[TylerE]

Moisture causes corrosion.

Rivers produce lots of moisture.

Corrosion weakens the steel. It's literally the metal flaking away.

All steel corrodes. Even stainless.

Once corrosion sets in and the outside of the metal starts to pit, the corrosion speeds up drastically. (The old area vs volume thing, same thing that makes lump coal difficult to ignite while aerosolized coal dust is practically an explosive).

Here's a modern US bridge - all steel - that collapsed in Pittsburgh two years ago. In some areas over 80% of the original steel thinkness was gone after 49 years.

https://www.youtube.com/watch?v=J-VnWB4fiFk

[Gare]

Paint protects steel. A frequently painted steel bridge can last centuries.

As always, lack of maintenance and poor design choices (use of corten steel) are the culprit.

[cwillu]

Ode to The Paint on Nuclear Submarines

    The purpose of the hull is to protect the paint.
    The purpose of the reactor is to drive the paint around.
    The purpose of the SUBSAFE program is to ensure the paint comes to the surface and will not be lost.
    The purpose of the cathodic protection system is to back up the paint.
    The purpose of the weapons is to defend the paint.
   The purpose of the Special Hull Treatment is to protect the paint.
   The purpose of the Vertical Launch System is to destroy those who would do the paint harm.

--Naval Research Lab

[Intralexical]

What's the purpose of the crew? Provide the paint company?

[cwillu]

The purpose of the crew is to reapply the paint.

[euroderf]

They are the only carbon-based life forms, otherwise useless to the paint, capable of touching up the paint.

[8bitsrule]

The Golden Gate in S.F. has been continuously painted since it was built in 1937 ... that when they've finished on one end, they move to the other end and start again. [0] https://www.goldengate.org/bridge/bridge-maintenance/paintin...

This page details the other continuous maintenance going-on. [1]https://www.goldengate.org/bridge/bridge-maintenance/

[joshstrange]

From reading the linked article it seems that they are continuously painting but it’s not really end-to-end then restart. It’s more of an ongoing maintenance as-needed, at least based on the description on that website.

Now that effectively might mean then combing over the bridge end-to-end and painting as needed so what I said might be a distinction without a difference.

[ElevenLathe]

There so far haven't been any steel bridges that have actually lasted centuries, even if we highly suspect that they are capable of it with the right maintenance.

[TylerE]

Good point. The really old ones are iron.

ETA: Also, the ones that have survived aren't road bridges. They're either railway bridges (and thus were built strong in the first place) or have been relegated to historic preservation/pedestrian use only. Modern cars and (especially commercial) trucks are far heavier than early vehicles. A road bridge built in the 1850s was only designed to carry foot traffic and horsedrawn vehicles.

[TylerE]

Something like 20% of the bridges in this country are 10 years or more past their design lifetime and mostly not due to be replaced any time soon. You think they're keeping on top of painting every one? Saying preventing corrosion is as easy as slapping on paint is like saying computer security is as easy as not leaking your password. There are a million things that could happen, many of which are outside what any single person could control.

[Gare]

I don't know in what poor country you live, but in my country bridges are well maintained and I don't recall a single critical failure. Even though we're one of the poorest EU countries.

[TylerE]

https://infrastructurereportcard.org/cat-item/bridges-infras...

"There are more than 617,000 bridges across the United States. Currently, 42% of all bridges are at least 50 years old, and 46,154, or 7.5% of the nation’s bridges, are considered structurally deficient, meaning they are in “poor” condition. Unfortunately, 178 million trips are taken across these structurally deficient bridges every day. In recent years, though, as the average age of America’s bridges increases to 44 years, the number of structurally deficient bridges has continued to decline; however, the rate of improvements has slowed. A recent estimate for the nation’s backlog of bridge repair needs is $125 billion. We need to increase spending on bridge rehabilitation from $14.4 billion annually to $22.7 billion annually, or by 58%, if we are to improve the condition. At the current rate of investment, it will take until 2071 to make all of the repairs that are currently necessary, and the additional deterioration over the next 50 years will become overwhelming."

[tharkun__]

I don't know in what rich poor EU country you live, but in my "rich country" bridges are basically left to their own devices until a critical problem occurs and then they hastily start repairing it. In some cases they've been repairing bridges for decades that way. Some have recently finally actually been replaced or are being replaced. But nobody is going out and replacing a bridge that isn't on the brink of collapse and held up by "essentially duct tape".

[iamacyborg]

Now do a quick internet search for “hammersmith bridge”.

[gpderetta]

Which is a shame as it is otherwise a beautiful bridge.

[smegsicle]

maybe they wouldn't be so poor if they didn't waste all their money on infrastructure maintenance

[wizzwizz4]

What else is a country supposed to spend money on‽ Transportation, utilities, healthcare, bureaucracy, government: a country is its infrastructure.

[arp242]

There are metal bridges over 100 years old still in service today, including a few you've heard of. Do you know this specific bridge was replaced for this specific reason, or are you just making stuff up?

[TylerE]

I’m not making any thing up. Why the hostile tone?

This is basic materials engineering.

[arp242]

The question was why this specific steel bridge was replaced. Not "what are possible reasons someone could replace steel bridges?" and it's absolutely not "basic materials engineering" that every bridge needs replacing after a fixed amount of time due to corrosion.

So again, do you know this specific bridge was replaced for this specific reason, or are you just making stuff up? Because bumping in on a specific question with a generic answer without any knowledge if that applies to this specific case is making stuff up.

The hostile tone is because you're talking complete nonsense with great arrogance.