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Can engineers save the megatower?

News analysis : Megatowers after WTC

Events in New York last month have cast a long shadow over megatower projects worldwide. Four weeks on, Dave Parker looks at the options for designers seeking to reassure clients.

MEGATOWERS have long been the focus of passionate debate.

Only in geographically constrained locations like Manhattan and Hong Kong can they be judged as completely rational responses to short-lived coincidences of high land prices and burgeoning customer demand.

Many towers, such the Empire State Building in its early years and the Petronas Towers today, struggle to find tenants willing to pay economic rents. Now, after the World Trade Center tragedy, it could be even harder for promoters to make the numbers add up, especially if extra safety measures are demanded by potential tenants.

The challenge for engineers involved in such projects, therefore, is to meet such demands with a package of economically viable solutions.

Preliminary analysis of the 11 September disaster (see box) shows that fire was the real killer. 'The main fire load probably came from the existing contents of the buildings, ' says BRE fire and risk sciences division managing director Jeremy Hodge. 'But the problem was that the aviation fuel in the planes started fires on several floors at once - and that the temperatures would have risen much quicker than in a normal office fire.'

Hodge believes that the fire's behaviour would approximate much more closely to the hydrocarbon fires offshore oil structures are designed to resist. He also suspects that much of the spray applied fire protection to the steel around the area of impact would have peeled off as the structure flexed.

Modern spray applied fire protection systems are routinely tested for resistance to flexing, and Hodge sees no reason why this type of system could not provide the extra fire protection that some clients may demand.

'But if they start demanding four hours' fire resistance for the whole structure, the only economic answer would probably be concrete cores and composite columns, ' says Connell Mott MacDonald (CMM) divisional director James Bennett. CMM is currently working on two concrete framed residential megatower projects in Australia, including the world's tallest, the 88 storey, 300m high Eureka Tower in Melbourne.

Bennett says these are less of a technical challenge than office towers due to lower occupancy.

He adds a warning: 'Concrete may be more inherently fire resistant than steel, but in the sort of temperatures that could be reached in a post airliner impact fire even concrete could be struggling.'

Improved fire resistance alone is not the whole answer. A hypotethical strike involving a 550t Airbus 380 rather than a 187t Boeing 767 would cause proportionaly greater structural damage, and preventing immediate progressive collapse would become more important.

Hodge warns that progressive collapse after structural failure on a typical megatower is very difficult to avoid. 'Once you get upper floors moving downwards there's not much you can do about it, ' he says. 'Dynamic loads increase exponentially.

'One possible solution in some situations is to use internal structural deflectors to shed collapsed floors sideways, away from the floors below. This would not be viable in dense urban environments though, obviously.'

Bennett points out that airrights buildings over rail lines are already designed to survive the accidental loss of a column.

'But how many columns do we assume we would lose in a plane impact?' he asks. 'Providing such structural redundancy doesn't come cheap.'

Arup's preliminary analysis of the disaster came to the conclusion - among others - that it might be possible to design in 'collapse' storeys every 10 floors or so, able to carry the debris load from the nine collapsing floors above. But Arup director Faith Wainwright says the real post-disaster challenge is faced by the building owners, not the engineers. 'Proper management of the occupants, especially in a simultaneous evacuation, is very difficult.

'It's no use sitting back and waiting for the emergency services to arrive and tell you what to do. Effective responses to emergencies like non-compartmentalised fires, gas attacks or bombs immediately outside the building should be worked out and someone trained to take charge and order the correct action.'

Massive, well-compartmentalised concrete cores to escape down would also be desirable, most experts agreed, with escape stairwells pressurised to minimise smoke intrusion. Providing increased escape capacity while retaining enough of the floor plate to make the project commercially viable will tax the designers' ingenuity, but this, and increased fire resistance, is probably the minimum reassurance that clients will demand.

One obvious consequence of the disaster, Bennett believes, will be the end of mixed use towers. The 100 storey Arupdesigned Kowloon Station tower in Hong Kong could still feature 15 storeys of six star hotel above the office floors - but this is under review. The same applies to projects with plans for residential accomodation on the upper floors. And while mega office towers will still be built, there may be a quiet revolution in the pecking order, with the high status accomodation now near ground level.

Fuel fireball

Aircraft, for all their speed and power, are fragile structures. Fly a 400t 747 into a granite cliff at 800km/h and a microsecond later all that will remain is a cloud of aluminium confetti and four flattened lumps of steel and titanium slag. Much the same happened to the 200t Boeing 767s which smashed into the World Trade Center towers on 11 September.

Comparatively lightweight structures by some standards the towers may have been, but they absorbed the colossal kinetic energy of the doomed airliners without suffering immediate catastrophic trauma. Some of the perimeter columns that made up the very strong structural facade were smashed aside and composite floor plates buckled by the initial impact, but the wings and fuselages of the 767s seem to have shredded almost immediately.

Survivors' reports indicate that some debris, most probably an engine, penetrated at least one of the central drywalled service cores, although most escape stairs appear to have remained functional until the final collapse.

But impact resistance is only half the story. What did the real damage in New York was the 80,000 litre of aviation kerosene each 767 was carrying.

As the aircraft disintegrated and fuel tanks ruptured the tonnes of fuel partially vapourised and fireballs erupted out through shattered windows.

Radiant energy from these fireballs would have ignited flammable materials on desks near windows on nearby floors. The rest of the fuel would have gushed out into the heart of the destruction, spreading downwards through buckled floor plates, igniting floor after floor. Within minutes, up to 10 floors of each building were burning furiously, with temperatures in each rapidly peaking at 1,300degreesC or more.

No office tower anywhere in the world is designed to survive such a fire. Normally it would take hours for a conventional fire to spread up 10 floors, by which time the building's population would have long since escaped. With multiple floors alight and fire protection to the steel structure partially degraded by the initial impact, columns soon began to buckle.

Catastrophic progressive collapse followed, in less than one hour in the case of the South Tower, which was struck some 30 floors lower than the North Tower. Following the 1993 bomb attack on the World Trade Center it took nearly four hours to get all the occupants out of the building. This time around 90% of the 50,000 or so hapless workers and visitors made it out alive - 5,000 were unlucky. Many died because they stuck to the phased evacuation plans designed to cope with conventional fires.

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