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Richard Haselgrove
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Message 102079 - Posted: 10 Dec 2020, 19:14:57 UTC - in response to Message 102078.  


I can understand why the 1 and 5 could be on the "line before", and the 4 and 8 on the "line after". But how do they get the 3s to the end?
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robsmith
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Message 102080 - Posted: 10 Dec 2020, 19:35:00 UTC - in response to Message 102075.  

Until the extent of the damage to the tower tops (if any) was fully known and understood no repair could even be considered. Cables of the size required (diameter, construction, length) would take time, as would designing the new tower tops (or tower top fittings)and making sure the towers themselves still had enough residual strength for the new loading (additional cable weight, new/modified tops and cable "pull" loads). Remember it was less than two weeks between the initial cable break and the final total failure, minutes after an earthquake struck the area....

Using a helicopter to add new cables? - Well yes, but the shear weight of the cables at about 4 tonnes per 100m - and each run is about 150m, so that's 6 tonnes (granted the whole cable wouldn't necessarily be lifted in each case), and some of the cables are much longer than that, so weigh a lot more - not many helicopters are capable of lifting & controlling such cable weights and lengths. The most probable way of rigging cables of this nature is to use smaller draw cables to pull in the full size cables, and rigging that sort of setup is far from child's play.

Time was against them, all that cold be done was watch, observe, study and try to work out what the viable alternatives really were - I dare say a fair number on the ground really wanted to save the telescope as it had been their source of income for the majority of their working lives.

I really hope that the local political forces are able to convince the US government that spending millions on building a new telescope of at least the size and performance of the old one is worthwhile. Just now, as far as I'm aware, there are very few RADAR telescopes around, never mind one of the performance of the Arecibo big dish (I think the current biggest one is the Lovell telescope at Jodrell Bank in the UK as I don't think either FAST or Effelsberg are radar capable)
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ProfileGary Charpentier
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Message 102118 - Posted: 13 Dec 2020, 4:44:44 UTC - in response to Message 102103.  
Last modified: 13 Dec 2020, 4:44:56 UTC

firemen have a risk
Which is why they "go defensive" all the time when the risk gets too high. Going defensive is running like hell from fighting the fire inside to standing around making sure other things don't catch while what you were called for burns itself out.

Once the first cable snapped it was time to go defensive.

https://en.wikipedia.org/wiki/Sikorsky_S-64_SkycraneUseful load 20,000 lbs.

Unfortunately you can't tie a knot around the platform with a cable that big, so you would have to have people put an attach point on the platform before the cable could be attached to take load. It isn't going to just be one person either and it is going to be a lot of man hours and every man hour increases the risk.
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ProfileDave
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Message 102121 - Posted: 13 Dec 2020, 10:09:57 UTC - in response to Message 102118.  

Unfortunately you can't tie a knot around the platform with a cable that big, so you would have to have people put an attach point on the platform before the cable could be attached to take load. It isn't going to just be one person either and it is going to be a lot of man hours and every man hour increases the risk.


And for that sort of job, you are talking about skilled construction workers where a lot has been invested in their training. I don't see many construction companies accepting a job which could easily wipe out four or six of their most highly trained people.
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Richard Haselgrove
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Message 102122 - Posted: 13 Dec 2020, 10:33:14 UTC - in response to Message 102121.  

And each extra worker, each attachment point hardware, each welding kit - all adds to the load being supported by the existing cables. And with the likely failure mode being a runaway cascade of individual strands breaking, there wouldn't be time to evacuate workers when the final collapse began. As the video confirmed.
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robsmith
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Message 102123 - Posted: 13 Dec 2020, 10:55:17 UTC - in response to Message 102118.  
Last modified: 13 Dec 2020, 10:56:30 UTC

While the Skycrane can lift about 9tonnes, that is on a fairly short line and as a solid mass. For most helicopters the safe load of a hanging cable at least 150m long drops very dramatically - from memory by a factor of between 3 and five for every 50m of hanging cable.

As Gary Dave says very few companies would ever even bid for such a job as they realise that shortly after having their best high-work crew wiped out the lawyers would start to circle.
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Message 102125 - Posted: 13 Dec 2020, 12:43:01 UTC

Many years ago, I worked with helicopter squadrons while in the forces. They are fine for lifting but very unhappy about doing anything that might involve a sideways pull on them. I don't see how they could have been used there without that.
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robsmith
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Message 102128 - Posted: 13 Dec 2020, 14:21:43 UTC
Last modified: 13 Dec 2020, 15:11:25 UTC

Ah.... our paths may have crossed ;-)

Not only do helicopter crews not like things that impose a side pull while lifting they detest anything that might change shape by whipping around, something that cables are infamous for.
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Message 102129 - Posted: 13 Dec 2020, 16:09:04 UTC

Today's Inspirational Quote:
"Kids should be allowed to break stuff more often. That's a consequence of exploration. Exploration is what you do when you don't know what you're doing."

-- Neil deGrasse Tyson
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Message 102130 - Posted: 13 Dec 2020, 17:14:10 UTC - in response to Message 102129.  

Today's Inspirational Quote:
"Kids should be allowed to break stuff more often. That's a consequence of exploration. Exploration is what you do when you don't know what you're doing."

-- Neil deGrasse Tyson


Perhaps that might help them not do it when they are big kids. (Politicians)
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robsmith
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Message 102178 - Posted: 15 Dec 2020, 18:14:49 UTC

You may have no care for your life but most others do care that they get home from work with all their bits intact.

Since you know everything that's wrong with the Arecibo big dish it is about time you got on a plane and went down to Puerto Rico and told them what to do and stopped being such an arrogant being.
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betreger
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Message 102180 - Posted: 15 Dec 2020, 18:33:47 UTC - in response to Message 102178.  

+1
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