REPRINTED FROM UK's 'FIRE' MAGAZINE NOVEMBER 2001

PAUL GRIMWOOD

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Built in the late 1960s and early 1970s the twin towers of the World Trade Centre NYC stood over 1,400 feet high with a total of 55,000 employees housed within the 110 storey structures. During the course of a working day this population might increase to over 100,000 as many visitors passed through. 

  With a natural sway of three feet from centre the WTC was designed to withstand excessive stress, perhaps even a direct hit from a small airplane. That design strategy was based on an incident that occurred 56 years ago when an air-force B-25 aircraft accidentally ploughed into the 79^th and 80^th floors of the Empire State building, killing 14 people. Although damage was excessive the structural integrity of the building was never compromised.

  The WTC towers strength came from a reinforced central steel core and from closely spaced load-bearing steel columns and beams sited around the perimeter in tube design.  According to US structural engineer Eugene Corley the collapse of the structures following such impacts and fires were inevitable although it was impossible to say when this would occur. The north tower was the first to be struck and stood for one hour and forty-four minutes before collapse occurred, whilst the second (south) tower was only able to maintain its stability for some forty-seven minutes following impact. With over 90,000 litres of aviation fluid burning within each structure and a large amount of structural support destroyed on impact it was only a matter of time before the remaining steel support girders lost their strength leading to an estimated 100,000 ton load pan-caking onto to the floors below. The subsequent failure of lower levels would again have been an inevitable result of such excessive loads falling from above. If the initial impact points had resulted around the midpoint or lower in the structures the collapse would have more likely been sideways.

In an interview for BBC News Online Mr John Knapton, professor in structural engineering at Newcastle University, was highly critical of firefighters for entering the structures in the first place and went on to say that “if they did decide to take the risk, they should have been pulled out after an hour”.  Mr Knapton acknowledged that the sheer scale of the situation probably overwhelmed the operational commanders but went on to say, “you would have thought someone with technical expertise would have been advising them”.

  Whilst Mr Knapton makes some valid points I consider his assessment of the situation is over-simplified in that he fails to take into account a fire commander’s risk analysis which under such circumstances must be based on a calculated risk. The New York Fire Department have extensive experience of high-rise firefighting and are extremely familiar with the logistics of evacuation from the twin towers having rescued 20 occupants suffering cardiac problems, 29 pregnant women and an untold number of disabled people and young children (many trapped inside the tower’s 208 lifts) amongst the thousands of people they evacuated during a serious incident there eight years ago. Expected evacuation time from the WTC in its entirety is around two hours. It is further calculated from experience that, when using stairshafts for access, firefighters ascend at an average rate of two minutes per floor.

Past experience of high-rise incidents has demonstrated the structural integrity of central core buildings is usually to be relied upon for several hours under normal fire loads. However, the twin towers had been severely weakened by the impacts and subsequent high temperature fires and the likelihood of collapse was always of concern. It goes against the psychological profile of the firefighter to arrive and watch people die without any attempts being made to rescue them or slow the spread of fire. So often, firefighters function under an element of calculable risk based upon their level of experience to date. Nobody could anticipate the time-scale involved before the structures collapsed. Indeed, Mr Knapton himself suggests that if the firefighters decided to accept the risks of entering the towers in the first place then they should have been pulled out after an hour. However, based on that advice the firefighters in the south tower would have been caught in the collapse anyway and those in the north tower would have watched helplessly as bystanders from the sidewalks for a further 44 minutes as people continued to jump from upper floors.

It appears that the vast majority of firefighters who lost their lives were located in and around the south tower and the sudden and unexpected collapse of this structure signalled an immediate reaction from the incident commander (IC) to recall his crews from operating within the north tower. In effect this allowed 39 minutes for firefighters to evacuate the structure and many did so, but it is reported that some would not leave a large number of burned victims located at a staging post around level forty.

There will, of course, be major lessons learned with hindsight from this incident as there always are from any large conflagration. However, in talking to several senior UK fire officers who had followed the incident live on television they all immediately confirmed to me their strategic approach would have closely followed that of the IC in New York, committing large numbers of firefighters into both towers to undertake firefighting and rescue work at close quarters whilst constantly assessing the structural stability inline with previous operational experience. We must now wait and see if and how this new experience will influence future firefighting strategy in heavily occupied high-rise structures, particularly under such circumstances of major terrorist activity.