One of the world’s largest mobile cranes has been instrumental in delivering an alternative design for a vital bridge on Scotland’s largest construction project, as Margo Cole discovers.
Europe’s largest crane made a fleeting visit to the UK last month to lift seven bridge beams into position over the River Clyde during construction of the 8.5km M74 Completion project, south of Glasgow. Contracting joint venture Interlink M74 brought in the crane, a 2,000t-capacity Demag PC9600, for a week as part of a strategy to avoid working in the river while it built the Auchenshuggle Bridge at the east end of the job.
The bridge’s original design consisted of a three-span structure with abutments set back from the water and two piers in the river adjacent to its banks. This design, to meet the clients’ requirement for a wildlife corridor along the Clyde, had a relatively short centre span and two 50m-long backspans.
At the tender stage Interlink - a joint venture of Balfour Beatty, Morgan Est, Morrison Construction and Sir Robert McAlpine - proposed an alternative single span design, which it felt offered considerable benefits, particularly by eliminating the river piers.
“We reduced the total span to 90m but, more significantly, we reduced the requirement to work in the river, which is not something we want to do unless really necessary,” says Interlink structures agent Ross Glendinning.
Along with the benefits, however, the single span option also brought its own challenges - not least how to design, fabricate and erect the 90m long superstructure. Reducing the total span resulted in a far longer river span, and far heavier deck beams than originally proposed. “The steel is spanning 90m, which is really pushing the design to the stage where it becomes uneconomical,” explains Glendinning.
“We reduced the total span to 90m but, more significantly, we reduced the requirement to work in the river”
Interlink’s preliminary designs at tender assumed the deck could be built using six steel girders, each weighing around 250t. The JV believed that the beams could be erected using the UK’s largest mobile crane, a 1,200t-capacity Gottwald AK-680, which has already been used in the construction of structures at the west end of the project. Under this proposal, half of the beams would be lifted from the east side of the river and half from the west.
But post contract award detailed design indicated that seven beams would be needed and they would be heavier than first envisaged, making the original craneage plan less viable.
“Between July 2008 and March 2009 we looked at all the options and had monthly meetings with [structural steelwork contractor] Cleveland Bridge to optimise the steelwork,” says Glendinning.
Those discussions led to a decision to incorporate microsilica in the mix for the structural concrete deck slab. This would increase the strength of the concrete to 70N, allowing the thickness of the slab to be reduced. This in turn, reduced the dead load on the steel beams, enabling them to be made slightly lighter.
Even with the design change, however, the girders were still weighing in at between 270t and 320t, depending on their length, which varies from 83.3m to 91.5m according to their position.
Interlink calculated that, to lift loads of this size over the reach required would involve using a crane with “superlift” - a large counterweight to balance the heavy load. Superlift is stacked on a tray that hangs from the back of the crane, and requires considerable space on the ground so that the tray can be loaded and unloaded.
Space is at a premium at this end of the job, especially on the west side of the river, and temporary works to support the crane, its superlift and the bridge beams themselves would have extended beyond the site boundary. “The need for superlift meant we would have had to build temporary works in the river and go breach the land allocated for the project,” explains Glendinning.
The best option, therefore, appeared to be finding a crane with a large enough capacity to reach all seven beam positions from one side of the river.
Enter the PC9600, a leviathan with the potential to lift 2,000t and a maximum boom length of 170m. Interlink calculated that the machine could sit in one position - behind the east abutment - and easily lift even the furthermost beam into place.
The crane belongs to lifting specialist Sarens, and was last seen in the UK in 2003, when it was used to position a 210m long rail bridge in Dumfries. Since then it has been in action on an oil refinery in Kazakhstan, a nuclear power plant in Finland and, most recently, on a five-month stint placing turbines at a power station in Rotterdam.
For the Auchenshuggle lifts the PC9600 sat at bearing level, on top of 5m of Class 1 fill and 500mm of ex-demolition material, with the bridge beams on top of the embankment 5m above.
CBR tests proved the fill material would take the load imposed by the crane, and bearing pressures were modelled to check that the abutment wing wall could cope with the extra pressure from the superlift, which sat just 1m away while the crane was picking up the beams.
Finding a crane that could lift all the beams from one side generated a range of benefits. It meant the girders could all be delivered straight from the existing road network onto the new road embankment; the crane only had to be erected and dismantled once - activities that each take one week.
Setting up on the east side has also enabled road construction to proceed without interruption on the west side, to the extent that surfacing was already under way while the deck was being built.
“I’ve done a lot of work with cranes, and the biggest I’ve had before was a 135t crawler. I thought that was big!”
The PC9600 arrived on 82 articulated trucks, and required six operatives and a 275t “slave crane” to piece it together. “I’ve done a lot of work with cranes, and the biggest I’ve had before was a 135t crawler,” says Interlink structures agent Craig Lafferty. “I thought that was big!”
For the Auchenshuggle lifts, the crane was configured with a 98m long main boom and 48m back mast. The boom was counterbalanced by 350t of back ballast, and during the lifts a total of 900t of superlift was added.
Each of the Corten steel bridge beams arrived from Cleveland Bridge’s Darlington works in five pieces, with each piece weighing around 60t. They are open box plate girders with a maximum flange thickness of 70mm. Most were bolted together on site, with the exception of the outer faces of the outer beams, which were welded at the request of the client. The bottom flange of the beams is curved, with the maximum depth at each end being 4.2m - the deepest they could be in order to be transported under motorway bridges.
Day in the life of a super lift
One beam was lifted every 24 hours, in a process that began by moving the girder from its fabrication position on the embankment to the lifting position using four self-propelled modular trailers.
“By 11am or 12am we would be ready to lift, which took until about 3pm. It only took about 20 minutes to get it into position, and the rest of the time is needed to get the cross bracing fixed in. Once this is in you can just use a screwdriver to lock the beam into position, and a 330t beam is placed to an accuracy of around 2mm.
“The crane lifts the superlift at the same time as it lifts the beam,” he continues. “It lifts to a height of about 20m then slews right round, and as it puts the beam down, the superlift tray also goes down,” explains Lafferty.
By the time a beam was sitting on its bearings and the load was taken off the crane, the superlift tray was back on the ground.
At this point, the slave crane was mobilised to remove the superlift so that the crane could move back around into its starting position. Only when 700t of weight had been taken off the superlift tray - a process that took up to eight hours - could the crane safely slew back around ready for the next day’s lift.
Once the crane was set up and the beams were in position, the Auchenshuggle lifts went like clockwork, with one going in every day during a week in mid-June.
Interlink is now half way through 12 weeks of further activity to complete the bridge construction.
Number of beams: 7
Maximum beam length: 91.5m
Beam construction: Welded/bolted open box plate girders
Maximum beam weight: 320t
Total beam weight: 2,100t
Maximum beam depth: 4.2m
Maximum flange thickness: 70mm
Skew of bridge: 40°
Volume of concrete in abutment bases: 1,050m3 and 1,205m3
Number of piles in abutment bases: 100 each side
Total concrete: 6,500m3
Crane: Demag PC9600
Type: Pedestal crane
Lifting capacity: 2,000t
Maximum boom length: 170m
Maximum back mast length: 70m
Number of outriggers: 4
Distance between outriggers: 18.5m
Height of cab from ground: 5m
Block & tackle weight: 20t