Six key questions about UK bridge maintenance

23.08.2018

COWI Director and bridge maintenance expert Joanna Bonnett answers six key questions about the maintenance of the UK's bridges in light of the collapse of the Ponte Morandi in Genoa.

1. Who is responsible for maintaining the UK's highway bridges? 

"For UK trunk roads, National Highways (or the devolved bodies) dictates the rules for maintenance and stipulates how often they have to be inspected.

In other cases it is local authorities or trusts who are responsible for maintenance.  In many cases the authority responsible uses private companies to undertake inspection and maintenance activities on their behalf."

2. How do we know if our bridges are safe?

"We have a rigorous inspection regime. The 'Design Manual for Roads and Bridges' requires that a bridge has to undergo a general (visual) inspection every two years and a principal (close visual) inspection every six.  Any concerns are followed up with special inspections.

Then there are special guidelines for higher risk structures. For example, for pre-stressed or post tensioned concrete structures (where you pass a high strength steel tendon tightly through concrete to compress and thus strengthen it) there are specific regimes for inspection. They can be difficult to inspect because the steel tendons are buried inside the concrete making them hard to see, so we have to use more advanced inspection and monitoring techniques. 

For major structures in this country, we have a regime of independent checking of designs called Category 3 Checks. It is recognised globally as one of the most rigorous systems for design checking as it has to be carried out by a completely independent organisation. Importantly, it applies to interventions in existing structures as well as the design of new structures.

The Construction (Design and Management) Regulations 2015 also place duties on clients, designers and contractors to ensure that all construction works (including maintenance activities) are designed, planned and carried out by those with appropriate expertise and adequate resources to ensure safety at all times."

3. How do we inspect bridges?

"Improvements in technology are helping us investigate what is happening within a structure. For example, drones allow you to inspect the external difficult-to-access areas more easily and endoscopes can be used to inspect various internal areas.  

There will always be areas that are hidden, making them difficult to inspect, but that type of issue is prioritised much more in the design of structures now. 

We are now using real-time monitoring systems on large structures, and acoustic emissions monitoring of the tension elements (such as suspension bridge cables) is proving particularly useful. This allows you to listen for deterioration so that more intrusive inspections can be targeted, rather than being random.

So we are getting better all the time and we are very aware of the critical areas we have to be able to monitor."

4. What type of maintenance do bridges typically require?

"It depends on the type of bridge, its components, materials and quality of construction. Much of it relates to the surfacing of the deck, which gets worn out over time. Drainage is another important area - if you don't maintain your drainage you are likely to get structural problems because of the water not going where it's supposed to. 

All large bridges have some moving parts and anything that moves wears. Many have movement joints and bearings, which are designed to handle both thermal movement and movement caused by wind and other changes in the loads they are carrying. These are designed as replaceable elements and require monitoring, maintenance and eventually replacement. 

Steel gets corroded by water (especially salt water) so steel structures need regular painting. Concrete structures are typically lower maintenance, assuming their design prevents water or de-icing salts getting to the steel reinforcement."

5. Is it a risk to have private companies maintaining our bridges? 

"Although private companies are responsible for maintaining some bridges, the standards and inspection requirements are specified by government bodies so there is no reason to believe they are less well maintained. 

These requirements are typically laid-out in the concession contract enabling scrutiny of what is being done with a clear mechanism for measuring performance."

6. What improvements have we seen in maintenance techniques? 

"There has been a positive change made recently to allow risk-based inspection. This means we can optimise inspection intervals depending on the risk of that particular structure and spend more time and effort looking at the areas that are more likely to be vulnerable to damage or degradation.  

On our longest bridges, lots of the steel suspension cables now have dehumidification systems which blow dry air along the cables and monitor the humidity levels to ensure they remain at a level low enough to prevent corrosion. That would be difficult to do if the cables were encased in concrete."