Roads & Infrastructure Magazine explores the significance of bridge maintenance and how proactive asset management leads to effective and long-lasting infrastructure.
Bridges are a critical link for communities and commerce and are under constant pressure from traffic loads, environmental erosion and the force of gravity itself.
The Australian Local Government Association (ALGA) reports that there are around 30,000 bridges managed by local councils and a report from Australasian road transport agency Austroads cites there are more than 18,000 state road structures.
Managing these bridges is a difficult task, especially when there is only a certain amount of funding available to stakeholders.
On top of this, ALGA reports that, in total, $1.4 billion worth of concrete and timber bridges are in poor or very poor conditions.
Joshua Seskis, Senior Professional Leader of Future Transport Infrastructure, at the Australian Road Research Board (ARRB), says bridge asset maintenance is expensive and demanding, with budgets that tend to restrict over time.
“The fundamental component of asset management and maintenance is weighing up the bridge’s performance with the risks of deterioration and the cost of repairs,” Mr. Seskis says.
“Not every organisation will have the funding required to replace everything, but there are strategies that can help monitor and reduce risks to the structure, such as implementing structural health monitoring and proof load testing with sensors on the bridge.
“This information can help asset managers keep a closer eye on the structure and allows them to step in at the right moment to be the most cost-effective,” he adds.
The US Non-Destructive Testing (NDT) Resource Centre says the average lifespan of highway bridges is around 70 years. However, significant environmental damage often occurs to the bridge before it reaches its mid-life, requiring repair.
Asset managers for state road authorities in Queensland, Victoria and NSW use a system of visual inspections, which is outlined in manuals that are available from their respective websites. These take place once every six to 12 months and assess the current status of a bridge. During these inspections, it is often common to perform basic maintenance such as cleaning debris from the bridges, clearing vegetation from the surrounding area or draining pools of water.
Each bridge may have different pressures that will impact the structure. For example, a bridge that crosses salt water will experience different corrosion effects compared with a bridge that crosses a valley.
Traffic also plays a large role in determining what kind of stress a structure will be under, with larger bridges handling heavy vehicles and freight as part of their day-to-day function.
Mr. Seskis says proactive asset management is critical to ensuring structures are well maintained.
“When a bridge is designed, typically a certain level of performance is expected from the structure to accomplish what it was designed to do. This could be the traffic volume or types of vehicle an organisation expects,” he says.
“Prioritising structures that have a higher demand and are under more stress means maintenance will have the most impact.
“Proactive management takes into account possible risks to the bridge and ensures these factors are handled well before it potentially becomes a more complex and expensive issue,” he adds.
An example of this is managing the waterway a bridge crosses. Mr. Seskis says one of the most impactful things that causes damage to a bridge in this context is flood events.
“Deterioration-based failures are very minor, making up only around five to six per cent. Flood and scour events, on the other hand, are much more impactful, and are the cause for around 50 per cent of failures,” he explains.
To reduce the amount of damage a flood event can cause, proactive inspections of the surrounding area can be performed to assess potential risks. One particular risk in this event is that nearby foliage can fall into waterways and then be caught by the bridge structure.
If a tree was to become lodged against a bridge support structure, there is a possibility for increased scouring of the banks of the waterway, pressure may build against a bridge not designed to support such an event and lasting damage can be caused.
Removing the tree before this occurs often means the bridge is less likely to sustain significant damage from a storm event.
Mr Seskis says this routine maintenance and proactive approach to keeping bridges in the best condition avoids costly repairs.
“Closing a bridge for repairs is expensive and often has flow-on effects to the community and commerce of the region,” he adds.
While routine inspections are important, a more in-depth examination of the structure is also required. Usually this is included within the design phase of the bridge and occurs once every year to every five years.
These deeper inspections examine the structure and materials for any signs of damage risks that need to be addressed. Some state road authorities rate components of the structure from one to four to determine the condition of the bridge overall and will assess the performance of the asset.
In timber bridges, girders are inspected for internal rot and often have pieces of the structure repaired or replaced. Steel bridges are checked for the paint integrity, as this is the first barrier to protect the structure from corrosion.
Mr. Seskis says concrete used in bridges tends to require less intensive maintenance and is simple to repair if caught early.
“Concrete can be very complex and be affected by a whole range of factors that can cause problems,” he says. “The issue with concrete is that if these problems are left unchecked, it can quickly become difficult and expensive to properly repair.”
Computer modelling can also be used to determine how the traffic will affect the structure and how to appropriately close the bridge for repair.
Maintaining the West Gate
Some bridges are large enough to need a dedicated team of engineers to ensure everything is functioning properly. In these cases, closing a bridge for maintenance simply might not be possible.
One such case is Melbourne’s West Gate Bridge, which spans the city’s Yarra River and connects the western suburbs to the CBD. It is the third longest bridge in the country, measuring at more than 2500 metres. It carries around 200,000 vehicles each day and is a critical connection for the city.
The West Gate Bridge is a 10-lane bridge made up of two concrete sections and a steel section in the middle. VicRoads has a 15-year maintenance plan which highlights the periodic inspection of the concrete structure.
Shoukry Marzouk is the Team Leader Civil and Concrete for VicRoads working on the West Gate Bridge. He says the team works diligently to ensure the bridge remains safe and secure. “We’re always looking at how we can mitigate any risks to the structure. This even includes the flags on top of the bridge, which are often under intense wind stress in winter,” Mr. Marzouk says. “We have 24-7 security which constantly patrols around the compound and uses a CCTV system to monitor the bridge as a whole.”
Traffic modelling data is used to determine the best times to close certain lanes on the bridge for routine maintenance to minimise delays to less than five minutes.
Mr. Marzouk says the team working on the West Gate Bridge works around the clock to keep one of Melbourne’s busiest road corridors open now and well into the future.
“Generally, we don’t find many significant defects and are able to fix any cracks in the concrete with a concrete injection.”
He adds the bridge’s design will mean the West Gate Bridge will last for a very long time. “There aren’t many bridges out there like it from a technical perceptive. From an operational perspective it’s a highly sensitive asset and our work maintaining it is critical.”
ARRB’s Mr. Seskis adds that while these larger bridges often have teams working on them every day of the year, the same principles apply to local councils and state road authorities.“Bridges are complex structures and their failures weigh heavily. Proper maintenance and asset management is fundamental to ensuring these pieces of critical road infrastructure can live past their intended lifespan.”