The Forth road bridge

[Bob Abell 2Participant @bobabell2]

Engineering problem

[Bob Abell 2Participant @bobabell2]

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

[Colin BishopModerator @colinbishop34627]

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 Geezer 1Participant @dodgygeezer1]

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 TParticipant @pault84577]

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 Abell 2Participant @bobabell2]

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 TParticipant @pault84577]

Bob

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

[Peter FitnessParticipant @peterfitness34857]

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? (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 by

Peter.

[Colin BishopModerator @colinbishop34627]

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

[shipwrightParticipant @shipwright]

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 Abell 2Participant @bobabell2]

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

[Paul TParticipant @pault84577]

Don't make the mistake of comparing this bridge with todays technology, it was designed in the late 50s just think if you had a car back then what was it like?

Many concrete and steel structures from the 60s that were intended to last 100 years have been demolished due to structural failure…the so called concrete cancer, so it is hardly surprising that this bridge is in trouble.

Lastly don't compare this bridge to the rail bridge as they are totally different types of structure and the rail bridge has constant maintenance to keep it open.

[Paul TParticipant @pault84577]

For those interested in this subject the BBC Coast programme visited the bridge in 2007 to discuss, amongst other things, how corrosion was affecting the structure.

This episode is being shown tonight on Yesterday at 7pm (freeview 19, sky 537)

[Bob Abell 2Participant @bobabell2]

Thanks for the info, Paul

Looking forward to that…………Only hope I can remember

Bob

Edited By Bob Abell on 10/12/2015 11:04:35

[Paul TParticipant @pault84577]

Ask Joyce to remind you….it starts when Emmerdale starts.

[Colin BishopModerator @colinbishop34627]

Perhaps they should have used Roman concrete, it was apparently harder and more durable that the modern stuff!

Colin

[ashley needhamParticipant @ashleyneedham69188]

I know what they should have used….

Ashley

​ ​

[Colin BishopModerator @colinbishop34627]

They don't do sheets that big Ashley.

Colin

[Paul TParticipant @pault84577]

Hello Colin

The question of the difference between Roman concrete and modern concrete is a perennial one and the answer lies in the different way we use concrete compared to the Romans.

Concrete is strong in compression, but weak in tension and bending. This means that by itself, concrete is of limited versatility. It’s easily used in columns, arches, and other things that will only be in compression.

But you run into trouble when you try to make long bridges, tall buildings, thin walls, or anything else that wants to bend in the middle. Concrete is also very brittle and when it reaches it’s breaking point, it shatters like glass. Because of this, if it fails, it fails instantly and catastrophically, without giving any warning.

In modern construction, both these problems are solved the same way: by putting steel bars or wire in the concrete at key locations. Steel, unlike concrete, is incredibly strong in tension. A concrete beam reinforced with steel in it will be about a fifth as big as an unreinforced one.

It will also be much, much safer. Unlike concrete, which is brittle, steel is ductile so when it fails, it doesn’t fracture, it stretches. This added ductility gives failed reinforced concrete the ability to absorb a great deal of additional energy before it collapses, and provides ample time for people to evacuate.

Because of this, including steel in concrete isn’t just a good idea: it’s the law. Nearly every major building construction standards requires concrete to have a minimum amount of reinforcing steel, outside of a few special cases.

But all this strength has a drawback: steel also severely reduces the durability of concrete. Because concrete is porous, over time chloride ions and other corroding elements work their way into the concrete and begin to corrode the steel inside. The amount of time this takes varies depending on how deeply buried the steel is, but it inevitably happens.

This corrosion both weakens the steel and causes it to enlarge, ultimately bursting the concrete from the inside out. Steel corrosion is the primary mechanism behind concrete decay, and one of the major limitations on modern concrete’s lifespan.

Because Roman concrete had no reinforcing, it has none of these problems.

Another major difference between Roman concrete and ours is the cure time. Modern concrete hardens and reaches its maximum strength very, very quickly. The “standard” time for concrete to fully cure and reach it’s capacity is 28 days, but it’s not uncommon for it to reach usable strength in just a few hours.

This rapid cure time, while helpful for speedy construction schedules, introduces thermal stresses as the reaction heats up. These stresses cause cracking, and ultimately reduced durability. To make matters worse, additional steel must be included to address these thermal stresses, exacerbating the problem of corrosion-induced failure.

Roman concrete, on the other hand, cured astonishingly slowly. In the breakwater the concrete was sampled from took two years to cool down completely. This extremely slow cure time means lower thermal stresses, and a corresponding higher durability.

Of course, all this is academic if we couldn’t produce similarly durable concrete today. And it turns out, when the situation calls for it, we’re capable of putting together an extremely durable concrete mix.

Modern concrete structures have been built that are designed to have 1000-year lifespans. In fact, the exact mineral that supposedly makes the Roman concrete so durable, aluminium-substituted tobermorite, was patented 30 years ago, for the exact use suggested in the recent paper. Ultra durable concrete is within our reach. But the requirements for making it – no steel, extremely slow cure time, high pozzolans in the mix – are very limiting, and very, very expensive.

The trick to making concrete that can last the ages isn’t some lost recipe or mystical method, but boring old economics.

Paul

Ash there is a manmade board that is super thin and 200 times stronger than steel……….yes its superhardboard…..and the only thing that can damage it is kraptonite.

 

Edited By Paul T on 10/12/2015 12:08:01

[Colin BishopModerator @colinbishop34627]

Topic tidied up. Thanks for the explanation Paul, I found it very interesting. My comment about Roman concrete was a bit tongue in cheek, I was aware of the balance between compression and tension and the long term corrosion issues.

Back in the 70s I was also close to the consequences of the notorious calcium chloride problem as several of our public buildings had roof beams which were affected by it.

Colin

[Paul TParticipant @pault84577]

Hi Colin

It is very likely that the Forth Road Bridge has calcium chloride in its concrete, as you will know calcium chloride was added to a concrete mix to accelerate the curing process and many contractors over specified its inclusion simply to speed up their construction programmes.

The maximum limit should have been 1.5% by volume but this was often exceeded especially during the winter months.

For those who don't know calcium chloride attacks the metal reinforcing within reinforced concrete, this was especially dangerous in pre-stressed concrete beams where corrosion of the pre-tensioned reinforcement could result in catastrophic failure, the affected beams could literally explode as the tension was suddenly released when the tensioned steel finally gave way.

Although the use of calcium chloride was effectively banned by building code in 1977 some exceptions do remain most notably in sub merged marine and piling situations.

So Colin how did you go about removing / repairing the beams in your public buildings….or were you involved in a 'near miss'

Paul

Edited By Paul T on 10/12/2015 17:01:44

[Colin BishopModerator @colinbishop34627]

It was 'near miss' as I recall, we were putting up 'homes for the elderly' to a common design at the time, there was much concern and testing.

Those premises are now being phased out as being no longer suitable for residential care these days.

Colin

[Bob Abell 2Participant @bobabell2]

Hello Paul

Enjoyed tonight`s version of Coast……….Nearly as good as the Hoylake one!

I was struck immediately, by the slenderness of the main cables!…………..Made me cringe!

The pinging sound of the wires snapping every 5 secs or so………was most alarming!

And the sight of all those wires disappearing into that dodgy concrete…….Had me reaching for the smelling salts!

On a lighter note……..I was impressed seeing Alice Roberts, smiling and chatting away…..Topside!

You would think, the bridge could be saved, with a few ambitious mods?

The programme had some interesting other stories……..Like the coal mine vent from the15th century

How did they dig the hole……Under the sea?

The Tay Bridge disaster, in 1876, must have been frightful!

Bob

[Paul TParticipant @pault84577]

Good Morning Captain Bob

A few clever mods might extend the bridges life by a couple of years and it could be extended further by limiting the amount of vehicles using the bridge but these are really exercises in futility and akin to using a sticking plaster on a person who has just had an arm ripped off.

I do know of one ambitious course of action that the Chinese used on one of their defective bridges which will defiantly work…………knock it down and start again.

As for the mine shaft in the sea a chap called Sir George Bruce did it by constructing an artificial loading island into which he sank a 40 ft shaft that connected to another two shafts for drainage and improved ventilation. The technology was far in advance of any coal mining method in the late medieval period and was considered one of the industrial wonders of the age.

The Tay Bridge was lost during a storm but the collapse happened so quickly and involved such a large amount of the structure that engineers believe that a rouge wave was responsible.

 

Paul

Edited By Paul T on 11/12/2015 11:53:27

[Dave MilbournParticipant @davemilbourn48782]

A rouge wave?? "Sacré bleu! Quelle horreur! Zut alors! La mer est transformée en peinture!" Defiantly the wrong word, mon brave…

DM

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