Sunday, 16 December 2012


Invisibly holding our civilisation together

I like bridges, geeky I know, but I like them. I am also in awe of the people who really understand them as they have a much better grasp of maths than I ever will!

Everybody knows what a bridge looks like and you see them everywhere - or do you? Of course, there are many high profile and "landmark" bridges - you only have to go to a city with a river and it will be criss-crossed with bridges.

The cable-stayed QEII bridge which carries the A282 over the River Thames between Essex & Kent (basically the M25), connects the two counties; and along with the 2 tunnels (Kent into Essex) keeps the motorway network of South-east London running (or not if you are stuck in the queues on a Friday!).

The arguments of congestion and tolling aside, I don't think anyone can deny it is an impressive piece of civil engineering and a vital piece of UK infrastructure.

Bridges are also sometimes a bit mad. Take the over-the-top (looks wise) Tower Bridge a few miles upstream. Built at the end of the 19th Century, it provided an important link to The City as London grew and the lifting bascules allowed cargo ships to access the Pool of London. Of course, today, it remains an important local link and is a huge tourist attraction. 

But, most bridges are not landmarks, most bridges are not high-profile and society continues without so much as a thought about them. The only time we every realise there is a bridge is when we cannot use it. 

Cast your mind back to the time before the 2012 Olympics to this time last year. The Hammersmith Flyover was carrying 90,000 vehicles per day along the A4 into and out of the heart of London. By Christmas, the bridge had been closed by Transport for London following concern about the risk of catastrophic failure (it could go bang at any time!). What followed was months of closures, weight limits and pressure on TfL because the A4 was an Olympic Route and the bridge had to be open in time.

The bridge is constructed from prestressed concrete consisting of a series of single columns holding up cantilevered "wings" which span either side of the columns which then carry the road above. Prestressed concrete basically relies on the reinforcement (steel tendons) being stretched, the concrete being poured and then once the concrete has cured, the tension on the tendons is released. The tension energy remains locked in the concrete and so gives it strength. It also allows the concrete elements to be quite thin (slender) and so can make very attractive structures - for an engineer, Hammersmith is quite elegant.

The trouble with this flyover is that the method was very new when it was opened in 1961. In order to keep the bridge deck free of ice in the winter, there were heating elements buried in the structure (important as bridges stuck up in the air will ice up more easily than the ground below). The heating was basically very expensive to run and packed up years ago. The traditional method of de-icing was then used for decades (salting) and because of a chemical reaction created by the wet, salty winter conditions, the tendons corroded inside the concrete (I am no chemist either).

Of course, Hammersmith was massively high-profile in terms of traffic loading and the Olympics and about £10m was thrown at it to extend it's life by at least 15 years. The thing that worries me is that to plan and build a replacement takes years and lots of money and so has the inevitable just been put off because of the Olympics? There is a whole other blog to be written on whether or not the bridge should be replaced!

The vast majority of bridges are lowly things. The quietly carry smallish roads over smallish rivers, roads over railways, railways over roads, they are footbridges. Many are not even seen by the people using them every day. 

This bridge carries the A1306 New Road over the River Beam between the London Boroughs of Barking & Dagenham and Havering. It carries traffic, buses, cyclists and pedestrians and used to form part of the A13 which has since been shifted to the south (on lots of bridges!). But, this is still an important local route and the photo is from an angle that most people will not ever see. It is not beautiful, particularly clever, but just sits there and does its job (I assume it is in a good condition, I don't know!)

What is clear, though, problems with these bridges can cause chaos, their improvement is woefully underfunded and we should be worried. I am not particularly worried about these bridges suddenly collapsing (most are not prestressed), but they wear out (over a long time). Bridge maintenance is not particularly exciting, but it is vital.

If being done properly, bridges (certainly on the public highway) are inspected by qualified people every two-years, with a detailed inspection every third time (6-years). This means that the first signs of problems are picked up early and repairs can be built into maintenance programmes and budgets - a bit like going to the dentist regularly. If there are problems, then additional inspections can take place, materials can be extracted and tested and a variety of temporary restrictions can be used to keep heavier vehicles off. Standards for new road bridges in the UK are for a life of 120 years, but even they need to be inspected and maintained.

The Asphalt Industry Alliance puts the maintenance backlog of roads in England and Wales over £750m, but this is just the roads themselves, not the bridges which connect them together.  There is no decent UK data on bridge maintenance backlogs, but it will be measured in hundreds of millions, if not a couple of billion. There are also the bridges owned by Network Rail and hundreds of little privately owned bridges which are still locally important.

If a bridge has to be closed, it is not the immediate point of closure which is the problem, it is the wider area affected by the closure and diversions. I was involved in the imposition of a 3 tonne weight limit on a railway bridge (carrying a local road) which went on over a year (the bridge is still not repaired, but a more detailed investigation changed the restriction method). It was not on a main road, but it meant that buses and emergency vehicles had to be diverted and for the local community it was disruptive. 

There are many local bridges like this around the UK which need basic routine and structural maintenance, but highway maintenance is the Cinderella of civil engineering and bridges are the Buttons (ouch). Politicians love big projects that everyone can see, but rather than blowing a billion on a handful of new road schemes, this money should be invested in what we already have and a majority needs to go to keeping our ageing bridge-stock in good condition.

As usual, the responsibility for the state of our essential infrastructure lies at the feet of the politicians. The loss of engineers at the higher level of decision-making is a ticking time-bomb which will one day blow up in our face. It will be our face as it is the taxpayer who foots the bill every time.


  1. The traditional arch bridge has a lot to recommend it, in the fine 'new world' way of thinking where ever greater technical tricks are used to stretch beyond using designs which are inherently safe.

    On the rail network, arches which were built when trains had 40 ton locomotives and travelled at less that 50 mph now have 90 ton locomotives thundering over them at over twice that speed, on the roads - including roads abandoned for centuries, arches remain standing - with famed examples like those on Wade's road from Carrbridge to Inverness.

    We even see it in the roads themselves. New roads built by the brute force of an ability to shift a mountain - then sinking, because unlike the old routes carefully built into the landscape and packed down over centuries, we have challenged nature to build them. Old road builders accepted flooding for a few days, often bot an annual occurrence, but built roads, and even bridges which could sit happily under water for those short periods and then immediately start up in use after the waters receded. Some features were even laid out to perform the function of a flood channel, as a road running 6" deep in water can still be used as a road, but it can clear water which would otherwise be building up to flood homes and businesses.

    Yes we have much to re-learn , a detail I reflect on as I use the flush setted street near me which still sheds water imperceptably into the self flushing side gullies, even with the lightest shower, still (where the utilities haven't wrecked it) functioning perfectly 160 years after it was laid with a sound tight bond on its puddle clay or cold tar bad, with no maintenance at all to speak of. makes you wonder at the false economy of using so much tarmac on our urban roads.

    1. I think "value engineering" is to blame, or as I call it "done on the cheap". Our forebears probably over-designed things, but this is why we are still using Victorian infrastructure!

  2. Hi, I am a chemist- the problem with the prestressed concrete reinformcement and salt is pretty much as you describe it. The only little bit of detail I'd add that's of any use is that as iron/steel corrodes, it swells, so that not only do you have less strenght in the metal, but it is forcing the already cracked (as the water has got in) concrete apart- making things even worse.

  3. Chloride iron exchange? Yes, the swelling of the reinforcement is a good point. It can create enough force to "blow" pieces of the concrete off (which is a potential danger to road users) and so expose reinforcement direct to the atmosphere.

  4. You have nicely explained about bridge work and all repairing that is to be done there for making it strong. You have described nice facts which we need to concentrate.