[alancalverd]

Access is no problem for the trace heating idea. You can use a motorised crawler to walk up the cables, or hoist a bloke up with a rope. Much better than dismantling and rebuilding the anchorages: "if it ain't broke, don't fix it"!

[Peter Dow (OP)]

Access is no problem for the trace heating idea. You can use a motorised crawler to walk up the cables, or hoist a bloke up with a rope. Much better than dismantling and rebuilding the anchorages: "if it ain't broke, don't fix it"!

The stay cable system is boasted by its manufacturers to be designed so as to individually replace the strand cables, without breaking the anchorage, the cable, or the flow of traffic.

Whereas the stay pipe which covers the cable is just a 6mm to 10mm thick of HDPE pipe - which isn't yet broken - but it isn't designed to take the weight of a "motorised crawler" or a "hoist" plus all the people, tools and parts they would have to lift up there to get the job of fitting the trace heating done.

So the design of crawlers and hoists would not only have to provide access for inspection, which is easy enough, but also to do all the work required, which would be problematic to be sure to not damage the relatively fragile stay pipe.

So if the stay pipe "ain't broke" then don't break it and then have to fix it.

I think there is more chance of the stay pipe getting broken or the crawler breaking down and getting stuck or a bloke falling out of his hoist or some tool or part getting dropped from a great height and breaking something or someone on the roadway below more severely than any lump of ice can do.

[Bored chemist]

So, the simple design would work OK as long as someone checked the weather forecast.

No. Someone "checked the weather forecast" on the Titanic.

That doesn't make any real sense.

[Peter Dow (OP)]

So, the simple design would work OK as long as someone checked the weather forecast.

No. Someone "checked the weather forecast" on the Titanic.

That doesn't make any real sense.

Having "someone checking the weather forecast" isn't a foolproof recipe for avoiding dangerous problems with ice, whether that's ice falling from a bridge or an iceberg a liner steams full speed into.

In order to have systems that work and are foolproof there needs to be a lot more complexity than you suggest.

[Bored chemist]

So, the simple design would work OK as long as someone checked the weather forecast.

No. Someone "checked the weather forecast" on the Titanic.

That doesn't make any real sense.

Having "someone checking the weather forecast" isn't a foolproof recipe for avoiding dangerous problems with ice, whether that's ice falling from a bridge or an iceberg a liner steams full speed into.

In order to have systems that work and are foolproof there needs to be a lot more complexity than you suggest.

And what you propose would add even more complexity.
Unless you plan to leave your system heating the wires all summer, it also needs to look at the weather forecast.

[Peter Dow (OP)]

And what you propose would add even more complexity.

Sure. Complexity is good.

Unless you plan to leave your system heating the wires all summer, it also needs to look at the weather forecast.

What my heating system needs is to have sensors which measure the surface temperature of the cables and to have computer control adjust the heating power according to the measured temperature.

As my oft-posted bar chart and line graph proposes, the cable heating control would adjust the "Heating Power" (pink bars) - Watts of heat energy per metre-squared of stay pipe surface area - according to the Temperature of the Stay Pipe Surface in degrees Celsius.



So helping you to read that graph, the heating control regime it proposes would be, from left to right of the graph -

• at a temperature of stay pipe surface of -1.4°C, the heating power is the maximum of 318 Watts/metre-squared.
• at a temperature of stay pipe surface of 0°C, the heating power is 150 Watts/metre-squared.
• at a temperature of stay pipe surface of 1°C, the heating power is zero, or off.

Of course there are many other possible control regimes that may serve better in practice than that one.

No doubt you could switch the system off altogether during the summer months.

What you couldn't risk doing is to simply employ a worker with an on-off switch with instructions from management to "check the weather forecast". You need something more complex, more automated, more foolproof than that.

[alancalverd]

You need something more complex, more automated, more foolproof than that.

The word you need is "reliable".  There is a complex, automated, foolproof system to prevent the 737MAX from stalling on rotation. So far it has killed around 350 people.

[Bored chemist]

Sure. Complexity is good.

No.
Just , no..

It's  also a pity that you didn't understand that I was kidding when I said it should be a person who checked the weather.
People are extremely complicated and dreadfully unreliable. That was part of my point.

My point was that there isn't a need to heat the cables quickly, because the forecast (or even the current temperature and rate of change) will let you turn on the heating in good time. (And you would need to do that, no matter what heating system you used.)

And, since the "big selling point" of your complicated system is that it is quick, -which isn't necessary- your system is,as I said, overly complicated.

[Peter Dow (OP)]

You need something more complex, more automated, more foolproof than that.

The word you need is "reliable".  There is a complex, automated, foolproof system to prevent the 737MAX from stalling on rotation. So far it has killed around 350 people.

Reliable is good. The UK has an unreliable Prime Minister whose foolish misgovernment of the pandemic so far has killed around 120,000+ British people.

[Peter Dow (OP)]

Sure. Complexity is good.

No.
Just , no..

It's  also a pity that you didn't understand that I was kidding when I said it should be a person who checked the weather.
People are extremely complicated and dreadfully unreliable. That was part of my point.

Dreadfully, especially BoJo the Clown.

My point was that there isn't a need to heat the cables quickly, because the forecast (or even the current temperature and rate of change) will let you turn on the heating in good time. (And you would need to do that, no matter what heating system you used.)

And, since the "big selling point" of your complicated system is that it is quick, -which isn't necessary- your system is,as I said, overly complicated.

Well for heating the cables whose stay pipe can cool down very quickly, it may be more efficient to be able to heat quickly and powerfully - like the power profile for doing a washing machine cycle - rather than like that for leaving the heating on all day.

I don't really see the purpose in dumbing down the design to the simplest you can get away with.

[alancalverd]

Where the pilots have switched off the automated anti-stall system on the 737, there have been no fatalities.

In the early days of manned space flight, NASA spent huge sums designing a pen that could write in zero-g, and freeze-dried  food that could be reconstituted with hot water in a pouch. The Russians used pencils and sandwiches.

[Petrochemicals]

I hate to be a kill joy, but what about a  mesh grid sloped above the road, it would reflect the ice and let the wind pass ?

[Bored chemist]

The UK has an unreliable Prime Minister whose foolish misgovernment of the pandemic so far has killed around 120,000+ British people.

That may be the only sensible thing you have said in this thread, but I don't think it's related to the topic.

[Bored chemist]

In the early days of manned space flight, NASA spent huge sums designing a pen that could write in zero-g, and freeze-dried  food that could be reconstituted with hot water in a pouch. The Russians used pencils and sandwiches.

https://www.snopes.com/fact-check/the-write-stuff/

[Peter Dow (OP)]

I hate to be a kill joy, but what about a  mesh grid sloped above the road, it would reflect the ice and let the wind pass ?

A reasonable suggestion but

1. A large part of the reason for building a bridge rather than a tunnel is for the architectural art of it - the form, not just the function. No doubt many would object to a net in that it would "spoil the look of the bridge", especially when that net got dirty with bird droppings, wind-blown garbage - plastic bags etc. There may not be suitable anchor points to attach the net in the correct position and so adding these may be glaring bolt-ons which again may detract from the bridge as art. The ice net would serve as a constant visual reminder not of the wisdom of the Scots as bridge-builders but as a reminder of the error of the Scots in the initial mistake in not designing to prevent ice fall from the beginning. Whereas my solution once completed is pretty much invisible from the outside.

2. How big would be mesh be? Too big and dangerous lumps of ice could fall through, too small and it could ice up, form a ice sheet which would present a wind loading problem.

[Petrochemicals]

I hate to be a kill joy, but what about a  mesh grid sloped above the road, it would reflect the ice and let the wind pass ?

A reasonable suggestion but

1. A large part of the reason for building a bridge rather than a tunnel is for the architectural art of it - the form, not just the function. No doubt many would object to a net in that it would "spoil the look of the bridge", especially when that net got dirty with bird droppings, wind-blown garbage - plastic bags etc. There may not be suitable anchor points to attach the net in the correct position and so adding these may be glaring bolt-ons which again may detract from the bridge as art. The ice net would serve as a constant visual reminder not of the wisdom of the Scots as bridge-builders but as a reminder of the error of the Scots in the initial mistake in not designing to prevent ice fall from the beginning. Whereas my solution once completed is pretty much invisible from the outside.

2. How big would be mesh be? Too big and dangerous lumps of ice could fall through, too small and it could ice up, form a ice sheet which would present a wind loading problem.

The mesh should not succumb to ice too easily as it is not as high, the wind I suspect has a lot to do with superchilling water and/or the surfaces it lands on. I suppose the angle may also be effectual as water will drip if too shallow angle is
Asked for. The mesh, perhaps in metal, would need to stop the ice falling, reducing its speed, so a certain size ice piece from a  height of 60ft  would be able to be calculated as to the damage potential it has.

[Peter Dow (OP)]

Or you can use a plastic boot and blow hot air up the tube.

FORCED CONVECTION

Other bridges subjected to much icier weather may require more powerful heating so strand overheating may be a greater risk and then in order to enhance the heat transfer from strands to stay pipe, one could force convection in the air space between the strands and stay pipe, by oscillating an air flow up and down the cable and creating air turbulence within the pipe.

I don’t propose to spend any time soon researching exactly how forced convection could be engineered because the Queensferry Crossing strands won’t overheat according to my calculations.

[Petrochemicals]

What is the corrosion danger from electrically charging a bridge structure ? How do they do it for power lines ?

[Bored chemist]

What is the corrosion danger from electrically charging a bridge structure ?

Nil, because the cables strands are coated in plastic, and the rest of the bridge isn't connected.

How do they do it for power lines ?

Often by ... coating the lines in plastic.

With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.

[Colin2B]

How do they do it for power lines ?

Often by ... coating the lines in plastic.

With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.

as well as that, they also use Al for some overhead which doesn't have the same corosion problems as the steel in bridges.