Failure

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.

Almost Isn’t Good Enough

October 24, 2018

That’s a picture of a handrail where the riverfront walkway meets a wood pseudo-pier. (“Pseudo” because I’ve never seen any kind of boat docked there but I have seen a fair number of sunbathers there.) The upper portions of the handrail are in good condition and have been repainted not so long ago; the embedded […]

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Public Involvement

October 19, 2018

A couple of current projects involving neighboring construction, combined with CROSS/SCOSS newsletter 52, got me thinking about how we prevent structural failures and how we prevent minor or incipient failures from become tragedies. Obviously the first line of defense against failure is the work of structural engineers (in all capacities, including design, review, and inspection) and […]

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Forensic Engineering And The Press

October 5, 2018

This article from the New York Times on the collapse of the central bridge of the highway viaduct in Genoa is worth reading. It covers the topic well and has excellent graphics to help explain some of the more difficult engineering topics. I generally do not want to spend a lot of time discussing current-day […]

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Three Paths, All Dangerous

October 2, 2018

I was looking at fire damage in an old industrial building in Brooklyn and noticed that there were three distinct patterns. The photos below are from the top floor and the remnants of the hung plaster ceiling has been removed. (I) First, in some places the fire simply burned through – exceeded the timed rating […]

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Food For Thought

July 26, 2018

SCOSS summarizes the full report on Grenfell. Some of the specifics are UK-only, but the general issues apply everywhere. Most architects and engineers are not experts in fire safety, so we follow the codes. But there are always gray areas at the edges (How much flame-spread is acceptable in a pre-manufactured curtain wall? How much […]

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Not-Really-A-Failure Portrait: Rubble

July 9, 2018

Given rubble masonry’s terrible reputation, you’d think I have loads of pictures of it failing, but I don’t. I have pictures of it in bad condition, I have pictures of incipient failure and I have pictures of it doing what it does, which is erode and crack peculiarly. But I have few pictures of it collapsing […]

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Repetition

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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A Subtle Symptom

July 6, 2018

It’s possible that the floor joists above were damaged by water from the tub.

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Aesthetics

June 24, 2018

I’ve already said elsewhere anything I might say about the rusted beams in this photo. But that is some pretty, pretty oxidation.

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