Case Study: Sully-sur-Loire Bridge Failure

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Concepts Shown:

brittle-to-ductile transition temperature, cyclic fatigue

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The Sully-sur-Loire bridge was originally built in 1832. The bridge crosses the Loire River about 150 miles south of Paris, France. Since 1832, the four 100m suspension sections have been reconstructed many times, but the masonry towers are original. The bridge was rebuilt last in 1946 after being blown up in World War II. In January, 1985, vertical hanger bars tore loose from the suspension cables on the upstream side of the bridge. The shock from the hangers caused the hangers on the downstream side to collapse. The failure of this section triggered the collapse of the remaining sections. [See photo.] Sully-sur-Loire was experiencing an unusual cold spell when the bridge collapsed; in fact, the cold period was the lowest recorded since the bridge's reconstruction in 1946. The bridge collapsed when a 77,000 lb. timber truck drove across it when the temperature reached -4�F.

The low temperature caused the steel in the hangers to drop below its ductility transition temperature; consequently, the hangers lost their ductility and became brittle. Figure 1 shows the relationship between toughness (a measure of ductility) and temperature. When the truck crossed the bridge, the hangers broke instead of stretching due to the decrease in toughness at -4 oF. As Francois Bouchard, chief investigator explained, "the hangers broke cleanly indicating fragility." Bouchard also explained that due to post war materials shortages, the less than perfect steel with which the bridge was constructed decreased the number of cycles the bridge could be loaded before it fatigued. Figure 2 shows the life of a material that is cyclically loaded. When the material reaches it endurance limit, the time to failure decreases. Bridges experience a lot of cyclic loading from the force of cars and trucks crossing them.

Both of these factors, ductility transition and cyclic fatigue, contributed to the collapse of the Sully-sur-Loire bridge.

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Author:
Dave Goodman
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