Temporary bridges can be incredibly versatile; from getting construction plant and traffic onto site to reconnecting flooded communities and helping keep traffic jams to a minimum in the event of major infrastructure repair works. Whatever their application, they can make a real difference to a construction project or community – but they’re not without their challenges. In this blog, Mick Woods, our Construction Manager explores the key things to think about when you’re in need of a temporary bridge.
A permanent bridge, whether spanning over road, rail or water, can be years in the making, from the initial engineered design and fabrication of the elements, through to its on-site construction. With temporary bridging, this whole process is shortened dramatically, with some emergency and reactive projects requiring a temporary bridge on site in as little as a few days.
While the process of designing a bespoke temporary bridging solution can be incredibly challenging, requiring a great level of expertise and experience, installing it safely and efficiently, with minimum disruption to the local community, can be just as much of a challenge. In fact, it may surprise some people to learn that, while the engineering and design stage accounts for around 30% of time and resources spent, 70% is logistics.
One of the first things your temporary works supplier will think about when it comes to temporary bridging is access. By its very nature, a bridge is there to carry people or vehicles over something, meaning that accessing the site could potentially be complicated and/or time sensitive. Installing a temporary bridge over a main highway, for example, would likely mean the short-term closure of the road, requiring collaboration with Highways England. Likewise, a railway bridge would require coordination with Network Rail and, potentially, the temporary closure of the rail network.
Of course, keeping closures and disruption to a minimum is a priority. Opting for modular bridging equipment that can be partly pre-assembled away from the main site, thereby reducing the amount of manual handling on site, is one option. Overnight installations, when traffic is quieter, is another.
Either way, this need for speed and efficiency highlights the importance of experience and expertise, with careful planning and methodology being critical.
Similar to access, available space on site is another consideration when planning a temporary bridge installation, as this can determine the install method used. Rivers, motorways and other buildings or bridges can all result in the available space for assembling and manoeuvring the temporary bridge into position being heavily restricted.
Broadly speaking, there are three types of installation methods when it comes to temporary bridges: lifted into position using a crane, launched (whereby the bridge is assembled on one side and then pushed to the other using rollers) and crane-assisted launch (a combination of the two), each with its own unique set of challenges. For example, crane-assisted launch is often used when space is limited and the bridge is too heavy to be simply crane lifted. Here, in addition to planning the crane positioning, you also have to carefully consider the bridge assembly sequence (due to limited space) and design the roller layout and loads, with the front rollers taking the majority of the bridge weight during the launch.
While it may sound contradictory, since we are discussing the temporary, consideration of the permanent structure can be just as important. A common application for temporary bridging is emergency or reactive projects, where the existing structure has suffered damage, such as that caused by a lorry strike. In these situations, a temporary bridge can be needed to provide traffic with an alternative route while structural repairs are undertaken, helping to keep disruption and any traffic jams to a minimum.
A key part of repairing the existing bridge, work that is often carried out alongside the install of a temporary bridging solution, can be to physically lift the bridge off its support piers, using a temporary propping and jacking system to do so. With there being many different types of bridges in the UK, each unique in its own right, it stands to reason that each bridge would also require a bespoke propping and jacking solution, with jacks installed on the existing abutments and props on ground level.
As with installing the temporary solution, the methodology of lifting the permanent can also depend heavily on what the bridge is spanning over. Water, for example, can pose a particular challenge, such as on the New Elvet Bridge project, where we had to adopt the unusual approaching of lifting the bridge from above, due to the access issues.
Of course, this is not to say that once the temporary bridge is successfully in-situ the work is finished. Applying structural monitoring solutions is an ideal way to monitor the structural behaviour of the temporary bridge and provide added assurance and confidence in the scheme, especially given that loading levels can change from day-to-day, depending on the amount of traffic (and type of traffic) using the bridge.
With structural monitoring, project teams can gain valuable insight and view live load data, as well as set up alert systems should the loading exceed a pre-determined level.
It’s clear that there are many challenges and considerations when approaching temporary bridging, with the logistics requiring perhaps just as much careful planning as the initial engineered design itself. Fortunately, with our end-to-end service offering and in-house install team, we’re here to take the stress away. Right from the initial concept and design stage, we’re on hand to help; not only designing, supplying, installing and removing the temporary bridge but also the corresponding propping, jacking and monitoring equipment to ensure a safe job well done.