Determining a Practical Bridge Design for a Wide River and Heavy Traffic
Hello everyone, I am thinking yet again about infrastructure. (my calendar project recently asked about... well, that's outside my expertise, I'll get back to that eventually)
And I am thinking of the limits of just how much a bridge can do.
So, some backstory. This main bridge (along with 2 others, many kilometres away, with much less load on them) joins a large city on the northern side with its brooklyn/queens/new-jersey suburbs on the other side, and this city is LARGE. The city is, let's say, 80-95% of the population of Tokyo. The river is very wide, nearly as wide as the Hudson River in NYC, and I'd like to retain as much shipping width as possible.
I would like to run a 6-track mainline railway across, plus a 6-track metro line(multiple "lines" would use it), and finally a 6-lane highway.
But, looking up the longest bridges, not many have anywhere near this much load on them. Which isn't great, because this river is a internationally important shipping channel that carves through a mountain range and provides very cheap bulk freight to a mediterrainian sea. And due to some locks constructed, this entire river must remain at least panamax, but a larger span is needed here due to the business of the ports around the bridge
So, how would I make a 500 metre bridge? In reality, would this just be done as three parallel bridges, each carrying a different type of vehicle? Or would one massive bridge be done?
And if it were to be 3 separate bridges, how strong would the 6-track mainline bridge need to be, assuming heavy north american-like weights? (they use a wide track gauge, and some double-decker EMUs would be weighing a bit more than a pullman heavyweight...)
So, to summarise what I am asking: 6 traffic lanes, 6 mainline railways, 6 metro lines, either all in one bridge or all in separate bridges. Bridge must not block shipping in the busy river below, so should be near to 500 metres main span length. How possible/feasable are these options?
2 answers
Neopanamax beam is just under 52m, with a height of just under 60m. Let's give it about 50% margins on each side, and that give you 100m between bridge supports. A suspension bridge with a 100m span would not appear on a list of notably long bridges in our world.
Suddenly this is not looking like a huge technological problem.
I suggest that you run two 6-wide bridges next to each other, one for heavy trains and the other for your metro trains and vehicle traffic. You might be interested in reading https://www.researchgate.net/publication/330482896_Live_Load_Spectra_for_Railway_Bridges_in_USA which discusses actual loading parameters.
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Look around the world to see what is possible with our current technology. The Golden Gate bridge has a deck 90 feet wide (27 meters) supporting 6 lanes, and its main span is 4200 feet (1.28 km) long spanning a major shipping channel. It's obviously plenty high and wide enough to support the large port facilities in the Bay Area. And, it was built in 1937 (86 years ago).
Other bridges support rails. I don't know if there is a bridge that carries 6 tracks. That seems excessive. Two tracks seems plenty. That allows one train simultaneously in each direction. Look at any chunk of 6 parallel tracks around the world. Where are 6 tracks carrying separate trains concurrently? Mostly you find a lot of parallel tracks in switching yards, not for carrying multiple independent trains for long distances.
It seems like with some planning, two tracks is enough. Look around where trains are used heavily on land. Once you get past train yards you see two tracks at most. This includes major train routes around the country, like running east/west out of Flagstaff AZ.
So yes, what you ask seems doable with some reasonableness applied first. And, if you can build a 2-track train bridge, you can build a wider one. The economics don't favor doing that, though.
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