Forth Road bridge closure until the new year

[whole diary]

[IndustryInsider]

Cracks found in the structure, and with the second bridge not yet finished the railways will see a massive increase in traffic:

http://www.theguardian.com/uk-news/2015/dec/04/scotland-scrambles-to-find-extra-trains-after-closure-of-forth-road-bridge

Great chance for Scotrail to help come to the rescue and provide as many extra and strengthened trains as it can.

[ellendune]

If only they can find the rolling stock.

[John R]

Somewhat strangely the Scotrail website is tonight showing major delays on the Fife Circle line "due to urgent bridge repairs between Edinburgh and Kirkcaldy".  Which at first sight would make readers think that it was a railway bridge causing the problem...

[LiskeardRich]

Can they not use the third bridge instead of the forth one? 

[ellendune]

Can they not use the third bridge instead of the forth one? 

Yes but that only crosses the firth of third.  If you are going to get all the way you need to cross the first, second, third and forth bridges.   

[stuving]

I noted this from the Guardian article (it's not attributed to anyone else):

The defect has highlighted the bridge^s age: it is now more than 50 years old and has survived longer and with far heavier traffic than originally expected.

Is that really true? Was it built and funded assuming less than 50 years working life?

[stuving]

I noted this from the Guardian article (it's not attributed to anyone else):

The defect has highlighted the bridge^s age: it is now more than 50 years old and has survived longer and with far heavier traffic than originally expected.

Is that really true? Was it built and funded assuming less than 50 years working life?

According to Wikipedia, no - it was 120 years. The traffic is higher than assumed, but by less than double (from the same source). So even the most extreme life reduction from that source (life due to counting fatigue cycles proportional to vehicle count) would not get it down to 60 years from 1964.

I suspect rust.

[grahame]

Is that really true? Was it built and funded assuming less than 50 years working life?

I don't know [I do now - thanks Stuving and Wikipedia]... but the problems aren't entirely out of the blue.

http://www.forth-bridges.co.uk/queensferry-crossing/history-queensferry.html

Despite significant investment and maintenance since it opened in 1964, the Forth Road Bridge (FRB) has shown signs of significant deterioration in recent years.

In 2004, inspections of the main cable found corrosion had resulted in a loss of strength of between eight and ten per cent, giving rise to fears of the need of significant restrictions in the future to allow for repairs. Some 60,000 vehicles use the bridge every day and it represents one of the most vital economic arteries in Scotland. Given these issues and the impact major maintenance works would have, the FRB is no longer deemed viable as the long-term main crossing of the Firth of Forth.

[chrisr_75]

I think the original design life was 120 years, but corrosion has taken an unexpected toll on the structure and it is now routinely carrying several times it's intended capacity of traffic, hence the accelerated wear and tear. The original Severn crossing (now the M48) also suffers from similar corrosion issues, with heavy vehicles restricted to 1 lane in each direction. I believe the corrosion issues were mitigated by covering the main cables in a weatherproof membrane and then drying them out - also being considered for the M48 bridge(s).

I suspect the replacement has turned out to be very very well timed!

I've read accounts of HGV drivers ignoring the restrictions put in place during the part closure earlier this week, so I'm not surprised the bridge has been closed in its entirety when stupidity like that is on display!

[JayMac]

And the rail bridge continues on regardless.

If you want to build something to last, get railway engineers to do it.

Excepting Thomas Bouch of course.

[TaplowGreen]

And the rail bridge continues on regardless.

If you want to build something to last, get railway engineers to do it.

Excepting Thomas Bouch of course.

  .......but if you want it delivered anywhere near on time and within budget, don't get Network Rail to do it!

[ellendune]

The design life of a bridge like the Forth will be based on the fatigue life of the structure. This is quite separate to any corrosion issues. Metal fatigue is a type of failure that occurs after repeated loading at stresses well below the ultimate strength of the material.  The lower the stresses in the material the longer the fatigue life.  The life of the component is related to therefore to the stress levels and the number of load cycles. 

If this is a fatigue failure (and the pictures are consistent with this) then the principal reason why it is happening earlier than the 120 year design life is likely to be that the traffic on the bridge is far more than had been anticipated by the designers. However increased stresses due to more traffic on the bridge at once is also a possibility. Structural analysis techniques at the time of the design were not as advanced as now, but this is unlikely to be an issue as I am sure that structure would have been re-analysed several times during its life. 

[Electric train]

And the rail bridge continues on regardless.

If you want to build something to last, get railway engineers to do it.

Excepting Thomas Bouch of course.

  .......but if you want it delivered anywhere near on time and within budget, don't get Network Rail to do it!

Oh I don't know about that, Borough viaduct bridge at London Bridge was delivered 5 years early 

 

[stuving]

Oh I don't know about that, Borough viaduct bridge at London Bridge was delivered 5 years early 

For the well-known Thameslink 2016 project?

[stuving]

The design life of a bridge like the Forth will be based on the fatigue life of the structure. This is quite separate to any corrosion issues. Metal fatigue is a type of failure that occurs after repeated loading at stresses well below the ultimate strength of the material.  The lower the stresses in the material the longer the fatigue life.  The life of the component is related to therefore to the stress levels and the number of load cycles. 

If this is a fatigue failure (and the pictures are consistent with this) then the principal reason why it is happening earlier than the 120 year design life is likely to be that the traffic on the bridge is far more than had been anticipated by the designers. However increased stresses due to more traffic on the bridge at once is also a possibility. Structural analysis techniques at the time of the design were not as advanced as now, but this is unlikely to be an issue as I am sure that structure would have been re-analysed several times during its life. 

That's pretty much what I was saying, only I was questioning whether it could have already reached the number of fatigue cycles used in its design. It turns out we were both wrong, apparently, and it is primarily an overstress issue due to the traffic live loading and/or inadequate design margins. Of course fatigue may still be involved in the failure mechanism, but it is not the main change from the design condition.

The big change since the design is in the worst-case traffic load - up from 2880 to 8430 tonnes. That's a triple whammy, due to heavier lorries (per m length), a higher proportion of them in the vehicle stream, and longer queues (now two lanes nose-tail over the whole length). In addition, the wind loading is higher than assumed, and so are the temperature differences. The towers have already been reinforced inside and in their bracing for those reasons.

These truss end links were already identified as weak points and in need of urgent replacement, or at least strengthening. That should have already been done, according to Amey, but maybe that is not quite up to date. There was a debate as to how much needed to be done now, given that in a few years all the heavy vehicles will be on the new bridge so these high loadings should not happen any more (maybe).