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Monitoring of Suspension Bridge Main Cables: The Forth Road Bridge

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

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Monitoring of Suspension Bridge Main Cables: The Forth Road Bridge

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  1. 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

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

  3. 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

  4. The Main Cable

  5. 11618 wires in 37 strands (4.98mm dia.) • Wire specification 100 – 110 Tons/sq.in (1544 – 1698 N/mm2)

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

  7. Access

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

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

  10. W E

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

  12. W E 1 2 3 4 8 broken wires Panel 1 (100–100E)

  13. Inspection findings – summary

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

  15. Factor of Safety 2 2014

  16. 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%

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

  18. Cable Dehumidification

  19. Cable Dehumidification

  20. Side view of whole platform

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

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

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