When Newport City Council decided to build a footbridge to link the east and west banks of the River Usk, and improve access to the city’s centre, it chose a structure consisting of a 140m single-span deck hung from two 70m-long V-shaped tubular mast resting on the west side of the river, a design that reflects the area’s heavy engineering heritage.
The completed River Usk footbridge
With no access from the river, restricted access on the east side, and buildings right next to the west side, assembling and installing the bridge components would be one of the major challenges for the main contractor, Alfred McAlpine, and the bridge fabrication and erection contractor, Rowecord Engineering Ltd. Rowecord called in Ainscough crane hire.
Ainscough’s initial proposal was to utilise a 1200t capacity Gottwald AK 912 strut boom crane to install the 220t back mast and the 317t front mast as single pieces. The bridge deck would be split into pieces. The AK 912 would install the 70t central span (at a radius of 82m) and the remaining sections would be installed by heavy telescopic cranes working from the respective abutments.
Whilst a car park behind the west abutment provided a suitable location for the cranes, on the east abutment there was no suitable existing access or hardstanding available.
The proposed erection scheme required the compulsory purchase and demolition of two houses for access, and the construction of a temporary crane pad projecting out onto the river bank.
It was at this stage that an unusual problem arose: otters. The River Usk and its banks are protected by environmental laws, so any works that encroach upon it have to be specially approved. And Ainscough’s proposal was rejected. The problem was that there was evidence that after many years of absence otters were beginning to repopulate the east bank, and it was considered that the proposed crane pad would discourage the otters from remaining in this area.
Ainscough therefore came up with an alternative that substituted a Gottwald AK 680-3 for the crane originally proposed. Whilst small enough to sit on the piled crane foundation already installed by the client, the AK 680-3 could install all three central deck spans, including the central 50t east span at a radius of 102m. This meant a smaller crane could be utilised to install the east abutment span, standing further back such that the crane pad would not need to encroach on the river bank. The otters could then happily continue to use the riverbank.
Ainscough worked with Rowecord to develop a site layout that allowed section and crane assembly. One of the first structures to go up on the site was a temporary A-frame, tied down to the bridge’s permanent foundations.
This frame was needed because of access problems. The site was too small to fit a crane that could lift the front mast into its final position. Instead, it would have to be lifted into place at a steeper angle, and then lowered out into its final position with strand jacks. These jacks would mount on the A-frame, which would also be the temporary support for the backmast.
This frame was unfortunately directly in the path of Ainscough’s first lift, the back mast. In order to pass in front of the A-frame, the backmast would need to be boomed into minimum radius, which meant two Maxilift counterweight trays, one for the initial pick-up and one for the final set-down. Even at minimum radius, the back mast only cleared the frame by being turned on the hook as it was slewed into place.
Ainscough raises the bridge’s assembled back mast with Gottwald AK 680-3 and a Liebherr LTM 1500 as tailing crane
Once the front and back masts were in place, the AK 680-3’s 77m main boom was extended by 30m to install deck sections. The Gottwald needed up to 600t of floating Maxilift counterweight to achieve some of the lifts.
Deck assembly was not straightforward either. The designers split the deck into five pieces, and installed the central section first, hanging underneath the front mast. The problem was, as soon as another section was attached, the centre of gravity would shift and the deck would rotate. The initial idea avoided this problem by employing cranes on either bank simultaneously. But the otters had scuppered that plan.
View of the first deck section, flanked by the front mast and Gottwald crane
If the riggers installed the west central span next, the east central span would be out of range of the crane. On the other hand, if they installed the east span next, the deck would rotate the bridge front mast directly in the path of the crane boom. To get around the problem, the team installed the east central span, and then anchored it to the east shore with chain hoists and wires.
Ainscough used a 500t Liebherr
LTM 1500 to install the west abutment span and a 400t Liebherr LTM 1400 to install the east abutment span and precast sill unit.
