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.

 
 
 
 

Five ways in which the rest of the world can avoid the homelessness crisis plaguing the US

Housing for all. Image: Nicobobinus/Flickr/creative commons.

Homelessness is a growing problem in the UK, where the number of people sleeping rough has doubled since 2010, yet it is dwarfed by the scale of the issue in the US. More than 500,000 homeless were found across the US during just one night, compared to the UK’s 2017 count of 4,751. Changes in the definition of homelessness and flawed methodologies suggest that the true number for the US could be anywhere from 2.5 to 10.2 times greater.

Millions more live in overcrowded or slum housing, forced to choose between the damage that poor conditions do to their physical and mental health, and the street. All of the US’s housing issues – from foreclosures to evictions to poor conditions – hit communities of colour the hardest.

This is due to a legacy of discrimination, which continues to undercut any commitment to safe and decent housing for all residents, whether in the private or public sector. In my recent book, City of Segregation, I explain how the long, violent history of creating spaces for the white and privileged classes is embedded in a number of practices, which continue in US cities to this day.

Exporting inequality

As private developers and investors seek out urban land in major cities around the world to secure their fortunes, real estate patterns and practices developed within the US are increasingly being observed elsewhere.

In cities as diverse as London, Sydney and Durban, community groups which have been working for decades to improve their neighbourhoods languish with little public or private resource. Meanwhile, developers create spaces for foreign investors and new residents, who anticipate certain protections and privileges such as greater security, high quality amenities and neighbours with similar interests and backgrounds.

This is a driving force behind rising evictions and the criminalisation of homelessness, alongside gated communities, hostile architecture, “broken windows” policing with its focus on prosecuting activities such as graffiti or jaywalking and the growing privatisation of public spaces through regeneration.

But there is still time for other countries to choose a different path. The UK, in particular, can build on the legacies of the post-war political consensus that all residents should have access to quality housing, and its acknowledgement of institutional racism and some history of government anti-racist campaigning.

Both legacies should be improved, but a renewed commitment to a programme of housing and anti-racism are central to increasing equality, prosperity and well-being for all. Based on my research, I’ve come up with five steps which the UK and countries like it can follow, to ensure that future development reduces – rather than drives – homelessness and inequality.


1. Build social housing

Unlike the US, the UK acknowledges a right to a home, and within living memory provided it for a huge swathe of British society. Social housing – whether in the form of traditional council flats, cooperatives or community land trusts – provides a variety of housing types and keeps rents from rising too far beyond wages.

When social housing is widely available, it makes a huge difference to people who – for one reason or another, and often through no fault of their own – become homeless. With social housing to fall back on, homelessness is a temporary condition which can be safely resolved. Without it, homelessness can become a life-destroying downwards spiral.

2. Preserve and expand community assets

Severe segregation in the US stripped entire communities of access to quality food, jobs, education, green spaces, services, banks and loans. Poverty is endemic, and can easily tip into homelessness. While far from perfect, the UK’s post-war commitment to universal provision of services, such as education and health care, and building social housing across all neighbourhoods underpinned a surge in upward mobility.

This achievement should be salvaged from the damage done by Right To Buy – a policy which sold off social housing without replacing it – and austerity, which has prompted a sell-off of public assets and land, as well as the closure of childrens’ services, libraries and community centres.

3. Decommodify housing

A market geared towards building apartment blocks for the portfolios of investors who will never live in them cannot produce the kind of housing and neighbourhoods which residents need, much less at a price they can afford.

While London has been badly affected for some time, this trend is now spreading to other areas of the UK and Europe. Local and national governments must act to prevent global demand for housing as investments from driving prices beyond the reach of those who need real homes.

4. Build communities, not walls

Gates, bars, armed security and homeowner restrictions are all ugly traits of private housing developed within the US context of desperate inequality and racism. The UK has a long and vibrant tradition of community development, creating a supportive built environment and social infrastructure of schools, libraries and other municipal services for residents.

Community assets. Image: Helen K/Flickr/creative commons.

This kind of development, and the social mobility and growing equality it fosters, safeguards public health and safety – not big walls, barbed wire and security guards. The private rented sector in the UK should be regulated to bring it more in line with Europe, where tenants prosper with security of tenure and strong regulation of rents and rent increases.

5. Raise your voice

Those who are bearing the brunt of our current housing crisis must be at the centre of efforts to change it. From tenants’ associations and renters’ unions, to campaign groups such as Justice for Grenfell, it’s vital to support those voices advocating fairer housing rights.

This also means rejecting austerity’s constant cuts to public services, funding social support for physical and mental health and ensuring that homes are safe, decent and secure, to create a safety net for those who are working to improve their communities.

The Conversation

Andrea Gibbons, Researcher in Sustainable Housing and Urban Studies, University of Salford.

This article is republished from The Conversation under a Creative Commons license. Read the original article.