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The Forth road bridge

Engineering problem

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Bob Abell04/12/2015 19:25:17
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Hello Paul

Any comments regarding the crack in a major load bearing girder and how it can be repaired?

It looks more than a 3 week repair job to me!

It`s strange, how it appears to be a tie?

Bob

_87041837_tower.jpg

_87041835_fracture.jpg

Colin Bishop04/12/2015 19:48:29
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Bit ironic (sic) isn't it. The relatively modern suspension bridge is suffering structural failure yet the much older railway bridge which has had big heavy trains thundering across it since 1890 just soldiers on.

Colin

Dodgy Geezer04/12/2015 19:53:12
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Oh, a couple of dabs of superglue and Bob's your uncle. I'd do the job myself, only I'm working on the containment vessel for Hinkley C...

Paul T04/12/2015 20:15:54
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Bob

I would do exactly what the engineers are doing, having located this problem I would look for further damaged parts throughout the entire structure then investigate the cause.

The bridge has frequently exceeded its maximum load capacity, don't forget that it was designed in the late 50s and opened in 1964 and since then its carrying capacity has been far exceeded.

There is also a steady degradation of the main cables that have lost 10% of their strength due to corrosion and proposals were already in place to limit the traffic to reduce weight.

Far from its original design lifespan of 120 years it is now expected that, without extensive repairs and rebuilding the bridge would close in 2020.

Basically the bridge is knackered.

Bob Abell04/12/2015 20:26:33
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Paul.....Why do you say, the bridge was overloaded?

Since the bridge was designed 50 years ago, it won't be on computer either

Unless early problems forced a design study?

Every time we cross the Menai Bridge, I can't help wondering about those wrought iron chains, disappearing into the concrete......Mmmmmmm?

Bob

Paul T04/12/2015 20:29:45
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Bob

When they designed the bridge they never envisaged the increase in traffic numbers that we now have.

Peter Fitness04/12/2015 21:42:16
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Paul, you say the bridge's carrying capacity has been far exceeded, do you mean the load on the bridge at any given time, or the volume of traffic now using the bridge? I am no engineer, but I would have thought that, when a bridge was being designed, the theoretical load would be calculated assuming that the bridge deck would be covered in vehicles, plus a percentage, perhaps as much as 100% for safety - or am I being naive, or simply ignorant?smiley (Probably both) To me, 50 years is not a long time, especially in terms of such structures as the Forth road bridge. I've been married for over 55 years, and the time has flown bylaugh

Peter.

Colin Bishop04/12/2015 21:58:36
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I would suspect that it isn't a matter so much of the static load on the bridge deck but more to do with higher than expected traffic volumes causing greater than anticipated vibration and flexing of the bridge components which has led to early metal fatigue.

The bridge is also in a very exposed location with frequent high winds although that should have been designed for.

Colin

shipwright04/12/2015 22:24:00
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I wonder whether they have a distributed sensor system (strain gauges, accelerometers) on the key parts of the structure of the Forth (and for that matter similar modern suspension bridges) ? As I'm sure you are all well aware Rolls Royce use real time monitoring on all of their engines on aircraft. Knowing the "normal" signature set, any deviation outside specified limits will cause alarms to be set at the monitoring centre and the engine will be investigated when the aircraft has landed.

Ian

Bob Abell05/12/2015 09:41:06
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Hello Ian

There is monitoring equipment installed......They have just detected a large number of wires breaking, in the last 3 months!.........Scary or what?

Bob

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