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

Theoretical Capacity

Forth Road Bridge

Average Daily Traffic Totals 2006 (Northbound)

Ve

hic

les

24,000

25,000

26,000

27,000

28,000

29,000

30,000

31,000

32,000

33,000

34,000

35,000

36,000

37,000

38,000

Monday Tuesday Wednesday Thursday Frisday Saturday Sunday

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

The Main Cable

• 11618 wires in 37 strands (4.98mm dia.)

• Wire specification 100 – 110 Tons/sq.in (1544 – 1698 N/mm2)

April 2002 Draft Guidelines discussed at NCHRP workshop

Oct 2002 FETA appointed consultants

April 2003 Investigation begins on site

Nov 2004 Guidelines published

Access

NE Tower Top Side Span (SS) Cable Sleeve

Internal inspection method

Wedging for inspection

Removal of wire samples

Turnbuckle ferrule

W

E

Wire Corrosion Stages

Stage 1 Stage 4Stage 3Stage 2

Panel 1 (100–100E)

8 broken wires1 42 3

W

E

Panel Stage 1 Stage 2 Stage 3 Stage 4 Broken

1: 100-100E 0% 8% 55% 37% 8

2: 00N-02NE 9% 86% 4% 1% 0

3: 00S-02SW 11% 73% 11% 5% 31

4: 100-100W 0% 36% 38% 26% 17

5: 00N-02NW 11% 75% 6% 8% 8

6: 98-100NE 0% 16% 59% 25% 8

7: 22-24NW 0% 43% 44% 13% 5

8: 18-20SE 19% 60% 19% 2% 0

9: 74-76SW 7% 58% 25% 10% 2

10: 58-60NE 22% 43% 26% 9% 7

Inspection findings – summary

Original cast diameter of 1524mm (5 feet) still apparent

Testing – Sample wires

CABLE STRENGTH VS BRIDGE AGE

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

1960 1970 1980 1990 2000 2010 2020 2030

AGE OF BRIDGE, YEARS

CA

BL

E S

TR

EN

GT

H,

kN Factor of Safety 2

2014

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%

Way Ahead

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

study.

Cable Dehumidification

Cable Dehumidification

Side view of whole platform

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

Acoustic Monitoring

Commission West Cable Nov’ 08East Cable Sept’ 09

Re-examine cables

FORTH ROAD BRIDGE MAIN CABLECRITICAL PROGRAMME

Wrap cables and fit equipment

Plan and construct new crossing

If Dehumidification Unsuccessful Heavy Vehicles Barred

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

De-humidification

Replacement/Augmentation Study

Dec 2006