News

Australia's tall building design fails terrorist attack test

Media Release, Monday 9 September 2002

University of Melbourne engineers have found Australia's tall buildings are likely to collapse following even a moderate bomb blast or collision with a light aircraft.

Associate Professor Pryian Mendis and PhD candidate Tuan Ngo assessed Australia's tall buildings and constructed a computer model of a standard 52-storey Australian-designed building. Using sophisticated computer software, they then investigated what would happen to the structure under extreme loading from bomb blasts or collision from a light aircraft.

They have presented their research at a number of national and international conferences this year. They will present a similar paper at the International Association for Bridge and Structural Engineering (IABSE) symposium in Melbourne this week.

"Under the extreme loading experienced during such events most Australian tall buildings would probably suffer progressive collapse, which occurs when a section of the building fails to support the load above and triggers a cascade of failures leading to the collapse of most or all of the building. The Murrah building in Oklahoma, which collapsed in 1995 after a bomb went off, and the World Trade Centre suffered this fate," says Mendis.

They recommend that guidelines on abnormal load cases and provisions to prevent progressive collapse should be included in the current Australian building regulations and design standards.

"Australian tall buildings are only designed to withstand the low intensity lateral forces from high winds or minor earth tremors. A bomb blast or collision is a short duration and high intensity load which Australia's buildings are not designed to withstand," says Mendis.

"It is universally accepted that it is unreasonable to design buildings to withstand an impact from a large jet such as was experienced by the World Trade Centre, but some extreme loads can be considered," he says.

"The ultimate aim is to localise the damage to the impact or blast site and prevent progressive collapse of the building, or at least delay collapse long enough to allow evacuation of the building.

"It was estimated that 80 per cent of the deaths in the Oklahoma bombing were due to the progressive collapse of the building rather than the blast itself."

Australia's tall buildings, and many others around the world, are designed around a central core that contains things such as the lifts and emergency stairs. This core is designed to take most, if not all, of the loading that comes from forces such as wind or earthquakes.

The external frame is designed to take vertical loading in the form of weight from the structure above and human and materials on each level only.

"When a building designed this way is hit by a plane or bomb blast it is the external frame, consisting of support columns, the beams between the columns and floor slabs, which gets knocked out," says Mendis.

"The building is left without anything to hold it up and it will collapse," he says.

Mendis and Ngo suggest engineers need to focus on strengthening the external frame to help prevent progressive collapse in extreme loading events. They say a range of simple, economical measures could be implemented today and add significant strength.

These include continuous top and bottom reinforcement of floor slabs. Currently, Australian standards only require top reinforcement. Under loads, the top reinforcing steel of floor slabs will rip out resulting in collapse of the floor. With bottom reinforcement steel running through the slab into the column, when the floor slab fractures, the bottom reinforcement should act like a net catching the floor and holding it in place.

Mendis and Ngo are now looking at ways of designing building so that its vertical load is distributed throughout a range of support areas.

"This way, if one or more support columns and beams are knocked out, progressive collapse is prevented as the load from above is distributed laterally and onto other columns and beams," says Mendis.

The University of Melbourne, the CSIRO and the Australian Defence Force Academy (ADFA) are also collaborating on research to find ways of improving the high strength concrete (HSC) currently the preferred construction material for the majority of Australia's tall buildings.

HSC is susceptible to shattering (spalling) under rapid and high intensity heat, for example following the ignition of hydrocarbon fuel. The collaborative research project will test the fire resistance and structural behaviour of HSC under impact and hydrocarbon fire.

"HSC has never been tested under these conditions before, so nobody knows if, in its current form, it is a suitable construction material to use to strengthen building against extreme loads," says Mendis.

Associate Professor Mendis has worked as a consulting engineer in Melbourne before joining the University of Melbourne in 1991. He has been involved in designing many tall buildings in Australia. Ngo has designed many tall buildings in Hanoi.

Mendis and Ngo's analytical work is supported by a grant awarded by the Victorian Partnership for Advanced Computing (VPAC).