Scotland’s largest infrastructure project, the vast new cable-stayed bridge across the Forth Estuary, is entering its final, most visual construction year.
And the unrelated sudden total closure, for nearly three weeks in December, of the structurally-troubled Forth Road Bridge, just 300m downriver, further increases pressure on the bridge construction team to ensure that this replacement, the £790M Queensferry Crossing with its extensive approach roads, does indeed open by the end of December.
“With bridge towers and their complex foundations complete, plus deck erection well underway, the most challenging construction work is behind us and I am now totally focused on hitting our goal of opening to traffic late this year,” asserts Michael Martin, project director for contracting joint venture Forth Crossing Bridge Constructors (FCBC). “Our main enemy now is the estuary’s angry unpredictable weather, especially high winds.”
An instant site visit, through binoculars from his quayside office window, confirms to Martin the successful erection of the 24th balanced cantilever deck section.
“That’s approaching a quarter of them up and all positioned to 2mm tolerances,” he says, with just the hint of a satisfied smile.
The 110th and final lifted deck section is due to “close” the bridge this summer, giving access across all of its 2.7km carriageway, and engineers are already studying a speed-up plan for the finishes – blacktop, painting and erection of deckside windshielding panels.
“We hope to overlap several of these final operations,” says construction director Alan Platt.
Platt is equally ebullient about even earlier acceleration of the ongoing deck erection schedule. “We are determined to beat our programmed target rate of completing three sections a week,” he says. “Weather permitting, I am confident we can soon achieve four.”
Platt and his boss are experienced British engineers in the multinational FCBC joint venture of Hochtief, Dragados, American Bridge International and Morrison Construction.
The 16m long, 750t composite deck segments are growing outwards across the estuary as balanced cantilevers from all three of the UK’s highest bridge towers. There is now little sign up-top of the 120 men who spent the last two years constructing, round the clock, these hollow concrete masts to their record breaking 210m height.
Our main enemy now is the estuary’s angry, unpredictable weather, especially high winds
The innovative jump-form climbing shutters, used to raise the towers in 4m lifts, remain in position but are eerily empty. These fully enclosed four level “birdcages” allowed much of the required 75,000m3 of concrete to be pumped up from barges and placed irrespective of the surrounding weather.
Planned seven day lift cycles were regularly slashed to five and the last tower was topped out in December.
For the final 50m or so of each tower, over a dozen 4m tall prefabricated steel boxes, in which top ends of the deck’s 288 stay-cables will be anchored, were craned up into the birdcages and incorporated into concrete lifts.
Around 80 of the 250mm diameter bridge cables, sheathed by white, high density polyethylene piping, are already fully stressed, with two cables supporting each deck box 100m beneath.
We need four hour weather windows for deck lifts, with wind speeds less than 11m/s
The project’s three tower cranes, rising even higher to 250m above the estuary, remain important during deck and cable erection; but are now less “programme dominating” compared to last year’s tower construction. Scotland’s wild 2015 summer, when tower-top winds regularly gusted to over 40m/s, triggered double the expected downtime during tower construction. Average speeds above 12m/s stop all crane movements.
“Wind mid-estuary is incredibly squally and our construction programme remains totally dependent on exactly when in our operation cycles it blows,” says Platt. “We need four hour weather windows for deck lifts, with wind speeds less than 11m/s, but near double that window for the full operation from tow-out to final segment positioning.”
With Scotland’s last summer significantly worse than this winter so far, there were weeks allowing only a single day’s real progress and wind-related downtime peaked to 60%. Engineers do though concede that the total downtime figure for all operations so far is close to the originally predicted 25%.
We set ourselves the most aggressive targets, planning to do everything humanly possible
Wind speeds, sea state, fog and even snow forecasts are monitored virtually non stop. Martin’s comment: “We now need ‘lucky’ weather where we have to take literally every opportunity and sometimes just go for it,” is cascaded down the ranks triggering innovation at every stage.
Platt’s intention to increase the programmed three deck boxes erected per week to four centres on barges being immediately loaded and ready to go from FCBC’s fabrication yard in Rosyth Docks 3km upriver.
A special barge capable of floating-out two sections instead of one is already in use. This increased readiness should ensure that any available break in the weather can be used for lifting.
The basic 30m wide steel-trough boxes are shipped from their Chinese fabrication yard to Rosyth Docks. Here, each section’s 460mm concrete deck is cast, cantilevering sideways to widen the boxes to 40m.
An extra, third, casting bay is now being added to ensure an ample supply of these so that 750t composite sections are always ready to be barge-loaded and towed by tug to the bridge site.
queensferry bridge v2
With barges anchored directly beneath the bridge to 200mm tolerances, a four point lifting frame, lowered from the single blue traveller crane sitting on the deck above, raises the box on an incoming tide. It is lifted 55m to hang just 200mm out from the last-positioned deck section.
Twin 580t strand jacks on the traveller crane complete the computer-controlled lift in 1m strokes; with level, inclination, and multiple load gauges constantly monitored from an inconspicuous green cabin sitting on the deck. Smaller jacks pull in the hanging deck to interlock with rebar and flange plates on the previous unit.
“We need at least 2mm accuracy at this stage to insert the first 60 of an eventual 1,600 bolts to secure the new deck,” explains Gerard Kiely who heads up the operation on the northern tower. “So far we have successfully inserted every bolt painlessly.”
Kiely then checks a labyrinth of ultra fine pre-set measurements between marker points on the existing and newly installed deck units. These check points were marked out many months before in China’s Shanghai fabrication yard where the entire deck was trial assembled five numbered units at a time.
This trial run was overseen by a dozen-strong UK engineering team from FCBC; its designers – Forth Crossing Design JV, a joint venture of Ramboll, Leonhardt Andrä und Partner and Grontmij – plus an independent quality assurance company and client Transport Scotland. The exercise ensured that, on site, crews could concentrate on the Meccano-like fits, knowing every unit’s differing loads, stress and inclination geometry remain secure.
Key engineers from FCBC, and its design JV, monitor every erection operation and, overseen by Transport Scotland engineers, sign it off. Meanwhile Kiely supervises a range of temporary and permanent bolting, extensive flange and web-plate welding, non destructive testing operations, plus pouring a 700mm wide carriageway-level concrete stitch across the two units.
Sheath pipe sections
A further crew welds 12m long cable-sheath pipe sections into average 260m length.
A single seven wire strand is threaded through the sheath pipe and two cables are hoisted up from parallel fixings on the deck carriageway’s future central reserve to the tower-top anchor boxes. The single strand is then stressed to 10t by jacks at both ends.
Up to 108 further strands are threaded through the white piping before the cables are fully stressed to a maximum 800t.
Only then, after this seven day multi-trade cycle, will traveller crane lifting ropes be released from the deck, and the slender crane rolled forward on rails run between the erected twin cables.
Kiely sees his prime acceleration aim as a logistics exercise, coordinating and condensing the work cycles of the 150 strong erection team which carries out half a dozen different trades in the congested deck box area.
Back on dry land, 2km from deck erection, Martin and some two dozen of his senior engineers, are equally consumed studying logistics – though over a somewhat larger arena. Every morning he chairs often several meetings reviewing the project’s daily and weekly, 20-plus key operations. These range from barge and tower crane movements, to deck erection and approach road realignments.
On the office floor above, Martin’s counterpart, Transport Scotland project director David Climie, shares a remarkably similar philosophy. “Given our fierce weather, the contractor is using his initiative impressively and is performing very well,” he says supportively.
Both engineers look back at last month’s dramatic closure of the Forth Road Bridge (FRB) as a worrying, if short, episode. FCBC’s only practical road link for concrete, aggregate or blacktop, was severed instantly. Concrete supply to the southern works from its north bank batching plant, reverted to multiple pumping, plus truck and barge carrying operations taking over three times longer.
Climie does, however, see in the FRB’s closure; “a small silver lining in a pretty black cloud. Any feeling from the public of our bridge being some form of vanity project is now firmly dispelled,” he says.
Forth Road Bridge
Sudden closure to all traffic of the 51 year old Forth Road Bridge on 3 December, following cracking in below-deck steelwork, created chaos across the region’s highways network and concern among bridge engineers nationwide.
At the Queensferry Crossing, just 300m up river, contractor FCBC immediately sent across barges to aid pier access for inspection crews and equipment. The project’s resident structural monitoring team from consultant Arup and subcontractor Strainstall also offered assistance.
forth bridge inspection landscape
Other site engineers, many initially struggling just to get to work, brainstormed contingency plans for materials and concrete supplies following loss of the only practical road link across the estuary.
Cause of the closure was the discovery, during routine strengthening work, of two cracks in vertical steel links connecting deck trusses to the northern bridge tower. Temporary repairs, involving steel splints welded across the cracks, allowed the bridge to reopen on 23 December – 12 days earlier than predicted – but only to light traffic up to 7.5t.
This offered little respite to FCBC as its materials delivery lorries could still not cross.
More permanent strengthening is expected to be complete late this month – weather permitting.
High winds have characteristically proved the only snag so far for the incremental launch of a 222m length of the northern approach viaduct, out from its bankside assembly area towards the nearest tower’s balanced-cantilever operation.
Planned for late this month, the 5631t launch – the most complex yet attempted in the UK – was delayed from last December by weather-induced crane downtime during erection of a vital 42m high steel-truss kingpost. Sited roughly half way along the single and twin box run of welded deck sections, and supported by cable-stays fixed to both ends, the kingpost’s role is to prevent the deck’s leading edge deflecting up to 200mm downward during the 10-day launch.
Shallow water, a steep cliff bank and marine environment concerns, ruled out conventional barge-mounted erection of this northernmost length of bridge deck. Instead twin 600t jacks, positioned at the back of the deck, with some 20km of bundled strand run forward to anchor points in a bankside abutment, will pull out the boxes, sliding them across 30 flat-plate bearings and over the bridge’s first two river piers.
This is challenge enough, without the launch’s unprecedented length and weight demanding insertion of a simple, yet novel, ‘rotation’ technique to ensure sufficient clearance over the furthest pier.
Towards the end of the launch, as the deck’s rear end passes over the bankside abutment, its support transfers to skid shoes running over two parallel concrete walls. The tops of these short walls slope 8° downward.
As the launch continues, the deck’s rear end is forced downward. Using the nearest river pier as a fulcrum, the deck will seesaw – with its leading edge, 222m away out over the estuary, rising an estimated 2m. This should exactly match the level of the balanced cantilever deck section approaching from the nearby north tower.
The launched viaduct will then pass, with 20mm clearance, over its second pier and stop exactly 16m past it.
“We need just three engineers to computer-control the entire operation,” says head of launch Jared Carlson. “We aim to halve this final clearance tolerance.”
The completed launch will sit exposed until this summer when the approaching northern tower’s landward balanced cantilever arrives to create the bridge deck’s sixth and final closure.
The complex 6km network of realigned and upgraded roads, both sides of the crossing, is over 60% complete.
The vast £11M Ferrytoll Viaduct, Scotland’s second largest current bridge project, stands structurally finished less than 1km north of its chart topper. South of the estuary, rare weekend closures of the A90 have allowed removal of a small, but once strategic, concrete B-road overbridge, demolished with important utilities remaining embedded in its embankment.
Priority now switches to realigning the main reclassified M90, routed to run over the new crossing. And here FCBC’s head of network connections Ross Glendinning, was last month dealt a surprise trump card when the FRB suddenly closed for 19 days.
As part of contractual efficiency incentives, the contractor is charged for any traffic disruption on any section of the remodelled network. And the extensive list of lane occupation charges (LOCs), always prominent on Glendinning’s desk, remains a constant headache.
These daily LOCs range from a few thousand pounds for speed limit reductions to a hefty £25, 000 for a main-road lane closure. The biggest challenge, though, is the realigned motorway itself, where approval for even minor disruption is tortuous and even more costly.
But FRB closure offered the contractor long stretches of totally empty M90, which Gendinning soon filled with flashing-light construction vehicles.
“Temporary freedom from LOCs has been a very welcome bonus,” he says.
Leading the client and contractor site teams are two Scottish engineers each offering a lifetime’s experience building bridges worldwide. Between them they have worked on a dozen complex crossings, including a massive Chinese suspension bridge and Scotland’s Kessock cable-stayed bridge, then Europe’s longest of its type.
It is the first time 54 year old Transport Scotland project director David Climie has worked on the client side. “There are times when I am itching to be out there helping to build our crossing, but thankfully I am based on site in the same offices as FCBC and we work as a partnership,” he says.
queensferry.profiles box.david climie
Behind him are Hong Kong’s Tsing Ma, Denmark’s Storebaelt plus Kessock and five other bridges. He even helped strengthen the FRB’s towers 28 years ago.
But most pride so far is reserved for China’s Jiangyin, then the world’s fourth longest suspension bridge. Its opening in 1999 was ‘unmovable’ to coincide with the 50th anniversary of the founding of China’s Peoples Republic.
“Chinese President Jiang Zemin, who was born in the region, told me I would be famous if we completed on time – but very famous if we didn’t,” Climie recalls. “I took that to mean punishment at the least.”
Heading up UK contractor Cleveland Bridge, he achieved completion four days early.
On the floor below Climie’s office, is the engineer he sometimes envies. FCBC project director Michael Martin only took on his current role a year ago: Though, as a past longtime director of Morrison Construction, he had represented the Scottish contractor on FCBC’s supervisory board for the previous two years.
The 63 year old cancelled his second retirement to take up the challenge at Queensferry.
queensferry.profile box.michael martin
“This is the ideal job for me as I get to still practice real civil engineering every day,” he says.
Martin has spent much of his time in front line engineering, with consultant Arup and then heading-up Morrison’s infrastructure business. “I’ve done the politics and finance stuff on previous jobs – but this is what I really enjoy.”
Paralleling Climie’s experience, Martin narrowly missed him on the Kessock crossing. Later he was chief engineer on both Dornoch and Kylesku bridges.
Time running both Morrison’s private finance initiative division, and Scottish Water Solutions in their delivery of Scottish Water’s capital programme, further broadened his experience – particularly in heading up major joint ventures.
Determined to retire again after Queensferry, he views it as iconic and a fitting professional epitaph.
“Every bridge of this size is unique and will always present you with challenges and problems to solve daily,” he concludes. “But I know I have the team here to deal with whatever is thrown at us.”