Concrete, Part 4

The Morandi Bridge by Bbruno

I’ve said that different materials lend themselves to different forms, and that there is such a thing as a concrete-inflected structural type. That idea leads, unfortunately, to a discussion of the collapse of the Morandi Bridge in Genoa last August.

The bridge had a number fo different components (click on the picture above to expand it) but most of them were relatively straightforward. The A-frame towers were mostly in compression, with some bending; the inclined struts below the roadbed were mostly in compression, with some bending; the main portions of roadbed were in bending, with some compression, and the suspended spans of roadbed were in bending. There’s more to the structural design of those elements than that but none of that jumps out as odd in any way. The part of the design that seems odd to an engineer is that the inclined stays were pre-stressed concrete. Concrete is not ordinarily used for structural members that will be loaded primarily (in this case, entirely) in tension. In theory, the pre-stressing was enough to keep the concrete in compression in these tension members, but that’s as odd structurally as it is in a sentence. The cables inside the concrete were known to have deteriorated and were considered to be in critical condition prior to the collapse. Their deterioration was most likely caused by water and acidic pollutants getting through the concrete, possibly through the concrete’s pores.

If we avoid the anthropomorphizing of discussing what concrete “wants” we get to the idea that concrete (a brittle material that is far stronger in compression than in tension and which is made ductile only be embedding steel in it) is better suited to structures that are designed to carry load in tension and (when reinforced) in bending. Nervi’s Olympic stadiums, for example, are daring concrete structures that have held up well in part because their designs emphasize compression. Pretensioned concrete tension members use greater tension than is needed for the design to create compression in the concrete so that it performs well in tension. It is, to say the least, counterintuitive. The use of concrete in tension members may also have contributed to the collapse of the under-construction pedestrian bridge at FIU last year. If you want to design with a material’s properties, you keep wood dry, you keep steel braced against buckling, and you keep masonry and concrete out of tension.

Finally, I also strongly disagree with the Guardian article that suggests that this disaster calls Italy’s engineering heritage into question. You can find examples of bad structural ideas anywhere, particularly during periods of experimentation like the 1960s. You can find deferred or missing maintenance everywhere. You can find dangerously deteriorated concrete everywhere.

Part 1 is here.

Part 2 is here.

Part 3 is here.