Structures

Innovations in bridge design

The latest movable formwork helped the mighty Mersey Gateway to open under budget, while new developments in precast systems are putting railway bridges on a faster track

The recent opening of the Mersey Gateway, the stunning 1km-long cable-stayed bridge linking Runcorn to Widnes, shows that innovation in concrete bridge construction is alive and well. The use of two 1,700-tonne movable scaffolding systems as formwork for the curved viaducts’ in-situ concrete decks helped the Gateway to open on time and under budget – not bad for a £600m project that involved more than 9km of new roads, seven junctions, 12 additional bridges and almost 5 million man hours.

And yet, when it comes to medium-span road and rail bridges, owners, consultants and contractors are often unaware of the wide range of suitable concrete solutions, choosing steel bridges almost by default. Just like their larger siblings, these bridges and bridge replacements need to be built as safely, simply and quickly as possible. What makes them more complicated is that they are often built across a live carriageway – traffic management costs can be huge and railway diversions are unusual, thus making speed and ease of construction perhaps even more important than on prestige projects. Fortunately, there are a growing number of concrete systems that can meet this challenge.

Bridging the Mersey Gateway

The Mersey Gateway project, which opened in October, is more than just a bridge. In fact, it includes 13 bridges of various descriptions, its 9km elevated route crossing a complex network of infrastructure. In Widnes, for example, the main north and southbound carriageways required a 99m-long skew bridge to span a double-track freight line.

The bridge was manufactured off-site by ABM Precast Solutions to permit high-speed installation with minimal need for on-site resources and labour. Despite having to work around continued rail operations and the vagaries of the British weather, ABM installed the 3,700 tonnes of precast concrete in just 15 working days. The bridge has a 5.86m clear height with each 14.5m span supported on precast wall sections.

The total length comprised 56 of these three-part portal units, which were stitched together with in-situ concrete to allow them to deal with the uneven loading imposed by the skewed crossing. The bearing-free design is intended to minimise the need for maintenance during the structure’s 120-year design life.

 
Here, bridge consultant Simon Bourne explains how a new medium-span railway UU-Bridge type uses prestressed, precast and in-situ reinforced-concrete elements to fully replace decks within a normal weekend closure.

New precast railway bridge - the “UU-Bridge”
This innovative solution uses the latest ideas in precasting, prestressing and construction. Precast U-girders are used as edge beams, before being filled with in-situ concrete to form solid edge girders. These are then stitched to transverse, precast reinforced-concrete slabs, forming a through-girder in an overall U-shape – hence “UU-Bridge”. The through-girders maintain an overall depth, and a deck depth below the track, comparable to any steel or existing structure. The stitch detail is formed using C85/100 high-strength concrete, which reaches 50MN/m2 in 18 hours, at which time train loads can be applied.

The U-girders are prestressed, with the tensioning and concrete strength determined by the serviceability limit states of frequent traffic combinations. The grade of the precast concrete has to strike a balance between the need for a thicker bottom flange to control stresses and a hinner area to control weight. The aim with these girders has been to keep the weight of the most typical lifts to less than 100 tonnes and to keep the maximum weight generally below 150 tonnes. The outcome is that high-strength concrete needs to be used, especially for the longer spans.

The transverse reinforced-concrete slabs are also precast, with C50/60 concrete. For single-track railways, the slab is formed as a single piece: 2.6m wide, 250mm thick and weighing 15-40 tonnes. For double-track options, the slabs are split in to 2.5m lengths for transport: these are 5.5-7.5m wide, 400-500mm thick and weighing 15-25 tonnes. All the precast slabs can therefore be lifted by the cranes already on site for the girders. The whole, monolithic single-span deck unit, comprising U-girders and slabs, then sits on four pot bearings.
 

Construction details

Figure 1a (below) show the double-track U-girder. The standard depths vary from 1.6m to 2.6m, accommodating all spans from 10-39m. For spans from 10-24m, the U-girder is 1.6m deep, allowing it to sit under the platform gauge. For spans up to 39m, the U-girders become progressively deeper – up to 2.6m. These new U-girders are similar to existing precast manufacturers’ large U-beams, but are bulkier to suit the railway requirements and are in-filled with concrete to control the large stresses. The whole solid section acts as a mposite unit for both bending and shear.

For the 10-24m spans, the U-girder needs strengths from C50/60 to C70/85, while for the 27-39m spans, the strength needs to be C80/95. The infill concrete also experiences large stresses and is required to be C40/50 to C50/60. The infill concrete could weigh up to 200 tonnes – hence the need to avoid lifting it. The U-girders vary from 30-75 tonnes for the 10-24m spans to 95-145 tonnes for the 27-36m spans, and reach 165 tonnes for the 39m span. They can all be erected by large mobile crane or cranes.

Figure 1b shows the single-track U-girder. The standard depth is 1.6m for all spans from 10-24m. The U-girder needs strengths from C50/60 to C70/85, while the infill concrete can be C40/50. The U-girders vary in weight from 20-40 tonnes and can all be erected by modest mobile cranes.

 


Skew layouts – where the span is not at a right angle to the obstacle it crosses – are very common for railway bridges and can be incorporated into the U-girder system (see figure 2). The creation of a single, monolithic deck requires stitches between the precast pieces. There are nominal transverse stitches between the reinforced-concrete deck panels, and main longitudinal stitches, carrying significant moments and shears, between the slabs and U-girders. These stitches are designed on the basis of overlapping hoops that generate the full tensile capacity of the section. They should be formed in C85/100 high-strength concrete in order to reach the short-term strength required.


Programme

A detailed construction programme has been prepared for a typical UU-Bridge. It incorporates the removal of an existing deck, replacement of cill beams, installation of U-girders and deck slabs, including all in-filling and stitching, waterproofing, and installation of the ballast and track. In summary, the whole series of operations can just be completed within a normal weekend possession of 54 hours, although a long-weekend possession of 72 hours would give greater time guarantees.

Simon Bourne is the author of the new Concrete Bridge Development Group (CBDG) Technical Guide 15 – Bridge Replacement Guide, which has just been published. Banagher Precast and Network Rail also collaborated on the UU-Bridge project.