World record-breaking bridge

The Forth Road Bridge has been successfully carrying road traffic across the Forth estuary since 1964, but with loads now weighing many times more than what it was designed for and concerns that cable corrosion would force premature closure, a replacement became an urgent necessity. Although many options were considered, the choice was straightforward in the end. A modern cable-stayed bridge was far cheaper and quicker to build than a tunnel or a suspension bridge, and a location close to the existing bridge made connections into the existing road network far easier. The clincher was Beamer Rock, a natural outcrop in the middle of the Forth that provided the ideal location for the central tower of a three-towered cable stayed bridge.

Modern design

The design for the new bridge was initially developed by Transport Scotland’s (TS) consulting engineers Jacobs Arup, led by Arup’s global bridge design practice leader Naeem Hussain (who was awarded the Royal Academy of Engineering’s Prince Philip Medal in 2012 for his groundbreaking work on bridges, including Queensferry). The final design was developed in a joint venture between Ramboll, Sweco, and Leonhardt, Andrä und Partner, while construction was the responsibility of Forth Crossing Bridge Constructors (FCBC).

The bridge’s form sits in harmony with its two well-known neighbours, each representing the pinnacle of bridge engineering of its generation. The design’s most distinctive – and unusual – feature is the crossover of the cables at the middle of each span. This stems from a peculiarity of three-masted cable-stayed bridges: as the central tower is not connected to a stiff back-span structure (which would reduce any twisting), an out-of-balance live load on just one of the main spans can cause large deflections in the central tower and deck, and large bending movements in the tower. This can be countered, for example, by making the central tower larger and stiffer, making the deck much deeper and stiffer, or by tying the tower tops together with additional cables. A much more elegant, and economical, alternative was to design the stay cables to overlap over almost a quarter of each 650-metre-long main span, which effectively adds greatly to the stiffness of the deck without increasing its size. This has never been done before on a large bridge.

This design formed the basis of consultation and planning, and the subsequent Bill received Royal Assent in January 2011. Meanwhile, bids were invited to do both the detailed design for the bridge and its approaches, and the construction. Tenderers were given a set of ‘definition drawings’ that indicated the requirements for the overall form and geometry of the bridge, including the shape of the ‘mono-towers’ and the arrangement of the cables in two lines down the centre of the bridge, together with a ‘specimen design’ that gave an example of how all other aspects could be designed. In early 2011, the contract was awarded to FCBC and its designers for £790 million, for completion by the middle of 2017.

FCBC’s early decisions included opting for a composite design for the trapezoidal box sections of deck for the bridge and approaches (steel with a concrete top, rather than all steel or all concrete); ordering all major steel components from overseas fabricators in China and Poland. Perhaps most surprisingly – and successful – the engineers chose to service the site and its remote locations in the sea by investing in a 30-strong fleet of floating plant, rather than (as expected) building causeways for road access from the shore. This approach saved both time and cost.

Working underwater

The first onsite task was to create the foundations, particularly for the three main towers. As with the bridge’s earlier neighbours, the foundations presented an even greater challenge than the more visible superstructure, and represented around 30% of the total cost. The central tower on Beamer Rock was relatively easy to construct, but the other two had to be sunk through deep water and soft ground to rest on the solid rock beneath. For these, FCBC chose some of the largest steel caissons (large cylinders that were lowered onto the seabed and had thousands of cubic metres of concrete poured into them, which the tower foundations could be laid on top of) ever used in bridge construction.

The largest, for the south tower, was 30 metres tall and 32 metres in diameter, double-skinned with a 400-cubic-metre volume annulus (a ring-shaped structure) between (capped with temporary steel plate so that the air inside provided buoyancy while towing and placing), and a cutting edge at the bottom. This was then carried out into the estuary on a semi-submersible barge, which was partially sunk beneath the caisson, hooked up to a large shear-leg crane. It was then towed into position where it had to be located with 200 millimetres accuracy; a feat that was not easy in a fast-flowing tidal estuary, even with the help of GPS. By slowly releasing air from the annulus, the caisson was gently lowered to the seabed. Concrete was added in the annulus so that it would sink until it was embedded firmly in the soft sediment deposit below. A separate 11-metre-tall single-skin cylindrical steel caisson was then bolted on top so that it would extend above water level as the caisson was sunk further.

The sinking stopped when a corner of the cylinder reached the sandstone bedrock. The perimeter of the caisson was sealed into the rock by a ring of interlocking jet-grouted piles and the remaining soft material was dredged out to expose the bare rock beneath. But then came the most difficult and time-consuming bit of all: vacuum cleaning and minutely inspecting the rock head (in deep, murky water with little visibility) to ensure it was clean and free of large cracks.

Once this was done, the bridge’s foundations could be created on top of the rock head by pouring tremie (underwater) concrete to fill the inside of the caisson. For the south tower foundation, this was a continuous pour of 16,900 cubic metres of concrete over two weeks, supplied by a fleet of barges that worked around the clock from the quayside batching plant. (This was a world record for an underwater marine concrete pour.)

This brought the foundation up to around 15 metres below water level. The remaining water was pumped out and a five-metre-thick reinforced concrete tower base was built on top. Once this was completed, construction of the towers could commence. The hollow, elliptical, tapering heavily-reinforced concrete towers were formed in ‘jump lifts’: a four-metre-high section would be poured using concrete ferried by barge from the shore and pumped in. The moulds were then lifted and re-fixed ready for the next pour. Each lift took about seven to 14 days.

Deck assembly

Meanwhile, all 122 steel troughs for the deck were taking shape in a Shanghai fabrication yard, overseen by a team of engineers from TS and FCBC. Each 250-tonne trough was match-fitted to its neighbour in the factory before being disconnected and transported to Rosyth Docks. Once it reached the docks, reinforced concrete deck up to 460 millimetres thick was added, increasing the weight to around 700 tonnes, before each 4.5-metre deep and 40-metre wide box-section unit could be barged to site.

One of the most awkward tasks was placing the first sections of deck, once the towers had risen above deck level. First, a giant 1,200-tonne-capacity floating shear-leg crane was used to erect a huge temporary structure around each of the tower bases. The structure consisted of two 146-tonne raking triangular trusses (supports) topped by temporary steel platforms. The crane raised the first deck sections and lowered them on to the platforms round the towers and then lifted a traveller crane on to either end.

Each section of deck was floated out from Rosyth on a barge, lifted up by the traveller crane on a rising tide, placed with precisely the right level and inclination (computer controlled to within two millimetres), and bolted into position. As the tower rose and the deck moved outwards, the first of the 288 cables were installed and tensioned. Each cable consists of up to 109 seven-wire strands encased in a white polyethylene weatherproof sheath up to 315 millimetre dia. The first strand was lifted with the sheath and stressed to 10 tonnes by jacking crews on the towers. The other strands were then pulled through the sheath and stressed one by one. Although stressing could be done within the towers, positioning the cables and threading the strands at the tower tops was done by workers in open baskets hoisting themselves on wire supports up the outside face of each tower, one of the most weather-sensitive operations of the whole project. Eventually, just before the decks met at mid-spans, the central tower and the deck on either side were recognised by Guinness World Records as the longest free-standing balanced cantilever in the world.

The crossing’s most dramatic moment was the closure of the mid-spans. Vertical differences between the two adjacent cantilevers could be as much as four metres before final balancing, which was achieved by a combination of repeated tweaking of the stresses in the cables and the less sophisticated process of shifting the location of plant and ballast on the decks to alter the balance. Using jacks, one side of the deck was ‘pushed’ landwards by 300 millimetres to make space for the final deck section to be lifted in place and fixed on one side, while several hundred measurements were coordinated by computer software that monitored real-time loadings, stresses, orientation and global positioning. Work was suspended if wind speeds reached over six metres a second. Releasing the jacks closed the gap to just five millimetres, and the join was completed by bolting, welding and a concrete ‘stitch’ at deck level. The crossover cables were then added, with a complex sequence of stress readjustment on nearby main cables.

Weather impact

The team had known from the start that weather – particularly high winds – would play a major part in the programme: for example, a deck lift required a four-hour weather window with wind speeds less than 14 metres a second, and less than 11 metres a second for positioning and stressing the cables at the tower tops. However, it was felt that good and bad periods would even themselves out, which proved correct for the first few years.

Caisson sinking was delayed by bad weather, and there were further delays in grouting the seals beneath the caissons and cleaning the rock head. Acceleration measures were introduced, notably increasing the number of traveller cranes from four to six so that deck erection around each tower could operate in parallel rather than in sequence. However, erecting the falsework structures around the towers was much quicker than expected, thanks to benign weather in autumn 2014. Up to the end of 2015, total downtime due to weather was around 25%, close to original expectations, and an early opening date at the end of 2016 was being confidently predicted.

In 2016, at a time when wind-sensitive operations were at their peak, tower construction and cable anchoring were delayed by unrelenting high winds. Deck erection and the dismantling of temporary works were also affected, and weather-related downtime in 2016 was 40%. The winter of 2016–17 was particularly frustrating: mild, but with periods of exceptionally high winds, which meant that 20 days were lost to wind in February and 16 days in March. The removal of the first tower crane, normally a two-week job, took 65 days. The opening date for the crossing was put back, which created an unfortunate impression of delays. However, completion is now scheduled to be only 10 weeks beyond the date agreed some six years earlier.

Bridge to the future

So far as possible, the crossing has been future-proofed. An extra lane or bus lane can be added to the road layout, currently a dual two-lane with wide hard shoulders. The bridge has louvred wind-shielding at 3.6 metres high along its whole length to prevent closure during high winds. A continuous walkway inside the bridge deck will help maintenance be carried out when needed, while the insides of both of the deck boxes and towers are continuously dehumidified to prevent corrosion of the exposed cable anchorages. The cables themselves are well sheathed, but individual strands or even whole cables can be removed, if needed, and replaced without interrupting traffic. The whole bridge is wired up with several thousand accelerometers, anemometers, temperature sensors, corrosion cells, GPS/biaxial tilt meters, and dynamic and static strain gauges so that a picture can be built up of the bridge’s performance in all combinations of wind, temperature and traffic loading. The bridge is intended to last for a very long time.

Like its neighbours, the Queensferry Crossing has paved the way in innovative engineering, and the three bridges across the Forth should provide a fitting monument to engineering achievement for many years to come.

Simple cantilever bridges, which project horizontally into space and are supported at only one end, are as old as civilisation, but in the late 19th century, engineers began to explore the potential for combining the cantilever principle with steel trusses to span much larger distances, notably on the Forth Bridge.

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This article has been adapted from "World record-breaking bridge", which originally appeared in the print edition of Ingenia 71 (June 2017).