The Fall of WorId Trade Centre I. Flashback of the disaster A. The twin towers

The Fall of World Trade Centre
   Well, my topic today is Architecture, and to be specific, what I am going to talk about is World Trade Centre — the skyscrapers.
   After watching the twin towers of the World Trade Center collapse, twice reducing 110 stories to five in less than a minute, it’s hard to see how the buildings could have ever been considered safe. It’s even harder to see how engineers can look at the events of September 11, 2001, as a relative success in terms of architectural strength. Consider this: the rubble at the site weighed a million tons. The site spanned a vast 16 acres. The nonstop cleanup, conducted day and night, still took the better part of a year. Even if the greatest loss — so many human lives — is excluded, there’s no arguing: this was, in all senses, an enormous disaster.
   And yet, for all the destruction — and for all the disbelief. Americans felt as they watched the towers collapse — engineers unanimously say that the buildings performed "magnificently" after terrorists crashed a plane into the side of each. Because the towers stayed standing for about an hour each, they gave rescue workers enough time to get tens of thousands of people out. The towers owed their resilience largely to their architect, Minoru Yamasaki and structural engineer Leslie Robertson. The men’s original design enabled the buildings to withstand the impact of the planes, only after jet-fuel fires weakened their steel beams.
   Constructed with cheaper methods, many other buildings would have collapsed immediately. Today’s most common skyscraper design places vertical support beams throughout a building. Others have diagonal beams as their means of support. But the World Trade Center was built before those were the standards. Raised in the 1960’s, the glass-and-steel towers were designed and built with innovation and special attention to safety and stability because of their extreme height. Engineers wanted to make sure the towers could withstand hurricane-force winds of up to 200 mph — on an especially windy day, the towers could sway up to several feet in either direction. With memories of a 1945 plane crash into the Empire State Building lurking in their minds, engineers and architects also designed the towers to withstand the force of a Boeing 707.
   There are several reasons to make the tower magnificent. Firstly, the engineers agreed that the secret to ensuring the buildings would stand tall under extreme circumstances rested in their supporting steel columns, which were set up differently from most skyscrapers’. Secondly, the majority of the beams framed the outside walls of the towers: 61 beams per 211-foot side provided maximum reinforcement. Engineers often refer to this as a solid "steel tube" design, and rightly so compared to most buildings, which may have vertical support columns spaced as far as 20 feet apart. Thirdly, the outer columns helped the buildings withstand a huge wind load, but another smaller group of steel beams shot through the centers of the buildings and bore the main load of supporting the towers. This "core", which housed elevator shafts and stairwells, was connected to the outer steel columns by still more steel running through each concrete floor. The "double support" may have been what ultimately kept the buildings standing for so long.
   What actually caused the twin towers to fall? All the same, no amount of reinforcement could have prevented the towers’ eventual collapse. Immediately after September 11, most engineers thought the fires, and not the planes’ impacts, were to blame. That theory has generally held up. A federal investigation was inconclusive, but it did conform with engineers’ suspicions. The steel columns didn’t actually melt. But as the fires inside the buildings consumed more and more jet fuel — about 24, 000 gallons of it — they overwhelmed the buildings’ sprinkler systems, started to weaken the concrete insulation around the steel, and then deliberated the steel itself. Most of the fluffy fireproofing material on the towers’ steel columns was blasted off by the plane crashes. With flames reaching temperatures of 1, 500 degrees Fahrenheit and higher, the steel columns started to warp, and the top floors buckled. From then on, the towers had no more chance of standing than a building targeted for demolition. Their collapse was the first known instance of steel buildings collapsing because of fire alone.
   In fact, the collapse played out just like a controlled demolition. To destroy a building, experts usually place explosives not just on the lowest three floors, but also on higher floors. With the top floors exploding, demolition experts can create what they call a "pancake effect", which you might realize is the same as a downward domino-effect. Unlike dominos, of course, the towers fell straight down rather than sideways. But their downfall was caused by the same forces that knock a line of dominos on their faces: Floors on the top gave in, crashing through the floors below them. As each concrete floor fell, it added force to the collapse — making it harder for each floor to hold up against the increasing pressure. Once the top floors gave way, the rest of the catastrophe was inevitable.
   To be exact, many lives lost, but many saved. If the towers were never going to stay standing, why all the extra support? Was it futile after all? Suppose for a minute that the towers hadn’t been built with all that reinforcement — that they’d collapsed a few minutes after being struck. The rubble at the side still would weigh a million tons. The site still would have taken as long. But how many more people would have died? It’s a figure no one wants to calculate. Thankfully, because a team of engineers and architects made the Trade Center as safe as it could possibly be.
   To sum up, I first recalled the disastrous episode of the fall of the World Trade Centre, then I explained how extraordinary the design is. After that, I summarized the reasons of its catastrophic collapse. That’s it for today.