Gary Carter investigates the use of finite element analysis (FEA) in the postmortems of three tragic events over the past thirty years.

Although designers and engineers work diligently to ensure the viability and safety of their designs before they are built, sometimes, unforeseen circumstances can occur. Whether they are the result of mechanical failure, natural phenomena or acts of sabotage, each year these incidents cause billions of pounds worth of damage and significant loss of human life to even the best engineered designs.
When these catastrophes occur, government agencies such as the United States Federal Emergency Management Agency (FEMA) often enlist the services of engineers charged to ascertain precisely what went wrong in an attempt to prevent a future recurrence. In many of these cases engineers need utilise simulation software as a 'forensic' investigation tool.
While there may not have been a way to predict or prevent these tragic incidents in the first place, it is worthy to note the significant role which finite element analysis software, such as ANSYS Multiphysics, plays in helping engineers comprehend the unique mechanics of design failure under extreme circumstances.
* Destruction of Space Shuttle Challenger, January 28, 1986. Seventy-three seconds after launching from the Kennedy Space Center in Florida, an ignition of mixed liquid oxygen and hydrogen fuel, brought about as the result of a faulty engine sealant, destroyed the shuttle orbiter Challenger.
Then-president Ronald Reagan appointed a special commission to investigate the cause of the accident and develop corrective measures. Among the members of the investigation team were engineers from Thiokol Space Operations - the original designers of the solid rocket boosters (SRB), where the failure was believed to have occurred.
Utilising ANSYS' nonlinear analysis capabilities, the Thiokol team was able to identify that unusually cold weather that day had caused the rubber O-rings-which seal the components of the SRB together-to stiffen. Subsequently, the SRB lost cohesion causing a chain reaction that resulted in the shuttle's destruction.
"At the time," observes Troy Stratten, principle structural analyst at Thiokol, "this was probably the largest ANSYS nonlinear model ever run." Indeed, the representation of the tang and clevis joint region (left), where additional high stress concentrations were detected, contained 30000 elements and 100000 degrees of freedom.
Once these design flaws were identified, NASA required the SRB to be re-certified for operation. The results generated by the ANSYS simulation allowed Thiokol's engineers to redesign the SRB's joint system to minimise gapping and characterise stress concentrations. Finite element analysis proved to be the key in ensuring the safety and success of future shuttle missions.
* The crash of TWA Flight 800, July 19, 1996. Fourteen minutes after taking off from New York City's John F Kennedy International Airport, a Boeing 747-100 - identified as Trans World Airlines Flight 800 - exploded and crashed into the sea nine miles off Long Island, New York.
To this day, the precise cause of the crash has not been identified conclusively. However, a joint investigation spearheaded by the Boeing Company, in cooperation with the National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA), yielded a 'most likely' scenario to explain the events of that fateful night.
A thorough inspection of the recovered wreckage-confirmed via simulations performed in ANSYS-led investigators to hypothesise that the fuel vapour/air mixture inside the nearly empty centre wing tank (CWT) was ignited by elevated temperatures. The ignition of this combination of gases ruptured the fuel tank which, in turn, caused the plane to violently break up. While fuel tank fires or explosions of this type are rare, two other previously documented occurrences confirm that they fall completely within the realm of possibility.
Patrick Safarian, then senior specialist engineer for Boeing, recalls the key to satisfactorily resolving this important issue was found in the robust structural and Computational Fluid Dynamics (CFD) capabilities in ANSYS. "Performing failure analysis at this level was, until then, probably unheard of," he enthuses. "It may still be unsurpassed."

Safarian's ANSYS rendering of Flight 800 (above) was based upon an ANSYS-modified finite element model of a 747-400 freighter, consisting of 120000 shell and beam elements with 750000 degrees of freedom. He observes that while the initial source of ignition was unknown. "We moved (it) around, remodelled, re-analysed and took careful note of the results. This gave us a good degree of confidence in the failure sequences."
As a result of Safarian's efforts, the NTSB advised the FAA to take numerous steps to prevent a recurrence. These include thorough examinations of the physical condition of fuel tanks and all related components on more than 850 aircraft currently in use throughout the world. Other recommended precautions-such as pumping inert gases into fuel tanks, refuelling from ground tanks (which store fuel at a lower temperature level), and carrying an 'appropriate' amount of fuel in tanks at all times (as full tanks are less likely to explode than empty ones) are also under consideration.
n The Collapse of the World Trade Center, September 11, 2001. While catastrophes that occur via natural or mechanical means are difficult enough to foresee, it is nearly impossible to predict those that result from a deliberate attack by an unknown enemy.
Following these events, FEMA formed a coalition with the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) as well as the city of New York and several other Federal agencies and professional organisations-to document the performance of buildings at ground zero. Their goal was to document the sequence of events, likely root causes, and methods or technologies that may improve or mitigate the observed building performance.
Although WTC buildings 1, 2 and 7 collapsed completely, other structures such as the Bankers Trust Building (located at 130Liberty Street) remained standing, sustaining only moderate localised damage. Robert Smilowitz, consulting engineer with Weidlinger Associates Applied Science division in New York City, led the team studying the Bankers Trust Building.
While not originally designed to sustain the loss of a column over a significant portion of its height, this structure's ability to arrest collapse demonstrated an inherent tenacity in the moment-connected steel frame lattice. To gain a better understanding of the building's response to the impact of debris from the falling WTC structures and identify specific design features that contributed to this performance, Smilowitz's team utilised simulation software from ANSYS Inc.
In order to represent the structural behaviour in the damaged state, the team had to develop nonlinear spring representations of the girder/column moment connections (left). Detailed plate models of the connections were developed and analysed parametrically to determine the appropriate nonlinear spring characteristics. These properties were then specified at the corresponding connections in the ANSYS model of the building.
Using ANSYS' static nonlinear analysis capabilities, the team was able to determine the diminished capacity of the connections resulted from the 'out-of-plane' bending associated with the damaged state of the structure. This partially explains the damage pattern, which was contained in the northeast face of the building, extending from the initial impact area on the 22nd floor down to the eighth floor.
"It is difficult to draw conclusions," observes Smilowitz in his report to FEMA. "More detailed study is required to understand how the collapse was halted." He believes that a complete FEA analysis on the Bankers Trust Building-conducted in ANSYS-will aid current and future builders in constructing buildings better able to avert catastrophic collapses in the event of abnormal loading conditions.

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Gary Carter is Managing Director, Ansys Europe, Riseley, Berkshire, UK .www.ansys.com