September 11, 2001, changed our lives 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 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 fortunately 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.
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.
整栋楼里所有的电梯 楼梯井 公用井
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.
1945年 一架 B-52 飞机在大雾天意外撞上帝国大厦
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’s
For our viewers who are just tuning in right now,
a twin-engine plane or possibly a 737 passenger
jet flying into the World Trade Center
It appears to be still embedded inside the building.
The impact damaged both the outer shell and the core.
But both towers kept standing because of redundancy
their weight was able to shift 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 all the contents of the building
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 that we’ll get to in a minute.
Building code requirements have not changed a lot.
The people who had a vote, 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, supertall buildings, are constructed all using concrete walls in the form
现在大多数高层建筑 超高层建筑 都是用混凝土墙建造的
And then steel framing around the perimeter
and then the floor system between the floor and the exterior wall.
Take the new World Trade Center, for instance.
It’s basically a hefty three-foot thick concrete core
with a glass skin.
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.
So concrete, typical concrete, conventional concrete we use all the time has a strength of 4,000 psi
所以混凝土 典型的混凝土 传统的混凝土 我们一直使用的混凝土强度为4000psi
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 have been constructed with it.
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 won’t talk[laughter]
我知道其中一些 但我并不知道全部 而且我也不会说的［笑声］
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
actually used a system that are called scissor stairs
But stairs were actually very close to each other within the core.
So when the plane 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.
September 11, 2001, changed our lives lot.