Bearing the load: Congleton link road bridges

04/09/2020

Highways reports on the  £90m Congleton link road and the bridge structures that make it so special.

Like many market towns that have become absorbed into a city’s commuter belt, the Cheshire town of Congleton was constructed for a time before cars and commuting became part of our daily routine.

However, Congleton’s location just 30 miles south of Manchester, means the town is on the route into work for many who live in the greenbelt but work in the city.

Over the past few decades, traffic has become a severe issue in Congleton’s narrow roads, with bottlenecks deterring trade for many local businesses and making life difficult for residents.

 

To address this, Cheshire East Council proposed a 5.5km link road to create a bypass, freeing the town centre of through-traffic and commuters. The plan aims not only to ease congestion, but also to boost the local economy by making Congleton more accessible to visitors and more appealing to small businesses.

The £90m Congleton link road will join the A534 Sandbach Road (to the west of Congleton) with the A536 Macclesfield Road (to the north of the town). Planning for the project began as far back as 2012, with the main earthworks commencing in March 2019, following ground investigations, utility diversions, ecological management works and detailed design. The scheme is due to open to traffic by the end of 2020.

Construction and civil engineering contractor GRAHAM is responsible for delivering the five-phase project, which includes two bridges: one to enable the diverted traffic to cross the River Dane and another over local B-road, Giantswood Lane.

The company worked with CCL, a specialist in engineered solutions for structures, on the bridge elements of the scheme, with CCL providing a variety of bridge bearings to meet the requirements of the bridge design and challenging local topography.

River Dane Crossing design development

The River Dane crossing is a two-span bridge of composite construction, with reinforced concrete abutments on the west and east banks of the river and a single concrete pier on the east bank, all constructed with 1200mm piles.

The original proposal for the bridge included an additional pier on the west bank of the river to support the larger span, but the challenges of the topography and the water table prompted a redesign of this initial proposal.

The steep bank that descends from the western abutment to the location earmarked for the western pier created issues with accessibility. Erosion of the riverbank was a further consideration. To overcome the buildability issues created by this topography, which were likely to include accessing the site for the proposed western pier from the east bank, the bridge design was altered to the single pier on the east bank.

This decision reduced the temporary works involved, aided the build programme and helped to manage the level of risk, thereby benefiting the project.

 

River Dane Crossing construction

The spans of the River Dane Crossing are of unequal length, with a 62m span crossing the water and a 25m span completing the path of the bridge from the pier on the east bank to the eastern abutment.

Although the GRAHAM team cut into the high embankment to construct the western abutment, there is a vertical difference of 20m between the west and east sides of the river, which has resulted in the 62m span being constructed at a 1/20 gradient.

The bridge has been designed to provide a floodplain, allowing for worst case scenario flood planning, because the River Dane experiences flash flooding during periods of heavy rainfall.

The permanent formwork for the bridge was pre-cast, but the abutments were all cast in-situ, beginning with the pier, prior to assembly of the deck. Once each of the abutments and the pier had been cast and allowed to cure to full strength, the bridge bearings were installed on the bearing shelf to enable some movement in the bridge structure and absorb the vertical load of both the span and the vehicles that will use the crossing.

The spans are composed of steel girders, installed first from the east abutment, then from the west and tied together before a carefully-sequenced concrete pour, aligned to the optimum loading conditions for the steelwork, to create the bridge deck.

 

Bearing engineering

CCL designed and manufactured the bridge bearings to meet the loads and forces stipulated by the project’s structural engineers. For the abutment on the west side of the river bank, where the span is designed to slope from the high embankment down towards the pier and the lower embankment on the other side, five free-sliding spherical bearings were installed to allow 2293kN of vertical load and movement in all directions.

These bearings were designed with a pre-set angle of 0.80 on the top plate to allow for the gradient of the span above. Alongside these free-sliding bearings, a single guided spherical bearing was also installed on the western abutment, capable of absorbing both 2251kN of vertical load and horizontal load, while allowing movement in one direction only. This bearing was also designed with a pre-set angle of 0.80, aligned to the angle of the span.

For the pier, CCL designed and supplied five free-sliding pot bearings, which will take 5816kN in vertical load and allow movement in all directions. One guided pot bearing, capable of taking both 5733kN of vertical load and horizontal load was also installed here and this has been designed to allow movement in a single direction.

There are no bearings on the east abutment because the steel girders are cast into the diaphragm, but at the western abutment they enable articulation of the bridge because the steel structure of the span will contract and expand longitudinally by as much as 100mm in response to warmer or colder weather. The free-sliding bearings allow horizontal movement and rotation in all directions. The guided bearing contributes to free movement while preventing any perpendicular movement that could damage the bridge structure.  

Giantswood lane overbridge

The other bridge included in the link road project is the overbridge at the intersection of the new Congleton link road route with the existing Giantswood Lane. Construction of this bridge, concurrently with the abutments for the river crossing, involved a six-week closure of Giantswood Lane, so carrying out this element of the scheme within the allotted programme was mission-critical.

Again, the CCL team designed the bearings to meet the loading and movement requirements calculated by the project’s engineers. A total of 14 free-sliding pot bearings were installed, with seven at each abutment, and each bearing is capable of supporting a load of 2062kN while allowing movement in all directions.

A guided pot bearing was also installed at each abutment to take both vertical and horizontal load, while allowing movement in one direction only. This bearing was manufactured with the addition of a steel weathering plate attached to it.

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