This article focuses on engineers who are making unique contributions behind the headlines of the World Trade Center disaster. To do the job, engineers at the site had been using technology created by distant colleagues. Excavators, specialized vehicles, and robots are navigating the area, and teams of workers and technologists are slowly removing the concrete and steel shards of the fallen buildings. Robots moved on crawler tracks to carry video and infrared cameras into the ruins. Some worked at the ends of tethers as they dragged power and communications links behind them. High-flashpoint fuels are being discussed as a way of mitigating the damage if all else fails. Additives, for instance, could preclude violent explosions, but they are also likely to make the fuels less efficient. There are also highly sensitive sensors that can detect minute quantities of substances such as explosives, and improved 3D luggage-screening technologies to help airport personnel interpret what they see.


Before the initial shock wore off, engineers were responding to the attacks on Washington and New York. They were coming to rescue the injured and to treat the wounds left in the landscape by unprecedented acts of malice.

A section of the Pentagon had been destroyed by the intentional crashing of a hijacked commercial airliner. Two others were diverted to deliver greater destruction to New York City. A fourth plane, taken by hijackers whose destination will remain unknown, crashed in Pennsylvania.

The place where the World Trade Center stood in New York has become an excavation site filled with rubble that stands six stories high. More than 5,000 died there, including 300 rescuers and firefighters who rushed into the buildings before they collapsed.

The challenges of moving around the area are formidable, the challenges of clearing the monumental wreckage even more so.

To do the job, engineers at the site are using technology created by distant colleagues. Excavators, specialized vehicles, and robots are navigating the area, and teams of workers and technologists are slowly removing the concrete and steel shards of the fallen buildings.

Could today’s technology have prevented the attacks? Or protected more people from harm? No one can say yet, given the improbable nature of the assault.

Research under way before the disaster was already addressing many of the issues raised by the use of commercial airliners as missiles. That research has been given new impetus and direction in the hope that technology will prevent a recurrence of the events of September 11.

—Harry Hutchinson

The Heavy Equipment Moves In

In the minutes following the collapse of the Twin Towers, Tim Mullally, who like so many couldn’t turn away from the television that day, understood intuitively that rescue workers would immediately need all-terrain vehicles.

Mullally sells John Deere equipment in Jeffersonville, N.Y. Within hours of the attack, he loaded up the five Gators at the dealership and headed for the city.

Gators are small, six-wheel all-terrain vehicles originally manufactured as military vehicles and now finding much use among farmers, hunters, and sports teams, according to Barry Nelson, manager of public relations for John Deere. The vehicles are well balanced and highly maneuverable, and though they only go 18 miles per hour, they’re useful for the back-and-forth hauling of heavy items over several miles of difficult terrain that farmers frequently must do. Sports fans have seen them carting injured football players off the field.

As Mullally recognized, a car or even a pickup truck would not be able to drive in and among the mounds of debris, though certainly vehicles would be necessary at the site, which is being called Ground Zero.

“It has the type of tires that go through almost anything,” Nelson said. The tires resist puncture, which is another consideration in the glass- and metal-strewn area.

The vehicles have made their way through small passageways, crowded streets, and piles of debris to haul food, water, and rubble.

John Deere also shipped 14 Gators to New York and provided wheel loaders, excavators, and backhoes. The company trained rescue workers to operate them.

Many other heavy equipment and vehicle suppliers also have helped with the effort by providing machinery. From Aurora, I11., Caterpillar brought in a 345 Ultra High excavator capable of 84-foot vertical reaches. Equipped with shears, the machine can lift heavy debris and cut through steel beams.

Caterpillar also provided an M320 wheeled excavator with a hydraulic cab from Aurora. The elevated cab places an operator high in the air to improve visibility in tight conditions. Two operators from the Caterpillar Demonstration Center in Edwards, I11., were in New York to operate the specialty equipment and provide backup support to other operators. —Jean Thilmany

Robots in Forbidding Places

As early as 6 p.m. on the 11th, robots began arriving at the site. Teams came from Foster-Miller Inc. in Waltham, Mass.; iRobot Corp. in Somerville, Mass.; the Space and Naval Systems Warfare Center in San Diego, and the University of South Florida in Tampa. They converged on the scene under leadership of the Center for Robot Assisted Search and Rescue based in Littleton, Colo. Of the 17 robots that attended rescue and recovery efforts, eight deployed at Ground Zero, and one was lost in action.

Inuktun Services Ltd. of Nanaimo, British Columbia, and the DARPA Advanced Technology Office in Arlington, Va., also lent robots and sensors to the lifesaving effort.

Robots moved on crawler tracks to carry video and infrared cameras into the ruins. Some worked at the ends of tethers as they dragged power and communications links behind them. Others—larger than the tethered breeds—carried their own battery power into the wreckage. Wireless Ethernet communications kept these roamers in touch with the world of light.


Heavy machinery has begun the slow and dangerous work of clearing the six-story-high debris that fills the site once occupied by the World Trade Center in New York.

By the 26th, robots were assisting in structural inspections of the slurry wall of the basement, the rescue having ended. —Paul Sharke

Propping Up the Bathtub

The slurry wall, known as “the bathtub” because of its basin-like shape, holds back the Hudson River from the basement of the World Trade Center. While engineers say that the danger of the river actually penetrating the basement wall is small, they still plan to use a complex tieback plan to reinforce the structure, once debris has been removed, a method similar to that used to support the wall when the Trade Center was built.

Dan Hahn, a senior associate with Mueser Rutledge Consulting Engineers of New York, is part of the team charged with clearing the debris and reinforcing the wall. Before he joined Mueser Rutledge, he worked 32 years for the Port Authority of New York and New Jersey, and did some of the design work for the foundations of the World Trade Center.


This topographic map of the World Trade Center was created in June 2000 using lidar equipment. Pulses of light projected from a low-flying aircraft create a 3-D image that is accurate to 15 cm.

According to Hahn, “We are removing the pile of debris above the ground level, but it will be at least another four months before we get to the wall and reinforce it.”

The wall, made of a clay and water mixture, was reinforced by the structure of six basement floors. After the collapse of the towers, debris provided support.

The way debris is being removed is very painstaking, Hahn pointed out.

“We remove debris at 10-foot intervals down to about 70 feet, then drill through the wall at a 45-degree angle, then go through the soil and 10 to 20 feet into the bedrock of Manhattan,” he said. “Then, we’ll install a high-stress cable, about 100 feet long, into the bedrock and grout it (with a mixture of water, cement, and sand) into the bedrock. When the grout attains the required strength, we’ll test it. If it is okay at that point, we’ll attach a cable to the wall and go down another 10 feet. We’ll repeat that process about five or six times. This is the same way the World Trade Center was built.”

This laborious process is expected to take up to a year, but can begin only after all the debris above ground has been cleared, Hahn pointed out.

Raymond Sandiford, chief geotechnical engineer for the Port Authority, said that when the Trade Center was built, 1,130 tiebacks were installed. He said the plan now is to use about 750 tiebacks of high-strength steel wire strands, bundled in groups of 15.

“We can do about one of these installations a day,” Sandiford said. —Peter Easton

Maps of Dangerous Terrain

One helpful tool in assisting with the work at Ground Zero is digital imagery provided in different formats. EarthData International of Washington prepares such images under contract with the State of New York Office for Technology for use by government agencies.

Using digital cameras and laser equipment in an airplane that makes two flights daily over the site, Earth-Data provides information in three formats; 2-D, 3-D, and thermal. “These images are like blueprints,” said Bryan J. Logan, chief executive officer at EarthData. “Workers on the ground only see mounds of rubble. These maps of the site help with the use of cranes and other equipment, as well as make people aware of any potential dangers that may arise there.”

The 2-D digital images are ready quickly and are used to detect movements in buildings and other structures in the area. This information lets structural engineers take the necessary precautions to secure unstable areas and protect workers in the recovery effort. Because the scene continues to evolve, the images enable authorities to make critical decisions.

Light detection and ranging equipment, known as lidar, is a laser imaging system that generates 3-D images of the site. From the aircraft, flying at about 5,000 feet, a laser shoots 15,000 pulses of light per second, which are bounced back when they encounter an object. The lidar equipment measures the timing of the returned light to generate a 3-D map of the area.

“The lidar is accurate to 15 centimeters and measures both vertically and horizontally,” said Logan. “It detects the size of rubble piles and the positions of surrounding buildings, and the depths of any pits or openings.”

Thermal images are developed from pictures taken in the morning. These detect hot spots where fires may continue to burn and pose a threat to gas lines or electrical equipment. Additionally, these thermal images are used so that firefighters know where fires may be and so recovery workers will know to keep away for their own safety.

“In the morning, the site has just come out of the cool of the night and hasn’t been heated by the sun yet,” said Logan. “This allows us to get a sense of where real dangers exist in the wreckage.” —-Jack Raplee

Terror vs. the Fire Code

“The two buildings stood up pretty darn good—one for 56 minutes, the other for about an hour and a half,” said Richard Gewain, a civil engineer and senior engineer at Hughes Associates Inc., a leading fire research firm. The fire code could not prepare a building to withstand a crash by a fully fueled commercial jetliner.

Gewain has been asked to serve on the American Society of Civil Engineers investigative team that will study the World Trade Center disaster.

He has repeatedly viewed the videos of the Twin Towers’ destruction and described several factors that may have led to their collapse.

“The World Trade Center was designed as a tubular structure with the exterior walls carrying the vertical and horizontal dead loads,” Gewain said. “The aircraft crash knocked out a sufficient number of the columns within at least two of the walls in two or more floors, which weakened the structural integrity of the building. One of the questions that should be answered is, would the building, as damaged structurally, have collapsed if there had been no fire?”

Bare open web steel joists in a floor construction like those in the World Trade Center can resist fire for only seven minutes, according to the ASTM E 119 fire test referenced by all building codes. That is why the steel joists in the floors of the WTC were protected with a spray-on mineral wool fiber having a density of 11 to 15 pounds per cubic foot, giving them a resistance rating of two hours under ASTM test conditions.

However, the material may have been knocked off the steel joists as the planes crashed into the floors of the buildings. The conditions experienced on September 11—the impact and fire involving jet fuel and instant explosive flashover of all building contents—on those floors went far beyond the conditions used as the bases of the ASTM fire test.

“The nearly full tanks of aviation fuel set multiple floors aflame—two, three, or as many as eight, from what 1 have read—and the sudden creation of a 2,000°F fireball caused a thermal shock that probably knocked additional sprayed fiber off the steel,” Gewain said.

The civil engineer noted that the ASTM E 1529 and Underwriters Laboratories UL 1709 fire resistance tests for hydrocarbon pool fires would be more in line with the temperatures of September 11.

“The hydrocarbon pool fire was developed for the steel supports of elevated vessels and pipe racks in the petrochemical industry,” Gewain said. “This qualifies a coating of 25 to 50 pounds per cubic foot to withstand physical damage, severe weather conditions, and instant continuous fire temperatures of 2,000°F.”

At Hughes Associates, Gewain and his colleagues have modeled roof constructions with steel joists protected for two hours based on ASTM E 119. Modeling the same construction exposed to a UL 1709 fire exposure reduces the fire resistance by nearly one-half, he said.

Gewain said that codes like the International Building Code have adopted field tests to evaluate density, thickness, adhesion, and cohesion of fire protection spray coatings. “Steel fireproofing materials are not evaluated to withstand the impact of a commercial aircraft, but the IBC has mandated third-party inspection of spray-applied fire protection material in the 2000 Edition of their code, and that’s a start we can build on,” he said. —Michael Valenti

Sensing the Hazards

The state of the Twin Towers just before they fell was unknown to everyone, including more than 300 firefighters and rescue workers who entered the buildings and died in the collapse.

Researchers say that, in the future, wireless sensor networks may help rescue efforts during emergencies in large structures by locating trapped people, warning of fires, and monitoring structural conditions.

According to Albert P. Pisano, director of the Electronics Research Lab at the University of California, Berkeley, and chair of the Executive Committee of ASME’s MEMS subdivision, there are research programs under way to develop networks of small, battery-operated sensor modules linked together by radio-frequency transmitters.

Research at Berkeley is focused on outfitting buildings with sensors at critical structural points to determine damage. Tiny, wireless networked sensors, known as “motes,” would be part of a self-assembling system; that is, they would be able to locate and communicate with each other automatically.

Berkeley is working with Crossbow Technology Inc. of San Jose, Calif., which is supplying its wireless monitoring system to research groups. “The networks are self-configuring,” said John Crawford, vice president of business development for Crossbow. “The network can identify whatever motes are around it and then talk.”

The sensors, distributed in rooms or a series of rooms, could find each other, communicate, and share information, he said.

Pisano emphasized that wireless sensor networks are still in the research stage, but could become commercially practical in the next few years.


This lidar image, created after September 11, enables recovery and emergency workers at Ground Zero to navigate the area safely by showing pits and debris. These images are updated daily.

He said that wireless sensor networks would be ideal for retrofitting existing buildings, because they would eliminate the high cost of wiring and be more dependable than wired networks.

Pisano envisions a future in which wireless networks of sensors can stream data on temperatures, smoke, vibration, or fire to a remote location. A possible use for the information is to be able to predict how a building may fail.

Nicholas Sitar, a professor of civil and environmental engineering at Berkeley, said that wireless sensor networks could help emergency workers assess what is going on inside a building to judge how dangerous the situation is. Sitar said that a simple prototype of such a system could be six to 12 months off, depending on research funding. A more complex system could be two years away.

According to R. Brady Williamson, a professor emeritus of engineering science in the Department of Civil and Environmental Engineering at Berkeley, older buildings present potential problems for fire safety because they may not meet newer standards.

“If you have an incident happen such as the World Trade Center, it would be useful to have lots of sensors of various kinds,” he said. —-John DeGaspari

Security for Air and Ground

In the wake of the events of September 11, keeping airliners out of the hands of hijackers has become a matter of intense concern.

Perhaps the easiest fix to implement in airliners is hardening the cockpit entrance—a practice said to be already followed by El Al, the Israeli airline. In general, current cockpit doors are flimsy “and do not represent a barrier to intruders,” observed Harry Armen, a member of ASME’s Board of Governors and director of technology development for a large aerospace defense company. One possible approach is to have a double set of doors, so that unauthorized persons would find themselves trapped between them.

Other, more complex responses involve changes in aircraft navigation systems. As Samuel Venneri, associate administrator for aerospace technology for the National Aeronautics and Space Administration, noted, certain lines of research already in progress, aimed at enhancing aircraft safety and accident avoidance, could be reconfigured to also address security issues.

“In nearly all accidents involving loss of control, the airplanes had some functional capability,” Venneri said. In some cases, pilots have been able to save an airplane and get it back to earth by using its capabilities and surfaces in nonstandard ways. Current research seeks to develop ways for computers to take over the airplane’s propulsion systems and reconfigure them to adapt to emergency situations.

A similar sort of override would adapt systems already tried on the F-16, which involve a database with a GPS-based terrain map that is designed to tell when a plane is about to crash into an obstacle and, if necessary, take control to prevent a crash.

This system could also detect deviations from the approved flight path, Venneri said.

He said that techniques could be developed to “harden” aircraft systems and avionics against the possibility of a virus.


The World Trade Center burns on the morning of September 11; according to an engineer, the buildings held up well under the fierce assault.

(Venneri, his boss Daniel Goldin, and Ahmed Noor of NASA are the authors of this month’s cover story. They look at technology for the future of air and space travel, including issues of safety and security, in an article beginning on page 48.)

High-flashpoint fuels are being discussed as a way of mitigating the damage if all else fails. Additives, for instance, could preclude violent explosions, but they are also likely to make the fuels less efficient. Or, as Armen put it, “You want to make sure, under normal conditions, that they work.”

There are also highly sensitive sensors that can detect minute quantities of substances such as explosives, and improved 3-D luggage-screening technologies to help airport personnel interpret what they see.

Since many of the ideas NASA is working on were already in development, Venneri observed, some results could be expected fairly soon. His team is currently putting together ideas and hopes to have a demonstration of the technology ready to go in four months.

—Henry Baumgartner