3.2 Subsequent internal inspections
In 2008 a second internal inspection of the main cables was undertaken with the aim to set a benchmark from which the effectiveness of remedial measures could be measured . Estimation of the strength loss in the main cables was of the order of 10 %. The inspection confirmed the 2004 / 5 inspection findings .
A third inspection of the cable was carried out in 2011 / 2012 to determine :
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Effectiveness of the dehumidification Another point on the loss of strength curve
The third internal inspection of the main cables confirmed the findings from the earlier inspections that the external and internal wires have suffered deterioration in a non-uniform manner in all eight panels inspected .
The report concluded that the cables should continue to be inspected in accordance with the NCHRP guidelines with the next internal inspection programmed for 2017 .
4 . ACCOUSTIC MONITORING
The rate at which wires break can be measured using acoustic monitoring techniques . It also enables to monitor the entire length of the cables .
Fully wired monitoring system was implemented in 2006 . This system utilised 15 No sensors on each main cable .
In 2015 , a replacement acoustic monitoring system was commissioned which utilises 58 No sensors on each main cable to provide more detailed information .
5 . DEHUMIDIFICATION
Dehumidification of the main suspension cables based on recent developments elsewhere was one of the possibilities investigated to prolong the life of the cables .
The system was installed to slow down the rate of corrosion . The process involves pumping dried air into the cable at various points , having first wrapped it in an airtight neoprene .
6 . REPLACEMENT OR AUGMENTATION OF THE MAIN CABLES
The first inspection of the main suspension cables opened the discussion whether and how it was possible to replace the main cables . As a precautionary measure , a study into the feasibility of augmenting or replacing the bridge ´ s main cables was commissioned .
This study examined the most appropriate construction methods taking into account structural options ( installation of new cables and hangers on the existing structure ) including the integrity of the existing anchorages , the impact of the works on traffic and the effects on the surrounding area .
The following options were considered : a ) Replacement above :
A new cable to be constructed above the existing cable with subsequent replacement of the existing main cables .
The following was to be considered : �
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Aerial spinning or pulling prefabricated formed from 91 PPWSs with each strand made up of 127 wires
Pulling the new cable was preferred option as its installation could be more controlled . The final level would be 5.8m above the level of the existing cable .
� Forming new ( it would most likely be adopted ) or reuse the existing anchorages
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Alterations to the footways at the side towers as the new cable will be diverted down to the anchorages over the side towers
The stability of the side towers is maintained and would not require any additional works
The main challenge in supporting the new cable on the main towers is how to transfer the loads into the highly stressed existing structure . Moreover , there is limited access and space .
The preferred option involved supporting the new structural steelwork at the existing saddle by a combination of strengthening and augmentation for the existing steelwork at the head of the existing main towers . It involved substantial alteration to the existing saddle .
1 / 2017