A different layout was used for each bridge since the cables were different . For the Eastbound Bridge , which has helical strand cables , the voids between the strands are larger than parallel wire cables , which results in lower airflow resistance .
For the Westbound Bridge , which has parallel wire strand cables with thousands of tiny voids between the wires , the airflow resistance is comparatively much higher . Therefore , a greater blowing length was used on the Eastbound Bridge cables in comparison to the Westbound Bridge to obtain efficiency and economy .
The tower tops were set as exhaust locations since these regions are more difficult to make airtight , as well as it would be difficult to install and maintain long vertical air pipe runs up the tower legs . The air from the cables was exhausted into the anchorages to dehumidify the splayed strands and cable anchor systems .
On the Eastbound Bridge , mid-main span was set for the injection point location , but on the Westbound Bridge it was considered more prudent to have shorter blowing lengths and inject at quarter-points given the higher air flow resistance of its parallel wire cables .
Potentially suitable plant room locations were limited on the Eastbound Bridge , since the cable anchorages were at water level . Having only 2-lanes , the Eastbound Bridge was also less accessible from lane closures in comparison to the Westbound Bridge . Therefore , a single plant room at the east end of the bridge under the approach span was considered to be most practical to facilitate future maintenance and reduce capital cost ( Figure 7 ).
Figure 7 : Eastbound plant room on platform ; backstay and approach span on left , suspension span on right
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Plant rooms on the Westbound Bridge were conveniently located inside each of the anchorages at deck level . This had several advantages including no traffic vibration , good maintenance access , shelter from the elements , more stable temperature conditions and lower cost lightweight plant room framing and wall components ( Figure 8 ).
Figure 8 : One of two plant rooms on Westbound Bridge within anchorages
On the Westbound Bridge , protective shroud walls were also erected to create exhaust chambers around the splayed strands and allow the dried-air from the main cables to keep the splayed regions dry . The shroud walls were constructed using a combination of a rigid wall system and a polyester fabric membrane for fitment to the top of the anchorage roof .
The Eastbound Bridge anchorages were smaller so the shroud enclosures were not required ; instead , the dried-air from the cables exhausted into the anchorages to protect the strands and their anchor plates .
3.3 Wrapping and Sealing Works
For main cable dehumidification to be successful , airand water-tight wrapping must be installed , including robust sealing at the cable bands and saddles . The elastomeric wrapping specified for the project was D . S . Brown ’ s Cableguard TM wrap system .
The new cable wrapping was applied spirally over the bridge cable under tension using a Skewmaster TM wrapping device ( Figure 9 ). The wrapping was performed uphill to create an overlapping shingleeffect so as to avoid water ingress from surface water running down the cable .