To his surprise, he felt nothing broken and no pain except for a burning in his eyes. That was widely the pattern of the day—survival as an all-or-nothing proposition. The room was dark, and so dusty that he had trouble breathing. He put a handkerchief to his mouth, which helped. Someone yelled to a fireman, “Could you please put on your flashlight?” The fireman did, to little avail. People stood up, saying, “Where are we? What’s going on?” They lifted a steel roll-up door, thinking to get out, and found a group on the other side thinking to get in. The two groups frightened each other. A fireman went out to explore, and with visibility limited to about two feet, he nearly fell into a giant crater. He found a way across it and returned, saying, “Come on, I see a street-light.” They went out in single file. Lombardi found himself on a sidewalk, but otherwise could see no change from the conditions that had existed inside. He lost track of his companions and walked down the street in confusion. He remembered the roar, and again thought of the bombing in 1993: had a device gone off in the underground? He passed the south pedestrian bridge, which he recognized, and headed south on West Street. He had a scratch on his forehead that was bleeding. People came up to him offering help, and someone gave him some water. Finally he got far enough away to look back. He saw the North Tower standing, but not the South. He thought, “Wait a minute. The North Tower is there. I know the North Tower is there. But what happened to the South?” It was confounding, and he could not conceive of an answer. He was an engineer, but human, too. He walked on for a while, until for the second time that day he heard a roar. He stopped and turned and watched in disbelief as the North Tower fell.
It took distance to sort things out. The technical speculation began right away—on the Internet, in engineering offices, and in universities around the world—and some of it was wild. Among the early embarrassments were statements by Van Romero, an explosives expert and a vice-president of the New Mexico Institute of Mining and Technology, who publicly opined that pre-set explosives could explain the collapses, and implied that the jet impacts were meant as bait for the rescue personnel. He was immediately inundated with e-mail from conspiracy buffs. After only a week he tried to take it all back, publicly renouncing his earlier ideas and signing on to the growing convention that fire and impact damage in combination were responsible for the collapses. Miserably, he said, “I’m very upset . . . I’m not trying to say anything did or didn’t happen.” But of course it was too late.
A Berkeley engineering professor by the name of Abolhassan Astaneh-Asl was more successful. He pulled down an early grant from the National Science Foundation, rushed to New York, checked into the trendy Tribeca Grand Hotel, got The New York Times’s ear, and began to show up on television inspecting twisted steel in the New Jersey scrapyards where it was being collected. Priscilla P. Nelson of the National Science Foundation helpfully explained, “It is important for engineers to be able to make the observations now that will lead to reduced building vulnerabilities in the future.” A skeptic told me that some of the specimens Astaneh-Asl had identified on camera as bearing scars from the initial impacts were later discovered, from the numbering on them, to have originated much lower in the buildings. If true, it was hardly the sort of mistake that a man like Astaneh-Asl could have been expected to renounce, or even to mope about. This was a big disaster, with all kinds of impacts, and he had important work to do. In December he announced that he would be collaborating with a computer programmer from the Lawrence Livermore National Laboratory to build a “full-fledged, realistic model of the whole World Trade Center—including other buildings, not just the towers.” He said, “Then we’ll bring the plane in—to the computer model—and hit the building with various scenarios.” Priscilla P. Nelson helpfully explained that the work “could lead to the generation of new computer models that will really enhance our ability to understand structural response.” Astaneh-Asl may have revealed more than he meant to when he said, “The most important contribution of my career was to go to New York right after the attacks.”
There was a lot of that kind of talk at the time.
But the main investigation had a different tone—cautious, preliminary, and publicity-shy. It was funded with $1 million from the Federal Emergency Management Agency (FEMA), and performed under the auspices of the venerable American Society of Civil Engineers (ASCE). The grant of such public authority to a private professional organization to investigate events potentially related to the competence of its own members would usually have raised legitimate questions about conflict of interest, but in this case it never quite did. Certainly there was an element here of a profession preparing to circle the wagons. The team was headed by an understated Midwesterner named W. Gene Corley, who holds a Ph.D. in engineering and is the senior vice-president of a modest research facility north of Chicago. Corley had led the official engineering review of the Oklahoma City bombing of 1995, and had an impeccable reputation as an independent investigator of building failures. He was a steady man, and as close to a disinterested insider as America was likely to produce. But he had limited control over a team that had expanded from a preliminary selection of about a dozen specialists—experts in metallurgy, fire, structural dynamics, and so forth—to an unwieldy group of twenty-two that included some of the profession’s biggest names. The expansion was not just due to an urge to contribute, or to an abstract interest in the subject matter. People were also concerned that the investigation not provide ammunition for future lawsuits. The team was set up so that each of the members would be able to influence the contents of the final report. The pattern was exemplified by LERA, where the Trade Center’s original engineer, Leslie E. Robertson, excused himself for obvious reasons of proximity—only to cede his place to his wife, Saw-Teen See, the firm’s managing partner. But these were exceptional times. There was widespread agreement that for the health of the United States as well as the companies involved, it was important to keep the hyenas at bay.
Everyone understood the deal. The attack on the World Trade Center was an act of war. Despite the occasional chatter in the press about shoddy steel or substandard fireproofing, the towers were as well designed, built, and maintained as could have reasonably been expected in America in the late twentieth century. But the context had changed now. The towers fell because they were severely maimed and sprayed with burning jet fuel; they fell as any building will, no matter how resilient, if it is hit by the next bigger missile in the escalating progression of war. In retrospect, their greatest failing was so obvious that it hardly required discussion: the stairwells had been clustered too closely together, and their simultaneous destruction had trapped people above the impact zones, causing more than a thousand unnecessary deaths. This was an error that would have to be avoided in future designs. But there was no point in wishful thinking here. Civilians die in wars, and always will.
Beyond that, however, it was important as much in principle as in practice to make an honest effort to understand the process and mechanisms of each tower’s collapse. This was to be a preliminary investigation only, with an emphasis on going public with the essentials as soon as possible. Corley knew that the effort was never going to be what enthusiasts in Washington were wishing for—something like an airline-accident investigation, controlled and contained, with the investigators on top, and the possibility even of a sexy reconstruction in a hangar as a finale. The Trade Center ruins were infinitely too large and chaotic to allow for that—and the emotions there, because of the continual discovery of bodies, were far too intense. The investigators spent some time at the site, but in practice their presence turned out to be of little help in the inquiry. Corley told me that given the weights and quantities involved, even the inspections of the steel that had been sorted at the New Jersey scrapyards proved to be an inefficient means of gathering evidence. This was certainly an argument against the critics who later claimed that all the steel should have been retained. More important were the videos and firsthand accounts from September 11, the
original plans that LERA was providing, and the application of methodical science. Corley had confidence that the catastrophe could be reasoned through. It was obvious that critical inner workings of the collapses would remain obscure, and that debates and experiments concerning them would continue for years; indeed, this was an important part of the creative turbulence that followed the attacks, and that in time would lead to improvements in structural design. But as Corley had expected, by spring, as the last remains of the Trade Center were being removed, the essential facts were known.
The death of the South Tower was better understood than that of the North, because its symptoms showed particularly well. The 767 that approached from the south, United Flight 175, was a machine designed for lightness and efficiency, a slick aluminum tube 159 feet long with graceful wings stretching 156 feet from tip to tip, two fuel-efficient engines, and a cockpit that offered the pilots a gorgeous view. It was minimally loaded that morning for the Boston–Los Angeles run, with only sixty-five people aboard and about half of the maximum fuel, and as it approached the building it weighed 137 tons. It was flying at nearly 590 mph, which was more than 150 mph above the airplane’s designed limit at low altitude. In the cockpit the overspeed warning must have been warbling loudly.
The tower that soared above the cockpit in those last seconds was in its own way designed for lightness and efficiency—and it, too, was essentially a tube, though squared, 1,362 feet tall, and made of steel. Each of its faces was 209 feet wide and consisted of a palisade of aluminum-clad fourteen-inch box columns, which when bolted and welded together and braced by horizontal plates formed the exoskeleton that carried half the building’s weight, and gave it all of its stiffness against side loads. The side loads were expected to come primarily from winds and, at the design limit of 150 mph, to amount to a maximum sustained 13-million-pound push. But the building was more resistant even than that, because the Port Authority, out of conservative impulse, had decided during the construction to go beyond the design and anchor the foundation structure directly into bedrock. Hidden inside the square tube of perimeter columns stood the other half of the support structure: a central core consisting of massive columns arranged in a rectangle much longer than wide, which was aligned in the South Tower on a north-south axis and contained the elevators and emergency stairwells. Though the core was not designed to resist side loads, its alignment meant that the north-south axis was, however marginally, the tower’s strong one. More significant, it also meant that the acre of column-free office space on each floor (the goal of this tube-inside-a-tube design) was wider on the east and west sides (think viewless cubicles) and narrower on the north and south. The floors were built on thin steel trusses that bridged between the core and perimeter columns and supported corrugated-metal decks on which concrete was poured. The floors were thin and light, and served as diaphragms to stiffen the exterior columns against buckling forces. Because the vertical load lessened significantly with height, the thickness of the steel in the columns was progressively reduced from the bottom of the tower to the top. But no matter how it was trimmed, the weight added up. Ignoring all the people, furniture, and equipment that the tower contained, its structure alone weighed some 600,000 tons.
By comparison, the oncoming 767 was a mere pebble. But as pilots know, the energy of an impact varies with the square of the airplane’s speed—meaning if you go a little faster, you hit a whole lot harder. And this airplane was moving ferociously fast. The killer at the controls almost missed, and would merely have clipped the left wing, but he hooked his intent last turn like a wolf after prey, and punched hard into the south face of the tower three fourths of the way up, on the east side of center. The airplane hit about nine times harder than it would have during a typical accident while on an approach to landing. The building never came close to toppling sideways, but it swayed as much as twelve inches at the top, and endured a violent impulse that ran vertically through it like a rebounding wave. The airplane entered the building with its wings banked left, stretching from the seventy-eighth to the eighty-fourth floor, slicing through half the south-face perimeter columns. Frame by frame, the videos show it heading inside as if intact. An MIT study later estimated that only 6 percent of the plane’s kinetic energy was expended against the south face. Almost all of the rest of the energy was expended inside—25 percent in tearing up the floors and pushing and piling the cubicles, and another 56 percent in damaging or severing as many as half the columns in the core. Two of the three stairwells were taken out, along with the people who were using them to evacuate the building in response to the earlier North Tower attack. The third stairwell was damaged but remained usable, and allowed several brave and desperate people trapped above the impact zone to escape. A few of the airplane’s heaviest parts—an engine, a piece of landing gear, a wheel—kept right on going, as far as six blocks into the city, and a section of fuselage hit the roof of World Trade Center Five. But most of the airplane remained inside the intended victim, piled into a dike of mixed debris against the tower’s north and east perimeter walls.
One of the many astonishments of that day is that the building was able to swallow an entire 767, and to slow it from 590 mph to a stop in merely 209 feet. According to calculations based on photographic evidence of the damaged perimeter, the structure had redistributed the gravity loads so efficiently that only twenty feet from the entry wound the demands being made on intact columns were hardly higher than normal. The building was certainly not safe. It had suffered serious if unknowable damage to the east side of the core. At the same time, for seventy feet along the inside of the south face, and perhaps a short distance along the east face, the floors had been torn loose from the structure, leaving perimeter columns unbraced under the gravity load of the roughly thirty-story building overhead. Still, if not exactly shrugging off the hit, the South Tower had absorbed it well: absent an earthquake or a strong windstorm, the building would have remained standing indefinitely.
But then, of course, there was the fire. As the airplane disintegrated, it released its fuel—roughly 10,000 gallons of volatile “Jet A” kerosene that sprayed and vaporized through the six floors of wreckage, poured into the elevator shafts, shot through broken windows along the north and east faces, and immediately ignited. Apocalyptic though it seemed, the huge fireball that blossomed over the plaza was actually to the building’s advantage, because it consumed as much as a third of the available fuel, releasing the heat harmlessly into the air. That left two thirds of the fuel inside the tower, however, and it was widely spread and burning.
The building’s sprinkler system had been destroyed by the impact, but in any case it would have lacked the pressure to operate effectively over such a wide area. More significant, much of the light spray-on fireproofing that coated the structural steel had been knocked away—and precisely in the same east-side areas that had sustained the worst damage and now were threatened by unusual and increased loads. On only one of these floors had a fireproofing upgrade that doubled the thickness of the coating been completed—a circumstance that provided for healthy debates afterward, though there was never direct evidence that the thicker fireproofing would have better endured the impact.
At any thickness, the coating was meant to insulate the steel and prevent it from overheating in a fire for at least two hours (by which time an office fire will typically have burned itself out in any given part of a building). The problem with overheating is that long before steel melts, it weakens. In structural steel like that of the South Tower the weakening begins around 350°. By 1,100° the steel loses about half its strength. Kerosene burns twice that hot inside the carefully crafted furnace of an airliner’s engine, but within the imperfect combustion chambers of the South Tower impact zone it is thought to have reached at most only about 1,500°—a temperature unlikely to induce the failure of such a tremendously redundant design, especially on a gentle, windless day. Moreover, the kerosene fire simply did not last long enough at any temperature to
overheat the building’s massive columns. Corley’s fire specialists believed that the jet fuel never collected into deep, slow-burning pools, and that it burned through entirely within four minutes. They concluded that jet fuel itself did not bring the tower down.
What it did do, however, was set off raging office fires simultaneously on six different floors. It was a conflagration that would have been impossible for the firemen to control, had they gotten to it—a fire large enough to create its own powerful winds, sucking oxygen in through all the perimeter holes and broken windows, generating energy three to five times greater than that of a standard nuclear power plant, and eventually heating the steel to temperatures as high as 2,000°. The fire fed on wrecked office furniture, computers, carpets, and aircraft cargo, but primarily it fed on ordinary paper—an ample supply of the white sheets that were so much a part of the larger battlefield scene. Without that paper, Corley’s experts believed, the fire might not have achieved the intensity necessary to weaken the steel beyond its critical threshold. It would be simplifying things, but not by much, to conclude that it was paperwork that brought the South Tower down.
On the debris pile in the northeast corner the fire melted the remnants of the shattered airliner, which half an hour after entering the building began to flow in a stream of molten aluminum down the tower’s outside. Still the tower endured. But then the southeast corner of the eightieth floor collapsed, triggering the progressive failure of the floor along the entire east side of the building. Dust plumes shot out through the broken windows. The eastern perimeter columns now stood unbraced for the space of at least two floors, adding to the dangers that the building already faced from the unbraced condition on the perimeter on the south side. Shortly afterward, in the southeast corner, near the entry wound, a cluster of exterior columns began to buckle. It was 9:59 exactly, merely fifty-six minutes after the airplane hit. Peter Rinaldi was living the last four seconds of his earlier life. Frank Lombardi was pushing into the hotel bar somewhere far below. With its support giving way beneath it, the top of the tower tilted east and then south, rotating in a clockwise direction, and suddenly slammed down. Even if it had been felled from below, the tower could not have capsized in a conventional sense, because like most other buildings, it lacked the structure to hang together for more than a few degrees off vertical. But it was not felled from below; it was hammered from above, and it accelerated as it fell, crushing the core and peeling back the exoskeleton with each successive floor. As the external walls peeled, they broke primarily at the bolted connections, allowing welded prefabricated sections of the columns to fall free. The upper sections fell east and south, and hit the Bankers Trust building; the lower sections fell north and west, and gave the Marriott hotel the first of its two fatal blows.
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