Developers of the new World Trade Center learned a slew of lessons from the numerous attacks and eventual collapse of the original buildings -- re-examining every aspect of the security and safety systems. Here's what they've figured out.
A Befitting Pedestal
The September 11 attacks were not the first successful assault on the complex. In 1993, a Ryder truck laden with 680kg of explosives and driven by Ramzi Yousef exploded in the North Tower's underground garage, killing six, opening a 30m wide gash in the building's base, and threatening the structural integrity of the Bathtub.
That scenario is extremely unlikely to occur again. Mostly on account of the building's new location 30m from the edge of the street (as opposed to the previous building's 8m setback from a state highway near the Hudson River) and ultra-strong reinforced concrete and 64-tonne steel beam base that the new WTC tower sits upon. The windowless pedestal is 60m by 60m, the same measurements of the original WTC, rising to a height of 57m and is designed to absorb and deflect the blast from even the largest vehicular bombs. Designers plan to cover this impenetrable pill box with a series of 2000, 4m tall glass prisms coated in the same plastic as employed in automotive safety glass and, though it won't allow much light in -- due to the massive concrete wall behind them -- the prisms should create an eye-catching refraction pattern.
Above this solid base rises a robust column of steel moment framing with multiple levels of redundancy -- beams and columns welded and bolted together and designed, according to the Lower Manhattan Development Corporation,
to resist lateral loads through bending of the frame elements. Paired with a concrete-core shear wall, the moment frame lends substantial rigidity and redundancy to the overall building structure while providing column-free interior spans for maximum flexibility.
This will help prevent the Total Progressive Collapse that caused the first Towers to fail. When Flight 11 struck the North Tower, the impact blew off the building's installed fireproofing and dumped 58,000kg of burning fuel into the building. This massive flaming load super-heated the WTC's steel support columns -- up to 815C -- which softened them to the point of buckling. The top floors then collapsed downward, drawing the exterior columns inward and smashing the floors below before being themselves destroyed as they struck the 816,000kg debris pile.
In all, one WTC will utilise more than 40,000 tonnes of structural steel -- as much as 22,500 Lincoln Town Cars, double what was used in the Chrysler Building -- and over 200,000 cubic yards of concrete like New York has never seen. According to Eduardo del Valle, Director of Design Management for One World Trade Center, it's "stronger than any concrete engineered in New York City -- ever".
Vehicle Security Center
Another reason that ground-based attacks against the complex are significantly less likely to meet with success is the The World Trade Center Vehicular Security Center and Tour Bus Parking Facility (VSC). This subterranian state-of-the-art facility will screen every vehicle -- every car truck and bus -- entering the underground parking complex for traditional as well as radioactive explosives. The facility's foundation utilises 9700 tonnes of structural steel reinforcing bars -- that's one-tenth of the total weight of the Eiffel Tower. Once through screening, vehicles will be able to access a network of underground roadways that serve all seven towers within the WTC site.
The pedestrian entrance process will be no less stringent. Visitors to both the WTC buildings and the 9/11 Memorial will first pass through airport-style metal detectors and X-ray machines and have every bag, briefcase and delivered parcel screened. Museum patrons won't even be allowed into the central plaza without passing security. In addition, the WTC building "visitors will be issued a photo ID for the day or for a limited amount of hours and the turnstiles will respond to those ID's only for that time frame in which card access is allowed," explained del Valle.
Occupants of the towers -- those that work there daily -- will receive CLEAR card-like trusted security badges that will allow them to bypass the normal screening lines. What's awesome is that as the worker passes through the security turnstile, a proximity chip in the ID card activates the Destination Dispatch System that controls elevator service. "So, if you work on the 38th floor, it will automatically tell you which elevator to go to that will take you directly to the 38th floor." del Valle said, "It's a security feature so that people who are in the building for a specific purpose get there only." Stair access will be restricted as well and record the time of entry, duration, and exit floor of workers who use them.
Watching over this process will be an undisclosed number of the 670 NYPD officers who make up the Lower Manhattan force -- more than in any other district in the city and more than the entire city of Oakland, CA. More than 400 closed-circuit surveillance cameras positioned in and around the complex will provide live feeds to a 24-hour monitoring command centre located nearby. The feeds will also be monitored by video analytic software that is designed to recognise suspicious people, activity and unattended baggage.
Security has been tight even during construction. At every entrance to the site, every worker must present his social security card for computer verification and have his iris scanned for verification. The Port Authority maintains 24-hour video surveillance of the site as well.
The lifts of the new WTC are as numerous as they are secure -- with 10 lifts servicing the Observation Deck and Restaurant alone. According to del Valle, the Destination Dispatch System works as such,
Basically, the elevator operation -- the individual cabs -- are controlled by software that determines very quickly -- within seconds -- which cab is most efficient to serve a particular call based on other calls that have been registered by the system at the turnstiles. So really it's the turnstile that sends the signal to the destination dispatch software and that then manages -- digitally-the elevator that is assigned to a group of people or you individually, depending on your destination. That maximizes the service capacity of the elevators and minimizes the use of electricity and power so that elevators are not dispatched at random or by the demand of a single caller
Every lift in the WTC is protected within the central core structure, which is basically a vertical concrete bunker. The Rebar reinforcements are as thick as a man's forearm, spaced very closely together and filled in with the same super-strong concrete as the building's base. In addition, the entire MEP system (mechanical, electrical, plumbing) as well as the life and safety systems is housed there as well. "So, if there was any event that compromises the ring, none of the critical systems will be compromised," del Valle concluded.
Even the stairs are designed in excess of New York building codes and built to prevent the issues presented by the 9/11 attacks. They're 50 per cent wider than required, which not only allows more people to exit down them at the same time, they also allow for people in wheelchairs to be transported down them as well. In addition, at every landing of the stairway, there is an "area of refuge" where a wheelchair-bound person could wait to be rescued by first responders or assigned building personnel if they cannot otherwise leave the building. What's more, the stairwells are internally pressurised to keep smoke out of the stairwells and will feature low level emergency lighting -- surprisingly not present in the original Tower -- as well as concrete protection for all sprinklers and emergency risers. Every stairwell is interconnected to provide redundancy and exits directly to the street.
First Responder Systems
The new World Trade Center is built with first responders in mind and addresses the problems present in the original Tower -- namely, NYPD and FDNY radios did not communicate with one another and the FDNY radios had poor reception within the structure, hindering rescue efforts and preventing warnings of an imminent collapse to reach those on the inside. Now, a series of antennae and repeaters will run the full height of the building -- from the basement to the 104th floor, providing a vital means of communication.
There's also a dedicated first responder stairwell so that if, say, the public stairs are inaccessible rescue crews can still reach the upper floors. And, for those responders that don't want to climb 104 flights, a dedicated, waterproof and fire-proof lift will allow access to the full height of the structure.
In the event of a fire, the building features water tanks with double the capacity required by NY building code -- again, for the sake of redundancy -- and the sprinklers and emergency risers are all protected by concrete sheaths. The WTC is even equipped with chemical filters in the air supply system in the event of a biological attack.
Obviously, no amount of technology can ever completely secure a building 100 per cent against external threats -- especially against a bunch of psychotic zealots who are willing to hijack planes and kill themselves in order to make a point, but the new World Trade Center complex has been designed with virtually every contingency in mind. So, while the new WTC remains squarely in the cross-hairs, it's become an incredibly hard target.
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A civilisation can distinguish itself by how well it responds to disaster, and 10 years later, 9/11 is as much a story about recovery and rebuilding as it a story of terrible loss and tragedy. As a nation, our political and economic response has been imperfect -- possibly even dead wrong -- but we're focusing on the mechanical marvels that have helped us bounce back.