Following the New York World Trade Centre disaster of 11 September 2001 when two jet aircraft were flown into the Twin Towers, and when one was flown into the Pentagon, these and many other questions have been asked. As part of the review an international summit meeting on tall buildings and a meeting of the International Council for Research and Innovation in Building and Construction (CIB) Task Group TG 50 – Tall Buildings – were held in London in April 2002, Kuala Lumpur in 2003 and Ottawa in 2004. The objectives were to discuss what happened during the 11 September incident in order to develop strategies for coping with any future such disasters.
General areas of discussion included structural performance, evacuation issues, insurance/reinsurance perspectives, fire service access and operations, megacities/megastructures, city planning issues, regulatory considerations, risk perception and fire engineering.
While the WTC disaster was the catalyst for the meetings, they took the form of a forum to improve building design, egress and disaster planning in a general sense.
Building design is all about risk management, the likelihood of certain adverse effects and the likely consequences of them happening. In the past such design events have been encapsulated in standards and building codes. The UK Building Regulations were amended after the Ronan Point collapse in 1968 that was caused by a gas explosion. This caused the progressive collapse of the 22-storey building. The subsequent enquiry found that the risk was “foreseeable”, and the Regulations were amended to stipulate that a building is to be so constructed that in consequence of an accident the structure must not fail or collapse disproportionately to the cause of the damage. The requirement applies to buildings of five storeys or more. This may well be considered all that is necessary in terms of additional wording in the New Zealand Building Code and Codes in some other countries. If such a simplistic approach were taken who would decide on the type and degree of hazard to be designed for and how would consistency in that regard be attained?
The Twin Towers were designed in the 1960s for the largest aircraft of the time getting lost in fog, and at a relatively low speed hitting one of the buildings. They were arguably designed to withstand the impact of a jetliner, as they withstood the initial impacts in the September 11 event and allowed 99% of those below the points of impact to escape the buildings. They eventually failed, however, from the effects of the subsequent blast effect, fireball and intense fires. There are no current standard tests available to evaluate the consequences of a sudden pressure pulse as experienced at the WTC site. This will be an area of future research.
The defeated safety systems included:
- fire proofing· sprinklers (with risers broken)
- compartmentalisation (a five-storey high hole in the side of the building)
- egress stairs (blocked above points of impact)
- pressurisation and lighting
- structural (with 40 columns destroyed)
Present engineering risk/statistical analysis techniques would put this disaster into an “acceptable” engineering risk paradigm. Society may, however, have greater expectations, and that is the challenge ahead.
The Future
The discussions were comprehensive and it is likely that some worthwhile recommendations will result from the conference and from the deliberations of TG 50 over its series of meetings.
The research agenda for the future is likely to include:
- Integration of building and infrastructure systems
- Effects of a sudden pressure pulse
- Redundancy of buildings and systems
- Egress
- Urban search and rescue
- Public education and education of insurers
- Performance based design standards
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