monitoring of suspension bridge main cables: the forth road bridge presented by alastair andrew...
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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