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.
Ronald Hamburger?
I mean Please. You can't name your kids that, guys.
"In 1945 a B-52 accidentally crashed into the Empire State Building." LOL Talk about lazy researching.