The Scottish Swing Bridge complex is nearing completion

Both machine and structure, a complex 184m-long, double-leaf swing bridge is nearing completion in Scotland. Her twin halves were built separately in Belgium and the Netherlands and hauled, on tension cables, to the River Clyde near Glasgow.

Pivoting on large cylindrical slewing bearings on either side of the river, the two decks of the bridge must project 65m towards each other over the river. Its 27 m long taillights incorporate 500 metric tons of counterweights. The two halves were installed this April and May and should be closed for the first time at the end of this month.

“I’d say it’s the most complicated drawbridge I’ve ever designed, and I’ve designed a lot of these things,” says Tallahassee-based Jim Phillips, principal bridge designer for Hardesty & Hanover LLC (H&H).

After securing approval and a budget for the crossing, Renfrew Council in 2015 engaged Sweco UK to help develop its design and procurement, says the owner’s project manager, Barbara Walker.

Although several types of crossing were considered, including tunnels, site constraints made a two-leaf swing bridge the best option, says Christopher Cardno, who has been part of the Sweco project team since from the beginning

The local port authority required a navigation channel 90m wide and 45m high, potentially to allow the passage of offshore oil rigs, he explains. The flight path of the nearby airport establishes a ceiling of just over 40 m. And space constraints on both banks ruled out a single-leaf swing bridge, he adds.

Sweco’s reference design, with architect Kettle Collective, was “gold-plated” and would likely break the budget, says Cardno, who is now head of design at Jacobs UK Ltd. Based on that plan, the 2019 offerings had been “quite significantly exceeded.” budget,” confirms Walker.

Three joint ventures had been selected to bid, but one quickly dropped out, much to Walker’s disappointment, he says. The remaining two were invited to bid for the reference design and also to negotiate a price after a period of value engineering.

In March 2021, a team led by Republic of Ireland-based John Graham Construction Ltd. won the $103 million design-build contract. It beat out a joint venture that includes American Bridge Company and Denmark’s Cowi A/S.

Graham, who declined to discuss the project, subcontracted the steel to a joint venture of Netherlands-based Hollandia Infra BV and Iemants NV of Belgium. This team engaged H&H for the detailed design, supported by Roughan & O’Donovan Ltd., Dublin.

The bridge halves sailed for about a week to the River Clyde. Photo courtesy of Smulders

With work about to begin, the owner exercised a break clause in Sweco’s contract and re-tendered the site supervision work, awarding it to Jacobs. But “we have some continuity,” Walker says. By then, Cardno had moved to Jacobs, with six years of project experience.

Recognizing “a lot of things that made the bridge a lot more complicated and a little bit more expensive,” H&H did “kind of a complete redesign,” Phillips says. For example, the original 30° inclination of the pylons in relation to the roof axis was removed. And the 30 ° inclination of the ends of the deck dropped to 6 °.

Instead of having seven braces in the front and three in the back, “we changed it to three and three, which was much more efficient and also balanced the loads on the pylons,” says Phillips.

The decks were narrowed and the pivot positions moved 2m forward to reduce the counterweights, making the entire bridge lighter. It has also “contributed to reducing the maximum tipping load on the slewing bearing,” he adds.

“On many swing bridges, the bridge is balanced, so the total dead load center of gravity is right at the pivot point,” Phillips explains. But in this case, unbalancing the structure was found to be optimal. “It’s part of a process we followed to balance all the loads on the slewing bearing in the different conditions.”

In the final design, the deck consists of 2.2 m wide edge beams that are usually about 2.4 m deep. The roof cross beams are 750mm deep and twice as deep as the riser locations.

At each pad, the deck structure consists of a full-width steel box, which is approximately 3.7m deep and 10m long. Internal stiffener plates connect the box to the edge beams and bollards. And in the center, a cylindrical diaphragm 6.7 m in diameter reflects the pad below. A 50 cm projection of this diaphragm below the flat bottom of the box is bolted to a swivel bearing, which is anchored within a concrete “pivot pier” on the river bank.

Diagram courtesy of Hardesty & Hanover

Due to space limitations, Hollandia and Iemants each built a full half of the bridge in their respective home workshops and shipped the structures to the site. On week-long voyages, the southern half was flown from Rotterdam this April and the north followed from Flushing in May.

To move the approximately 1,600 metric ton, 38m high, 18m wide and 92m long structures, heavy lift specialist Sarens NV used a combination of multi-wheel self-propelled modular trailers (SPMT) and floating legs. “Site conditions in Rotterdam and Flushing required different SPMT configurations due to space and ground pressure constraints,” recalls Saren Technical Solutions Leader Kenny Decoster. And “extensive ballast was required to compensate for tidal and load variations during Glasgow operations”.

The south bridge section was rolled to the Rotterdam dock with six 10-axle SPMTs and loaded onto a 122 m long barge by three floating legs. In Flushing, site conditions allowed the structure to be rolled directly onto the barge in 32-axle SPMT. Sheerlegs and SPMT were used to place the structures on their bearings when they arrived on the Clyde.

A bridge half rolling on its bearings. Photo courtesy of Roughan and O’Donovan

For the sea trip, the contractors emphasized that the cables remain at about 80% of the final values, says Phillips. “We left it to ourselves to make the final adjustment…on site,” he adds.

To accommodate temporary and varying loads during SPMT transport and leg lift, “the blades included diaphragms and reinforced webs,” Phillips adds. “Both conditions are quite different from the final condition.”

The successful marriage of the two halves of the bridge has been “pleasantly surprising,” Walker says. And after intensive value engineering, the crossover stays close to budget, allowing for design changes, he adds. “We hope it opens up [this] October, and as far as we’re concerned that’s very close.”