The Maintenance of Long Span Bridges Barry Colford Chief Engineer & Bridgemaster Forth Road Bridge

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1 The Maintenance of Long Span Bridges Barry Colford Chief Engineer & Bridgemaster Forth Road Bridge Heriot-Watt Industry Day Sustainable Development Infrastructure Solutions and Challenges 26 February 2013

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3 Akashi Kaikyo, Japan

4 The relationship between design, construction and maintenance Good design must take account not of only safe and cost effective construction but also safe and cost effective future maintenance. If we fail to do this then we face the prospect of retrofitting expensive monitoring and perhaps replacement of the structure or structural elements.

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8 Inspections 8

9 The Main Cable

10 Cable construction Cable comprises wires in 37 strands (4.98mm dia.) Cables constructed using aerial spinning techniques Cable diameter 600mm Wire specification Tons/sq.in ( N/mm 2 )

11 Cable construction Traditional corrosion protection: galvanising on each wire, compaction, red lead paste, wire wrapping and painting. Cable has been regularly inspected externally and repainted

12 Forth Road Bridge Cable Investigation Load changes during life of bridge Dead Load Main Span 152 kn/m 158 kn/m Side Span 204 kn/m 211 kn/m Live Load Carriageway 15.5 kn/m 27.5 kn/m Footway 4.5 kn/m 0.3 kn/m

13 Background to inspection FETA have been involved in early ICSBOC conferences and workshops & had awareness of emerging problems with some US suspension bridges FETA aware of NCHRP Guidelines and recommendation to carry out first inspection at 30 years Forth Road Bridge was approaching 40 years and although there were no outward signs of internal corrosion, the decision was taken to undertake an internal inspection

14 Internal Inspections First Inspection 2004/5-40 years since opening Second Inspection 2007/8 - limited benchmarking for dehumidification Third Inspection 2012 post dehumidification limited by funding

15 Forth Road Bridge Cable Investigation Site work access platform High Level Low Level

16 Forth Road Bridge Cable Investigation Site work access platform

17 Costs First Main Cable Internal Inspection 2004 Total cost = million for 10 No panels. Cost/panel approx. 237K Third Main Cable Internal Inspection 2012 Estimated cost = million for 8 No panels. Cost/panel approx. 475K

18 Wedging for inspection

19 Inspection Findings

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21 Wire Corrosion Stages Stage 1 Stage 2 Stage 3 Stage 4

22 Forth Road Bridge Cable Investigation Broken wires

23 Panel 1 ( E) broken wires

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25 Forth Road Bridge Cable Investigation Internal inspection method Compaction Rewrapping

26 Conclusions A good external inspection and maintenance regime was not sufficient to prevent deterioration A good external condition doesn t mean it is good on the inside Widely varying levels of corrosion in different areas were difficult to explain Need to rely on new research and development such as dehumidification to prolong cable life and future inspection and acoustic monitoring.

27 Acoustic monitoring Internal inspection only looked at a small proportion of wires. No guarantee that the worst deterioration has been found. Rate of deterioration is uncertain. Acoustic monitoring can provide information on the whole cable, and over a long time period.

28 Acoustic monitoring installation

29 Oil the cable? Extensively used in US as means to prolong life of cable Appears to have a number of drawbacks: Expensive as it requires full length cable opening Leakage at low points Will slowly dry out making reoiling a challenge Lack of evidence of long term success

30 Main Cable Dehumidification 30

31 Main Cable Dehumidification

32 System Layout Injection sleeve Exhaust Sleeve Dehumidification Plant Room / Plenum chamber Objectives of Cable dehumidification System Introduce dry air into the cable to remove moisture and to maintain a level of condition that will suppress further corrosion Improve air tightness of the main cables and prevent moisture ingress Installation without closing the bridge to traffic, i.e. minimum inconvenience to bridge users Plant designed to provide economy of operation Remote web-based control and monitoring systems to record key data from a safe distance

33 Installed with minimum inconvenience to bridge users An airtight cable is achieved through the application of a proprietary wrapping material which is expected to have a long service life with virtually no maintenance It is necessary to access the entire length of both cables but given the large daily traffic flows, only limited overnight carriageway closures are available A sophisticated platform has been developed that, once installed on the cable, is able to move along the cable from panel to panel. It provides a well contained and safe workplace

34 Web based control and monitoring system It is possible to monitor all the vital information regarding the system s functionality as well as adjust settings remotely from the website or central computer Air condition is monitored at injection, exhaust and intermediate monitoring points Trend / historic logging and monitoring is provided Monitors and reports faults and indicates conditions when maintenance is required, e.g. when air filters need to be changed Allows remote integration and adjustment via WEB based system Weekly analysis reports are provided indicating progress of the drying process Secure user access system requires different levels of authorisation for monitoring and control Alarm notification by and text messaging Sophisticated self learning component control

35 Media hype The main cable dehumidification has attracted considerable media attention, including coverage in the national press, international engineering press and national television From The Sunday Times January 29, 2006 Giant hair dryers to save Forth bridge Jason Allardyce ANTI-CORROSION devices which operate like giant hair dryers could be fitted to the suspension cables of the Forth Road Bridge to extend its life. Ministers believe dehumidification equipment could make the crossing safe for several more years and, at 12m, would prove to be a cost-effective option. While they believe that total replacement may be inevitable, after engineers discovered severe corrosion on some of the wires inside the 4km-long suspension cables, they are concerned that this is likely to cost at least 600m. It blows warm air into the cables of bridges, extracting moisture which causes rust and corrosion. The cables would be wrapped in neoprene, the material used in wetsuits, to make the process more effective. Study into Forth bridge dry-out A study into the possibility of installing a drying-out system for suspension cables on the Forth Road Bridge is expected to get the go ahead. It follows a report which warned the bridge could be shut to all traffic in under 14 years unless action was taken. Consultants are to look into fitting 12m de-humidification equipment, which would pump dry air onto the wet cables. The Forth Estuary Transport Association (Feta) is set to appoint engineers Faber Maunsell later this month. Excellent track record "The team Faber Maunsell has put together to carry out this study brings together the very best in experience across the world in dealing with similar situations. "They have an excellent track record in understanding the complex issues involved in tackling corrosion in suspension bridges and, most importantly, providing the right solutions."

36 Third internal inspection 2012/13 Appears to demonstrate that the rate of deterioration of cable strength has been reduced and the factor of safety against failure of the cables has not materially diminished. This is giving strong comfort that the newly installed dehumidification system is retarding the corrosion of the bridge wires. Existing cracks may eventually lead to the wire fractures but the rate of breakages is expected to slow significantly. This should result in a further slowing down of deterioration of the cables and lead to a reduction in the loss of magnitude in the factor of safety. It will be important for the future resilience of the cables that the dehumidification system is maintained and the existing acoustic monitoring system is augmented. Future inspection and monitoring is essential

37 Cable FOS Cable Factor of Safety Year

38 Cable replacement/augmentation study Need to plan in case cable dehumidification is not effective Major technical issues to be resolved, including: Main towers and saddles already strengthened Concerns over existing anchorages New anchorages required Stiffening truss is theoretically overstressed Effects on cross Forth traffic flows

39 Main Cable Anchorages

40 Main Cable Anchorages

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42 Main Cable Anchorages

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46 Main Cable Anchorages Stage 2 Excavation Superficial deposits Area to be excavated MAIN CABLE ANCHORAGES

47 Main Cable Anchorages Stage 2 Exposure of Ducts Exposed ducts/strands MAIN CABLE ANCHORAGES

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50 Main Cable Anchorages Stage 3 Exposure of tendons Visual inspection and X-ray Removal of ducts Removal of grout Visual inspection MsS survey MAIN CABLE ANCHORAGES

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