6 . CABLES
The stay cables used on the Queensferry Crossing consist of a parallel strand system . This system is more expensive than the alternative – a parallel wire system , but was specified for ease of future maintenance . Parallel strands can be replaced individually as a maintenance operation , wheras parallel wires would require the replacement of an entire cable . Stay cables range from 109 strands down to 45 strands depending on their location within the structure and the stay lengths vary from 97 meters to 422 meters in length .
The strands were supplied in reel-less coils thus reducing the waste associated with having to scrap the typical wooden reels . The initial pilot strand and HDPE pipe was pulled up from the deck level with the tower crane and secured to the anchorage inside the tower anchor boxes . The cables were stressed at the live anchorage which was always located within the towers . Strands were pulled up to the tower anchorage one at a time with a typical pulley and shuttle system .
Due to the tight geometry within the tower anchor boxes it was only possible to install one set of stay cables on one face of the tower at a time . One set of stay cables was typically installed over a 3 day duration meaning that the opposing balanced segments were always offset from the segment erection by at least the same time frame .
Some of the stays were installed with temporary shims creating higher erection stage tensions in order to deal with the complex closure geometry imposed by the crossing stay feature on the project . The temporary shims were later removed with a large multi-strand jack after the span closures were completed .
After final tuning of the stays was completed in conjunction with calibrating the structural health monitoring system and the bridge ’ s global geometry , friction dampers were supplied to be engaged at a work point approximately 1.5m above the deck level .
In addition to the stay cable installation the permanent tie down cable and anchorages at the first piers were also supplied and installed on the back side spans of the flanking towers .
7 . MAINTENANCE
The design and maintenance philosophy are in accordance with Eurocodes , which establishes principles and requirements for safety , serviceability and durability .
Maintenance activities are performed during the working life of the bridge , limited allowance is made in design for corrosion of some components and wearing of moving parts such as bearings and movement joints , over time . These components are designed to be replaceable .
The future access to the deck is provided via underslung gantries from abutment to abutment . Internally there will be a shuttle running the entire length of the structure , 2633m , to transport personnel and small hand tools . Demountable cradles will provide external access to the towers and cables .
Figure 27 : Cables
1 / 2017