Most of the major developed economies have a similar problem with their transport infrastructure: it is getting old.
“Ageing infrastructure is a common problem in the developed world, because of the age at which a lot of the infrastructure development was carried out,” explains Flint & Neill associate Joanna Bonnett.
But instead of knocking down and replacing ageing bridges and viaducts, transport authorities and operators are instead turning to engineers to get extra capacity out of them.
“These structures are now very heavily trafficked, and are often embedded in urban areas where the possibility of diverting traffic to carry out a major replacement project is very constrained,” says Bonnett. “And, increasingly, operators are typically working within constrained budgets – whether they are government-based spending plans or DBFO contractors whose spending plans are tied up with their borrowing requirements.”
One thing that makes it possible to consider increasing the capacity of many of these structures is the fact that the technologies for assessment and inspection have “come on in leaps and bounds” over the last 10 years, according to Flint & Neill director Robert Percy.
“Since around 2005, technologies like laser scanning and GPS have became much cheaper and more widely available, and their accuracy has improved so that they have become applicable to our field,” he explains. “And we now have the computing power to be able to compute huge amounts of data.”
Detailed assessment and inspection
As a result, says Bonnett, clients now realise that, by investing in detailed assessment and inspection, they may avoid expensive replacement or reconstruction costs: “The balance is changing because the cost of monitoring and detailed inspection is coming down, while the cost of doing works on structures is going up. And we can do more with the monitoring and smart infrastructure side that is more economic than a major structural intervention and the disruption that goes with it.”
Percy adds: “It is now almost inconceivable that you can close a structure while you are carrying out the works. And even shutting lanes is difficult, apart from for very limited periods.”
The improvements in assessment, monitoring and inspection technologies mean engineers can now carry out extremely detailed analysis of what is going on within a structure. From this analysis, they can identify any immediate action that needs to be taken to meet the current capacity requirements, and prioritise replacement of elements like movement joints and bearings; as well as calculating whether the structure is, in fact, capable of carrying even more load or how to add more capacity. “We typically use a number of techniques, for example video walkthroughs, 3D modelling using photo rendering, and laser surveys to give accurate geometric data about the as built structure,” explains Bonnett.
Tech from other sectors
Percy adds: “Some of the techniques give you a visual representation and some are for movement or accurate geometric information. And there are all sorts of other technologies – such as radar surveying – that are used in other industries like automotive and aerospace, and that are now finding their way into civil engineering.”
This visual and geometric information is supplemented by technologies associated with measuring strains, explains Bonnett: “Strain measurement using very high resolution cameras is a promising technique being developed, and also there are fibre optic technologies that have been used in tunnelling that are now being applied to bridge structures. And we still use conventional strain gauges.
“What we often find is that the survey, strain gauging and detailed analysis techniques need to be used together to get the most accurate picture of what’s actually happening in the structure.”
Percy adds: “These technologies let us analyse structures in a far more detailed way than ever before. And that information feeds into the management of the structure according to the owner’s resources. It is driven by the need to get the last ounce of capacity out of a structure.
“Very often this is about providing a management strategy for a structure. Our job is to plot the optimum way to keep the structure in operation so that the client is spending money when it is most efficient to spend the money.”
Bonnett and Percy agree that this type of analysis requires different skills to those needed to design new structures.
“It’s quite a different mind-set,” says Bonnett. “Often the focus is not an immediate fix to the problem; it’s on managing and optimising the overall solution. Which means our task is very much about understanding the client’s drivers and what constitutes an optimum strategy – which can be their contractual obligations and budgets, as well as the safety of the structure.”
But is all this just delaying the inevitable? Not so, says Percy: “The technology is continuing to develop, and we will be in a different position in 15 years’ time.
Laser surveys are getting to greater accuracy, to the point where we should be able to go through the whole structure and produce a full structural model of every bolt size and plate size to create a complete as built structural model.
“And in future we will be able to send a drone throughout a structure and create a full structural model with every plate thickness represented.”
He adds: “There are also developments in the tools we might have to intervene, including technologies that are being used in other areas, such as structural adhesives.”
Flint & Neill has undertaken a number of projects to improve capacity on the Queen Elizabeth II Crossing since 2010.
The crossing, which opened in 1991, is made up of an 812m cable stayed bridge between 1km long approach viaducts to the north and south. The bridge carries southbound traffic from the London orbital M25 motorway over the River Thames between Essex and Kent to the east of London. It is one of the most heavily used and most critical pieces of infrastructure in the national road network.
Flint & Neill was initially commissioned by M25 DBFO operator Connect Plus to carry out a structural load assessment of the crossing and a bridge live loading assessment. The consultant had previously identified that, under some limited circumstances, actual traffic loading was capable of exceeding the design code.
Specialist investigations were also conducted into vibration in the towers and cables. Vibrations have been observed throughout the bridge’s history, and the aim of these studies was to identify the cause, quantify long term effects, and propose measures to gain better knowledge for future mitigation.
A survey was undertaken to review and confirm the geometrical profile of the towers and the deck of the main bridge, as well as on site testing and measurement to enable the current cable stay tensions to be calculated.
Flint & Neill have designed the scheme to replace the large movement joints on the structure. An innovative temporary ramp system was developed in conjunction with Connect Plus and contractor Jacksons to minimise disruption. Laser scanning was employed in the development of a 4-D model to refine the sequence of works and minimise site risks.
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