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.

 
 
 
 

Cycling on London’s Euston Road is still a terrifying experience

Cyclists on the Euston Road. Image: Jonn Elledge.

The New Road, which skirted the northern boundaries of London’s built up area, first opened in the 1750s. Originally, it was intended to link up outlying villages and provide a route to drive sheep and cows to the meat market at Smithfield without having to pass through the congested city centre. 

As with bypasses and ring roads the world over, however, it increasingly became congested in its own right. Today, you won’t often find livestock on the route, which is now Marylebone, Euston and City roads. But you will find up to six lanes of often stationary buses, cabs, and private vehicles. In a city whose centre is largely free of multi-lane highways, London’s northern ring road has long been the sort of abomination that you avoid at all costs.

But now, somewhat surprisingly, the road is seeing yet another new use. Earlier this week, the first phase of a temporary cycle lane opened on the Euston Road, the middle section of the route which runs for roughly a mile. As London rethinks roads throughout the city, this addition to the cycling map falls solidly into the category of streets that didn't seem like candidates for cycling before the pandemic.

It is, to be clear, temporary. That’s true of many of the Covid-led interventions that Transport for London is currently making, though those in the know will often quietly admit to hoping they end up being permanent. In this case, however, the agency genuinely seems to mean it: TfL emphasized in its press release that the road space is already being allocated for construction starting late next year and that "TfL will work with local boroughs to develop alternate routes along side streets" when the cycle lane is removed.

At lunchtime on Friday, I decided to try the lane for myself to understand what an unlikely, temporary cycle lane can accomplish. In this case it's clear that the presence of a lane only accomplishes so much. A few key things will still leave riders wanting:

It’s one way only. To be specific, eastbound. I found this out the hard way, after attempting to cycle the Euston Road westbound, under the naive impression that there was now a lane for me in which to do this. Neither I nor the traffic I unexpectedly found myself sharing space with enjoyed the experience. To be fair, London’s cycling commissioner Will Norman had shared this information on Twitter, but cyclists might find themselves inadvertently mixing with multiple lanes of much, much bigger vehicles.

It radically changes in width. At times the westbound route, which is separated from the motor traffic by upright posts, is perhaps a metre and a half wide. At others, such as immediately outside Euston station, it’s shared with buses and is suddenly four or five times that. This is slightly vexing.

It’s extremely short. The publicity for the new lane said it would connect up with other cycle routes on Hampstead Road and Judd Street (where Cycleway 6, the main north-south crosstown route, meets Euston Road). That’s a distance of roughly 925m. It actually runs from Gower Street to Ossulton Street, a distance of barely 670m. Not only does the reduced length mean it doesn’t quite connect to the rest of the network, it also means that the segregated space suddenly stops:

The junction between Euston Road and Ousslston Street, where the segregated lane suddenly, unexpectedly stops. Image: Jonn Elledge.

 

It’s for these reasons, perhaps, that the new lane is not yet seeing many users. Each time I cycled the length of it I saw only a handful of other cyclists (although that did include a man cycling with a child on a seat behind him – not something one would have expected on the Euston Road of the past).


Though I hesitate to mention this because it feeds into the car lobby’s agenda, it was also striking that the westbound traffic – the side of the road which had lost a lane to bikes – was significantly more congested than the eastbound. If the lane is extended, it could, counterintuitively, help, by removing the unexpected pinch points at which three lanes of cars suddenly have to squeeze into two.

There’s a distinctly unfinished air to the project – though, to be fair, it’s early days. The eastbound lane needs to be created from scratch; the westbound extended. At that point, it would hopefully be something TfL would be keen enough to talk about that cyclists start using it in greater numbers – and drivers get the message they should avoid the Euston Road.

The obvious explanation for why TfL is going to all this trouble is that TfL is in charge of the Euston Road, and so can do what it likes there. Building cycle lanes on side nearby roads means working with the boroughs, and that’s inevitably more difficult and time consuming.

But if the long-term plan is to push cyclists via side roads anyway, it’s questionable whether all this disruption is worth it. A segregated cycle lane that stops without warning and leaves you fighting for space with three lanes of buses, lorries, and cabs is a cycle lane that’s of no use at all.

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