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Analysis of a disaster

US engineers are pushing for a full computer simulation of the World Trade Center collapse to understand how it happened. Margo Cole reports.

The collapse of the World Trade Center (WTC) towers has driven engineers and researchers to use all the tools at their disposal in an attempt to understand why the buildings failed in the way they did and to make sure it does not happen again. Computer modelling could help with this process, according to a leading engineer who testified at the US House of Representatives Science Committee's hearing on the collapse.

Dr Abolhassan Astaneh-Asl, professor of civil and environmental engineering at the University of California, Berkeley, showed the committee a computer analysis of a Boeing 747 hitting a generic steel framed high rise building, and is convinced similar techniques could be used to learn more about the failure mechanisms that resulted in the collapse of the twin towers.

All he needs, he says, is full access to all the structural data that has been gleaned from the investigations in the form of drawings, videotapes and physical evidence.

'The example demonstrates the power of advanced technology developed in aerospace and mechanical engineering that can be brought to bear on this problem, ' he told the committee.

'We plan to use the drawings and the data and the software used in the example to build a computer based realistic model of the World Trade Center towers and analyse their response to simulated impact of the 767 planes and the ensuing fire.'

Astaneh-Asl used a variety of products from global computing company MSC. software to carry out the analysis of the 747 crash.

First he created finite element models of both the plane and the building. Concentrated masses were added in select locations within the aircraft model to simulate cargo, passengers and fuel, giving a total weight of 304,000kg, which is representative of a fully loaded 747.

The building was a six storey steel structure with tubular beams and columns and rigidly fixed foundations. Typical office loads were applied to the upper and lower floors, but in the central location - where the impact was to occur - the floors were not incorporated in the model because the engineers wanted to study the effect of the impact on the beam/column connections.

In all, the entire simulation model of the plane and the building included 61,000 finite elements.

The impact event analysed by Astaneh-Asl involved the plane crashing into the building between the 3rd and 4th floors at a speed of 200m/s. It was modelled using MSC.Dytran, a finite element program designed specifically for simulating events that involve both structures and fluid systems. It is typically used for predicting high speed events like crashes, but more often in mechanical or aerospace applications than situations involving building structures.

Predefined excessive strain levels were built into the model for both the aluminium of the plane and the steel of the building frame, and the finite elements in the two structures failed once they reached these levels. Once an element failed in the simulation, it no longer provided structural strength or stiffness, so the engineers were able to see how the progressive disintegration occurred.

After the impact simulation, a heat transfer and thermal stress analysis was carried out using a program called MSC.Marc, to model what happened once the fuel tanks in the plane ruptured and the fuel began to burn.

The software company admits the exact cause of the WTC collapse cannot be determined by this preliminary analysis, but says it does support the general consensus that the failure was thermally induced. The crash substantially weakened the structure but, because of structural redundancy, the gravitational load was redistributed, according to a paper by the MSC Corporation's Casey Heydari and Ted Wertheimer.

'Once members were further weakened due to the high temperatures, the load could no longer be supported, ' they say.

'Also, buckling in the trusses was possible, especially after the damaged cross beams no longer provided any lateral support.'

While accepting that it might be too costly to design and construct buildings to withstand this type of damage, they say: 'Non linear analysis may be used to determine the amount of redundancy required to mitigate the problem and to predict the time before full collapse occurs.'

They say the software could be used to carry out 'what-if' studies on both thermal parameters - such as insulation and wall thickness - and on structural parameters, including spacing of beams and columns, to analyse a full spectrum of load conditions.

Professor Astaneh-Asl spent 25 days conducting field investigations and collecting data in New York, including talking to the structural engineers involved in the WTC design, and studying the steel from the collapsed towers. However, he also wants access to full engineering drawings, design and construction documents and photos and videotapes taken during and immediately after the collapse.

He believes that feeding this information into the computer model could prove vital in establishing what happened to cause the structural collapse.

'There is a need for a comprehensive, in depth and researchoriented study of the WTC buildings from the time of plane impact through the ensuing fire and the final collapse, ' he told the House of Representatives science committee.

Astaneh-Asl's work to collect basic data from the site was one of eight research projects funded immediately after the attack by the US National Science Foundation (NSF). Grants went to both engineering and social science researchers to conduct post-disaster assessments at the terrorist attack sites.

Other projects gaining NSF funding included the use of handheld technology for data collection on structural damage at the WTC, and the use of a land-based laser system to produce high resolution 3D 'maps' of the interior and exterior of damaged buildings.

Data collected on all eight grant funded projects will be used to help improve the structural integrity of buildings, utilities and other infrastructure during fires, earthquakes, explosions and other hazards, and also to improve the country's response to such threats.

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