As just one other tragic example of a different kind, the collapse in 1985 of a ceiling above a swimming pool in Switzerland showed how a simple structural concept could be sensitive to the loss, through corrosion, of support from one of many hangers. After thirteen years of use, the roof of an indoor swimming-pool in Uster, Switzerland unexpectedly collapsed. Twelve people died on May 9, 1985.

As just one other tragic example of a different kind, the collapse in 1985 of a ceiling above a swimming pool in Switzerland showed how a simple structural concept could be sensitive to the loss, through corrosion, of support from one of many hangers. After thirteen years of use, the roof of an indoor swimming-pool in Uster, Switzerland unexpectedly collapsed. Twelve people died on May 9, 1985.

The Federal Materials Testing Institute, based in Duebendorf, Switzerland, and the Federal Materials Research and Testing Institute of Berlin concluded that the collapse was the result of chloride-induced stress corrosion cracking. The steel rods had been pitted, causing the roof to cave in. The roof collapsed in a zipper-like fashion, starting with the corroded rods. The collapse continued as the remaining rods were unable to bear the increased load. Chloride is a major factor in corrosion of reinforced concrete, as in the case at Uster. The chloride was either already present in the concrete or came from the pool via water vapor.

Chloride can overcome the passivity of the natural oxide film on the surface of the steel. The steel, lacking its passive film, readily releases iron atoms into solution (in this case, moisture is present in the concrete due to the humid environment).

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