September 13, 2001
AFTER THE ATTACKS: MONITORING THE FLIGHTS; Controllers Say Flow of Information on Hijacked Planes' Course Was Slow and Uneven
By MATTHEW L. WALD
The controllers assigned to United Airlines Flight 175 on Tuesday suspected that it had been hijacked as it flew off its assigned route. But they did not learn that another plane had been hijacked and had hit the World Trade Center until a minute or two before Flight 175 struck the center, people involved in the air traffic system said.
In contrast, controllers at the Washington Air Route Traffic Control Center had much more warning that something was wrong. Those controllers, who handled American Airlines Flight 77, which dived into the Pentagon, knew about the hijacking of the first plane to crash, even before it hit the World Trade Center, those involved said. That was more than an hour before they watched another hijacked plane, United Flight 93, cross their radar screen on its way to the Pentagon.
Advance knowledge made no apparent difference in the response; nobody intercepted the plane.
''We issue control instructions,'' one controller said. ''Any procedures beyond that point don't lie with us.''
Those procedures would, in fact, lie with the Air Force. The question of giving the Air Force notice of hijackings and authorization to shoot down civilian planes is likely to be a major concern for security officials in the next few days. A spokeswoman for the Federal Aviation Administration said today that there was a policy for when a civilian plane could be shot down, but the agency would not discuss it. The military routinely refuses to disclose its rules of engagement. Nor was it clear today exactly when, or if, the Pentagon was notified on Tuesday.
As the crisis took shape, information flowed unevenly within the F.A.A. The agency has broken up air traffic up into sectors small enough for two controllers to handle, and grouped the sectors in different air traffic offices. Such compartmentalization allows the agency to handle several thousand flights simultaneously, but may also have prevented information from flowing quickly enough.
The F.A.A. has refused to give details, saying that the way the information flows within the agency is part of the F.B.I.'s investigation into Tuesday's attacks. But people involved describe a haphazard flow.
For example, at the New York Air Route Traffic Control Center in Ronkonkoma, which handles long-distance traffic around the New York metropolitan area, the first inkling of a hijacking that most controllers had was when a supervisor came to the cafeteria and asked if he could change the television channel to CNN.
''Our TV's are always tuned to ESPN,'' one controller said.
The television screen showed one tower of the World Trade Center with a hole in it. ''We didn't know what kind, what airplane. There were rumors it was a 737,'' the controller said. ''We said, 'No way, it would be a much bigger hole.' We were watching, and we saw the second one go in.''
In the darkened, windowless cavern that is the operations floor of the center, most controllers did not learn of the twin hijackings until their colleagues came up from the cafeteria.
At the control tower at La Guardia Airport, the first definitive information for controllers was the sight, viewed through binoculars, of the second plane plunging into the building. On the other hand, as soon as controllers in Boston heard that a plane might have hit the World Trade Center, they knew what had become of American Airlines Flight 11, which they had been tracking since it began behaving erratically, people involved said.
At the New York Terminal Radar Approach Control, the air traffic office whose airspace American Flight 11 entered soon before its crash, a conclusive report of what happened to that plane reached the room only a minute or two before the United plane hit the other tower, controllers there said. ''We had 90 to 120 seconds; it wasn't any 18 minutes,'' said one controller, referring to the actual elapsed time.
Another controller said: ''They dove into the airspace. By the time anybody saw anything, it was over.''
After the two World Trade Center crashes, controllers at the New York traffic center were briefed by their supervisors to watch for airplanes whose speed indicated that they were jets, but which either were not responding to commands or had disabled a surveillance device called a transponder. Controllers in Washington got a similar briefing, which helped them pick out hijacked planes more quickly. Two of the four planes had transponders that had apparently been tampered with in flight.
In fact, though, transponder failure is an ambiguous sign, even if the plane then strays from its assigned altitude and course. Controllers do not assume the worst ''if something weird happens with the airplane,'' said one; an electrical problem could be responsible and the pilot might be headed back to the departure airport. Standard procedure is to ''give him room and watch what he does,'' another controller said.
Transponders are robot radios, carried in a plane's tail, that respond to queries from ground-based radar by giving the plane's identity and altitude. Ground radar can calculate, based on the timing of the transponder response and the direction from which it came, the plane's latitude and longitude. American Flight 77, which hit the Pentagon, had had its transponder shut off, so controllers had less information on the flight. On United Flight 175, the second plane to hit the World Trade Center, someone changed the code that the transponder was sending, which had the effect of confusing the air traffic control computers. On the controller's screen, the data block, three lines of letters and numbers that give the plane's identity and other details, cut loose from the blip and drifted off.
Controllers also say they were told to watch for planes heading for Camp David, the presidential retreat in Maryland. Unlike the World Trade Center and the Pentagon, Camp David would be hard to spot from the air, but it is clearly marked on charts, because the airspace below 5,000 feet is off limits to civilian planes. But because Camp David is close to Washington and a hijacked plane's target is unknown, it would be hard to say whether a plane headed toward that location actually intended to go there, controllers noted.
September 20, 2001
A NATION CHALLENGED: THE SKYSCRAPERS; Engineers Say Buildings Near Trade Center Held Up Well
By ERIC LIPTON AND JAMES GLANZ
Though buildings near the World Trade Center show evidence of damage like blown-out windows or gashes on their facades, none appear to be slumping, leaning or subsiding, and none are in danger of imminent collapse, engineers said yesterday after a detailed series of inspections.
Isolated structural repairs will be required on at least three large office buildings: Bankers Trust, 30 West Broadway and 3 World Financial Center. Many others, like 22 Cortlandt Street, will require more modest exterior repairs, as well as extensive work to windows, heating or ventilating systems, or other damaged parts.
But it is unlikely that any of these or other nearby buildings will need to be demolished, said Richard Tomasetti, president of LZA/Thornton-Tomasetti, the engineering firm the city hired for the assessments.
''All of them, in my opinion, there's no reason in the world that they can't be saved,'' Mr. Tomasetti said. ''I can't tell you what the owners will decide for the few buildings with limited structural problems, but these buildings don't need to come down.''
Two reporters from The New York Times obtained a copy of the city's preliminary assessment of 195 buildings in the area yesterday, and were allowed to view structures in and around what has come to be called ground zero. The contrasts there evoked the capricious power of an earthquake, with structurally sound buildings standing across the street from smoking mounds of rubble five or six stories high.
Through much of the area south of Park Place, north of Albany Street and west of Broadway, there are stretches where hundreds of windows facing the towers have been blown out. Roofs have been damaged from falling debris, including airplane parts found on the Federal Office Building at 90 Church Street and another building across Barclay Street, at 100 Church Street. Other building exteriors are singed by fire or just caked with soot.
But the preliminary results of the city's detailed building-by-building assessment of the area support the conclusion that while damage to facades and interiors is occasionally severe and plainly visible, many of the buildings could be occupied in several weeks to several months. There is no evidence that any buildings have been compromised structurally in a way that would require demolition.
In part, the survey suggests, that is because the twin towers collapsed almost straight downward, a circumstance that the engineers said might have reduced the death toll from the terrorist attack.
''It's like controlled demolition,'' said Matthys Levy, a founding partner at Weidlinger Associates, a structural engineering firm in New York. Mr. Levy, the co-author of ''Why Buildings Fall Down'' (Norton, 1992), said the collapse of the towers was ''an uncontrolled demolition project, but it acted like a controlled demolition project.''
If the buildings had tipped or tumbled sideways instead, Mr. Levy said, ''you would have seen tremendous damage outside the zone, and you would have had those buildings possibly collapse.''
The eight-page Department of Buildings survey of 195 structures in and around the World Trade Center complex again and again reports ''building is safe'' or ''minor cleanup required'' or ''O.K. to occupy.'' Buildings with those kinds of findings include 4 World Financial Center, occupied in large part by Merrill Lynch & Company. and 75 Park Place, the home to the city's Office of Management and Budget.
Even 1 Liberty Plaza, a building with more than 50 stories across the street from 4 World Trade Center, which is partially collapsed, is listed as ''structurally sound, including facade,'' despite rumors during the last week that it was nearing collapse. (The 4, 5 and 6 World Trade Center buildings are largely wrecked and will have to be demolished, an engineer said.)
The Millenium Hilton Hotel is also structurally sound, the survey says, though it is missing some front windows and its lower facade is damaged.
These assessments do not mean that tenants will necessarily be able to move back soon. The 75 Park Place building needs an extensive cleanup, as does 4 World Financial Center, where employees may not able to return for two to three months, a Merrill Lynch spokesman said.
Engineers also cautioned that after the main structural questions have been addressed, a wide range of other cleanup and repair work would remain, including repairing or replacing heating, cooling and ventilation systems; replacing hundreds of windows; restoring electricity and installing or repairing fire alarms and security systems.
''Merrill Lynch will not put one person back into any of our buildings until we are absolutely sure that they will be safe,'' said Rich Silverman, a Merrill Lynch spokesman.
John F. Hennessy III, the chairman of Syska and Hennessy, a engineering firm that specializes in mechanical and electrical systems, said the problems inside the buildings were likely to be similar to those encountered after the 1993 bombing of the trade center.
''One of the big problems there was getting the building clean because there was a tremendous amount of smoke that went through the building,'' said Mr. Hennessy, whose firm worked on the trade center after the bombing and has been hired to rebuild systems in the interior of the Pentagon.
The appearance of the residential buildings where thousands of people live south of Gateway Plaza in Battery Park City supports Mayor Rudolph W. Giuliani's assertions that they have suffered no major damage, although dozens of windows on the east facade of Gateway Plaza have been blown out.
On the northern edge of Battery Park City near the Embassy Suites Hotel and the movie theater, there is no apparent structural damage.
A handful of buildings have not been inspected thoroughly yet, including three on Cedar Street, in part because of their proximity to the center of the impact zone.
The effort to assess the damage to the 195 buildings in the area has involved about 100 engineers making the inspections, officials said. On average, about 2,000 workers, using everything from large cranes to metal cutters, are at the site each day, removing debris and preventing damaged facades from tumbling. For example, a beam from one of the World Trade Center towers pierced the facade of 3 World Financial Center, and workers have tied it down until it can be safely removed.
The building assessment and cleanup effort is led by Michael Burton, executive deputy commissioner, of the city's Department of Design and Construction, in cooperation with the Port Authority of New York and New Jersey and private sector consultants.
The command center of the project is on the second floor of Public School/Intermediate School 89, at the intersection of Chambers and West Streets. Boxes of hard hats and goggles sit in the entrance hallway.
Six major construction companies in the New York region have been recruited to help with the work: Tully Construction, Amec Construction Management, Bovis Lend Lease, Turner Construction, Plaza Construction and Tishman Construction. LZA/Thornton-Tomasetti Group is overseeing the individual building assessments. Mueser Rutledge, another engineering consulting company, is working to ensure that the massive underground walls that surround the World Trade Center complex and hold back the Hudson River are not compromised.
This work is under way as firefighters continue to douse smoldering debris. In a classroom at the school used by the engineers and city officials as a meeting room, bins of colored building blocks sat on shelves and children's books sat abandoned. A large spiral notebook propped up in one corner had been carefully lettered with this message:
''Jack be nimble,
Jack be quick,
Jack jump over the candlestick.''
September 18, 2001
A NATION CHALLENGED; Dust Is a Problem, but the Risk Seems Small
By ANDREW C. REVKIN
As thousands of workers streamed back into Lower Manhattan yesterday for the first time since the terrorist attacks, federal officials said they faced no significant health risk.
Low levels of asbestos were detected in some dust and debris close to the wreckage of the World Trade Center, the officials said, but there was no evidence of danger, except to search crews moving the rubble.
''We haven't found anything that is alarming to us,'' said Mary Mears, a spokeswoman for the Environmental Protection Agency.
As an extra precaution, officials recommended that businesses in the area clean the filters on air-conditioners and use vacuum cleaners equipped with filters for fine particles -- those labeled HEPA -- to avoid scattering any hazardous dust.
Officials recommended similar precautions for apartment dwellers, saying they should use vacuums with particle filters, mop floors and use wet cloths to dust, and wash clothing soiled by the ash and dust separately from other laundry.
Over all, though, officials said, the only significant health risk remained near the destruction. Workers there should wear masks and protective gear and clean their shoes before heading home, they said.
Some officials expressed frustration because many of the workers -- most of them hard-bitten construction workers -- were ignoring their recommendations.
''In the early hours of a rescue, the urgency of these efforts leads people to forget their own health and safety,'' said Dr. Neal L. Cohen, New York City's health commissioner. The city will add more safety officials to its teams this week, he said, to make sure that more searchers wear protective attire.
Tina Kreisher, a spokeswoman for the E.P.A., said that ample gear was available at the attack site but that because of the heat and stress, workers commonly refused it.
''There are small pockets of asbestos,'' Ms. Kreisher said. ''The concern is there -- not for the city, not for residents, but definitely for these workers.'' Many workers may be there for months, she said.
Federal officials said they would set up at the site equipment able to clean 1,500 workers twice a day.
Agency officials and independent experts tried to quell rumors about other hazards, including the possibility that the fires might have turned freon from air-conditioners into a poisonous gas called phosgene.
The chemical reaction that generates phosgene is possible in extremely hot flames, but not in fires like those still burning, agency officials said. Any gas generated by the initial inferno has dissipated, they said.
When rescue crews prepare to enter buried pockets where survivors might be found, they generally test the air for organic compounds like freon, which can be suffocating because it is heavier than air and can build up in pockets, officials said. Suspect air samples are sent to a mobile laboratory for analysis.
Dr. Cohen said that continual monitoring of the crash site and rubble with Geiger counters had turned up no evidence of radiation, which might be emitted, for example, from medical X-ray equipment destroyed in the attacks.
Many officials and experts added that the decomposing bodies of victims of the attacks posed no danger.
''A dead body is not going to give you a disease,'' said Dr. Paul Blake, the state epidemiologist for Georgia. Dr. Blake, who was chief of the enteric diseases branch at the federal Centers for Disease Control and Prevention, added, ''You have a multiplication of bacteria in decomposing bodies, but they are the same bacteria we all have within us, in our bodies and on our skin.''
To try to catch any surprises, the Environmental Protection Agency deployed monitoring and testing equipment around Lower Manhattan over the weekend, including a mobile laboratory used during the Persian Gulf war to check for gas or biological attacks, officials said.
There were no signs that the attackers had dispersed any toxic agents, officials said, and the laboratory was focused on identifying conventional kinds of pollution.
Many dust samples collected near the attack site last week contained 1 percent to 2 percent asbestos, agency officials said. That concentration is not high enough to create a short-term risk of lung disease, the officials said. Nonetheless, the agency sent 10 trucks into the area over the weekend equipped with filtered vacuums that suck up contaminants without spreading them.
For residents and people working nearby, the air appeared to present little risk, officials of the Occupational Safety and Health Administration said. The agency sent crews around the financial district late last week wearing small devices that sample air and found no significant signs of asbestos. Other tests were done for air inside buildings in the financial district.
''Our tests show that it is safe for New Yorkers to go back to work,'' said a statement from John L. Henshaw, the assistant secretary of Labor for occupational health. ''Keeping the streets clean and being careful not to track dust into buildings will help protect workers from remaining debris.''
Over the weekend the Environmental Protection Agency parked five air sampling systems near the crash site and put another one on Canal Street to monitor any drift uptown. The systems will measure asbestos, lead, PCB's and other harmful compounds during the cleanup, agency officials said. Other samples were being taken at established pollution monitoring stations in Lower Manhattan and Brooklyn.
Dr. Philip J. Landrigan, the chairman of the department of community and preventive medicine at the Mount Sinai School of Medicine, generally agreed with the federal officials' views, adding that rescue workers who did not wear protective gear ran the highest risk.
Dr. Landrigan, who has often worked to highlight health dangers in the environment, said the public faced little risk. ''People who live in Lower Manhattan and who work there are certainly going to be exposed to dust,'' he said. The dust, from pulverized building materials, could cause bronchitis or asthma attacks in children or vulnerable adults, he said.
''But having said all that,'' he added, ''I don't think we are looking at a situation that is in any way life-threatening.''
Some office workers were taking no chances. One woman rode the commuter train from Westchester County yesterday morning carrying a personal air-filtering machine for her office in the city.
September 13, 2001
AFTER THE ATTACKS: AFTERSHOCKS; As Remnants Collapse, Workers Run For Cover
By JENNIFER STEINHAUER
The stalagmite remnants of the fallen World Trade Center towers collapsed entirely yesterday, sending rumbling debris and clouds of smoke billowing again through Lower Manhattan and prompting rescue workers to flee from the site of the destruction. Officials declared a zone of roughly eight more blocks in the area unstable.
City officials confirmed last night that the steel and concrete wreckage of the south tower, which had been toppled in a terrorist attack, and 5 World Trade Center, felled in the aftermath, crumpled to the ground in the late afternoon.
Fire Commissioner Thomas Von Essen said last night at a news conference that engineers were busy inspecting neighboring buildings in response to reports of a crack in 1 Liberty Plaza, the 64-story high rise. That plaza has sustained structural damage, but officials said last night that although they had not determined the extent of the damage to that building or others on Liberty Street, they did not believe that it was in imminent danger of collapse.
All day yesterday, rescue and emergency workers battled through the destruction, confronting ruptured gas lines, raining debris and constant rumors of other buildings said to be weakened from the attacks.
''This is a very dangerous rescue effort,'' Mayor Rudolph W. Giuliani said last night. ''The men and women who are doing it are literally putting their lives at risk.''
The fragile search and recovery efforts were hampered intermittently for several hours, and precautionary evacuations led to moments of panic among rescue workers. Police and emergency workers in the areas around the destruction barked into their radios, arguing with pedestrians trying to cross barriers and telling them that more and more buildings were unstable.
Officials also said yesterday that it did not appear that the residential buildings in Battery Park City had sustained structural damage, but the city was still assessing those buildings to decide whether to allow those who have been evacuated to return.
The seeming aftershocks began about about 5 p.m. yesterday, while workers ploddingly cut through twisted steel and heavy forklifts moved rubble across the plaza in front of the fallen towers. Firefighters and police officers were standing around, gazing toward the clouds of gray smoke wafting up from the jagged heaps of wreckage. Nearby, workers had set up a triage center near the World Financial Center.
First came a rumble, and then one firefighter yelled: ''That part will go! We are waiting for it to collapse.'' Moments later, the remaining floors of the south tower of the World Trade Center fell to the ground in a heap of rubble.
Rescue workers and medical personnel bolted up Broadway and Church Street away from flying debris, concrete and smoke as ambulances began to scream from all directions, responding to the new collapse.
''Everyone started running,'' said Jonothan Schwartz, a Red Cross worker from Rockland County who stopped at last at Canal and Broadway. ''We were told there was more danger of another building falling. Everyone ran and ran -- kept going and didn't look back.
''I started running, and I didn't look back,'' Mr. Schwartz said. ''And I'm not going back. I'm going home, because it's too dangerous here.''
About the same time, the city's engineers yesterday expanded a safety zone around buildings that they believe had a greater chance of collapsing than earlier believed. Emergency personnel were temporarily evacuated from several blocks surrounding 1 Liberty Plaza at the southwestern edge of the World Trade Center.
Frantic calls to the police and Fire Department workers came from all directions, with reports of swaying buildings at John Street and the intersection of Greenwich and Liberty Streets.
Over at the West Side Highway, hundreds of people, frightened of falling debris, raced south, away from what they believed to be a collapsing building. They pushed past police barricades and dodged rescue equipment that was hastily being thrown into reverse. Many searched for a car to dive behind.
Firefighters and police officers led the stampede, struggling to race along streets thick with dust, empty water bottles, bits of metal and wire. Firefighters in heavy bunker gear yelled at colleagues, who stood looking toward a rolling pillar of smoke to move. ''Get out of here!'' screamed one investigator. ''Run! Run!''
September 18, 2001
Defending Skyscrapers Against Terror
By KENNETH CHANG
No one designs a skyscraper to withstand the direct hit of a fully fueled 767, and construction engineers agree that such an attack would have doomed almost any high-rise.
Each World Trade Center tower absorbed the impact of a jet with a shudder, as each was designed to do, and stood.
Inside, though, 2,000-degree infernos started burning, fed by thousands of gallons of jet fuel.
It then became a question of time. Would the fuel burn up first or would the steel columns weaken and buckle under the heat?
For the people on floors above the crash site, there was another critical factor: an ordinary fire would take two or three hours to burn through the gypsum wallboard around the stairwells -- but projectiles of plane wreckage almost certainly pierced through, letting in the fire and smoke. That trapped people on the upper floors.
The south tower collapsed 56 minutes after impact. The north tower lasted an hour and 40 minutes.
Someone probably could build a fortress skyscraper. ''Given enough money, we can design anything,'' said Dr. Charles H. Thornton, chairman of the Thornton-Tomasetti Group Inc. of New York City, the structural engineering firm that worked on the 1,483-foot-tall Petronas Towers in Kuala Lumpur, Malaysia.
But no one would pay to build one, and no one would want to work there. Such a building would probably have the aesthetic appeal of a containment vessel of a nuclear power plant, which is designed to survive the crash of a falling 747.
In the decades since the World Trade Center was built, however, new materials and building techniques -- some used on the more recent super skyscrapers like the Petronas Towers -- may have given people more time to escape.
The key would have been slowing the fires. The sprinkler systems offered little help.
Even if the pipes survived the impact, the sprinklers of a typical skyscraper put out a few hundred gallons of water a minute for half an hour, Dr. Thornton said, and water would have been useless against a fuel fire in any case. (Water and oil don't mix; droplets of water sink into the fuel, turn into hot steam and explode, and the fuel continues burning.)
By contrast, an anti-fire system at an aircraft hangar can unleash a deluge of 120,000 gallons a minute of water and foam -- which sticks to burning fuel -- for two hours straight, Dr. Thornton said.
Since extinguishing the fire is impossible, ''You have to build a more rugged building,'' said Dr. R. Brady Williamson, an emeritus professor of civil engineering at the University of California at Berkeley.
The main ingredients of any skyscraper are steel and concrete. Both are strong, but in different ways. Concrete bears more weight; steel can bend without breaking. The World Trade Center's supporting columns were made of steel, and the intense heat would have caused the girders to expand, distorting their shape and sapping their strength, leading to the collapse.
''It's better to build in reinforced concrete,'' said Dr. Mir M. Ali, a professor of architecture at the University of Illinois. ''If there is an impact, crash or explosion, it can absorb the energy better. That makes the building less vulnerable.''
But reinforced concrete -- concrete with steel bars inside -- is heavier. When construction began on the World Trade Center in the late 1960's, concrete was not a viable option because it would have required huge, unwieldy pillars to support the towers' weight. But high-strength concrete developed in recent years has made it more practical.
''The trend is toward more concrete,'' Dr. Mir said. ''The technology has substantially improved. An all-concrete structure would have lasted longer.''
Each of the two Petronas Towers has an outer ring of 16 7-foot-wide columns made of concrete and at the center of each tower is a 75-foot by 75-foot concrete core -- almost a building within a building -- that houses the stairwells and elevator shafts.
Concrete -- a mix of cement, sand and gravel -- is not impervious to heat. The cement expands at a different rate than the sand and gravel, causing cracks. Under intense heat, some types of concrete can flake apart at about three-quarters of an inch an hour, eventually exposing the steel inside.
''It ultimately would lose its strength,'' Dr. Thornton said.
The concrete core of the Petronas Towers may have remained intact under a similar crash and provided a better escape route than the gypsum-walled stairwells of the World Trade Center.
''In our buildings, most of the stairways are in the core, which is a very safe haven,'' Dr. Thornton said. The cores of the Petronas Towers are also pressurized to keep smoke and fire out of the stairwells.
In addition to building more fire-resistant structures, another protection against crashing airplanes would be to keep the jet fuel from entering the interior of the building.
At the University of California at Berkeley, Dr. Abolhassan Astaneh-Asl, a professor of structural engineering, has been developing a new construction technique -- bolting half-inch steel plates to six-inch concrete walls -- to create buildings that can better survive earthquakes. ''The concrete wall prevents the steel from buckling,'' Dr. Astaneh-Asl said. ''The steel prevents the concrete from cracking and shattering. When you marry them, they become very good.''
In tests, a half-scale, three-story building proved capable of surviving four magnitude-9 earthquakes. While the wreckage of the 767's flew into the interior of the World Trade Center, the extra mass of concrete and steel walls would have absorbed much of the planes' momentum.
''Most of the fracturing of the plane will take place outside of the building, not inside,'' Dr. Astaneh-Asl said. That is the same fundamental physics that make the S.U.V. the lesser damaged in a collision with a motorcycle.
Much of the fuel would have then splashed against the outside of the building instead of igniting inside, Dr. Astaneh-Asl said.
The World Trade Center attack may lead developers to regard a terrorist attack as a risk to be planned for instead of an unthinkable one-time tragedy.
''The perception of the terrorist threat is where earthquake hazards were in the mid- to late 1960's,'' said Dr. Jeremy Isenberg, president and chief at Weidlinger Associates, a consulting firm that once helped design resilient military bases and missile silos, and now offers its expertise for federal and commercial buildings. ''It took a series of three or four damaging earthquakes to drive home to owners of buildings that they had financial assets at risk.''
Developers may now request that more resilience be built into new buildings and into old ones being remodeled, Dr. Isenberg said.
While perhaps not much can protect against kamikaze jetliners, other simple steps may help protect against lesser attacks. Large, heavy cement flower pots, like those placed around the World Trade Center after the 1993 bombing, keep a car bomb a safe distance from the structural columns. Concrete walls around loading docks and mail rooms can be thickened to protect against bomb blasts. Jackets of graphite fibers wrapped around columns make them less likely to collapse. Protective glaze can be added to windows to make them less likely to shatter.
In planning for new buildings, structural designers are now more likely to add more redundancy where the collapse of one column does not lead to the collapse of the entire building, as occurred in the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City.
Dr. Thornton said that in one sense, taller buildings are safer than midrise buildings; taller buildings are less likely to topple because their builders generally provided more redundancy into the structures. The design of skyscrapers 50 stories or more, has been ''generally very robust,'' said Dr. Thornton. ''For 40 or less, it's not.''
But the taller buildings make a more tempting target for terrorists.