Tonnage fan club
12 October 2015
The three towers of the Queensferry Crossing have nearly completed their methodical - concrete pour by concrete pour - climbs towards the heavens. Only a handful more pours to go and soon each of the three elegantly slender structures will stand, independent of each other, at up to 210m high above the water.
A magnificent sight for sure, a modern wonder of the world, like the tines of some gigantic underwater fork, poking out of the water.
But it’s not a bridge, not yet.
However, installing the deck sections and stay cables is now well and truly underway – a complex process which will last well into 2016. Technically speaking, the bridge deck began at the end of 2014 when our old friend Taklift 6 installed the starter segments and support trestles at each tower. Since then, these segments have been fabricated, reinforced and concrete poured.
The first stay cables have also been fitted and tensioned to lift and support the weight of the starter segments.
But we are now into the phase everyone associated with the project has been anticipating the most. From these first dramatic lifts, each accompanied by the fitting of new stay cables, three “fans” will start to appear with each tower at their centre. With every segment and cable installed, the more spectacular and unusual the vista to the west of the Forth Road Bridge will become.
Carson T. Carney, FCBC Cable Stayed Bridge Technical Manager, is the man leading the operation to install all 110 deck segments.
“By filling the gaps between the three towers and connecting to the viaducts, these segments will, for the first time, actually begin to make a bridge out of the structure we are building.
"At the North Tower, the initial deck segments have recently been lifted off the temporary support trestles and rotated to give them the required geometry to fit the graceful arc which the final, completed road deck will form.
“This rotation was achieved by an innovative lifting technique that used only the stay cables to lift the deck segments to elevation. The first stay cables took the weight of the deck for the first time in late August which really was a significant milestone for the project.”
All subsequent steel segments are being fabricated and having reinforced concrete decks added in FCBC’s on-shore fabrication yard in Rosyth Docks. The deck boxes have been delivered by sea to Rosyth already weighing a healthy 250 tonnes each. The pre-cast fabrication process turns them into a gargantuan 750 tonne segment ready to become part of one of the world’s most impressive modern bridges.
Lifting, positioning and fixing
Each deck segment is transported out from the dockside fabrication yard on one of two huge barges and positioned at the tower by tugs. On arrival, the barge is anchored – to within a 20cm tolerance - beneath the blue traveller gantries at deck level (so that’s what those blue ‘wings’ either side of each tower are for!). These travellers will lift the deck segment up to deck height (approximately 55 metres) in a delicate three hour operation which is very sensitive to wind, sea and tide conditions. Each deck segment is then rotated by a few degrees in order to match the final geometry of the bridge design and a series of interlocking plates are joined together to hold the structures in place.
Once the segment has been correctly positioned, it has to be permanently fixed in place. Carson says: “This is achieved by a huge amount of welding to the perimeter of the steel box structure. This is a lengthy operation but a critical one as a perfect top weld is vital to allow the reinforced concrete “stitch” to be poured on top of the steel structure.“ This stitch, along with the welds, permanently secures the segment to its neighbour. These operations are constantly monitored by non-destructive test inspections to ensure an extremely high quality product is achieved. After the welding is inspected and passed, permanent bolts complete the steel connections.
As the reinforced concrete stitch is being poured, the stay cables have to be prepared. The cables are one of the signature features of the Queensferry Crossing design. They consist of a bundle of strands (a varying number depending on location - up to 109 for the largest cables) which are threaded through an external, white pipe (essentially a covering or sleeve). Each strand is made up of seven high tensile, galvanised steel wires, 5.2 millimetres in diameter. Six of the wires, coated in wax, are wound in a helix pattern round a straight central king wire. The strands are each contained in a high density polyethylene (HDPE) coating. The external white pipe is cut to the correct length and a single strand is threaded through. The pipe is then lifted off the deck and into position at the tower anchor box using the enormous, yellow Tower Crane.
The remaining strands are threaded through the pipe using a winch and shuttle system which brings the strands through the pipe one at a time. Each strand is cut to length and wedged into a steel anchor block at either end. The final result is one of the strongest steel cables in the world, capable of supporting the Queensferry Crossing road deck for many decades to come and, crucially, being straightforward to replace in the future without any disruption. The stay cables – two per deck segment – have been fully assembled and tested but the weight of the deck segment is still primarily being carried by the blue gantries. The stay cables are now tensioned up to final tension at which point the weight of the deck segment is transferred from the blue erection traveller gantries to the stay cables.
7 wires = 1 strand | 109 strands = 1 stay cable | 288 stay cables = 1 Queensferry Crossing
The hydraulically powered erection traveller, each weighing 250 tonnes, can now move forward (around 16.2 metres) on rails to the leading edge of the newly installed deck segment ready to lift the next segment making its way out from the land – and the above cycle is repeated.
On completion of all 110 deck segment installations, the stay cables will be finely adjusted to achieve the “global geometry” required by the design of the new bridge.
Carson adds: “Taking into consideration the massive weights and loads involved, the dramatic heights at which we will be working and the variable elements which Mother Nature will doubtless throw at us in this exposed, maritime location, the operations to install the road deck represent leading edge civil engineering at its most raw and exciting.”
Safe to say, we’ll all be watching.