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Spot the difference

A planned footbridge across the Clyde in Glasgow has uncanny similarities to Gateshead's Millennium Bridge. Or has it? Ian Lawrence reports.

At first glance the new £12.5m bridge planned to cross the Clyde in Glasgow could be mistaken for Gateshead's Millennium Bridge over the Tyne. Both feature a curved deck and inclined arch, and both structures were designed to play a part in regenerating rundown areas of their respective home cities.

However, the team working on the Broomielaw crossing in Glasgow insists similarities between this project and the 'blinking eye' bridge are merely superficial. Atkins, alongside architect Richard Rogers Partnership, beat five other shortlisted bidders to land the contract after a public vote last autumn.

Atkins head of structures Mike Otlet says: 'To the uninformed eye it might look similar but it's a very different arrangement. Our bridge is completely on one side. The Gateshead bridge is balanced. It's also symmetrical whereas ours is asymmetrical with a cable to hold it up. We also have a much, much longer walkway.'

Iain Macnab, Glasgow City Council's project manager, adds: 'Both public and politicians chose the same scheme. There has been the odd concern that it looks similar to the Gateshead bridge but most people were pleased with the fact that it's a complex and exciting structure.'

The steel bridge, named Neptune's Way, is designed to give the illusion of floating above the river as it spans the 100m between Broomielaw on the north bank and Tradeston in the south.

To achieve the necessary 5.4m clearance over the shipping channel, the 160m long walkway will slope at a constant 1:20 gradient towards midspan, while the supporting arch will reach a peak of 35m.

Otlet says: 'They wanted something you would notice. They wanted something iconic and with Glasgow being quite a brash place we knew we didn't need to hold back. It's a shape people will instantly recognise.'

Dominic Pask, the Atkins associate attached to the scheme, said the design should be very user friendly and compared it to another bridge the firm had worked on.

'Richard Rogers Partnership pointed out the Hungerford Bridge which has steps and lifts. I've been there and seen people trying to carry children's buggies up the steps. They wanted to avoid all that sort of thing and of course lifts can break down. They wanted to make it all one experience. It's pretty unique. [Structurally] it's almost a combination of lots of different structures.'

The timber-surfaced deck is 3.5m wide on the riverbank, but expands to 6.5m at its apex, offering views down the river.

Cyclists are accommodated on a slightly lower level, with a metal grille providing a reliable surface in all conditions.

The steel box arch, likely to be angled at between 25infinity and 30infinity, will support the deck via steeply inclined cable hangers. Tieback cables restrain the arch and take loads down to the foundations.

Sophisticated structural analysis will be needed for such an extreme design.

'You can do a back of an envelope calculation for most structures that are designed, ' says Otlet. 'This is much more complicated so we are quite reliant on the analysis. It takes it up to another level. You aren't going to find a formula for this.

There are too many elements. It's non-linear so it's unpredictable.'

Pask outlines the challenges ahead: 'If you have a cable stay bridge you know how it behaves but if you try something different then there are no precedents. We have to test and check everything.

Even 10 years ago this would have been a problem but now, because of the speed and power of computers, it means we can have a go at these designs.'

He says that altering one parameter would affect the bridge's shape and appearance.

Although all elements need to work together, much of the team's current focus is on the tie-back cable.

Pask says: 'We are trying to make the deck as light as possible.

The cable forms its own shape so if we tension it up more it will form a new shape. The load is generated by the weight of the bridge so to put more on will influence the shape. Whichever parameter we change will affect the cable, but we are pretty sure we have the geometry fixed and the deck fixed.

'The general feeling is that we should be okay with the arch and the deck, but we have to be very careful with the cable.'

Having narrower abutments will restrict traffic flowing onto Neptune's Way. Otlet adds: 'We aren't worried about horizontal wobble, which is what the Millennium Bridge had. Our work will concentrate on vertical excitation.

'Our bridge is quite wide and very stiff horizontally. However, we have allowed for the cost of fitting tuned mass dampers to stop any vertical bouncing. These would be positioned underneath the deck and there is another provision for dampers on either side of the arch's base. Live tests will be needed to check if the dampers are needed.'

At this stage it seems likely that once construction starts the arch will be assembled in three sections. It will be lifted using landbased cranes and then held in position using cables connected to the foundations.

Then the tieback cables can be installed, but not yet tensioned, before sections of the deck are hung, starting from either bank.

Once the hanger cables are attached the central deck section can be put together, lifted into position and the hangers attached.

Otlet is unsure how long the erection procedure will take, as the extent of the necessary temporary structures is not yet known.

The Broomielaw bank foundation will be connected to the adjacent quay by a small link bridge, minimising the amount of work required on the existing quay structure. The Tradeston foundation to the south will be formed on land.

Both will consist of a combination of vertical and raking piles and ground anchors to cope with the large forces.

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