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

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

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

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

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)

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

I know what they should have used….

Ashley

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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

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

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

DM