When it comes to bridge installation, nowhere is time more critical than on the rail network.
While installing, renovating or repairing bridges on the road network brings its own inconvenience, there are a number of workarounds that include contraflows, the use of alternative detour routes and other traffic management techniques. These are luxuries not as readily available on the rail network, and the installation or repair of bridging invariably requires a blockade – one that will be as short as it can realistically be.
To further complicate the issue, many rail bridge sites present the additional challenge of access. Rural bridges are often away from road access, and many urban bridges are in built-up areas where access is limited. When this is factored in with the need to deliver the project within a specific blockade, it becomes clear that planning, design, and time management are the fundamentals that underpin successful delivery.
Indeed, planning, design and time management – specifically time management with regards to delivery and installation methodology – are inextricably linked and each has an influence on the other.
Consider the installation
Steel bridges offer several advantages over concrete. Suitable for rapid installation, they can be prefabricated off site and can undergo full trial erection to avoid surprises during final assembly.
One of the key influencers in the design and manufacture of a steel rail bridge will be its installation. That may seem an over- simplification, but it’s often not fully appreciated how much the installation technique and specific site and time challenges have a bearing on everything from the form of the bridge to its manufacturing detail. With rail bridges, which are usually of utilitarian design, function will almost always take precedence over form, and the nature of the installation will often dictate the design.
There are several installation options available, including build in-situ, crane installation, self- propelled modular transporters (SPMT), and launching. Launching is only used where no other method is suitable and the decision to launch or lift-in is usually dictated by whether or not a heavy crane can access the site, and whether there is actually room to launch the bridge.
However, even on the seemingly simplest of installations, if the bridge is to be launched the devil is always in the detail(ing). The location of splices, bolts and other elements, if not designed with enough of an eye on installation, could mean the structure may not travel over its temporary slide supports easily during launch, creating problems that could potentially result in damage, delay, or the need for rectification on site.
By considering in detail the launch methodology, these problems can be addressed at the design and manufacture stage and avoided completely. Something as simple as the inclusion of a gap between splice plates on the underside of the bottom flange can make all the difference to smooth running during launch.
Planning well ahead
Early manufacturer involvement is critical in ensuring the design process is steered along the most efficient and effective path to success. The aim is to avoid any unexpected late design changes, any unforeseen challenges on site that require additional temporary works, or even any possible cosmetic damage to the structure if the installation methodology does not suit the design.
Herein lies the potential for error if the manufacturer is not brought in to manage the launch process. The manufacturer will have considered the launch methodology and designed the bridge accordingly to suit a specific launch schedule, therefore it makes sense to hand over the launch management entirely to the manufacturer.
If launch is not feasible or suitable, and if crane access is not an issue, lift-in offers usually the quickest and simplest installation method. With speed of installation almost always the primary consideration, this becomes even more acute when the challenge is installing a bridge over multiple lines, as was the case with the Mallard Bridge in Doncaster.
So named as a mark of respect to the legendary Mallard world steam speed record train which was built in the town, the challenge facing Balfour Beatty Civil Engineering was installing a new road bridge over the East Coast main line – a total of 17 lines of track, some of which are electrified – with as little disruption to the railway system as possible.
A steel solution was the only viable option for bridging the rail tracks as it would be quickest and could be installed with minimum disruption. Mabey Bridge worked closely with Balfour Beatty to deliver input on the value engineering, with the order to commence manufacture only given on the basis of value engineering completed.
While the bridge design is not necessarily complicated, the main consideration was the need for quick installation of the core steelwork so the design and manufacture of the steel sections reflected this. Mabey Bridge proposed and ran the installation scheme for the main girders. To deliver a final span of 180 metres, the bridge steelwork consists of three braced pairs, each 84 metres long and each weighing 210 tonnes. The lift plan was vitally important in minimising the amount of work over the railway during possession.
Prior to installation, the steel girder sections were manufactured at Mabey Bridge’s Chepstow the company’s yard. The components were then delivered to site by road before being bolted into full lengths to suit the proposed lifts.
The first braced pair was lifted-in during a 56- hour blockade over a weekend to minimise the risks and inconvenience presented to traffic. The remaining works were completed over a number of subsequent weekends. Following installation of the girders, the rest of the bridge structure could continue uninterrupted.
On completion of the bridge the Mayor of Doncaster, Peter Davies, was quoted as saying examples of engineering at its finest. I am proud of Doncaster’s railway heritage and it is fitting to honour it with this scheme.”
Regardless of the scope of complexity of the bridging challenge, the secret to a successful – and timely – installation and completion is to take a holistic view of the project and to ensure that there is a sequence in place that runs through design, manufacture and installation and, where all three fundamentals are related, within the thought process.
As with all things train-related, it’s all about making sure everything runs on time.