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From Building Design to Prime Time: Forensic Engineering
(Released July 2008)

 
  by Marianne Stanczak  

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Usually, an effective way of getting a point across is by using "everyday" examples. Not so with forensic engineering, which deals with the extraordinary. While thankfully the following are not everyday occurrences, their prominence in the media and in history has made them excellent examples of forensic engineering in practice.

COLUMBIA SPACE SHUTTLE

The break up of the Space Shuttle Columbia on Feb 1, 2003 caused much heartache and prompted many questions. In order to answer these questions, forensic engineering was performed:

  • FELISA unstructured Computational Fluid Dynamics (CFD) Solver (Bibb)
  • delta aerodynamics
  • wind tunnel tests
  • RCC panels (Reinforced Carbon Composites) analyzed
  • flight data
  • forensic examination of shuttle debris
*For more information, see Yng-Ru Chen's Discovery Guide "Columbia Shuttle Tragedy." http://www.csa.com/discoveryguides/shuttle/overview.php

HURRICANE KATRINA

The devastation of Hurricane Katrina in August of 2005 created a lot of finger pointing and extensive investigation. To make sense of the tragedy's scientific aspects, the Interagency Performance Evaluation Task Force (IPET) (Sills) examined geotechnical data. Radar and satellite data were also examined (Chen) and used to create mathematical models and computer simulations. One interesting note is that the damage was similar to that found in the areas hit by the December 26th, 2004 tsunami in the Indian Ocean (Robertson). It was obvious that the levees had failed, but why? Investigators from the University of California Berkeley and the American Society of Civil Engineers "blame weak links for New Orleans levee failures…floodwall failures…occurred at weak-link junctions where different levee or wall sections joined together (Investigators)."

water overtopping floodwall
Water Overtops Flood Wall

In addition, by using not-so-high-tech methods, looking through old records, it was discovered that some structures' building codes - if there were any at all - for Alabama, Louisiana and Mississippi were not updated appropriately (Summary report). This fact may not have been discovered using computer generated models or intricate physical tests, but is nonetheless extremely important.

I-35W BRIDGE, MINNESOTA

destroyed bridge
Destroyed Bridge and Navy Divers
Many forensic civil engineering studies deal with causes of bridge failures. Of recent note is the collapse of the I-35W bridge in Minneapolis, Minnesota in August of 2007. The immediate cause was fatigue and cracking due to age, but long-term causes date back to the bridge's construction.

Even though most forensics occur after a failure or accident, there are preventative tests. For example, Bridge 9340 (ironically, the bridge that collapsed) on I-35 (near downtown Minneapolis) had been tested by monitoring the strain caused by the weight of traffic flow. This information was then used in a Finite Element model (O'Connell).
gusset close-up
Overburdened Gussets May Have Played a Role in Bridge Collapse
This particular test, which took place in 2001, stated "Mn/DOT [Minnesota Department of Transportation] does not need to prematurely replace this bridge because of fatigue cracking." However, as we know, this bridge did collapse. As Finite Element Analysis (FEA) advocates state: "Had FEA been around when the bridge was built [in 1967], it would have caught the errors that seem to have led to the bridge's collapse" (Would FEA). Of course, since this is still a relatively recent event, investigations are still taking place, examining factors such as corrosion, cracking, and the welded joints (Cho).

OKLAHOMA CITY BOMBING (APRIL 19th, 1995) - Alfred Murrah Federal Building

No one expects a horrific act of terrorism to occur, let alone one caused by a fellow countryman. Such is the case of the Oklahoma City bombing, which had the impact of four thousand pounds of TNT. Not expecting a homegrown terrorist may be the reason that when the reinforced concrete Murrah building was built "(Corley) there were no safeguards against such things as bomb blasts or even earthquakes"(Corley). There was, however, wind resistance properties that did help lateral load resistance. Forensic seismology, in addition to assisting with origination of gunshots (see Basic Tools section), may also be used in the study of chemical explosions, specifically the timing and size of the bomb. This use helps to quash the conspiracy theory that the bomb used by Timothy McVeigh and Terry Nichols was in fact a cover-up to disguise a U.S. government bomb blast.

9/11 AND THE WORLD TRADE CENTER TOWERS

Even when we know who is criminally responsible for a crime, there are still many lessons, either to prevent its recurrence or at least to lower the severity of subsequent events. Such is the case regarding the collapse of the World Trade Center Towers on September 11, 2001. One of the main elements in these investigations is fire and the subsequent damage to the steel structures. One project by fire protection engineering students (Marshall) examined how the fire on the 96th floor of the North Tower related to the building's collapse. Construction design, fire properties of materials and a 1/20th scale model were used in this project (Usmani).

diagram of world trade center and fire impact
World Trade Center Structure
An important question proposed by Finite Element Method was: "Had there been no structural damage [i.e., that due to aircraft impact] would the structure have survived fires of a similar magnitude?" Properties such as thermal expansion, material loss of strength and material loss of stiffness were examined using computer-based structural analysis. The structure of the WTC was shown to make it more vulnerable to fire, especially one of the magnitude of September 11th's.

Another way of looking at the collapse is through structural damage caused by the aircrafts' impact. Fire is still taken into consideration, as are natural forces such as gravity, compressed air and kinetic energy. These, combined with the aircraft impact, worked together to create a chain reaction collapse of the buildings (Leary). Many simulations have since been performed, including LS-DYNA and other Finite Element models, elastic analysis models, collapse stability index, and others.

To prevent such a catastrophic failure from occurring again, one plan calls for the use of a "heavy-duty metal based honeycomb energy absorbing structure" (Zhou). It has also been hypothesized that if the building were constructed like the Pentagon, with its reinforced concrete, more of the impact from the aircraft strike would have been absorbed. (The Twin Towers were made from lightweight steel)

Quite like the city devastated by an earthquake being a good testing ground, "[m]ore is learned from buildings that fall down than from those that do not" (Rybczynski).

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