August 05, 2004

Paris Airport Collapse Update

I faced some criticism in May when I attributed causes of the Paris Airport's "ultra modern" terminal collapse to France's socialist government. I charged that using square cornered windows in a concrete tunnel structure was a bad idea, and inferred that a bad design resulted from a politically powerful architect dictating over subordinate engineers who would likely have chosen differently.

In June, the focus of the search for the root cause failure had narrowed to the concrete tube structure. Now a commission of inquiry has issued a preliminary report. The report is inconclusive, and contains a fair amount of speculation. The French Transport Ministry said it was still not completely clear why the accident happened. Some of the many news accounts I read on this story claim that the commission of inquiry has still not been given access to the scene, and has worked exclusively from photographs.

The focus of the investigation currently points to the source of the failure being in the concrete roof, as I predicted, and not in the foundation. Based on photographs showing a steel supporting strut pushed through the concrete shell in line with a large crack in the shell, there is speculation that the failure started at the supporting struts. But this could easily be a secondary failure, occuring only after a prior crack resulted in higher loads at the steel strut.

The report states, "It is likely that this perforation was facilitated by the prior and gradual weakening of the concrete." Well, concrete doesn't weaken as it ages, it gets stronger. What can happen gradually is stress cracking. While it's certainly possible that unsafe loading of the concrete at the steel struts produced cracks that eventually brought down the roof, it seems more likely that the fatal first cracks originated elsewhere. I'm still predicting the source to be corners of the square windows.

At any rate it remains clear that the use of concrete in this tubular, light transmitting roof was a poor choice of material.

Posted by JohnGalt at August 5, 2004 08:30 AM

There is still no way around the fact that to initiate cracks from the corners you need tension loading. If the concrete was loaded in tension then there was a failure elsewhere or a transition loading was not adequately accounted for in the design. When I speculate that the root cause was ground movement I am really just playing the odds. Ground movement (expansion and contraction) causes more structural failures then all other effects combined, including wind, flood and earthquakes.

Posted by: Silence Dogood at August 5, 2004 10:07 AM
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