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Comment by wizardOfScience | Hacker News Reader

A wind turbine tower is essentially a cantilevered beam resisting the bending moment from lateral wind loads. The loads can come from any direction. Bending induces stresses in any beam that increase with distance from the centre. A thin walled hollow tube is the most material efficient design theoretically as it concentrates the load bearing materials along the perimeter of the beam. Any deviation from this incurs material that is not fully utilised.

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Your explanation raises the follow-up question, which svantana already hinted at: Why don't construction cranes use hollow tubes instead of their typical truss structures?

The crane manufacturing business case is not driven by material efficiency to the same degree. It is a tool that needs to be reliable and have performance in operations. Limit the need for man hours through ease of use etc. It should also be able to take many assembly/disassembly cycles.

Thus does the amount of material not matter as much in a crane.

For wind turbine towers the material cost can be >>50% of the installed cost.

Working for a crane manufacturing company. While raw material costs are maybe not that drastic, we consider material efficiency the most important metric for cost and a lot of brain power was spent to optimise the amount of steel used. There are multiple reasons for this. The ones I can think of are:

  * The margins for cranes are thin and steel is expensive.
  * Thicker steel is harder to work with, increasing manufacturing cost.
  * Each kg of dead weight may decrease the performance of your product, e.g. max. Live load. This is especially true for the jib.
  * More weight at the top of the crane may necessitate a sturdier structure below, amplifying cost even more.
  * More weight may require more ballast blocks, which are costly (especially transport)
  * More weight means higher transport costs
  * More weight means more wind area, which is the critical factor for high constructions.

Because at cranes the force is not the wind that comes from any direction, but the directed payload hanging at the crane arm. Almost all cranes I now moving also the pole around.

Certainly makes them easier to disassemble and move elsewhere.

Yeah it's pretty essential that construction equipment be pretty easy to construct and deconstruct. There are some videos [1] which are worth checking out.

[1]: https://www.youtube.com/watch?v=vx5Qt7_ECEE

My guess: They want the wind to pass through the crane, while they want wind turbines to capture the wind.

This is the answer.

Interestingly, wind/storm loads are oftentimes the limiting factor for the configuration height of a crane.

This is because, when adding another tower segment, not only the total area increases but also the wind forces. The other loads stay roughly the same

This is the reason why bottom-slewing cranes, which are commonly used for small buildings, sometimes are built with solid walls. Top-slewing cranes, which are used for high buildings, always use a steel framework.

This is what I come to HN for, kudos

This is almost a "spherical cows" level gross over simplification. If you weren't defending it in other replies I'd think you were satirizing people who only have book learning and no real world experience.

At the limit the failure of your statement obvious. If instead of a thick walled wood tube hundreds of feet tall this structure were an orders of magnitude wider cylinder of thin plies the same height it wouldn't even be able to hold itself up, it would flop over, tear and all fall down under its own weight if not from manufacturing variances then from the wind and differential expansion/contraction from the sun and if by some miracle it survived that it would flop over

The material has to support itself and tolerate undefined small (relative to the main load) loads in other directions as well as point loads from fastening it to whatever you are using it to bear the loads of, going all in on "large and thin" fails to optimize for this for more or less the opposite reasons that going all in on "solid" does.

The parent is explaining why it’s hollow instead of being a truss. They accurately explain that the hollow tube is optimal to counter the wind loads. Your criticism is that they didn’t discuss gravity loads, but that wasn’t “in the prompt” — the gravity load doesn’t explain why they’re a hollow cylinder. There are a wealth of other irrelevant things that could be discussed, but have been neglected because they are irrelevant to the question.

Frankly your comment disappoints me. wizardOfScience gave a _great_ answer, that would make any solid mechanics professor proud.

You're strawmanning bot me and the comment I'm replying to. Whether you're using a truss design or a solid material wall what I'm saying holds true. You can't just go all in on "put the material at the perimeter" because it'll have other problem. Gravity is just one of those problems.

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