Brooklyn Bridge Engineering question about wood

Factual Questions
1

I’m reading “The Great Bridge” a really good book (good, not great) on the building of the Brooklyn Bridge.

In the book I was surprised to learn that the New York tower is built on 31 feet of pine. (The Brooklyn town has wood as well.) In other words, there is sand, then concrete, then 31 feet of wood then stone. Obviously, the bridge has withstood the test of time, so clearly the think works.

I have three questions. Why can pine survive there, but if I put it in my back yard it will begin to rot in a season? Is it the lack of air and creatures that keeps it safe down there?

Do modern engineers have any concerns about all that lumber now?

Lastly, can I presume that we wouldn’t pine/wood now?

As always, thanks in advance.

2

not only that bridge but many tall and heavy buildings in big cities that were on made on reclaimed land are on wood. little air and constant wetness keep the wood from deteriorating.

3

My father, a structural civil engineer, told me that if you keep wood either always wet or always dry then it will last a long, long time.

Buildings were sometimes built on a mat of willow or other wood to prevent it from sinking. As long as it stays wet, it’s okay.

4

Weren’t those wood caissons that were filled with concrete?

5

Yes. I just read an article from Forest Finance about retrieval of wood from flooded lakes (for dams) - in many instances, they just flooded the whole region with a standing forest, no chopping down before.

The wood retrieved was tested by an institute and found to be in perfect condition, even better receptable to paint.

There was neither rot nor bark beetle, the two great afflictions for trees with oxygen and land around.

Also, harvesting with machines is not significantly more expensive than harvesting on land.

If all that wood would be harvested, then a bit less wood from original forests would be needed.

6

Yes they were filled with concrete. It was like a giant box without a bottom. Once it reached the correct depth (bedrock on the Brooklyn side, sand on the New York side) they filled the inside of the box so to speak with concrete.

7

Reclaimed wood that has been submerged for decades is a carpenter’s wet dream, especially if it is ‘old growth’ lumber. Then ya got yer woods like Chestnut that are pretty much extinct.

Pull that up from a lake where it’s been submerged for a century and it’ll be perfect… or better. Can wood be better than perfect? Ask a cabinet maker or furniture builder. They’ll say yes.

8

Just a little side factiod.

IIRC it was in caissons where they first discovered “caisson’s disease” AKA the bends AKA decompression sickness.

9

I read about this a long time ago. IIRC the caissons were floated out to location, then masonry towers were built on top of them at rate that slowly pushed the caisson to the river bottom. Then workers descended through shafts in the towers to work in the submerged caissons to dig out the river bottom so the caissons would sink down even further. Finally the whole thing was filled with concrete and masonry so that the weight of the bridge is held entirely on the masonry and concrete. The wood won’t rot in that mud. But even if it did would it affect the structural integrity of the bridge?

10

Above bolding mine.

I don’t think is is entirely true. It’s my understanding (and I could be wrong) that the weight of the entire structure does indeed rest of the wooden roof of the caisson, which as I noted in my question is ~31 feet in height.

sand - concrete - wood (roof of the caisson) - granite

This is what surprised me, that the tower rested on wood.

11

There are 3600 tons of wood making up the caissons which were bored/checked recently and are as fresh as new. Many of the homes in Venice have wood footings.

If the timbers used are structural, the fact that they are submerged does not contribute to their decay, and may, in fact, delay any.

12

But the actual roof itself is probably only a short thickness of wood. IMHO the wood could probably slowly rot (even though the opinion here is it won’t or at least will do so very slowly), eventually being compressed into nothingnesss by the mass above it, and still cause no significant structural problems.

13

Not to kick a dead horse on this, but the roof is at least “22 feet of solid timber.” per “The Great Bridge” That is what concerned me. If that was compressed to ‘nothingness,’ there’d be issues.

14

Okay,

Well, If they say its 22 feet thick, its 22 feet thick. Though given my impression of how you use a cassion I can’t imagine WHY you’d need it that thick or want to make it that thick given that your going to fill the hollow part with concrete when your done. Actually I can imagine why, but thats just because they wanted to fill it easily, not because they had too.

Also 31 feet tall vs 22 feet thick mean different things. .

15

snerk

I know, I know, my sense of humor is adolescent…