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For those interested, this article [1] discusses the history of cable-stayed bridges and touches on why they've become popular for a certain length of bridge. ("Improvements in stress tolerance, corrosion resistance and computer modeling around the same time helped their popularity grow as engineers built them cheaper and more efficiently.")

[1] https://www.scientificamerican.com/article/popular-cable-sta...

that is a truly beautiful bridge. there's a very nice looking one in dallas as well, though i didn't have the words for its type until just now: https://www.google.com/search?q=hunt-hill+bridge+dallas&clie...

And the new public transit/pedestrian bridge in Portland http://image.oregonlive.com/home/olive-media/width620/img/op...

Thank you, in Scotland beside the Forth Road Bridge they are building the same kind of bridge and I wondered why all of a sudden, this was the go to bridge.


One word - Cost! Advances in Post-tensioning methods and technologies make cable stayed bridges the most economical type to span such long distances.

So unless it is a very long gap that needs to be spanned like Golden Gate Strait or the Strait of Messina, which would require a suspension bridge , the cable stayed type will be the "go to bridge"

Source: personal experience. I have built 2 such bridges before. Led the planning and installation of the actual stay cables. I know all about it :-).

Had to make a lot of the jigs ourselves. Lots of room for innovation and automation ;-)

Hey Arjun - I'm fascinated by the human processes that underly big projects like building bridges.

Any chance we can talk about your work, and perhaps specifically about the room for innovation and automation?

ok. my email is <username>@GMAIL

You forgot trains. Suspension bridges can't handle trains well.

What about buckling though? With all that force from the cables pulling the bridge deck toward the towers, it seems like a bit of unexpected bending or weakness (breeze, earthquake, minor damage) could cause the deck to travel inward to the tower while being crushed.

I do have to agree that I am tired of cable-stay bridges. They are a distant third for looks, behind suspension and obviously the stone arch. More stone arches please! Do a big one, a mile or two in height.

Good point about trains. But I know one of the first suspension bridges did carry trains. https://en.m.wikipedia.org/wiki/Niagara_Falls_Suspension_Bri... .When we bid for the Tappan Zee bridge, the community wanted us to design it to take trains in the future. (Our team did not get the project) . Not sure how the executed design turned out.

Actually there is very little stress on the cables. The cables are stressed just enough to take the sag out of em. i.e. All the cables are actually supporting is their self weight! All you need is a hand-held jack and a lot of clever math, to install them :-).

The deck on one side of the tower is held in place by counter weight of the deck on the other side.

About earthquakes, it's designed for those 100 year quakes, hurricanes and floods. When we were building the Audubon Bridge across the Mississippi River, we were in fact hit by a 100-year flood and 2 hurricanes. Other than some stored material floating away and having to watch out for alligators in odd places. We got through fine :-)

The bridge type is rarely chosen for beauty :-) especially when contractors have to bid competitively for tonget a chance to build them. Governing factors are technical feasibility (design and constructability) and cost.

You mentioned the Golden Gate - in Lisbon, we actually have a very similar suspension bridge, but ours does have a train running underneath it, added years later after construction. Supposedly, it was the first aerial spinning of additional main cables on a loaded, fully operational suspension bridge.


Very cool! Now I know it may be possible to add trains to existing bridges :-). Thanks for the link.

Found a video of the company adding the extra cables: https://vimeo.com/45236687

I can't believe how much I am enjoying seeing engineers speaking in regular chat about such large projects. I thoroughly enjoyed this thread. Appreciate the sharing.

As a former mechanical engineer, I'd be interested in more details about the same thing but the referenced article is the best I've been able to come up with absent doing some serious research.

In the case of the Queensferry Crossing I wonder if ease of cable replacement is one of the reasons behind the design choice. The Forth Road Bridge [0], a suspension bridge over the Forth to the East of the Queensferry crossing, has suffered from cable corrosion [1]. There is no easy way to replace the main cables in a suspension bridge without first removing the deck it suspends, causing years of service interruption. In a cable-stayed bridge it seems you could perhaps replace one cable at a time while the surrounding cables take the load.

I live with a view of the bridges, it has been fascinating watching them construct the new bridge.

[0] https://en.wikipedia.org/wiki/Forth_Road_Bridge

[1] http://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-32...

The cable are designed and installed in such a way they can be replaced whenever needed without disrupting traffic that much. (Lane closures may be needed)

In fact, it is designed with enough redundancy that if lightning or an accident takes out 2 (maybe 3 , can't remember) of the cables, the bridge will still stand.

Me too! It look's really pretty at sunset.

Quora has a great answer to the question:

> What is the difference between the cable-stayed bridge and a suspension bridge?

Complete with pretty graphics.


Personally, I prefer the look of old-fashioned suspension bridges.

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