2. Monitoring of Suspension Bridge Main Cables: The Forth Road Bridge Presented by Alastair Andrew B.Sc, C.Eng, MICE. General Manager & Bridgemaster Cambridge 15th May 2007

6. Theoretical Capacity Forth Road Bridge Average Daily Traffic Totals 2006 (Northbound) Vehicles

7. Structural Challenges Design based on 22 ton lorries The 38 tonne lorry introduced from 1983 The 44 tonne lorry introduced from 2001 Traffic increase from 4 to 24 million Bridge presently carrying 5620 tonnes Design load 2880 tonnes

9. The Main Cable

10. 11618 wires in 37 strands (4.98mm dia.) Wire specification 100 – 110 Tons/sq.in (1544 – 1698 N/mm 2 )

12. April 2002 Draft Guidelines discussed at NCHRP workshop Oct 2002 FETA appointed consultants April 2003 Investigation begins on site Nov 2004 Guidelines published

13. Access

14. NE Tower Top Side Span (SS) Cable Sleeve

16. Internal inspection method Wedging for inspection Removal of wire samples Turnbuckle ferrule

17. WEWE

19. Wire Corrosion Stages Stage 1 Stage 4 Stage 3Stage 2

20. Panel 1 (100–100E) 8 broken wires 1 423 WEWE

21. PanelStage 1Stage 2Stage 3Stage 4Broken 1: 100-100E0%8%55%37%8 2: 00N-02NE9%86%4%1%0 3: 00S-02SW11%73%11%5%31 4: 100-100W0%36%38%26%17 5: 00N-02NW11%75%6%8%8 6: 98-100NE0%16%59%25%8 7: 22-24NW0%43%44%13%5 8: 18-20SE19%60%19%2%0 9: 74-76SW7%58%25%10%2 10: 58-60NE22%43%26%9%7 Inspection findings – summary

22. Original cast diameter of 1524mm (5 feet) still apparent Testing – Sample wires

25. Factor of Safety 2 2014

26. Conclusions An excellent 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 Current strength loss estimated to be 8% to 10%

27. Way Ahead Fitted acoustic monitoring 2006 Fit de-humidification 2007/9 Inspect and re-evaluate 2009 and 2013 Cable augmentation/replacement feasibility study.

31. Cable Dehumidification

33. Side view of whole platform

34. Close up of travelling rail (turquoise) and main hanging strap that runs along this rail.

35. Acoustic Monitoring Commission West Cable Nov’ 08 East Cable Sept’ 09 Re-examine cables FORTH ROAD BRIDGE MAIN CABLE CRITICAL PROGRAMME Wrap cables and fit equipment Plan and construct new crossing If Dehumidification Unsuccessful Heavy Vehicles Barred 2006200720082009 2010201120122013201420152016201720182019 De-humidification Replacement/Augmentation Study Dec 2006