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Engineers complete Queensferry deck

Queensferry Crossing last segment lifted into place

The last of the 110 deck segments on the Queensferry Crossing is being lifted into place today marking a milestone in the run up to completion of the bridge in May 2017.

“It’s the last of 110 segments to be lifted into place,” Forth Crossing Bridge Constructors (FCBC) construction director Alan Platt told New Civil Engineer. “In total we’ve lifted 77,000t of deck segments.

“All being well the storm forecast for the weekend will move south instead of north allowing us to go ahead with the lift,” said Platt.

The £1.35bn crossing has been under construction since summer 2011 and has been hit by a number of weather related delays. The process of lifting the individual deck sections starting in September 2015.

The section itself is made up of a 450t steel, open topped box girder 40m wide and 12m long, and once lifted into place a precast concrete deck section will be placed on top to form the road deck.

It is being lifted by cranes mounted on the end of the cantilevering deck. Two strand jacks will then be lowered down to the section waiting on a barge below. They will lift the section from water level to its final position in an operation taking roughly four hours.

“It’s been a cyclic process from September last year, there are details where you go over piers and do connections between spans, but in principle, lifting the segment from the barge into position is a known operation,” said Platt.

Once the segment is slotted into place, it will be lined and levelled and large temporary Macalloy bars will join the sections together and hold it in a stable position while it is welded and bolted into place.

“It’ll take us about seven days to connect the last segment to the cable stayed bridge,” said Platt. “Then we’ll go through the alignment process to connect the cable stayed bridge to the viaduct and we’ve got about 10 days in the programme to get that joined up.”

Queensferry Crossing last segment lifted into place

Queensferry Crossing last segment lifted into place

The last opening to be filled is on the most southerly section of the cable stayed bridge which will adjoin the bridge to the approach viaduct on the south Queensferry side. To facilitate lifting the final piece into place, the approach viaduct was launched 400mm shy of its final position allowing a gap for the section to be fixed in place.

After the section has been fixed to the bridge side, the viaduct will be moved away from the land over its bearings to meet the deck segment, opening out an already installed expansion joint on its other side.

“The viaduct is on bearing across the piers and set 400mm towards the land,” said Platt. “When the segment is in place, we’ll just push it 400mm out to make it connect. The land side has the expansion joint in which will open up as it slides out. Effectively we’ve squashed the expansion joint together as if it had 400mm of compression.”

Prior to a concrete deck being installed on top of the segment, cables running from the control room will be fed through the section to allow the final connections to essential services to be made.

“One of the issues is that the control room for the crossing is in south Queensferry side,” he said. “All the data from the north side has to come south across the bridge and we can’t pull all of those cables and finish those systems off until we make the connection. Before we put the concrete deck on we’ll pull those cables through to the fibre optics and power supply so we can get on and finish the connections off.”

With the last segment in place, the team will have access right across the bridge making the ‘fitting out’ of the bridge easier.

“At the moment we have access from either end, but when it’s in, we’ll have access right the way across,” said Platt. “The work that is going on at the moment, we’re waterproofing the deck, we’re starting to asphalt the deck this weekend.

“We’re putting the vehicle restraint barrier system up, putting the wind shield in, taking the tower cranes down, removing temporary work under the deck, doing the scour protection, removing the cofferdams and then we’re doing the drainage. We’re putting in the mechanical and electrical systems, the fire protection, CCTV, the power system in general and the structural health monitoring system.

“That’s all going on at the same time.”

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