Why do bridges collapse – and how can we prevent it?

The Ponte Morandi bridge, Genoa, after its collapse, which has claimed dozens of lives. Image: Luca Zennaro/EPA.

As rescue workers look for survivors in the concrete rubble that used to be part of the Morandi bridge in Genoa, Italian authorities are starting their investigation into the possible causes behind this terrible tragedy.

It is too early to determine what may have caused the catastrophic collapse of more than 100 metres of the multi-span, cable-stayed suspension bridge, completed just over 50 years ago. But it’s important to understand that bridge engineering does not end when construction finishes and traffic starts to flow. In fact, properly looking after a bridge during its long life is as crucial as having a good design, using high-quality materials, and ensuring sound workmanship during construction.

Modern bridges are designed for a life of 100 years, though many centenarian bridges – such as the Forth Bridge in Scotland, which opened in 1890 – still provide sterling service, and of course there are smaller bridges built of stone to more ancient designs that have stood for many hundreds of years. Considering the number of bridges built in Europe during the expansion of the motorway networks from the late-1950s onwards, we should expect, and be prepared for, many to exceed their planned lifespan in coming decades. Facilitating this is ambitious but necessary, and made possible thanks only to regular inspection and maintenance that ensures that building materials have not degraded, and that structural elements are fit to bear the traffic and environmental loads they face.

The Forth Bridge outside Edinburgh, one of Britain’s iconic bridges, is more than 100 years old. Image: Andrew Shiva/The Conversation.

So what are the factors that affect the strength of a bridge and may compromise public safety?

Environment and climate

The climate in a bridge’s location, taken alongside atmospheric pollution common in cities, can have an adverse influence on the material of the bridge – for example, the corrosion of steel reinforcement or pre-stressed steel tendons embedded in concrete. Regular inspections are typically scheduled every six years for large bridges to identify any degradation, and to take appropriate measures to replace cracking concrete and corroded steel, or to introduce protective coatings.

In England, the Midlands Link motorway viaducts, comprising 13 miles of elevated motorway carrying the M5 and M6 motorways around Birmingham, suffered from chloride-induced steel corrosion early on in their life from exposure to salt used to de-ice the roads. This required an extensive application of corrosion protection measures in the early 1990s. More than 700 structures have benefited from this action, demonstrating the cost savings that can be made if appropriate action is taken at the right time.


Stress and fatigue

Fatigue caused by use is another factor, and inspectors will look out for tell-tale signs of failure often associated with the cyclical stress produced by passing vehicles, particularly heavy trucks. This type of failure is especially relevant for metal bridge decks and the cables of suspension and cable-stayed bridges. Traffic has increased ever since these bridges were built, which inevitably leads to the need for more maintenance and strengthening work, such as additional steel, glass or carbon fibre-reinforced plates on critical parts in order to restore or enhance their strength compared to what was deemed necessary during their design. For example, Network Rail in the UK used fibre-reinforced polymers to strengthen more than 20 bridges carrying highway or railway traffic between 2001 and 2010.

Consider how we all tend to react to a road sign bearing the words: “Essential Bridge Works – Expect Long Delays”. One such situation prompted this comment from a member of the public: “We are doomed. I am going to buy a tent and pitch it outside work for the three months while the misery goes on.” Perhaps knowing why this is necessary – and the consequences of not doing so – might persuade people to reconsider such views.

Money and willingness to spend it

Equally, we must understand that maintenance budgets need to be set at levels that far exceed those that would allow engineers only to “firefight” the most severe problems, as is becoming worryingly commonplace. Instead, budgets need to allow for planned interventions and necessary upgrades over many decades. That requires public and government support, as well as skilled engineers committed to ensuring the safety of an ageing structure.

There are challenges in devising improved methods to assess bridge strength, developing new repair techniques, and new ways of collecting and using inspection and monitoring data to provide advance warning of problems. These constantly push technological boundaries, making it possible to operate existing bridges safely during their long service lives. And the experience gained feeds into new designs that will become reality in years to come.

The ConversationThose investigating the collapse of the Morandi bridge will look at inspection and maintenance matters. Other lines of enquiry will no doubt include the unusual design of the multi-span bridge, with only a few cable stays to transfer deck loads to the towers, the ongoing work to shore up the foundations, and the heavy rainfall at the time of the collapse. In the shadow of this terrible loss of life, it is worth remembering that bridge inspection and maintenance may be annoying for commuters – but it is crucial.

Marios Chryssanthopoulos, Professor of Structural Systems, University of Surrey.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

London’s rail and tube map is out of control

Aaaaaargh. Image: Getty.

The geographical limits of London’s official rail maps have always been slightly arbitrary. Far-flung commuter towns like Amersham, Chesham and Epping are all on there, because they have tube stations. Meanwhile, places like Esher or Walton-on-Thames – much closer to the city proper, inside the M25, and a contiguous part of the built up area – aren’t, because they fall outside the Greater London and aren’t served by Transport for London (TfL) services. This is pretty aggravating, but we are where we are.

But then a few years ago, TfL decided to show more non-London services on its combined Tube & Rail Map. It started with a few stations slightly outside the city limits, but where you could you use your Oyster card. Then said card started being accepted at Gatwick Airport station – and so, since how to get to a major airport is a fairly useful piece of information to impart to passengers, TfL’s cartographers added that line too, even though it meant including stations bloody miles away.

And now the latest version seems to have cast all logic to the wind. Look at this:

Oh, no. Click to expand. Image: TfL.

The logic for including the line to Reading is that it’s now served by TfL Rail, a route which will be part of the Elizabeth Line/Crossrail, when they eventually, finally happen. But you can tell something’s gone wrong here from the fact that showing the route, to a town which is well known for being directly west of London, requires an awkward right-angle which makes it look like the line turns north, presumably because otherwise there’d be no way of showing it on the map.

What’s more, this means that a station 36 miles from central London gets to be on the map, while Esher – barely a third of that distance out – doesn’t. Nor does Windsor & Eton Central, because it’s served by a branchline from Slough rather than TfL Rail trains, even though as a fairly major tourist destination it’d probably be the sort of place that at least some users of this map might want to know how to get to.

There’s more. Luton Airport Parkway is now on the map, presumably on the basis that Gatwick is. But that station doesn’t accept Oyster cards yet, so you get this:

Gah. Click to expand. Image: TfL.

There’s a line, incidentally, between Watford Junction and St Albans Abbey, which is just down the road from St Albans City. Is that line shown on the map? No it is not.

Also not shown on the map: either Luton itself, just one stop up the line from Luton Airport Parkway, or Stansted Airport, even though it’s an airport and not much further out than places which are on the map. Somewhere that is, however, is Welwyn Garden City, which doesn’t accept Oyster, isn’t served by TfL trains and also – this feels important – isn’t an airport.

And meanwhile a large chunk of Surrey suburbia inside the M25 isn’t shown, even though it must have a greater claim to be a part of London’s rail network than bloody Reading.

The result of all these decisions is that the map covers an entirely baffling area whose shape makes no sense whatsoever. Here’s an extremely rough map:

Just, what? Image: Google Maps/CityMetric.

I mean that’s just ridiculous isn’t it.

While we’re at it: the latest version shows the piers from which you can get boats on the Thames. Except for when it doesn’t because they’re not near a station – for example, Greenland Pier, just across the Thames to the west of the Isle of Dogs, shown here with CityMetric’s usual artistic flair.

Spot the missing pier. You can’t, because it’s missing. Image: TfL/CityMetric.

I’m sure there must be a logic to all of this. It’s just that I fear the logic is “what makes life easier for the TfL cartography team” rather than “what is actually valuable information for London’s rail passengers”.

And don’t even get me started on this monstrosity.

Jonn Elledge is the editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.