The last few sections of the new footbridge over the River Aire in Castleford, near Wakefield have just been lifted into place, with the hope that it will spark regeneration in the area. The bridge is S-shaped in plan, 4m wide, stretches for 130m over three, V-shaped steel piers and is truly breathtaking.
However to bring in an outstanding bridge within a modest budget is no easy task. Client Wakefield Council decided that the contract should be awarded on a design and build basis.
But to ensure that the end product was of the highest possible quality, it stated that there should be a set of fully detailed architectural drawings from the outset. Structural working drawings were left to the contractor.
"The architect had a fixed vision and we had a fixed budget" explains Wakefield Council project manager Tim Wheeler.
"We put the bridge out to tender as a designed bridge and the tender brief was to get the costs down."
The provision of a set of drawings, which contractor Costain adopted almost entirely, meant that there was far less risk associated with the job Đ an uncommon scenario for most design and build contracts. But this put the pressure on Costain to look for cost savings.
Building the bridge's three piers in the deep and often fast flowing river was one of the main challenges. A cluster of piles beneath each pier was initially considered, but constructing a cofferdam would have been tricky Đ not least because the ground conditions consisted of cracked and weathered mudstone.
Instead, Costain decided to support the piers on single, cased piles measuring 1800mm in diameter. This immediately reduced the construction footprint and minimised the duration of piling.
The pile casings provided a mini-cofferdam within which preparation works for installing a precast pilecap could be carried out safely in the dry.
With the piles in place a, prefabricated box known as a "limpet" was clamped onto the pile cap before installation. Once the pile cap and limpet were in position on top of the pile, the area inside the limpet could be pumped dry to create a water tight working area for the connection of the pier bases below water level.
The pile cap, pier base and pier are held down by four high-tensile Macalloy bars, positioned 4m deep within the pile.
All three foundations were completed in time for the arrival of the steel legs in October, despite the site team having to contend with two major floods in 10 days during June. These swelled the river to record levels, and swamped the site for over three weeks making work in the river impossible.
Of primary concern was keeping hold of the crane mounted jack up barge which aided all lifting and piling operations during the floods.
"We appreciated that the river would rise, but it wasn't the level that was the problem so much as the velocity," recalls Costain site agent, Andrew Langley.
"We had to make sure that it wouldn't drift downstream and hit the only other existing bridge in Castleford, a 200 year old road bridge."
Fortunately, Costain had considered such unforeseen events at tender stage.
"With Costain, it wasn't just about the price but how they wanted to build it", says Wheeler. "Their detailed process at tender stage impressed and gave us confidence."
The £4.8M bridge will be officially opened at the end of December, providing the local community with a safe and attractive link between the north and south banks of the Aire.
Architect: McDowell & Benedetti
Structural Engineer: Alan Baxter Associates
Civil Engineer: Arup
Contractor's structural engineer: Tony Gee & Partners
Client: City of Wakefield Metropolitan District Council
Funding: English Partnerships, Yorkshire Forward, Wakefield MDC
The main beam was originally a multicell box girder spanning between piers. One cell gradually increased in depth and rose out of the decking to form seats between the piers. This provided the extra depth at the midspan to satisfy the maximum bending moment in the structure.
"We needed the extra depth for the longer spans" explains architect Renato Benedetti. "The idea was to create public space in the middle of the river, to which the benches were integral. We liked the synergy of needing the depth and turning it into a place to sit."
Arup undertook hydrological modelling for the project, which involved creating a 1:50 scale model. The team worked in close cooperation with the Environment Agency and British Waterways to ensure that the bridge would not increase flood risk by trapping debris and allowing water to back up, nor interfere with British Waterways' weir maintenance operations.
"Initially we carried out numerical modelling but some of these issues are best demonstrated physically," explains Arup engineer, Will McBain. "The model allowed the various bridge options to be assessed with regard to flood risk, debris impact, boat impact risk, siltation and erosion."