The World Trade Center collapse
Reporter S. Alexander
The following is a report by Stuart Alexander of a meeting held at the ICE on 25 June 2002 organised by the ICE’s Structural and Building Board and chaired by Gordon Masterton. The meeting was an excellent example of the ICE’s role as a learned society, bringing together practitioners and experts to consider an issue of global concern for the benefit of advancing knowledge and improving standards. John Roberts, Chairman of the UK based study group on ‘Safety in tall buildings’ gave the vote of thanks.
On 25 June 2002, Ed DePaola mesmerized a capacity audience of 350 for a full two hours with his lecture on the World Trade Center collapse of 11 September 2001. Ed is Principal of Severud Associates, the current President of the Structural Engineers Association of New York, and a member of the FEMA–ASCE (Federal Emergency Management Agency American Society of Civil Engineers) investigation team. The printed word cannot describe the impact of more than a dozen video clips and a hundred still images, described and illuminated by a leader of the New York Structural Engineering community, who, like so many, lived with the collapse and its aftermath so closely that he only had two days off from continuous seven day working in the first four months after the tragedy. The broad facts are well known, but many of the details are not. The north tower—WTC1—was hit by a Boeing 767 carrying 10 000 US gallons (38 000 litres) of fuel flying at 470 mph centrally on the north face between the 94th and 98th floors. Fifteen minutes later the south tower—WTC2—was hit by another Boeing 767 with 10 000 US gallons of fuel, but flying at 586 mph, on the south face towards the southeast corner between the 78th and 84th floors. The different orientation of the rectangular central core—East–West in WTC1, North– South in WTC2—combined with the different speeds, heights and angles of impact is what made the modes of collapse different. Examination of videos and photographs showed that the aircraft had sliced through 36 of the 59 steel facade columns on WTC1 and 32 of them on WTC2. However, both structures were able to redistribute the loading, probably helped by a two-way system of ‘hat’ trusses at roof level, and therefore did not collapse immediately, although some loads were probably close to ultimate. Both aircraft broke two or even three floors on impact and ploughed into the central core areas, pushing furniture and other debris in front of them. Jet fuel was sprayed over a wide area and immediately ignited. Some burned in the orange fireballs visible outside the building facade, some drained down the lift shafts, igniting in the entrance lobbies with lethal effects, and the rest acted like lighter fuel on a barbecue, so that although it burned up within 3–4 minutes, the temperature had risen to 1000 C and several floors, each an acre in extent, were alight. This in itself overwhelmed the sprinkler system, although the supply pipes were ruptured as well. The lift shafts and stair enclosures were formed with four layers of 16 mm ‘sheetrock’, and of the three staircases in each tower, only the northernmost one in WTC2 was not completely destroyed in the impact. As a result, only four people from the floors above the impact levels in either tower survived. The sprayed mineral fibre fire protection was installed to a two-hour standard to the open-web truss floor system and three hours to the columns. However, the impact dislodged this protection extensively, so that the steel gradually degraded as the fires raged and collapse became inevitable. WTC2 failed first, 56 minutes after the impact, starting with the perimeter wall at the South-East corner. WTC1 survived for 102 minutes; the rooftop antenna moved first, showing that the initial failure occurred in the central core. The floor trusses were supported at each end on cleats and fixed with two bolts; these weak connections failed, leaving the columns unrestrained, and progressive collapse followed. The next largest building on the site, the 47-storey WTC7, burned unchecked for seven hours before ultimately collapsing due to the failure of two-storey high transfer trusses located between the 5th and 7th floors over a major sub-station. The failure to burn out within the usual two hours was put down to highly flammable transformers plus fuel provided for the 16 or more generating sets in the building, boosted by confiscated contraband stored by the secretive government agencies occupying the building. One other building, WTC5, also collapsed from fire, and these two are the only buildings anywhere in the US to have collapsed from fire alone. Many of the surrounding buildings suffered major ‘collateral damage’, but did not collapse. Examples included facade columns destroyed for part of their height and a huge pile of rubble perched six storeys above the ground. The whole is a success story; of 58 000 people working in the complex at the time, only 2923 died, although that includes over 300 emergency workers. The twin towers stood long enough for everyone who had an unimpeded route to get out, and most of the surrounding buildings remained standing in spite of considerable damage and loading from debris. But the criteria have now changed. It is not practicable to design structures to resist impacts from all current aircraft, let alone those planned for the future, but nevertheless structures need to be more robust and redundant: Ed recognised the knowledge available in the UK. Passive fire protection needs to be able to resist impact and rapid temperature rise, and to protect structural members and their connections even when distorted. Egress systems need to be able to resist damage. Phased evacuation is probably no longer enforceable. Thought needs to be given to stairway lengths, fatigue and the disabled—should lifts be used? Structural drawings need to be available at short notice and stored so that they can be viewed walking around with a laptop, and fire fighters should be given lessons in structural engineering. Above all, keep the threat away from the structure. September 11th was not a failure of structural engineering but of security—or even of politics. The full FEMA–ASCE report is available free to study and download at www.fema.gov/library/wtcstudy.htm. It is advisable to note that it runs to over 170 pages plus 70 more in the appendices, and takes a while to download over a telephone line. The audience were silenced and the few timorous questions asked only served to show how all-embracing the talk had been.
The World Trade Center collapse