Explainer: How to ensure structural resilience

The performance of infrastructure is coming under increasing stress from a range of factors, including population growth, urbanisation and climate change. If a structure is to continue providing the vital facilities on which society depends, it must be resilient to predictable stresses and able to respond effectively to unforeseeable and unavoidable threats.

The term resilience is used to describe how an asset behaves before, during and after it comes under stress. It comprises:

• Robustness before the hazard occurs
• Resistance during the disruptive event
• Recovery after the event

“The first part of resilience is ensuring that the structure is robust before it’s subjected to the stressor,” says Stergios-Aristoteles Mitoulis, associate professor and head of future structures at the University of Birmingham’s School of Engineering. “If its capacity has been reduced, it will be less able to provide resistance during the event. 

"For example, if a bridge is 30 years old when it is hit by flooding, it may have been affected by scour over that time, making it more likely to be damaged.”

Mitoulis adds that the third element of resilience – a structure’s ability to be repaired rapidly after it has been damaged – is key.

“This could mean having all of the materials – and the finances and other necessary resources – in place to effect a quick recovery,” he says.

Sotirios Argyroudis, associate professor of infrastructure engineering at Brunel University London, agrees. He notes: “Resilience is not only about robustness. It’s also about how quickly you can recover the structure after a failure. There are two elements to this: organisational resilience – contingency plans and resources – and how the structure is built, so that components can easily be replaced.”

He cites the case of the Great Kanto Highway in Naka, Japan. This major artery was severely damaged by the Tohoku earthquake on 11 March 2011, but the highway authority already had a reconstruction plan in place, complete with all the materials, labour and finance required to execute it. The repairs began on 17 March and the road reopened six days later.

“All the resources and plans were there, so the infrastructure operator could make very quick decisions,” Argyroudis says. “Sometimes you can’t avoid failures, so it’s important to understand which infrastructure is the most valuable and assess it in advance against potential scenarios.”

Climate change is increasingly subjecting assets to potentially catastrophic hazards that they weren’t designed to withstand. These include:

• Extreme temperatures – for example, the buckling of rails in hot weather
• Extreme rainfall, which can cause scouring and subsidence (for example, the 2009 River Crane bridge failure), landslides (such as the 2020 Stonehaven train derailment) and flooding (the 2019 Toddbrook Reservoir dam failure)
• Rising sea levels, leading to storm surges, flooding, erosion (for example, the 2014 railway closure at Dawlish) and impacts on airports
• Wildfires owing to droughts and heatwaves
• Extreme wind – for example, its impact on electric power systems

Structures can also be subject to non-climate-related stressors, such as: 

• The deterioration of ageing materials – for example, the 2009 Stewarton train derailment
• Inadequate monitoring and maintenance
• The imposition of loads that the asset hadn’t been designed to support
• A change of use from that envisaged when the asset was designed
• Sociopolitical or demographic changes that render the asset unfit for purpose

Structures can be exposed to several hazards concurrently and must be resilient to them all. Many assets are designed to withstand an earthquake in regions where this is common, for instance, but they could fail because of flooding, say, if that risk had not been adequately managed.  

The Polcevera Viaduct was a three-span viaduct and multi-span overpass that carried the A10 motorway over the Polcevera River, a railway line and several buildings, both commercial and residential.

Opened in 1967, the bridge had an unusual design: its main spans were supported by cable stays in which the steel cables were encased in prestressed concrete. The theory was that this would protect them from corrosion, but that would prove incorrect. What's more, enclosing them in such a way made it impossible to monitor their condition properly.

On 14 August 2018, one main pylon and a 250m length of deck collapsed in a rainstorm, killing 43 people and severely injuring 12. The cause was found to be a cable breakage inside the failed pylon.

The official investigation concluded that the stability of the structure had been compromised by corrosion that had started “from the first years of the life of the bridge and continued without ever stopping until its collapse”. 

Severe corrosion had been observed in the upper parts of the stays during renovation work in the 1980s. An inspection in 1992 had also found many strand breakages. However, a proposal to add extra cabling was rejected and the bridge would continue to be repaired on a piecemeal basis.

While also highlighting design and construction problems, an independent review deemed the inspection and maintenance processes inadequate. Its report stated: “If, in any of these phases, the proper checks were made, it was highly likely that they could have prevented the tragedy.”

The concept of design life is embedded in the thinking of structural engineers. It has no legally agreed definition, so it must be defined carefully in contracts. It’s usually taken to be a structure’s period of use as intended by the designer, after which it may need to be replaced.

Eurocode 0 – Basis of Structural Design (EN 1990) is a European construction standard that sets out indicative design lives for various types of structure:

• Category one – temporary structures (not including parts that can be dismantled for potential re-use): 10 years
• Category two – replaceable structural parts (for instance, gantry girders): 10 to 25 years
• Category three – agricultural buildings and similar structures: 15 to 30 years
• Category four – building structures and other common structures: 50 years
• Category five – monumental buildings, bridges and other civil engineering structures: 100 years

“Design life is a notional concept that is as much economical as it is physical,” says David McKenzie, chair of COWIfonden, the parent foundation of engineering consultancy COWI. “It is the assumed period for which the structure is to be used for its anticipated purpose with an appropriate level of maintenance. But most structures should continue beyond their design life as long as they’re adequately maintained.”

David Balmforth, ICE past president and a visiting professor at Imperial College London, believes that design life is “possibly not a helpful term at all. Infrastructure doesn’t suddenly stop working at the end of its design life. It doesn’t cease to exist; it can be recovered and made good. That’s why it’s more useful for us as engineers to think in terms of resilience.”

A resilient design will reduce the following, according to Argyroudis:

• The time it takes for the asset to recover from an incident
• The demand on emergency services and the threat to responders’ health and safety 
• The amount of damage and contaminated material to be disposed of
• The recovery costs incurred by the asset’s owners and the wider community

“In some cases, it is realistic and simple to design resilience into structures,” he says, citing the example of a hospital placing all of its back-up generators on the upper floors rather than in the basement, so that they should still work in the event of a flood. 

While this is a more complex and costly matter in cases where “you’re considering the interdependencies of infrastructure systems or accounting for multiple hazards, we have the know-how, methods and metrics to account for the resilience in our designs nowadays”, Argyroudis adds. “Resilience-based assessment and design are gradually being adopted in practical applications and they’re expected to be incorporated in the next generation of provisions and guidelines.”

Key points for engineers to consider:

• Design infrastructure in such a way that enables components to be replaced quickly and easily.
• Understand that any changes made during construction must be worked through with the design team.
• Be aware of how the structure will be monitored and maintained – and build in enough access to enable these processes.
• Understand which parts will need substituting over time. On a bridge, for instance, the stays will have to be replaced, so the structure should be designed to operate safely with one or two cables missing while those works are in progress.
• Consider how any possible changes might affect the asset’s intended uses. For instance, will a building remain habitable if summer temperatures continue rising at the predicted rate? Think in terms of functional resilience and consider potential changes of use to prevent obsolescence.
• Understand the capabilities of the structure, not only while it’s being commissioned and built, but at all stages of its life.
• Understand potential deterioration and failure mechanisms and work out how these can be managed to lengthen the asset’s life.

After the EJ Whitten Bridge opened in 1995 as part of the M80 ring road around Melbourne, accidents and congestion would regularly delay the 165,000 motorists and 22,000 lorry drivers crossing the bridge daily, as it formed a bottleneck. 

In 2017, a project began to increase the capacity of the bridge and make it safer. Its two separate viaducts, both carrying three-lane carriageways, were joined in the middle to create a single structure providing five lanes in both directions.

When the bridge was designed, provision had been made in the foundation and pier design for the gap to be filled with a third concrete box girder. But the engineering consultancy on the project, COWI, proposed a lighter design, which meant that the existing substructure wouldn’t need to be reinforced.

Future maintenance access and durability were important considerations in the design of the new structure and upgrades.

“Ageing infrastructure is a significant issue for the developed world,” says David Hirst, co-founder and managing director of Ainsty Risk Consulting. “A failure to monitor and understand the condition of an asset and maintain it over its lifetime is a major issue, as structural failures can be catastrophic.”

Infrastructure built half a century or more ago was not designed for today’s conditions, Argyroudis notes, because those engineering them couldn’t predict that factors such as increased traffic or new stresses caused by climate change would impose such extra loads.

“At the time they were designed, data and models for various hazards didn’t exist,” he says. “Now we have that knowledge. We can project what the expectation is for these structures – for example, the likelihood that a flood will affect them.”

Changing functional requirements can also cause infrastructure to ‘fail’ – for instance, buildings that are no longer comfortable or even safe to occupy because people are larger than they were 200 years ago.

Lessons learnt from catastrophic structural failures of ageing infrastructure include: 

• Change of use/modification/load path (e.g. Grenfell)
• Inadequate inspection
• Poor maintenance
• Poor understanding of inspection/maintenance requirements at design stage
• A combination or one or more of the above

Toddbrook Reservoir in Derbyshire was formed with the construction of a 24m-high embankment between 1837 and 1840 with a central puddle core and outer shells of granular earth fill. This dam originally had a single (primary) spillway, but an auxiliary concrete spillway was added in 1970.

After rainstorms between 27 July and 1 August 2019, the newer spillway failed. The rain that led to the failure was forecast accurately and fell in two separate events. Although these were rare occurrences, with the second spell of rainfall having an estimated annual probability of about 1%, the resulting flood was still far smaller than the probable maximum flood the spillway should have been able to handle. It had coped with significant floods before without incurring any apparent damage.

On 1 August, a single slab of the spillway chute collapsed into a void that had formed underneath it. The void expanded and more slabs further down the chute were dislodged, jeopardising the dam’s integrity. As a precaution, 1,500 people living immediately downstream in the town of Whaley Bridge were evacuated from their homes.

In his independent review of the incident, Balmforth concluded that the most likely cause of the failure was poor design, exacerbated by intermittent maintenance.

“There is evidence to show that it had deteriorated over its life,” he wrote. “With consistent good-quality maintenance over the years leading up to the event, the spillway might not have failed… However, it would have been unlikely to survive the probable maximum flood, which is many times greater than the flood in which it failed.”

Recent technological developments have made it far easier for today’s engineers to monitor the integrity of infrastructure assets.

“Monitoring is very important, as it gives you real-time data on the possible deformation of a structure,” Argyroudis says. “It can be done with sensors or with satellite imagery that reveals which areas are most exposed to hazards such as coastal erosion.”

McKenzie says: “Structural monitoring gives you a good understanding of how an asset is working. You can tell whether it’s been damaged or is not performing as well as expected. When engineers design structures, they must make assumptions – about how stiff the ground is, for example. Incorporating monitors in a new structure enables engineers to compare its performance against those assumptions."

He adds: “Monitoring can help you to set trigger limits. You want it to warn you in good time when a component will cease working or the structure is going to fail.”

Drones are being used routinely to inspect infrastructure where it’s impossible to gain physical access. The photos these take can be analysed efficiently by artificial intelligence systems that have been trained to spot defects.

Adaptation. Anticipating the harmful effects of potential hazards and taking appropriate action to prevent or minimise damage. 

Climate risks. Risks associated with climate change, including: 

• Increases in mean precipitation and more extreme precipitation
• Increases in mean temperature and more extreme highs and lows 
• Harmful interactions of temperature and precipitation 
• Rising sea levels
• Increases in storminess (including wind speed and lightning)
• Increases in the incidence of fog
• Changes in snowfall patterns
• Solar radiation

Critical infrastructure. Assets that are essential for the functioning of a society and economy.

Design assumptions. Assumptions stated or implied in the design. 

Design life. The period of use of a structure as intended by the designer, after which it may need to be replaced.

Disruption. An interruption to the normal course of an activity.

Fitness for purpose. The ability of an asset to be used for its intended function.

Functional capacity. The structure’s ability to work as intended.

Hazard. An event with the potential to cause fatalities and/or severe economic damage.

Interdependencies. Structures, assets, systems or organisations that connect with, and/or rely on, other structures, assets, systems or organisations to function properly.

Mitigation. Handling potential hazards by eliminating or reducing their possible impact.

Rapidity. The speed at which an asset recovers after a disruptive event.

Recovery. The ability to efficiently restore a structure, asset or service after a disruptive event.

Redundancy. A structural property concerning the ability to redistribute loads in case of damage. It increases when alternative paths and load-transfer mechanisms are available.

Resilience. The ability of a structure, system, service or community to withstand, absorb and recover efficiently and swiftly from unforeseen, abrupt changes in its capacity, operability and function.

Resourcefulness. The ability to respond to an external threat by identifying the problem and applying alleviation measures using the necessary resources. 

Risk. A potential event that may, should it occur, cause a structure to miss one or more of its objectives.

Robustness. The ability of a structure to resist the impact of a hazard.

Structural capacity. The designed loading and stress capability of a structure.  

Structural collapse. The failure of an asset or component to maintain its structural integrity.

Structural deterioration. A change to the material or properties of a structure that harms its structural performance.

Structural failure. The loss of a structure’s (or component’s) load-carrying capacity.

Vulnerability. The likelihood that a structure will be affected by a hazard – a combination of exposure and sensitivity.