Editors Note: Ray Bernard spent 11 of the 18 months prior to September 11 working on floors 65 and 71 in Tower One of the World Trade Center as a consultant on airport security related projects for the Port Authority of New York and New Jersey. Fortunately, when the September 11 attacks occurred, Ray was working on various consulting projects out of his home office in California, where he was also finishing an article on employment opportunities for large government agencies for this issueincluding job opportunities with the Port Authority.
Initially, the news media estimated casualties from the WTC attack at 20,000 and higher, but based on his knowledge of the design and structure of the buildings as well as the evacuation practices, Ray told us that the real number would be closer to 5,000. He predicted that close to 90% of the Twin Towers should have been evacuated, and thankfully he was right. We asked Ray to reschedule his original article and write this story for us.
Firefighter Mike Kehoe ascends a still-lit Tower One stairwell. He, as well as photographer John Labriola, were able to escape before thetowers collapsed. Photo courtesy of AP / Wide World Photos / John Labriola
More than 25,000 lives were saved on September 11, 2001 at the World Trade Center, according to New York City Mayor Rudolph Giuliani. Those saved include colleagues and friends of mine; those lost include a few Port Authority people with whom I had contact and one very good friend.
The heroic, life-saving efforts of the police, firefighters and other emergency personnel will live on in our minds and hearts. The images will always be there. They are for me, replayed over and over.
But part of what saved those 25,000 were efforts made days and even years before the towers fell in September. It was unnoticed work accomplished by unseen hands, from the engineers and architects who designed and constructed the buildings, to the engineers and technicians who operated and maintained them. Without them, many other life-saving efforts that day would not have been possible.
Withstanding the Forces Unleashed
On a foggy July morning in 1949, Lt. Colonel William Smith was piloting a U.S. Army B-25 bomber through New York City. He was on his way to Newark Airport in New Jersey to pick up his commanding officer, but through error and accident found himself flying over LaGuardia Airport. Because of the poor visibility, the LaGuardia tower asked him to land. But Smith persisted, requesting and receiving permission from the military to continue on to Newark. The last transmission from the LaGuardia tower to the plane was an ominous warning: From where Im sitting, I cant see the top of the Empire State Building. His plane crashed into the famous skyscraper, killing 14 and injuring 26.
As the engineers and architects of the World Trade Center Towers began their work some 30 years ago, they feared that a similar disaster could occur. So they imagined what would happen if a 707 (the largest airliner of the day) were to strike into the side of a tower and designed accordingly. Their findings: the structure would remain sound; the towers would stand. It was more of an academic exercise at the time frankly, says Ron Klemencic, chairman of the Council on Tall Buildings and Urban Habitat, an international organization of engineering, architectural and construction professionals who help set up international guidelines for skyscraper design. No one really thought that someone was going to crash an airplane into the side of the building.
Evacuees escape down a Tower One escalator. Photo courtesy of AP / Wide World Photos / John Labriola
But those engineers could not have foreseen the awesome force unleashed by the impact of the 767s that struck the towers. The Boeing 767s used in the attacks were significantly heavier, faster and equipped with more powerful engines thana 707. The kinetic force released by a 767 (which weighs approximately 350,000 lbs) traveling at an estimated 400 mph is equivalent to the force generated by 93,333 six-thousand pound wrecking balls moving at 10 mph, according toLeRoy Alaways, Ph.D., a managing engineer in Accident Reconstruction at Exponent Failure Analysis Associates in Menlo Park, Calif.
To stand up to that kind of energy, it would take a resisting force of 400 million pounds to prevent crippling structural damage, says Klemencic, who is also the current president of the engineering firm that designed the World Trade Center. The Twin Towers, he continues, were only designed to withstand a hurricane-force wind of 15 million pounds.
However, it wasnt the sheer force of the impact that brought the towers down. Instead, it was perhaps the only vulnerability in their design: They couldnt withstand the unthinkable heat released by the combustion of 60,000 pounds of jet fuel.
But then, the people who designed the fire-resistive systems did notcould nothave contemplated the subsequent heat fueled by thousands of gallons of jet fuel. I dont think you want to design every high rise building to resist a huge impact load, followed by an explosion, followed by an hour or so of burning jet fuel, says H. Scott Norville, a civil engineering professor at Texas Tech University in Lubbock. Considering normal threats of accidental fires, I think most structural members are sufficiently insulated. This was a very abnormal circumstance.
Professor Carl-Alexander Graubner of Darmstadt University of Technology in Germany, told SkyScrapers.com that, had it not been for the jet fuel, the towers might have stood up to the planes... Buildings cannot be made to resist that intensity of fire, heat and burning. The World Trade Center did not collapse because of the crash, but rather as a result of the catastrophic effects of fire on this particular type of steel-tube construction, in which the outer walls carry the building loads while the core contains only service functions.
The fuel-borne fire blazed as high as 2,000°F. When steel reaches a temperature of 800°F, says Masoud Sanayei, professor of civil and environmental engineering at Tufts University in Boston, it begins to weaken. When it reaches 1,200°F, structural steel has only one-third of its capacity. And one-third of the capacity was not sufficient to carry all the stories above where the airplanes hit, he continues. As a result, the steel melted and bent, giving way to floor upon floor falling in on each other, thousands of tons of weight collapsing and pancaking to the bottom.
Watching it was like watching engineers decommission a building, placing dynamite around the edges, making sure it implodes rather than falls over. But if the terrorists had their choice, it would have been to make the buildings fall over, causing more casualties and damage to other buildings, says Sanayei.
In spite of the nature and magnitude of the forces set against them forces well beyond anything any office buildings could be expected to endurethe Twin Towers held their structural integrity 102 minutes for Tower One and 56 minutes for Tower Two, allowing an estimated 90% of the buildings occupants to evacuate.
In retrospect, the building did perform heroically. It performed amazingly well, says Klemencic. In fact, most other buildings in the world faced with the same type of scenario would have fallen down immediately, or at least a portion of them would have collapsed immediately.
Indeed, many engineers from around the world were amazed at how well the towers held up to the initial impact of theairliners. I highly commend the design engineers on the fact that the towers performed as well as they did, says Norville. World War II brought the beginning of more flexible building designs at all heights... I also suspect that the WTC design was, most likely, cutting edge at the time.
Klemencic says that there were two primary reasons the towers held up so well against the initial impact of the airlinerstheir size and the specific characteristics of their exteriorstructural tubing.
Anyone who has ever been close to the Twin Towers has been awed by their sheer size. They are both beautiful andpowerful, stretching endlessly into the sky as you look from the streets below. However, it wasnt their height, but their width that provided a distinct advantage during the attacks.
Site plan of the World Trade Towers.
According to Klemencic, each floor of the towers had an area of approximately 42,000 square feet, significantly larger than the 20-25,000 square feet of a typical high rise. And the width of each tower was an astounding 209 feet. Significant, Klemenic says, because the wingspan of a 767 is about 150 feet. When the 767s slammed into the towers, not all of the columns were damaged or destroyed. Theoretically, Klemencic continues, a building that was less than 150 feet across could have been cut in two.
But the fact that each tower was wider than the wingspanof a 767 was a lucky coincidence. The strength and redundancy of the structure itself, however, was the direct result of careful planning by the engineers who designed the buildings. The WTC towers utilized tube in tube construction, which provided primary support to the structures from external sheathing.
And their columns, Klemencic says, were very closely spacedonly 40 inches on center compared with a distance of 30 feet on center on a typical high rise. All the columns and all the beams on the face of the buildings were welded together, Klemencic continues. When the plane created this hole in the side of the buildingsand photographic evidence says that the hole was about 140 feet widea vierendeel truss developed.
In other words, the building was able to span around this big gaping hole and redistribute the load of the building and carry it to the ground. Without this design, the initial impact would have caused at least a partial collapse of the tower.
Imagine a beaten boxer taking punch after punch to the face. When he falls, hes not going to crash inward. But when the towers fell, they pancaked, saving countless lives. The reason is the tube design. The measure of what we saw after the airplanes crashed is that both of these towers rocked back and forth and then stopped, says Sanayei. The towers, he says, were much like a steel pipe. Even if you cut a piece out of the pipe, it will still stand strong. The columns below served as a guide for the building to collapse withinlike an implosion, and the whole building came down within itself.
The WTC was considered a very innovative design at the time by having this tube concept, says Eve Hinman, president of Hinman Consulting Engineers, a civil engineering firm that specializes in the design of structures to resist the impact of explosions. And it helped make the structure a very efficient design, enabling it to be so tall and withstand the heavy wind loads that it had to withstand.
What It Takes to Build a World Trade Center
It takes over a dozen teams of engineers to create a complex the size of the World Trade Center. Civil Engineers for land survey, hydraulic engineering, geotechnical work (building foundation), structural design, traffic and roadways, parking, transportation (train, subway and bus), and environmental work; Electrical Engineers to handle the electric power distribution, lighting, telecommunications and electronic security; Mechanical Engineers to handle heating, ventilation and air conditioning, elevators, escalators; Materials Engineers for the concrete, steel, glass and acoustical design.
inoru Yamasaki, chief architect of the World Trade Center, with his model of Lower Manhattan. Photo courtesy of Minoru Yamasaki Associates.
And whats amazing about the accomplishments of the engineers who designed the WTC were the tools they didnt have. Today, engineers designing city buildings use desktop computers that can perform complex analysis in a matter of minutes, or even seconds. While the engineers who designed the WTC Towers did have access to a computer, it was a far cry from the PCs found in todays average college dorm room.
Instead, Klemencic says, the engineers who designed the WTC used a computer that analyzed punch cards. They would go down to Boeing [the design was actually done in Seattle] where they would run the analysis on one of Boeings ‘super computers. It would take overnight to run the analysis for one load case, Klemencic says. It took many more hours and a lot more sweat for sure.
Ordinary Building Features Saved Lives
Then there are the ordinary, everyday elements that go into the design of skyscrapers that go unnoticed. Most of the trade center occupants probably never thought twice about the emergency telephones located on each floor prior to September 11. Nor was much thought ever given to the high level of stairwell lighting, or to the maintenance of those lights. Escalators, lobby lighting and ventilation are among the systems that most building occupants take for granted.
Yet the routine but faithful maintenance of all the building systems in each tower¬from the lighting, heating, ventilation and cooling, to the emergency systemshelped ensure that systems such as lights, emergency phones and escalators stayed running until the moment of the buildings collapse.
During evacuation drills in the buildings, most people gave little thought to the fact that the stairwells were built in four separate sections. To go from one section to another you had to go through two metal fire doors. Separate stairwell sections meant that smoke, water or other material that might hinder evacuation in one section would be confined to that section of stairwell only.
Greg Brown, a consultant and project manager for the Technical Services Department of the Port Authority, recalls how each stairwell section was different during the evacuation. We started down from Floor 71. The first section of stairwell was fairly clear, says Brown. The next section had smoke in it. We covered our mouths and noses with whatever we could. When we got to the next section of stairs water was flowing in a stream down the stairs. Toward the bottom the water was ankle deep.
If the stairwells had not been built in separate sections, he says, we would have had both smoke and water at the same time and this would have significantly hindered the evacuation.
In the Wake of September 11
On October 15, the Council on Tall Buildings and Urban Habitat held an all-day meeting in Chicago. Their purpose: to find ways to make skyscrapers safer against various forms of terrorist attacks, ranging from suicide bombers with dynamite strapped to their chests, to car bombs, to biochemical attacks. Klemencic, who led the meeting, says there was plenty of disagreement among these experts, but they all agreed on one major pointdesigning buildings to withstand airline impact is not practical or feasible.
Instead, Klemencic says, it is the responsibility of the airline industry and the FAA to implement security measures to ensure that such a catastrophe never happens again.
But these experts also agreed that there were steps that could be
taken to improve skyscraper design, and guarantee the future safety of
their tenants. Their suggestions include:
- Equipping future skyscrapers with pressurized elevators for firefighters that would keep smoke out, and allow rescuers quicker access to fires.
- Designing wider fire stairs to facilitate quicker evacuation, and building heavily fireproofed refuge floors where people could gather.
- Banning underground parking garages, which could facilitate car bomb attacks, such as the 1993 attack on of the WTC.
- Using new structural techniques, such as the super-strong concrete cores like the ones used in the Petronas Twin Towers in Malaysia, which some experts believe would hold up better against the incredible heat that ultimately resulted in the collapse of the WTC towers.
- Increasing the use of redundant building systems. Multiple sources and independent distribution routes would better withstand disruptions caused by extreme events.
These are only a few of the committees suggestions, and Klemencic says that the debate must continue and that engineers, architects and other building professionals must continually strive to create safer and more terror-resistant structures. As unlikely as the idea seems now, new skyscrapers will be built. Not only are they an enduring symbol of urban culture, but they have practical features as well. They are extremely energy efficient, and expensive real estate prices mean that it is more economical for developers to expand upward instead of outward. Ultimately, it is up to you, the next generation of engineers, to analyze and implement the committees suggestions and to come up with new ones of your own. Thousands of future lives may depend on it.
WTC on the WWW
A list of links that provide architectural and engineering information related to the World Trade Center.
FEELING THE HEAT
The 18,000 gallons of jet fuel on each plane provided 2,430,000,000 BTUs of heat, enough to soften 2,500 tons of sheet steel. That heat would completely soften all of the steel on three WTC floors (outer walls, inner beams and core).
According to the Energy Information Administration 1995 Annual Energy Review, the average single-family household uses 98,000,000 BTUs per year. It would take the average U.S. single-family household 24.8 years of thermal energy consumption to use the amount of heat provided by 18,000 gallons of burning jet fuel.