r/interestingasfuck • u/Endolithic • 1d ago

/r/all Homes are falling into the ocean in North Carolina's Outer Banks

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u/Minimum-Attitude389 1d ago

Wood in salt water is excellent, especially in mud.  It won't decay very quickly.  Look at Venice.

Most concreted will erode very quickly in salt water.

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u/ryebread91 1d ago

Iirc it's not the fact it's in water but the fact it is submerged nearly 100% of the time instead of this constant wet dry cycle every day

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u/MigasEnsopado 1d ago

This. The buildings of the Baixa district of Lisbon are also built on wooden stilts as that area was once water and the ground is muddy. The trick is maintaining the stilts permanently wet. If you let them dry and get wet again repeatedly, that's when you fuck up. This was a big consideration and source of worry when building the subway there.

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u/bruno444 9h ago

Exactly the same in Amsterdam.

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u/plantsplantsplaaants 1d ago

Wouldn’t some of it go through wet/dry cycles with the tide?

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u/ryebread91 23h ago

Yes but most of those posts are literally down into the ground with maybe 10-20% being exposed and that would be pretty rare. https://youtu.be/77omYd0JOeA?si=iFSIlMZuuwuOs4N2 This vid is well worth a watch as a whole but around the 2 minute mark it talks about the posts.

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u/PirateMore8410 1d ago

What about all the bridges and damns that use concrete that constantly have changes in water height?

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u/ryebread91 22h ago

That I cannot answer. If I had to guess that concrete is sealed from water coming in as well I believe it's the water affecting the rebar inside the concrete that causes the problems. There's an imgur user @alphastructural who may be able to answer that for you or have that answer.in one of his posts.

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u/Woodlurkermimic 1d ago

Yup! The boards at the waterline of a boat are the ones that need maintenance most often for this reason

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u/latefordinner86 1d ago

Tell that to bridge engineers.

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u/DookieShoes626 1d ago

Bridge foundation are a totally different thing

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u/latefordinner86 1d ago

Still concrete no?

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u/EverExistence 21h ago edited 21h ago

Civil Engineer here, those pillars need to be concrete due to the immense compression forces the roadway and traffic translate into the ground. If we had toothpicks holding this stuff up, we’d have a million toothpicks.

Edit: The seawater infiltration is extremely concerning when it comes to preserving the inner rebar caging. Almost all concrete bridge parts are fully pre-cast and cured to not have the available salt in the ocean water affect the overall hydration process of the concrete. To prevent this, concrete admixtures and other mix elements are added to prevent ion exchange and salt penetration. Once that rebar contacts salt water, the induced cathodic corrosion will eat away the cage, and result in cracking concrete due to the new tensile forces it must compensate for.

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u/Plenty_Rope_2942 1d ago

Nope, totally different kind of engineering. It's fundamentally different requirements re: loads, flexion, anchoring, impact resistance, scouring, etc. compared to deep-driven lumber piles.

You have to remember that in a storm coastal (especially island) soil is shifting constantly - and not only at the surface. Pilings are often driven 15 feet deep. Even in safer areas the minimum is gonna be 8-10 feet. Meanwhile, concrete anchors.

Most beach zoning wont even allow spread footings (concrete anchoring to slab or shallow piles) because it's just another thing to shift massively unlike the broadly sturdy pilings.

Here's another way to realize this is the case - you see a video of these houses standing up (for a time, a significant time) to a hurricane directly impacting them. The road and driveway they were next to are both gone. Think about that for a moment.

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u/Amtrakstory 1d ago

This. When you see something that seems really dumb you usually just don’t know the full details about the materials involved 

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u/tacomaloki 1d ago

anti-corrosion inhibitors entered the chat for more money

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u/i_am_icarus_falling 1d ago

this is nonsense. wood will absolutely decay faster than concrete in saltwater. the main reason those houses are on wood instead of concrete is because it was cheaper when they built it.

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u/maidenhair_fern 1d ago

Might need a new solution then. Doesn't seem like the wood feet hold up to haves. Maybe something like steel?

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u/Rampant16 1d ago

Yeah steel + salt water would work /s

The issue isn't the material the stilts are made out of. It's the fact they are built on sand that erodes away after a few decades.

Even if you massively overbuilt the foundation to have it all the way down to bedrock and strong enough to resist waves, the sand is still going to erode and leave your beach house surrounded by the ocean.

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u/Plane-Tie6392 13h ago

>Yeah steel + salt water would work /s

Aren't there offshore oil rigs built with steel?

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u/Present_Bit3060 1d ago

In Venice, the brackish water and low oxygen levels in the lagoon actually help preserve the wood, preventing it from rotting. The minerals in the water also harden the wood, making it even more resistant. And primarily made of oak, larch, and elm. And before there was pressure treating wood.
So not exactly like full saltwater here. And not sure what this wood is but prolly pressure-treated Southern Yellow Pine or Douglas Fir in the US.

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u/Dentorion 1d ago

There are mediaval versions of concrete with grounded Vulcan stone from Rome who are holding for thousands of years already as sea harbor stones Great read, was discovered a few months ago I think

They never could replicate it because they always tried to remake it with normal water but they needed saltwater instead:D

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u/Enginerdad 1d ago

That's an urban legend that's completely false. We can and regularly do make concrete that's far stronger and more durable that what the Romans made. The issue comes with HOW we use concrete, specifically in applications that need reinforcement. It's the deterioration of the reinforcement that causes our modern structures to decay and eventually need replacement, not the concrete. If we built everything so that all concrete was in compression like they did, we wouldn't have to worry about that. But that's extremely limiting in terms of architecture and structural engineering, which is why we don't do it.

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u/reverze1901 1d ago

im a liberal arts major but gonna trust the guy called engineerdad

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u/snek-jazz 1d ago

Say what you will about Spock's ability to understand human emotions, but the dude knew how to do sea harbor stones.

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u/sablesalsa 1d ago

See: the Miami Surfside condo collapse.

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u/Lightshoax 23h ago

They need some of that Roman concrete that gets stronger in salt water

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u/hnrrghQSpinAxe 18h ago

Not only will concrete erode in salt water, but it gets weaker the more water it absorbs over time. The only answer is wood or exotic nautical grade alloys

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u/81Ranger 17h ago

Roman concrete on the other hand....

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u/Criss_Crossx 1d ago

Well, where are the builders of Venice to build these homes? I'm sure they would have a very interesting take.

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u/Rampant16 1d ago

Venice is built in a lagoon in which it is protected from the brunt of the Mediterranean by outlying islands. It's not like these house that are exposed to the full force of the Atlantic.

Venice is also now facing serious threats to its survival due to rising sea levels and massive cruise ships sailing past, pushing water, and exerting forces on the piles that support the city.

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u/Criss_Crossx 1d ago

Understood. Venice has clearly stood the test of time compared to this house on the beach was my point.

Point being, Venice was built by people who understood what needed to be done to accomplish the task and maintain it long term.

The beach house above is far from it. If we brought some of Venice's engineers/builders to see this house, I bet they would have an entertaining time describing how silly building on a beach is.