers were built with a mixture of masonry and steel
frames with vertical columns spread throughout the building.
Picture the Empire State Building - practically a fortress, with 210 vertical columns throughout
the building, and virtually no column-free spaces.
The Twin Towers, on the other hand, were basically “steel tubes.”
Steel columns lined the exterior perimeter, connecting to the core of the building via
the floors.
The floors themselves were made of concrete poured on a steel frame - note that this concrete
wasn’t providing vertical stability. Just flooring.
That was the work of the core’s 47 steel columns braced with sheetrock.
Finally, also in the core were all of the buildings’ elevators, stairwells, and utility
shafts.
And the reason they did that was...so that you could have this massive space of open
floor space where you can put some desks and workstations that would not be interrupted
by columns, Each building was about 95% air - so light
that they swayed in a strong wind.
Now, a super-tall lightweight building may sound risky in retrospect.
But the towers were actually quite strong.
The design was highly redundant.
Structural engineers talk about redundancy much like a person wearing both a belt and
suspenders.
Either one of them can hold up your pants, but if one of them fails, you have the other
present that can do the job.
And this building was highly redundant.
Additionally, both buildings were designed with the prospect of an airplane impact in
mind.
See, in 1945, a B-52 accidentally crashed into the Empire State Building on a foggy
day.
15 years later, two planes crashed into each other above the city, raining debris over
Staten Island and Brooklyn.
So designers were aware that aircraft crashing into buildings could happen.
The World Trade Center towers were designed for the state of the art aircraft of its day,
which was a Boeing 707.
But designing a building to resist a 1960s-era plane getting lost in the fog isn’t the
same as designing one to resist a larger plane being steered at top speed to intentionally
cause damage.
Near major airports jetliners are limited in speed to 180 miles an hour.
The aircraft that went into the Twin Towers were traveling in excess of 400 miles per
hour. I don’t think anyone thought that was a credible event prior to the World Trade Center.
The impact damaged both the outer shell and the core.
But both towers kept standing because of redundancy - the weight shifted away to unaffected columns.
But here’s the problem: the impact had scraped the fire-resistant coating off the steel columns
and beams, and the jet fuel had ignited a raging fire.
This heated the steel to an unsustainable temperature.
The jet fuel burned itself off in a few minutes But what it did is it ignited the contents
of the building - the desks, the chairs, the carpeting, the wall coverings, the paper files.
The heat of the fires could not and did not melt the steel.
But it’s not actually necessary to melt steel to make the buildings come down…
When you reheat steel to about 500 degrees Fahrenheit, it starts to lose some of its
strength and stiffness.
Eventually, the floors above the impacted areas became too heavy for the weakened steel
to support, and both buildings collapsed.
You might think this tragedy would have compelled us to make drastic changes to the rules of
how we build skyscrapers.
And we have in some ways we’ll get to in a minute.
But structurally...
Building code requirements have not changed a lot.
The people who had a vote, as a body, felt that the added
cost of the measures that were proposed did not make sense, given what was perceived to
be...the extreme rarity of such an attack.
The years since 2000 have seen a rise in supertall buildings around the world - nearly 9,000
skyscrapers were added from 2000-2020.
And even though resisting aircraft impact isn’t a requirement, many of them are much
stronger anyway because of different designs and materials.
Rather than using only steel construction.
Now most high-rise buildings are constructed using concrete walls in the
core of the building.
And then steel framing around the perimeter.
Take the new World Trade Center, for instance.
It’s basically a hefty three-foot thick concrete core with a glass skin.
And this wasn’t done to provide structural robustness, it was done because it was found
this was a more economical way to build these structures.
Still, the end result is a stronger building.
This is because concrete is far more fire-resistant than steel, but it’s also because concrete
itself has become much stronger.
Traditional concrete, conventional concrete we use all the time has a 4,000 psi
Quick pause - psi stands for “pounds per square inch.”
It refers to the total weight the concrete can support before failing.
The concrete used in the floors of the Twin Towers ranged from 3,000-4,000 psi.
But that pales in comparison to the strength of newer concrete we use today.
The core of the new 1 World Trade Center, for example, uses concrete ranging in strength
from 8,000 to 14,000 psi.
And that’s not even the strongest concrete that exists!
This is a relatively new type of concrete called ultra high performance concrete.
The high-strength concrete is roughly from 15,000 psi to 30,000 psi
So greater safety protocols and stronger materials have together created a wave of robust new
skyscrapers - even if protecting against future airplane hijackings isn’t explicitly required
But Hamburger told us that some of the world’s newest high-rise buildings have been constructed
with measures to protect against terrorist attacks.
No one will tell you which buildings...No one will tell you what weapon they’ve been
designed to defend against, but some of them have been voluntarily designed to be better
able to resist such events.
Karin: do...do you know which ones?
I know some of them.
I don’t know all of them, and I’m not talking [laughter]
Post-9/11 code changes actually revolved less around structural choices and more around
“means of egress.”
In layman’s terms: exits.
The designer of the World Trade Center used a system that are called scissor stairs...the
stairs were fairly close to each other within the core.
So when the aircraft went into the building, they managed to block both sets of stairs
- meaning that, even though people trapped inside theoretically had time to escape, they
couldn’t.
So one of the most significant things we’ve done is we’ve changed the building code
to require more separation between the places where stairways are located, so that it is
more probable that...there will be at least one stairway available.
Additional code changes included widening stairways; self-luminous exit pathways; third
stairways in buildings over 420 feet; boosting overall fire resistance; and more.
These changes reflect less of a focus on saving the buildings, and more on making sure the
people inside have time to get out in an emergency.