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Comment by peterwwillis | Hacker News Reader

So many concrete bridges fail for so many reasons, it's kind of crazy that we keep building them. It's much easier to inspect and repair steel than concrete, and it fails more gracefully.

Also, I'm not even an engineer and I can tell you that uneven strut design would give you uneven loading on the span. Christ. Why didn't they just build a damn cable stayed bridge?! If traffic was a concern, they could have built it half cantilever, half guyed. No formwork, strong, still visually appealing, no stupid truss design, and no concrete.

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> I can tell you that uneven strut design would give you uneven loading on the span

A structural engineer is quoted in the article that this uneven strut design would have worked if if that end beam was more solid.

It still bugs me. Yeah, that part would work, at least until it got loaded weird by high winds or by being hit by a truck or something.

They were very unconventional in this design (new type of concrete, single row of struts, prefabricated construction, concrete instead of steel, uneven struts, computer-only analysis instead of civil engineering 101 static analysis) that it demanded extreme care.

Conventional bridge design patterns are robust as in they still work when they're old, under unexpected loads, analyzed by below-average engineers, or built with less-than-perfect materials. Deviating from them demands extra extra care, or people die. :/

As you said, the problem was not necessarily the idea, but that they were not careful enough.

I'm sure you noticed that sky-scraper shapes are getting weirder and more unstable-looking:

https://www.designbuild-network.com/wp-content/uploads/sites...

The struts varied in thickness according to their net loads, so their loading should have been the same.

It was a cable stayed bridge. The bridge span was just put up. The tower and the cables were to be installed over the next few weeks. They allowed traffic under an incomplete bridge because money.

It specifically was NOT a cable-stayed bridge. It was a non-redundant truss bridge with a tower and faux stays that contributed nothing to the structural capabilities of the bridge. The arrangement of the truss structure mimiced the stay angles purely for aesthetics, not for any other reason. The choice to use a single truss structure rather than a more traditional redundant set of trusses was due to the desire for a striking design.

Adding the tower or cables would have done nothing to strengthen the bridge. The engineers in the article suggest that the second shorter section of the bridge to be built afterwards may have alleviated some of the stresses on the north truss beam, but the truss itself was unorthodox, so they cannot say anything for certain.

The stays were real. They were needed for hurricanes and large crowds.

The worst case would be a marching parade, on just one side, during a hurricane.

No, the article is pretty clear that the stays were fake, and intended solely to dampen vibrations. They provided no structural support whatsoever.

If it is "to dampen vibrations" then obviously it isn't "fake". Vibrations are serious business; many bridges have collapsed due to vibrations, and many more (Millennium Bridge for example) have had severe trouble. Humans are particularly prone to causing vibrations because humans react to bridge movement in a way that amplifies the movement.

Technical description of the bridge from the engineering firm:

https://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal...

Page 3: "The stay cable pipes increase stiffness for pedestrian loads."

Page 18: "The stays and pylon provide the required structural design to meet the pedestrian loads for harmonic conditions of natural frequencies"

Page 64: "HORIZONTAL FREQUENCY In the lateral direction, the fundamental frequency of the pedestrian bridge shall be greater than 1.3 hertz. VERTICAL FREQUENCY The fundamental frequency in a vertical mode shall be greater than 3.0 hertz to avoid the first harmonic."

Page 65: "The pylon stiffness was designed to provide significant benefit to the overall stiffness of the structure."

You're reading from the proposal, which is not the bridge that was built.

The bridge that they built did not use the stays as structural elements, and any vibration dampening would have been minimal, i.e., a side-effect of stiffening the bridge rather than the reason for including them.

Got an as-built engineering document which contradicts the engineering proposal document? An article written by a journalist doesn't count.

Unless you can find such a document: the stays were structural elements.

But FIU's bridge was designed to mimic the dramatic look of a cable-stayed bridge, where the deck is suspended from cables fanning out from a tall mast. In FIGG's design, though, the "cables" — actually metal pipes — were mostly just for show. The diagonal, v-shaped struts of the truss did the structural work.

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