Removing the old decks is an interesting exercise in reverse engineering, says Benaim managing director Mark Raiss.
'As you cut and remove parts of the structure you destroy or alter the structural system. You have to anticipate the effects of each phase, ' he says.
Starting at the edges of the deck, longitudinal beams will be severed one at a time at their mid-point and then 'munched' back to the piers.
'Cutting the beams will turn them from spanning structures into free cantilevers, ' says Raiss. 'Prestressing was put into the decks to help hold them up. Once the cuts are made, forces in the deck will become exactly the same as those applied by the prestressing.' Spanning between the piers, beams are subject to tension in the bottom and compression in the top ange. The 20 prestressing cables per deck were installed to counteract these forces. As cantilevers, the forces will reverse.
Raiss observes: 'We are fortunate that the bridge wasn't particularly economical, so the cables aren't very highly loaded and the ducts aren't very eccentric.' There will be no problem maintaining the structural integrity of the deck, he reassures. A debris mat will be oated across the river with side nets to catch small pieces of falling concrete and polystyrene void former.