. A lot. Political differences seemed to disappear. At least briefly. New security measures filled the worlds airports., Thousands of families, mourned the loss of loved ones. And, of course, the New York City skyline lacked its two tallest buildings., But theres. One change that you probably didnt notice because it took place inside new high rise. Buildings. And its probably not what youd 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. Doesnt 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 wasnt. 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 wasnt providing vertical stability. Just flooring.. That was the work of the cores 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 isnt 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 dont 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 heres 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 its 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 well 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 9000 skyscrapers were added from 2000 2020. And even though resisting aircraft impact isnt 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. Its basically a hefty three foot thick concrete core with a glass skin., And this wasnt 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 its also because concrete itself has become much stronger.. Traditional concrete, conventional, concrete we use all the time has a 4000 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 3000 4000 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 8000 to 14000. Psi. And thats, 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 15000 psi to 30000 psi, So greater safety protocols and stronger materials have together created a wave of robust new skyscrapers, even if protecting against future airplane hijackings isnt explicitly required. But Hamburger told us that some of the worlds 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 theyve 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 dont know all of them and Im not talking laughter Post 911 code changes actually revolved less around structural choices and more around means of egress. In laymans 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 couldnt.. So one of the most significant things weve done is weve 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..