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By Jeff Wing
September 11, 2016
On a July night in 1945, a B-25 bomber pilot returning to his training base became disoriented in a thick Manhattan fog and crashed into the Empire State Building between the 78th and 80th floors. 14 people were killed – the three crew and 11 people in the building. Elevator operator Betty Lou Oliver plummeted 75 stories into the building’s subbasement when her elevator cables snapped. She miraculously survived, married, had a happy life and many children. She passed away in 1999, two years before 9/11.
Following the Empire State Building accident, the construction sector did what it does best, moving quickly and expertly to draw lessons from the accident and enact protections for the common benefit. Among several other changes, building codes were thoughtfully but quickly revised to ensure that tall structures could withstand the blow of a Boeing 707 at cruising speed, that day’s largest airplane.
On the morning of September 11, 2001, four airliners were overtaken by hijackers and used as weapons. Two buildings at the southern edge of Manhattan were struck by Boeing 767s, and they fell – a combined 200,000 metric tons of steel and 780,000 metric tons of concrete plummeting straight down, each tower releasing energy equivalent to a 3.5 magnitude earthquake in the course of the rapid collapse. Nearly three thousand people died that morning – office workers, responders, bystanders – with many more tens of thousands of victims to follow in global events the 9/11 attacks precipitated. The World Trade Center construction project had taken 10,000 workers about 6 years to complete. When the towers fell, one came down in 10 seconds, the other in nine.
The immensity of the structural cataclysm of 9/11 inspires its own awe. Engineers and designers took lessons from the behaviors of the towers on that terrible day, and their findings continue to inform regulations and building codes these 15 years later. The Twin Towers featured redundant load-bearing systems so that no single failure would threaten the buildings.
Each tower had 244 columns around a central steel core, and from the 107th floor up, a so-called “hat truss” of horizontal and diagonal I-beams spread the weight distribution even more effectively, using the central steel core as a brace to guarantee structural integrity at that great height.
The World Trade Center towers did indeed absorb and withstand the impact of the 767s that day. The buildings did what they had been carefully and deliberately engineered to do. As various parts of the structures began to weaken, loads were shifted around accordingly.
The volatile jet fuel burned quickly away after impact, the smashed columns of the immediate impact zones redistributing their loads to the undamaged redundant columns, as designed. But the incendiary contents of the office spaces themselves – the carpeting, modular walls, plastic computer housings, the furniture, and the tons and tons of workplace paper – fed a runaway internal firestorm that only grew as it found more fuel, eventually superheating and weakening the redundant steel support columns sufficiently to make them deform, the perimeter supports finally bowing inward until they were unable to support the floors above. Once the upper part of each tower fell 10 feet through a single failed level of supports, the “pancaking” collapse was an engineering inevitability.
As expected, the construction community – our builders, our engineers, our designers – took the hard lessons and rolled them into a new era of design safety. Today’s post 9/11 building codes include more and wider stairways, reinforced elevator shafts, stronger stairway walls. The tallest buildings are equipped with an extra, dedicated set of stairs for emergency responders only, so firemen and other rescuers can race up to where they’re needed in the building without colliding with evacuees trying to get out.
One World Trade Center
One World Trade Center, the elegantly modernist glass and steel tower built to both replace and commemorate the twin towers, has at its base a decoratively disguised 187- foot concrete blast wall. Inside, 3-foot thick reinforced concrete walls wrap the stairwells and elevator shafts, and powerful, redundant sprinkler systems guarantee a doused fire if or when that occurs. The stairwells in One World Trade Center are extra wide to accommodate fleeing crowds, and are specially pressurized so they can also provide breathable refuge in the event of an interior toxic event or fire. There are biological and chemical air filters throughout the building’s ventilation system, and the west-facing windows are made of a specially tempered blast-resistant plastic.
What we know now about safer, more secure construction was learned at great expense. It could be said that the whole of our built environment today stands on the shoulders of those original Twin Towers, the men and women who built them, and those who perished the day they were struck. The clarity of purpose and collaboration that grew from the ruins of that terrible day, now 15 Septembers past, still drives our robust construction industry. Glass and steel, optimism and ingenuity – these are construction’s stock in trade, now more than ever.
Today's challenge may be to bring to our common social structures something of the passion, innovation, and dedication that define the construction expertise. In the wake of 9/11 there’s a lot of building yet to be done, of every kind. From this September forward, let’s make sure the coming generation of builders have all the tools they need to start patching up the remaining fissures that still threaten our shared foundation.
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