Observant commuters on Scotland’s Forth Road Bridge may this month notice a third deck closure has just taken place on the main balanced cantilever spans of the new Queensferry Crossing just up river.
And, as low winter sunshine highlights more than 200 glistening white cable supports for this elegant cable-stayed bridge, the public could be forgiven for thinking the massive structure is approaching completion.
The last of 110 composite steel and concrete deck segments will soon be lifted from barges in the estuary, and the first of the site’s three 235m high tower cranes is being dismantled. Technically, construction work – involving some of the most innovative cutting-edge bridge design and erection yet achieved anywhere – is proving virtually flawless.
But there remains an uncontrollable gremlin in the works. The seemingly never ending, extreme, unpredictable mid-estuary weather conditions are continuing to affect progress. Ongoing high winds, rain and now low temperatures are all aggravating weather-sensitive critical path construction operations.
Near double the expected weather-induced downtime throughout the last two years means the long held aspiration of contractor Forth Crossing Bridge Constructors (FCBC) completing the crossing early this December will not now be achieved. Bridge opening is currently estimated at mid-May next year; a target still, however, a month ahead of the contracted completion date.
“We always expected our exposed site to be vulnerable to bad weather leading to wind induced delays,” explains FCBC project director Michael Martin, a joint venture of Hochtief, Dragados, American Bridge and Morrison Construction. “We researched this carefully, but have experienced more delays than we predicted.”
“The last two summers have been particularly difficult, with severe gales even this August – a first in my 40 years of working in Scotland.” he says. “There are though, no technical reasons for the programme change, with the slippage due entirely to the weather.”
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Martin’s site-based opposite number at client Transport Scotland (TS) backs totally the contractor’s statement.
“I continue to have every confidence in FCBC’s high quality engineering expertise,” says TS project director David Climie. “We still have several technically complex, wind-sensitive areas to complete but none of us can control this ferocious weather.”
The bridge forms the major part of the £790M Forth Replacement Crossing contract which includes extensive motorway-standard approach roads on both sides.
As its 2.7km deck nears structural completion, Martin and Climie point to the extensive follow-on critical path finishes about to start – waterproofing, surfacing and painting – all sequential and all now with winter programmes at the mercy of not just wind but rain and low temperatures.
Eventually 3m high louvred wind-shielding will be placed the full length of the bridge deck. And, as Martin points out, its function and sheer scale point to possible difficulties erecting it during winter months.
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“We had hoped to do most of this work during summer, but now face additional winter working challenges,” he says. “We are continuously searching for smarter ways to save time and buy ourselves a bit of float.”
Virtually all the 750t deck segments, fabricated in China and topped with surface concrete in casting bays at nearby Rosyth docks, have now been towed 3km down river to the bridge site. Anchored in the fast flowing estuary to within the required 200mm positioning tolerance, all have been successfully lifted into the deck by cantilever-end traveller cranes.
Despite restrictions on working when winds speeds are more than 14m/s during the four hour lifts, experience gained over the 14 month programme has resulted in the planned segment erection schedule often being bettered. This has allowed FCBC to recover some of the time lost when weather conditions prevented operations from taking place.
Lifting round the clock at all states of the 6m tide, developing the ability to lock-off a suspended box at any stage, plus always having loaded barges on standby, regularly increased weekly totals to four and once five.
The follow-on programme of welding, bolting and pouring a deck-level concrete stitch – all to secure the new section to its cantilever end – has also benefited from site-gained experience.
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A redesigned access gantry – suspended from deck soffits, environment controlled mobile tenting over key welded areas that needed primer paint, plus an ongoing logistics exercise organising the 150-strong multi-trade workforce per tower all seeking the same space at the same time, has shaved at least a day off this critical path cycle.
“Final segment positioning on live, constantly moving deck cantilevers has benefited from both lots of preplanning and having key design engineers based on site to advise us,” says FCBC north tower section head Gerard Kiely. “So far all our connecting bolts, eventually to total 150,000, have fitted with only occasional adjustments.”
Kiely attributes much of the positioning success to extensive trial erection of the entire deck years ago at the Shanghai fabrication yard. Supervised by a dozen-strong engineering team from contractor, client and designer Forth Crossing Design Joint Venture – a grouping of Ramboll, Sweco and Leonhardt, Andra und Partner – up to 11 deck sections at a time were fully preassembled. Scores of locating marks, with their required measurements, were established for later use by site engineers.
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This has considerably eased the erection team’s time-consuming task of achieving correct loading, stress and orientation of deck sections – every one slightly different. “It is noticeable that in the few special areas not trial assembled we have had to do a bit more tweaking to get it right,” Kiely recalls.
Not such smooth sailing is conceded for the linked massive critical path operation of cable stay erection and stressing.
This work is subject to some of the most stringent limits for working in the wind of any operation – just 11m/s while cable strands are secured at tower tops. The roughly 250 cables so far positioned have notched up the site’s most weather-induced downtime. And a dozen of the maximum 420m long cables have still to be threaded into the highest tower-top anchor blocks.
Each deck section is supported on two centrally located cables within an outer up to 315mm diameter polyethylene sheath hoisted in one length by tower crane up to the anchor blocks.
The first of the seven-wire cable strand is pre-positioned, raised with the sheathing and stressed to 10t by jacking crews inside the tower tops. A maximum of 109 strands per cable follow, individually pulled up inside the white tubing.
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The tower-top cable positioning and strand threading demands up to six wokers in three small open baskets hoisting themselves on wire supports up the outside face of each over 200m tall tower. It is this totally exposed operation that, not surprisingly, demands such rigorous working constraints.
Despite near continuous Meteorological Office weather reports, plus some 60 wireless and hand held anemometers spread along the site, the basket crews have to individually take wind speed readings as they attempt to climb the towers. New Civil Engineer watched while one basket operation repeatedly stopped and started, leaving cable crews able only to watch, as localised wind gusts buffeting around the towers ruled over the day’s progress.
“Last week we could do no main cable work at all,” Kiely recalls. “This can push our critical path downtime to over double the expected 25%.”
Equally severe are the wind speeds controlling key operations during the four operations to join up main deck sections. With the third completed successfullly last month in the northern main span, each extremely complex two week multi-phase connection demands repeated cable load adjustment and massive inter box temporary stressing to achieve the required vertical and horizontal positioning accuracy.
Raising, to just 50mm accuracy, the 6.1m long deck closure section – a third the length of a normal box – into the narrow gap between northern and central tower balanced cantilevers, demanded that the the deck, which is cantilevered from the northern tower, is temporarily “pushed” 300mm landward by a 400t jacking operation at deck level.
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Normally, immediate connection between this shorter deck section and the northern cantilever would follow. But for this closure the standard procedure was replaced by a special locking arrangement.
To allow final alignment adjustment, a more flexible connection is used involving 38 Macalloy bars threaded horizontally between the two deck sections. With segment positioning finalised, the 75mm diameter bars are temporarily loaded to 9,000t and, after follow-on welding between deck sections, are then immediately de-stressed.
Vertical differences between the two cantilevers can initially reach 4m before final balancing. Several days of repeated stress tweaking to already erected cable stays in up to seven deck sections around the closure area is needed to ensure the spans can mate exactly.
This will soon be one of most iconic bridge vistas anywhere, with three such innovative river crossings elegantly illustrating engineering developments over the last three centuries.
Design JV project director Peter Curran
To stabilise this final levelling, a large temporary works steel frame, containing small adjustment jacks, is laid across the remaining gap between the cantilevers. Movement of the frame must at this stage be zero, so work must be suspended if wind speeds exceed 6m/s.
Releasing the jacks holding back the northern tower deck reduces the final closure gap to just 5mm.The join is completed by bolting, welding and pouring the normal deck level concrete stitch.
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The 10 day closure operation involves cutting-edge bridge technology. With the central tower’s 644m balanced cantilever claiming a world record length, each closure’s several hundred measurements are overseen by complex computer software monitoring real time loadings, stresses, orientation and global positioning surveys.
The fourth and final main closure – linking the far southern end of the bridge’s four spans to its approach viaduct – is now planned for early next year. This would originally have triggered a full scale attack on finishing operations across the whole bridge; waterproofing, painting, blacktop, weather-shield erection and threading a multitude of services through deck sections.
But FCBC construction director Alan Platt, conscious of these now wintertime operations mostly being sequential, is keen to get started early.
“All are acutely weather sensitive, not just to wind but now rain and low temperatures,” he explains. “We intend to remove tower cranes early to free up deck space, open up multiple work fronts and put tents over sensitive operations such as waterproofing.”
The Crossing’s crossover
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Centre-stage of the bridge’s aesthetic and structural elegance will be the highly visible crossover stay cables covering a 160m length of deck, mid point along the two main river spans.
This innovative solution to the structural challenge of stabilising the central mid-estuary tower, will add considerably to the slender cable-stayed crossing’s visual impact.
The northern and southern towers are stabilised primarily by back-span deck cables anchored directly over strategic V-shaped piers in the two sides’ approach viaducts. These concrete piers are further strengthened, and the deck prevented from uplift, by six stressed tie- down cables fed through the hollow pier legs from deck soffits to pier base anchor points.
But no such stabilisation is possible for the central tower, standing unsupported on top of the mid estuary volcanic outcrop known as Beamer Rock.
This the reason for the clever use of the 10 extra overlapping cable stays in each of the two main spans. These double to four the cable supports to each of the 10 mid span deck sections both sides of the central tower.
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By stiffening these segments with the extra cables, the effective structural depth of the deck mid span is increased. This stiffens the entire deck sufficient for it to stabilise the central tower.
“We believe no other bridge has adopted this novel solution,” claims Martin Romberg senior design engineer for the contractor’s Forth Crossing Design Joint Venture. “And it allows all three towers to have the same slender profile.”
Without crossover cables, the deck itself would have had to be much deeper with the central tower a different shape, possibly an A-frame.
Carefully timed installation of these extra cables, both before and after deck closures, is now underway. They contain about half the normal number of strands, but demand a complex sequence of stress readjustment in numerous nearby main cables.
They also require the very longest 420m cable lengths hoisted up to the relevant tower’s highest anchor boxes.
At 210m, the central tower is the tallest of any UK bridge – with the other two towers 202m high. Not by coincidence they mirror and complement both the three massive steel-trussed cantilever support towers of the Forth Rail Bridge and the slender suspension cable towers of the Forth Road Bridge, both impressive crossings less than 1km down river.
The project boasts an unusually large 12-strong site-based design team. With up to six cantilevers being erected simultaneously, two senior design engineers are dedicated solely to the control of superstructure construction.
“This will soon be one of most iconic bridge vistas anywhere, with three such innovative river crossings elegantly illustrating engineering developments over the last three centuries,” says the design JV’s project director Peter Curran. “It’s our role to ensure the latest crossing achieves the world-leading status of its neighbours.”
Ahead of schedule, despite delays
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Last June the Scottish Government announced that the crossing’s long hoped for December completion date has been put back to mid-May next year.
On site, senior engineers from contractor and client were quick to stress that a December opening was only a target date; that the setback is entirely weather related and the now expected spring opening is still a month ahead of the six year project’s contracted June 2017 completion.
“Back in March we were not exactly where we wanted to be on the programme but we still thought December was achievable,” explains Transport Scotland project director David Climie. “However the horrendous unpredictable weather over the following two months cut the legs from under us and we achieved hardly any critical path operations.”
Some 25 days were lost during this April and May period pushing total weather-related downtime over the last year to 40%, compared to the contractor’s original estimate of 25%.
The then two month net delay has now been severely compounded, by the just-starting extensive programme of finishing works say engineers.
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Pushed to winter months, instead of the originally planned summer timetable, are jobs whicha are arguably more weather sensitive than even earlier critical path operations: deck waterproofing; blacktop across the total 2.7km carriageway; plus painting and wind barrier erection. Rain, low temperatures and early darkness can now be added to work-stopping high winds.
Worse though, these final operations are largely sequential and this has more than doubled the total setback time, says FCBC’s project director Michael Martin.
“I am acutely aware that the pressure is on us to complete as soon as possible,” says Martin. “But winter working is particularly challenging and the safety of our 1,200 workforce will always remain paramount.”
From the start of the £790M design and build contract back in 2011 a large chunk of that pressure to complete by December has been linked to ongoing corrosion and resultant weakening of suspension cables on the nearby Forth Road Bridge.
Discovered a decade ago, it was long thought the deteriorating strand wires would force the banning of all heavy goods vehicles (HGVs) from this vital bridge link as early as next year.
So the pressure was then acute for a December 2016 completion of the new “replacement crossing”. It was this concern that directly prompted the fast-tracked approval of the crossing using, for the first time in Scotland, a parliamentary hybrid bill.
But more recent insertion of sophisticated dehumidification fans into the Forth Road Bridge’s cables has markedly arrested strand deterioration. Climie now estimates the bridge – currently carrying around 70,000 vehicles every day, six times design flow – can safely accept HGVs for another year.
Opening the new bridge in May will totally change the traffic flow scenario on the Forth Road Bridge. Virtually all traffic will immediately transfer to the new “weather-resistant” crossing.
This will leave the Forth Road Bridge as virtually a car free bus lane for public transport vehicles, taxis, cyclists and pedestrians. Gradual follow-on and extensive maintenance on the Forth Road Bridge, under its considerably lighter loading, will offer easier repair access and should, Climie predicts, allow the bridge to live out its full design life of at least another 48 years.
Queensferry Crossing | Key facts
- 288 – total number of cable stays
- 110 – total number of deck segments erected
- 11m/s – maximum wind speed at which cable erection is permitted
- 644m – world-record length of the central balanced cantilever
- £790M – fixed price cost of the main bridge and approach road contract