Concrete, Part 4

by Don Friedman on March 8, 2019

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.

Concrete, Part 3

March 7, 2019

Concrete has become something of a scapegoat for carbon emissions in building, but it’s amazingly difficult to pin down how bad it is, as a material, compared with other options. The problem, simply, is that it is difficult to make apples-to-apples comparisons of efficiency and carbon use across different structural systems. Let’s start with weight. […]

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Concrete, Part 2

March 6, 2019

The first article I read in the Guardian’s Concrete Week series was last Monday’s piece “Concrete: the most destructive material on Earth.” It’s an odd piece in terms of its organization, jumping around a bit. Eventually I realized that the problem I was having is that the article is discussing several quite different topics, linked […]

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Concrete, Part 1

March 5, 2019

I mentioned once that engineers tend to develop personal styles in their designs, which is something obvious to engineers but occasionally surprising to other people. Part of those styles is preference for some materials. For example, I prefer steel for building frames, although I’ve also worked with concrete. In any case, the Guardian’s “concrete week” […]

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Unclear Boundaries

January 15, 2019

That beautiful concrete is the undercroft, for want of a better term, at the London Bridge rail station. The station has a top-heavy configuration, with the entrances at street level and the tracks above. This makes sense for a constrained-site urban station, and London Bridge’s operational set-up reminded me of Jamaica Station. The concrete groin […]

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Set In Stone, So To Speak

January 7, 2019

I happened to look up while in a garage and was able to take a photo that shows a several interesting, if minor, topics about concrete in a small space. This garage was built in the mid-1960s and has been kept in a reasonably good state of repair. First, and most obviously, we have a […]

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A Rare Failure

August 24, 2018

That’s the underside of a large, built-up steel girder encased in concrete. (You can see the edges and grain of the wood boards that were used as formwork when the concrete was placed.) You can see the steel because the concrete is broken on the bottom, maybe because it failed spontaneously and maybe because someone […]

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July 8, 2018

That’s a concrete floor that’s been sealed against water entry by the simple and effective method of someone painting it. Someone painted it green. Then later, someone painted it blue. The red, then orange. (I may have the sequence wrong – it’s been a while since I took the picture and it’s hard to tell […]

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Too Perfect

June 23, 2018

How can you tell when you’re dealing with cast stone (architectural precast concrete) of the type spread in the NYC area by the Coignet Stone Company? The texture that seems to be a beautiful tooled surface is exactly the same in block after block, with maybe two or three patterns for all of the stones. […]

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Concrete Shells In New York

June 13, 2018

The short response to the title is that we haven’t got many. Part of using concrete shells is to have them exposed, since what’s the point of fascinating structural gymnastics if no one can see them? Concrete buildings here tend towards the utilitarian, with the material being a way to pack floors into a given […]

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