The 1,550t, 100m tall steel and concrete main pylon which will support the deck on the New Wear Crossing will be erected in one 24 hour lift today.
The pylon is the main tower for the new £117.6M cable stayed River Wear crossing in Sunderland.
Each leg of the A-shaped structure is typically 3m by 5m in cross section and is a steel hollow tube with a steel composite structure at the bottom of each leg made up of 20m in height of reinforced concrete. When in place, the pylon will sit on 8m high reinforced concrete foundation ‘tusks’, which have been built in cofferdams in the river.
The pylon was fabricated in Belgium. At the site, its feet will be attached via hinges to the tusks and then raised using a mix of tried and tested lifting methods and technologies, said Farrans Victor Buyck Joint Venture (FVB) project director Stephen McCaffrey.
“Quite a lot of planning has gone into the whole operation,” McCaffrey told New Civil Engineer. “It will be the first time anything of this size has been raised in this way since the London Eye was lifted in 1999.”
The giant pylon structure is on site and connected to the tusks ready to be lifted by four hydraulic strand jacks anchored 30m into the ground on the south side of the river. The strands are connected to a spreader beam which is in turn connected to the top of the pylon. A backmast – a 50m high crane boom taken from a Sarens’ SGC-120 super crane, affectionately named “Big Benny”– has been temporarily fixed to the pylon for the raising operation, with one leg on each of the pylon legs.
Hydraulic strand jacks attached to the frame from the south end of the bridge will pull it up until it reaches an angle of 70º, their operation counterbalanced and steadied by other strand jacks at the southern end of the bridge. Strand jack cables will then be disconnected from the frame and before the pylon is pulled into its final, vertical position.
As the pylon approaches its final position, the backmast will be retracted to stand vertically alongside it and the north side cables will be tensioned to ensure the pylon is supported from both sides until it is fully secured.
Once the pylon is raised, the backmast will gradually be removed and lowered to the ground and work will begin to secure the pylon permanently to the tusks. It will take three weeks of intensive work to ensure the pylon is fixed in place.
A series of 20, 6m long, 65mm diameter bolts will then be used to temporarily connect each leg of the pylon to the tusks.
“There is a holding down bolt arrangement in place,” said McCaffrey. “The bolts are currently recessed so we will have to pull the bolts up and put them into position before we can bolt the pylon down.”
The weather has also played a large part in the timing of the lifting of the pylon.
“The erection of the pylon will take place over 24 hours to 30 hours. The weather and the wind will play a factor in it has to be the right conditions before we start. We have to have a window of more than 30 hours because we have the process of raising the pylon and then fixing the pylon into the foundations.”
After the bolts are fitted, tensioned and the gaps grouted, the base of each leg of the pylon will be filled with reinforced concrete to create the permanent connection. The rigging and cables used for the raising will then be released.
The pylon will then become free-standing until the other half of the bridge deck is launched into place in the spring and the cable stays connecting the bridge deck to the pylon are positioned and secured over the summer.
The erection of the frame is due to start at 8am and is expected to last until Saturday evening.
“The big challenges are lifting, transporting and erecting the pylon,” said McCaffrey. “One of the advantages for methodology is that the pylon has been able to be manufactured off site in a fabrication factory in Belgium.
“That has removed the need for working at height and the construction of the pylon in exposed conditions. There is a significant quality aspect when it comes the challenge of transporting such a large component and getting it to site. We’re very happy that that has gone so well and smoothly.”