Terminal 2E of the Charles De Gaulle International Airport (Paris) collapsed unexpectedly in the early morning of Sunday, 23-May-2004. Four unsuspecting travelers died in the process, simply because they were in the wrong place at the wrong time. The $900 million (US) structure, which is designed to handle as many as 25,000 passengers in a single day, may be beyond all hope of repair. The big question: why?

Multiple investigations are currently underway to answer that question, but definitive answers are not expected for at least a year. Furthermore, investigation and cleanup efforts have been hampered by continuing failure of the structure. On Monday, personnel on the scene heard cracking noises and found new fissures in the supporting concrete pylons. Most workers were evacuated from the site at that time.

Failure Details

Available information is sketchy -- what is known is that a section of ceiling 30 metres long and 20 metres wide collapsed suddenly after standing for nearly a year without any reported problems. Based on technology adapted from tunnel-building, the terminal is essentially a long, elliptical tube formed from interlocking concrete rings. The structure itself has no internal load-bearing members, instead relying on the external walls to provide support.

The Sunday collapse occurred in a section of the terminal containing access points for three boarding walkways, a difference when compared to the rest of the structure. In addition, the Monday observations of cracking sounds and new fractures occurred in a symmetrical section of the terminal that shares this feature.

During the construction of the terminal, there were problems with hairline cracking in some of the concrete support pylons away from the area of the collapse. Measures were taken to strengthen these pylons, and the process used to build the remaining pylons was altered.

There are reports that passengers noticed cracks in the walls of the structure shortly before the accident. In addition, some observers of the collapse reported having seen puffs of dust accompanied by cracking noises just prior to the collapse.

Changes and Distinctions

Several theories have been put forth to explain the collapse, though no smoking gun has been identified. Currently, every avenue of investigation appears to be open -- design, materials, construction, quality control, etc. There is a real scarcity of useful information at this time. However, it should be possible to gain some insight by considering changes and distinctions suggested by the known data.

• The roof collapsed in a section of the terminal that is structurally different from the rest of the building, in that it contains access points for boarding walkways. This suggests the potential of an interface problem between structurally dissimilar portions of the building.
• Later observations of cracking noises and concrete fractures in a structurally similar area symmetric to the collapsed location, just a day later, is highly suggestive of a common design or construction flaw.
• The concrete support pylons away from the collapsed location had to be strengthened after development of early hairline cracks during construction. Later pylons (at or near the collapsed area?) were built using different techniques. Do the earlier and later pylons have different strength characteristics? If so, this is suggestive of a differential load-bearing capacity.
• By all accounts, the collapse occurred suddenly for no apparent reason. No obvious reasons exist that can explain the collapse. This suggests that perhaps some environmental factor changed which brought about the failure... the weather?
• The entire structure is built in a unique fashion that does not require internal load-bearing members. All of the building's strength and stability is provided by the thin-shelled exterior. This implies a lack of redundancy, and tight coupling of stresses through the body of the structure.

Conclusions?

Note: This was a very early and utterly naive attempt to provide a plausible theory of causation for the incident. Remarkably,  many of the elements I suggested were pretty correct! Compare with the explanation given at the Failures Wiki.  -Bill, 21-Sep-2014

If one were inclined to connect the dots, a picture might begin to emerge... speculative at best, but very, very suggestive. Could a recent change in the weather have affected the load-bearing capacity of the concrete pylons? Are some of the pylons different from the others? Did this cause some parts of the terminal structure to shift slightly relative to others? Did this introduce differential stresses in the building at an interface between structurally dissimilar sections? Was the unique design, deriving all strength from the exterior shell of the structure, able to withstand these differential stresses? Did the lack of internal load-bearing members allow a potential failure to develop into a full-blown event?

It is much too early to draw any conclusions about this accident. Everything in the previous paragraph is based on an incomplete analysis of insufficient data, done by a single person (me) while also watching television. (In other words, don't call this a root cause analysis.) One thing is certain however... had this event occurred in the middle of the day instead of the early morning, there would have been significantly more fatalities.

This story is being carried by many major news outlets. Information used to produce this blog article came primarily from the following (links updated/verified 2014-Oct-05):

• New York Times: article
• Channel NewsAsia: article (no longer available)
• New Scientist: article
• BBC News: article
• Guardian Unlimited: article

by Bill Wilson

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