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From CAD User AEC Magazine  Vol 15 No 05 - MAY 2002

MSC.Software Corp has provided simulation software tools MSC.Dytran and MSC.Marc to a US Congressional Committee to simulate and understand the collapse of the World Trade Center buildings.

On March 6, 2002, Dr. Abolhassan Astaneh-Asl, PhD., professor of Civil and Environmental engineering at the University of California, Berkeley, testified before the Committee on Science of the U.S. House of Representatives regarding his investigation of the WTC collapsed structures. His testimony included a series of simulations, developed in conjunction with MSC.Software using MSC.Dytran, of a 747 jetliner crashing into a steel structure.

"The tragic events of September 11th prompted us all to ask what we could do to help," said Frank Perna, chairman and chief executive officer of MSC.Software. "MSC.Software has a long history of working with researchers and governmental entities around the world to recreate complex engineering problems, including those dealing with forensics, in the hope of preventing tragedies like the World Trade Center collapse in the future. We are proud to have played a small part in the efforts up to this point and hope more governmental agencies will take up our offer to provide our software and services free of charge to any government entity investigating the attacks."

"Understanding how and why the World Trade Center buildings collapsed will help us prevent this type of building collapse in the future", said Dr. Astaneh-Asl, "To simulate the very complex issues and non-linearities involved in analysing the impact of an airplane and the ensuing fire you need the most powerful and advanced software, such as MSC.Software's simulation tools. The contributions of MSC.Software to our research program have been invaluable. We will continue to use these tools in our investigation to gain insight to what might have caused the collapse and to learn valuable lessons that can be applied in the future to prevent such catastrophic collapses."

The images accompanying this article show an analysis of the performance of a generic steel high-rise structure subjected to the impact of a 747 jetliner and the ensuing fire. The example demonstrates the power of advanced technology developed in aerospace and mechanical engineering that can be brought to bear on such a problem. Professor Astaneh has suggested during the investigation (which is still ongoing, with a fuller report due out in September this year) that a possible cause of the collapse was the buckling of the exterior columns of the WTC towers after being subjected to intense heat. He also investigated fracturing of the steel columns and the break up of the plane that occurred during the dynamic impact.

MSC.Dytran helps engineers predict how structures respond to real-world, high-speed events such as crashes. Typical applications include airbag deployment and occupant interaction in vehicle crashes, sheet metal forming, bird strikes on aircraft, explosive containment within aircraft structures, ship collision and grounding, projectile impact and penetration, drop tests, sheet metal forging and fluid-sloshing effects on structures. MSC.Marc is a nonlinear analysis tool for advanced engineering simulation. It can be used to simulate temperature effects such as during fires.

Professor Astaneh's investigation, whilst focusing on the events surrounding the tragic collapse of the WTC due to terrorist activity, was put into sharper focus by more recent events in Milan, where a small aircraft crashed accidentally into the Pirelli building in the center of the city. Being able to forecast the damage that would be caused by deliberate, accidental or environmental events on high-rise buildings enables civil engineers to mitigate the potentially catastrophic results by modifying designs. Whilst not wishing to pre-empt the final results of the investigation it would appear that although considerable damage from such an action could not have been avoided, the eventual collapse of the building was entirely unforeseen.

A comparison was also made about the efforts made, 30 or 40 years ago, by the National Science Foundation, funded by Congress, regarding the hazards of other 'extreme events' such as earthquakes and their effect on buildings and the subsequent loss of life. Research and engineering communities have been able to develop more efficient and economical technologies to mitigate seismic hazards. Professor Astaneh has been the Principal Investigator in conducting research on damage and collapse of several major buildings and bridges in the aftermath of earthquakes. The approach taken in earthquake engineering can equally be applied to investigation of damage due to terrorist attacks, as well as to minimising the consequences of such attacks. He argues that similar efforts are needed to counter the equally devastating effects of terrorism against the built environment.

Other comparisons with earthquake studies were made by committee member Dr W Gene Corley, the Senior Vice president of CTL Engineering, representing the American Society of Civil Engineers. ASCE has been involved in five recent studies, in El Salvador, India, Seattle and Peru, and, in 1995, examined the Murrah Federal Office Building collapse in Oklahoma City.

Among the important points made, he stated that one of the main difficulties was in establishing, as accurately as possible, the physical attributes of the buildings prior to the collapse. Doing this, he said, was a monumental task. The construction of the building was documented by literally thousands of engineering drawings. In addition, there were numerous changes to them over their lives. Such data has to be used to construct detailed computer models of the structures. Besides the hard work involved in scouring libraries and other data repositories for the information, ACSE ran briefings for the investigation team using the principal designers to explain how the buildings were constructed.

A principal cause of the collapse of the Twin Towers has been attributed to Progressive Collapse. Dr Corley explained that the likelihood of such an occurrence is dependent upon two inter-related though separate behaviours - the event or load to which the subject is subjected, and the strength or redundancy of the structure. At present there is no rational technical basis to specify either the initiating event or to evaluate the effectiveness of alternative mitigating strategies, and there is a need to develop engineering-based tools to guide the profession in the future.

Dr Corley added, also, that while events like September are rare, fires in buildings are not rare. To improve the performance of structures in a fire environment will also require the development of new tools and design methods through collaboration between fire engineering and structural engineering communities for application to both new and existing buildings.
Further testimony by Dr. Arden L Bement Jr, of the National Institute of Standards and Technology (NIST), discussed progressive collapse as well, regarding it as the spread of failure by a chain reaction that is disproportionate to the triggering event - responsible, besides the WTC, for the large number of deaths in the bombing of the Federal Building in Oklahama City.

He admitted that the United States has not yet developed standards and practices to assess and reduce this vulnerability. He also stated that the problem for modern buildings is their smaller margin of safety - and the reserve capacity to accommodate abnormal loads - due to increased efficiency in the use of building materials and refinements in analysis techniques. Professor Astaneh's, and others, testimony before the Committee on Science as well as a video of the entire hearing can be found at www.house.gov/science/welcome.htm. CU www.msc.software.com

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