Astro Hacker News - Zinc creep in cable spelter sockets brought down the Arecibo telescope in 2020

dzdt |next [-]

The forensic report on the failure of the cable sockets is at https://www.thorntontomasetti.com/sites/default/files/2022-0.... Unlike the linked article this includes detailed diagrams and discussion explaining how these sockets worked and how they failed.

My attempt at a quick summary: the Arecibo instruments were held up by heavy steel cables, which are composed of a bundle of 126 to 216 individual wire strands twisted together, where each wire strand is about 1/4 inch diameter. At the cable ends these were anchored in sockets. The socket is conical, widening out towards where the wire ends go. The wire strands of the cable would be separated in a "broom" shape to fill out that conical area, and then the whole thing filled in with molten zinc.

The failure mode of these zinc-filled sockets is for an inner core cylinder, about the same diameter as the cable body, to pull out from the socket while outer strands of the cable which are broomed out of that core either snap or pull free from the zinc plug.

This failure mode hadn't really been seen prior to the Arecibo collapse, partly because it is a long term process where the zinc slowly flows under stress over decades.

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BonoboIO |root |parent |next |previous [-]

Thanks for the quick summary

0xbadcafebee |root |parent |previous [-]

The pictures on page 25 seem to show how this socket design fails. First because they didn't evenly distribute the wires in the socket (seriously??)... And second, the shear on the zinc from the compressed wire bundle is essentially a narrow rectangle in the direction of the cable with almost all the force concentrated in a narrow forward bit, not really distributed evenly through the socket, because of the wire ends splaying outward and not being equally distributed. I guess the idea is the rest of the zinc towards the back provides a counter force on the front part, but there's a whole lot more zinc in the back pulling on a smaller amount in the front. The tight bundle and the conical shape is great for a tight install, but doesn't help distribution of load. Compare to the Golden Gate Bridge wire termination (https://c7.alamy.com/comp/2RY9PKW/golden-gate-bridge-plans-i...). The zinc creep is definitely bad, but it seems made worse by an uneven distribution of load in the socket, in addition to all the other problems...

tl;dr once the outer wires break in the socket (in part due to zinc creep, but also potentially more load on them), the "glue" keeping the wire bundle connected to the socket is broken, the former wedge keeping the core in place becomes a straight line, and you have a core ready to pop out.

r721 |next |previous [-]

In case you didn't see the video of collapse at the time (don't miss the drone footage which starts at 0:55):

>Watch the Arecibo Observatory's catastrophic collapse!

https://www.youtube.com/watch?v=u34uwFOCM-4

EdwardDiego |root |parent [-]

When those cables go... dang that's a lot of tension.

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snypher |next |previous [-]

(2024). Here's the report 'Failure Analysis of the Arecibo Observatory 305-Meter Telescope Collapse'

https://www.nationalacademies.org/read/26982/chapter/1#vi

ChuckMcM |root |parent |next [-]

Much better, AI generated "summary" pages are not a good link for this story.

jacquesm |root |parent |previous [-]

This is the better link. The meat is on page 2.

aaron695 |next |previous [-]

[dead]

metalman |next |previous [-]

not good, not good at all, as this is how ALL bridges and towers are supported, the basic engineering bieng that any cable end will be stronger than the cable itself and will last longer than the cable itself, except now, when it isn't, not good, not easy to inspect, not easy to service.

ceejayoz |root |parent |next [-]

> Long-term low-current electroplasticity, induced by the electromagnetic waves from the Arecibo telescope itself, may have sped up the zinc creep, it says.

I presume we aren't using most bridges as interstellar antennas.

billfor |root |parent |next [-]

The main paper goes into the detail on page 2. The cables were on beam path.

“The only hypothesis the committee could develop that provides a plausible but unprovable answer to all these questions and the observed socket failure pattern is that the socket zinc creep was unexpectedly accelerated in the Arecibo Telescope’s uniquely powerful electromagnetic radiation environment. The Arecibo Telescope cables were suspended across the beam of “the most powerful radio transmitter on Earth. National Academies of Sciences, Engineering, and Medicine. 2024. Failure Analysis of the Arecibo Observatory 305-Meter Telescope Collapse. Washington, DC: The National Academies Press. https://doi.org/10.17226/26982.”

tejtm |root |parent |next |previous [-]

> I presume we aren't using most bridges as interstellar antennas.

any like area of metal will receive as much interstellar emissions but I guess they did run a MW transmitter sometimes.

thenthenthen |root |parent [-]

MW as in Mega Watts? (I read it as Medium Wave the first time)

namibj |root |parent [-]

Yeah it was a radar that could collect the echo on the next rotation of Earth.

robotresearcher |root |parent |next |previous [-]

Arecibo didn’t know it was a telescope. Every wire in the world is an antenna.

AngryData |root |parent |next |previous [-]

No, but there is a lot of potential for large structures to act like an accidental antenna and/or receive low level voltages from the environment or other equipment.

Likely won't matter in most cases, but there could be a few disasters waiting around.

xattt |root |parent |previous [-]

That’s what the government (… and most rational people) wants you to think.

grim_io |root |parent [-]

how could the government control the birds without using bridges as antennas?!

pfdietz |root |parent |next |previous [-]

My guess is the safety factor here was lower than typical, causing the failure mode to occur.

I expect future bridges will design these joints even more conservatively because they were skating closer to the edge than they realized.

msandford |root |parent |next |previous [-]

They also mentioned that this is the first example of the unknown phenomena that caused the creep. This technology has been in use quite successfully for more than 100 years.

jryb |root |parent |next |previous [-]

This would predict that we should be seeing far more mystery collapses than we do though

sanex |root |parent |previous [-]

Bridges probably shouldn't be subject to the same electromagnetic radiation that gets focused by the telescope though so ideally not a problem there.

drum55 |root |parent |next [-]

Where the cables were there’s nothing focused, just wires, it was outside of the dish.

billfor |root |parent [-]

They were across the dish. From the report: “The Arecibo Telescope cables were suspended across the beam of “the most powerful radio transmitter on Earth””

ButlerianJihad |root |parent [-]

The role of the cables was for supporting the equipment. The cables that failed, and are examined in the report, were designed to hold the equipment up, while it moved around. These cables weren't involved in carrying signals, data, or power.

There is no denying that energy was induced because of their placement at the focal length of the dish, but that was incidental to their purpose. The bundles of wires were spirally bound around a core. There were certainly ground connections and lightning arrestors involved. The EM effects and properties are discussed on pp 40–47 of the report.

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phyzome |previous [-]

Well, that and the reductions in funding.

fsckboy |root |parent [-]

it had already been decommissioned prior to the collapse. Yes, decommissioning does indicate funding is being cut, but it was unrelated to the collapse.