3D model of a modern high-rise building

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The idea of building a tower to
touch the sky goes back thousands of years. And within the past century, architects and engineers have designed seemingly impossible structures that stand a quarter-mile high — a tribute to humanity’s need to test the limits, as well as a way to alleviate congestion in crowded cities. But after terrorists crashed two hijacked passenger planes into New York City’s tallest buildings on September 11, leveling both of the Twin Towers, iconic skyscrapers around the world suddenly gained a new label: target.

That shift has inspired builders with a new goal-creating the safest tall building in the world. In the aftermath of the recent tragedy, the brightest minds in architecture and structural engineering are pooling their expertise to figure out how a building can survive the newest forms of terrorism. Will novel kinds of concrete be able to withstand bomb blasts and the 2,000-degree temperatures of a jet fuel fire? Can sensitive laser detectors enable a building’s emergency staff to locate poisonous chemical agents before they even reach the building? Is it possible to create a fireproof evacuation system that will enable people on the top floors to descend to safety when the middle of the building is in flames? The answers to these questions and many others may be coming soon to a skyscraper near you.


Builders are exploring new ways to
make office workers feel safe. Options include protective steel plating on the building’s facade, blastproof escape routes, safety floors where people can wait out a fire, and laser-based
devices that can identify dangerous chemicals.

1. Refuge areas: Located at 15-floor intervals, these concrete-reinforced bunkers have high heat resistance.

2. Bombproof elevator shafts: These will enable firefighters to quickly reach the problem area.

3. Pressurized stairwells: Located within the building’s concrete core, these fireproof stairwells connect to the refuge areas and provide a smoke-free escape route.

4. Emergency command center: The building’s security facility is located on a floor above the lobby where it is less vulnerable to car bombs.

5. Concrete core: A central vertical column of concrete suppots the building’s weight and provides a fireproof shell for emergency stairs and elevators.

Concrete-encased steel columns: Concrete’s heat resistance will delay the melting of structural columns during a fire.

Exterior steel plating: Covering a building’s concrete facade with steel plates will help deflect a high-speed impact.

Wireless fire alarms: Sensors on each floor are connected independently to the emergency command center.

Poison and explosives detectors: A laser spectrometer continuously analyzes air samples and alerts security staff to chemical dangers.

Sprinklers: High-tech sprinklers emit a mist that extinguishes fires without damaging sensitive equipment.


Though the World Trade Center towers ultimately fell, they each withstood a massive impact and remained standing for an hour or more — time that enabled thousands of people to flee to safety.
Part of the reason the towers held up was their immense width, 208 feet. That girth meant that some of the outer steel columns remained in place even after the jets lodged inside the buildings. The severely damaged tops of the towers leaned on those remaining columns for support. If the ensuing fire hadn’t weakened those columns, the buildings would most likely be standing today.

“It’s time for people to stop asking what went wrong at the World Trade Center, and to start asking about what went right,” says Ron Klemencic, president of Skilling Ward Magnusson Barkshire, a Seattle engineering firm, and chairman of the Council on Tall Buildings and Urban Habitat.

But despite the World Trade Center’s solidity under attack, many engineers believe there’s room for improvement. In the early 1970s, when the Twin Towers rose to the top of the New York City skyline, any building more than 20 stories tall was made primarily of structural steel. Back then, even the strongest concrete would never have been able to support the weight of anything higher, let alone skyscrapers that stood 104 stories above the ground. Today, however, builders have at their disposal concrete that’s 10 times sturdier — its every square inch can withstand 10,000 pounds of pressure before crumbling.

Innovative kinds of concrete lend further structural benefits. For example, one new concrete contains recycled stainless steel fibers. The fibers increase the concrete’s strength and its ability to absorb energy. In case of a building-shattering catastrophe, this type of concrete would cling together in larger chunks. The better the concrete stays together, the less likely it is to break from the structure and plunge down onto pedestrians below. Steel-mesh concrete was pioneered as an earthquake-proofing technique, but it could also protect against the lateral force of an airplane crash.

Concrete is far more heat-resistant than steel; it’s also less flexible, which makes it relatively blast-proof. As a result, engineers say, encasing a skyscraper’s steel beams in concrete will better protect the building’s frame from intense heat and explosions.

In addition to fire protection, concrete provides stability. Whereas the World Trade Center towers derived their structural support from the steel columns around their perimeters, many engineers now endorse a skyscraper blueprint that features a concrete core that runs down the center of the building. Such a core serves as a building’s spine, supporting its weight. It can also serve as a safe haven within which designers can place emergency escape routes: fireproof elevators and stairwells.
A case in point is the Petronas Towers in Malaysia, twin buildings that opened in 1998 and that at 1,242 feet are the tallest in the world. Each tower possesses a vertical inner core, 41 feet in diameter, which houses the building’s exit routes, wrapped around a smaller concrete cylinder.

Anyone who remembers reading survivors’ accounts of escaping down the World Trade Center stairwells, only to encounter exhausted firefighters making their way up, will understand the utility of a fireproof emergency elevator, accessible to rescue workers. “The idea of building a blast-proof elevator shaft so that emergency personnel can get to a fire quickly isn’t new,” says Arthur E. Cote, senior vice president and chief engineer of the National Fire Protection Association, which establishes fire codes for skyscrapers. In light of recent events, though, he says, “I’m sure it will receive a lot more attention.”

Meanwhile, at the University of California at Berkeley, structural engineer Abolhassan Astaneh-Asl is experimenting with bolting half-inch-thick steel plates to the outer walls of concrete buildings. Though Astaneh-Asl’s work aims to lessen earthquake damage, many engineers believe such a facade could have absorbed the enormous energy of the recent airplane crashes, preventing a jet fuel fire from igniting inside.

The balance between building a secure structure and one that’s inviting is always tenuous at best. Creating a skyscraper that’s a beacon of safety is expensive, for one thing, and it also tends to destroy the allure of prime real estate. Redundant columns block luxurious views. “We could build fortresses,” says Klemencic, “but would people want to work in them and would builders want to pay for them? I’m not so sure.”

One need only to look at a structure designed to withstand a 767 crash — the nuclear power plant — to understand the aesthetic argument. Nevertheless, in today’s climate, tenants will be unlikely to sign leases for high floors without some assurance that the building is as secure as possible. “The commercial world will look into this more seriously,” Klemencic says, “and they’ll begin to use safety to enhance their marketing position, quite frankly.”


When a truck bomb went off in the basement of the Trade Center during a terrorist attack in 1993, the explosion knocked out much of the power to the building’s emergency operations center, which was located beneath the ground floor. After that, the security center was relocated to a higher floor — a sensible move, according to many engineers, who deplore the fact that the security headquarters of most office buildings are in the lobby. “You can drive a car full of explosives into the lobby, or a suicide bomber can walk in,” says Klemencic. “If you lose that command center, you lose your ability to operate the building and communicate with people.”

That is, of course, provided that everyone can hear you. Most alarm and loudspeaker systems are connected to the emergency operations center by cables that run vertically through the building. A fire or other catastrophe can sever those cables, causing the command station to lose contact with all the floors above the area of concern. A solution already in place in some new buildings is to equip each floor with a wireless remote fire alarm. The alarm has a sensor that provides two kinds of information to the command center: whether there is fire on the floor, and whether the sensor is in working order. Those remote signals are transmitted continuously, so any interruption should trigger an immediate response. “These devices are so sophisticated, they can even cut down on the prospects of false alarms,” says Frank Carista, an electrical engineer for Turner Construction.

In the event that people are unable to get out, other measures are being explored. In China, “areas of refuge” are popular with many builders. Every 15 floors or so, a floor or portion thereof is constructed of concrete slabs and designated as a place where occupants can go to wait out a fire. These areas are equipped with mechanized ventilation systems that clear the air of smoke. They are also connected to pressurized stairwells, should people need to escape a potential collapse. “The idea of building an area of refuge is particularly helpful to people who are handicapped or not in good physical condition,” says James Milke, a fire protection engineer at the University of Maryland. “The prospect of walking eight floors is a lot less daunting than walking 40 or 50 floors.”

Moreover, high-tech systems are in the works that would enable security guards to detect a planned attack on the building before it even happens. A device known as a laser spectrometer can sniff out explosive chemicals or poison gas within a mile of a building, depending on the wind direction. It can be placed in an innocuous-looking box about the size of a fax machine and mounted on the outside wall of a building.

A pump inside the box continually sucks in gas samples from the ambient air. Those samples are fed into a chamber where they are exposed to infrared laser light. Every chemical possesses a unique fingerprint, based on the way its molecules behave on the light absorption spectrum. The spectrometer’s sensors are set to recognize the specific frequencies of suspect chemicals, including the ones most commonly used in bombs. If such a frequency is detected, an alarm is tripped, alerting emergency personnel.

If a laser spectrometer had been installed at the federal building that was attacked in Oklahoma City in 1995, the device would have been capable of detecting the ammonia compounds in Timothy McVeigh’s truck bomb. “We already have a device that can detect ammonia at 100 times below the odor threshold,” says Patrick McCann, president and CEO of Ekips Technologies in Norman, Oklahoma. “We’ll soon be able to create laser detectors that can test for a dozen chemicals or more.”


Within hours of the collapse, the debate began over what — if anything — should be built to replace the World Trade Center. Some suggested rebuilding exact replicas of the original towers — a signal to terrorists that the city’s spirit is intact. Others suggested creating a park as a memorial to the attack’s victims. The World Trade Center’s current manager, Larry Silverstein, has suggested erecting something new: a cluster of four 50-story buildings. Such a plan would confirm the old adage that there is safety in numbers. Grouping the buildings around a central courtyard would protect the sides of the buildings that face inward from an airplane attack — and cut the cost of facade — strengthening measures in half.

But before any plan can be put into action, the painstaking and precarious process of clearing rubble from the site must be completed. Like players in a life-or-death Jenga game, the cleanup workers are carefully removing twisted metal and debris from the six floors below the towers, hoping their actions won’t cause the entire foundation to collapse.

The six floors beneath the Twin Towers — which contained shops and restaurants, a parking garage, the subway, and PATH trains — were surrounded by an underground retaining wall. Known to engineers as the bathtub, the wall kept out the surrounding soil as well as water from the Hudson River, which flows just a few blocks away. When the World Trade Center was intact, steel beams in the floors of the subterranean levels acted as struts, propping the bathtub from within so it would stay upright. The question now being asked by forensic engineers is whether the role of those struts has been usurped by the rubble that fills the bathtub. If that’s the case, then the cleanup effort could end up causing the underground retaining wall to collapse — flooding the site and endangering workers.

But Dan Hahn of Mueser Rutledge Consulting Engineers is confident it’s basically intact. “We check the retaining walls for water constantly,” says Hahn, whose firm is a member of the city-appointed task force charged with ascertaining the stability of the World Trade Center’s foundations. “As long as the walls are dry, we’re doing fine.”

Another challenge is the damage to the train tunnels that run beneath the World Trade Center complex. The Cortlandt Street station, a stop on the IRT subway line, must be rebuilt, as must 1,000 feet of track — a process that could take years. Meanwhile, the PATH train, which connects lower Manhattan with New Jersey, was also significantly damaged. “Between the broken water pipes and the water from the firemen’s hoses, there was water in the PATH tubes as far as New Jersey,” says Hahn. Mueser Rutledge created two concrete plugs, 16 feet in diameter, to seal the PATH tunnels and prevent further flooding. Once leaks have been investigated and the excess water pumped out, the plugs will be removed and tunnel rebuilding will begin.

Given the nature of the attacks, the threat of bioterrorism has been revived. In response, the Federal Transit Authority is developing so-called Urban Chemical Release Detectors, devices that can detect fire, smoke, biological agents, and other hazards within underground tunnels. As many as 10 of the devices could be placed in camouflaged locations around a subway station to alert station personnel of potential peril.

But even if all these technological advancements in building and transportation are implemented, will we be safe? Would concrete cores, steel-plated facades and bomb-sniffing detectors have prevented the tragedy that occurred on September 11th?

Ideally we’ll never know, because there will never again be such an attack. That hope, however, is no reason to skimp on safety measures, says Cote, the chief engineer at the National Fire Protection Association. According to Cote, the public’s attention span for safety concerns is painfully limited. A flurry of improvements often takes place after a tragedy, but all too soon the event recedes in people’s memories and builders begin chipping away at safety measures, complaining that they cost too much. “Sadly,” he says, “almost all of our advances in things like mandatory fire code come about after disasters.”


Within 24 hours of the attacks on the World Trade Center, teams from the Center for Robot-Assisted Search and Rescue, a national group of academic, military, and industrial robotics experts, arrived on the scene.

One team, led by University of South Florida computer scientist Robin Murphy, carried shoebox-size robots in backpacks up the mountain of rubble where the Twin Towers once stood. Five times in eight attempts, the robots succeeded in crawling into spaces beneath the wreckage. The machines located the bodies of five victims.

The robots moved on caterpillar treads, like miniature tanks. All were operated remotely by radio or a cable tether, but the goal is to develop robots that explore autonomously — finding their own way through the rubble.

In addition to locating victims, robots could map disaster areas and deliver emergency supplies. Small and expendable, robots can be sent into areas that people cannot reach or that are too unstable to be explored safely. Even the simple act of climbing atop a rubble pile can be dangerous, so Murphy is developing what she calls a “marsupial” team: a “mother” robot that can carry a smaller “baby” onto a heap of rubble, then deploy the baby to search under the debris.

Will robots replace human rescuers? Definitely not. “The purpose of robots is to do the things people and dogs can’t,” says Murphy. “The true heroes remain the firemen and rescuers. We’re just techno-geeks that help them.”

Paul Beck


The damaged remains of Building 4 of the World Trade Center shake and shimmy as passing trucks and nearby cranes rattle its very foundations. But thanks to a new laser-based motion detector, cleanup crews will have ample warning if the shattered building begins to collapse.

The device, known as a laser Doppler velocimeter, was originally created to detect land mines in third world countries. But on the night of September 11, the U.S. Army rushed the velocimeter and its creator, James Sabatier, to ground zero in lower Manhattan.

Under normal conditions, the building might sway 25 microns — about one-quarter the width of a human hair. But with an armada of heavy machinery removing rubble all around, Sabatier has recorded oscillations in Building 4 of nearly 300 microns — about one-third of a millimeter. Thankfully, that’s still not enough to topple a structure, and none of the metal workers, garbage carters, and rescuers at the site have had to be evacuated so far.

“It takes displacements of a few centimeters for windows to start falling out,” says Sabatier, a research scientist at the U.S. Army Night Vision Electronic Sensors Directorate in Fort Belvoir, Virginia.

Sabatier’s device works by bouncing a laser beam off the building’s facade. As the building sways, it alters the frequency of the reflected beam, much the way a train whistle appears to change pitch as it moves through a station. Conventional survey equipment, like theodolites that measure how far a building leans, are accurate to a few millimeters. But Sabatier’s detector makes continuous real-time measurements down to a few millionths of an inch.

Trevor Thieme


In a breath-stopping scene in the blockbuster movie Entrapment, actors Catherine Zeta-Jones and Sean Connery clamber between two towers, 1,000 feet above the ground. That scene was fantasy, but it was filmed at what is, in real life, the tallest building in the world: the Petronas Towers in Kuala Lumpur, Malaysia.

These twin towers, which opened in early 1997, are a reflection of both Asian and Western influences. Architect Cesar Pelli took his inspiration from geometric shapes in Islamic art, and the scalloped faade is reminiscent of lace — albeit lace made from glass and steel. It took 36,910 tons of steel to build the Petronas Towers. The buildings have 32,000 windows between them; it takes window washers an entire month to wash each tower just once!

Linked by a bridge at the 41st and 42nd floors, the Petronas Towers rise 1,483 feet off the ground, but they are only 88 stories. Thus though they squeak past the Sears Tower in Chicago by 33 feet, the Sears Tower can still claim the highest occupied floor (110 stories). The World Trade Center, which was built in the early 1970s, was the world’s tallest only briefly–until the Sears Tower was erected in 1973. New York loyalists insisted, however, that when the Trade Center’s antenna was included in the measurement, it still retained its world’s-tallest status. Now, of course, that’s a moot point, and all other such wrangling should halt for some time if the Center of India Tower planned for Katangi, India, goes up as planned in 2008. It will be 2,222 feet tall.

Aude Lecrubier


Robotics and the World Trade Centers**

by Angela Palmer

Scientists affiliated with the Center for Robot-Assisted Search and Rescue (CRASAR)
brought robots to the World Trade Center site immediately after the Sept. 11 terrorist attacks.

The robots were sent into the rubble to search for survivors and human remains.

CRASAR’s Web site features still images and videos of the robots at work,
including “bot’s-eye view” footage taken by cameras mounted on the robots
themselves. Following are a few examples. More still images are available here, and *.mpg movies
can be found here.


A robot view onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>into a conduit (one of the horizontal supports for the floors).

A robot view onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>inside the conduit.

A robot view looking onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>out of the conduit. This picture comes from the final video shot before this robot was lost.

A look at onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>Building 7 prior to its collapse.

A robot onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>view of the hole it was entering. This film shows that the area was unsafe for workers to enter.

A picture of onClick=””,’popup1′,’height=440,width=500,scrollbars=no,resize=no’)”
target=”popup1″>one of the robots that captured most of the images and video.


target=”popup1″>Robot view of the conduit. This particular video proved that the area was unsafe for workers who were about to be lowered down — the robot was lost after 40 feet.

target=”popup1″>A robot view of the parking garage about one block from the World Trade Centers.

target=”popup1″>A robot view as it looks for victims. The robot runs into a conduit at the bottom.

target=”popup1″>A robot view during a simulation (inside a bank).

target=”popup1″>Robots performing building reconnaissance. These are larger robots (most of the footage was shot by smaller robots).