Temporary restraint cables Since June 2004, the arch has been supported by five giant restraint cables anchored into the ground to hold it at an angle of 112-.
Final depropping involves transferring the arch's weight from the restraint cables to forestays and backstays connected to the north roof.
But these cables could only be erected when the entire roof was completed.
Releasing the stress in the temporary restraint cables via hydraulic jacks allowed the arch to rotate and the backstays and forestay cables to be connected and stressed. This process was completed in December, allowing engineers to take down the last of the five restraint cables before dismantling the roof support towers. Further roof struts and panels were being erected between January and February.
(A) Backstay and the main cablenet, connecting the arch with the prismatic perimeter truss (PPT) are slack and ready to be stressed.
(B) Rotating the arch from 112- to 109.5(corresponding to a movement of 4m at the top of the arch) allows CT17, the main cable in the cable net, to be connected to the ends of the northern half of the PPT and forestay cables to be connected back to the arch.
(C) Rotating the arch back from 109.5- to 111.1(corresponding to a 3m movement at the top of the arch) stresses the cables as it stretches. At this point the east and west ends of the PPT contract up to 60mm.
The stress in the restraint cables reduces as CT17 transfers the arch load to the PPT.
Back stay and forestay cables are connected up.
(D) Rotating the arch from 111.1- to 111.7- means that CT17 and forestays can take the entire arch load and the restraint cables are now redundant.
(E) Stressing up the back stay cables rotates the arch back to 112-.