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No cause for complacency over Pirelli disaster


Thursday 18 April was the 'thinkable' disaster. A light plane, its pilot lost, confused or suicidal, smashes into a moderately tall building. Four lives are lost, a small fire is soon extinguished. Damage to the curtain walling is spectacular but non-critical. Evacuation procedures and emergency services response seem to work well.

Once the terrorism possibility is ruled out and the world's stockmarkets recover from their knee-jerk response, the lessons from Milan seem all to be positive. An iconic, pioneering structure, as innovative in its day as the World Trade Center, has been put to the test - and performed well. Or has it?

A definitive verdict is premature, but certain conclusions can be drawn immediately. Multiple escape stairwells, separated from the central service core, is a design option that could be much more attractive in the future. Last week those in the Pirelli building had a safe, unobstructed escape route well away from the area of impact. And once again, sprinklers have worked as intended, confining the fire to a small area and minimising the risk to the building's occupants and the emergency services.

But the sagging of the 26th floor slab, reported in News this week, is worrying. At the time Pierluigi Nervi began work on the design in the 1950s, the world famous post-tensioned concrete guru must have been well aware of the 1945 tragedy at the Empire State Building in Manhattan. Then, an 11t B25 bomber, lost in the fog, slammed into the steel framed structure at more than 300km/h - without catastrophic results. WTC structural engineer Leslie Robertson certainly was aware of the incident when he began to design the Twin Towers a decade or so later.

In the event it was two 180t Boeing 767s which brought down the twin towers. Comparisons with the Milan disaster are illuminating. In terms of kinetic energy the impact on one of the towers was at least 400 times greater than that in Milan and probably more. The main fuselage of the single engined Rockwell Commander 112 aircraft seems to have smashed straight through the flimsy 1960s curtain walling on one side of the building and out the other without encountering any serious solid obstruction.

And there was probably little more than 50 litres of petrol in the wing tanks of the Commander, compared to 50,000 litres or more in each of the 767s, although proportionally more seems to have exploded.

In both cases it was the fuel which seems to have done the real damage. Robertson admits he never considered the possible effects of an aircraft's fuel load, so his towers stood up well to much larger impacts than originally envisaged but failed to cope with fires starting on many floors simultaneously. Nervi's creation was barely tested by kinetic impact - but the damage from what was not a serious fuel explosion does seem disproportionate.

Or was the explosion simply the proverbial straw on an overloaded camel's back? Were Nervi's 50 year old assumptions about office floor loading simply wrong, with the floors already operating with very low factors of safety? One thing is certain. It is too early to be complacent about what happened in Milan on 18 April.

Dave Parker is NCE's technical editor.

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