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The Forth Road Bridge:
Where Are We Today?
Dr Paul Roffey BSc, CEng MIMMM
Forensic Engineering & Materials

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Overview
• Brief history of the Forth Road suspension bridge
• Basic construction
• Inspection – practices, previous, current
• Findings from 2012 inspection;
– Coating integrity
– Tensile strength evaluation
– Fatigue test data
– Fracture mechanisms
• Future

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History & Construction

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Timeline
Forth Rd Rail Bridge
(opened 1890)
Forth Rd Suspension Bridge
(opened 1964)
Forth Rd Cable Stay Bridge
(opening 2016)

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Aerial spinning of wires
Construction
• Constructed in 1964 at a cost of £19.5
million
• 1006 metre span between towers
• Main cables have 11,618 high tensile
strength wires
• The main cables are approximately 60
cm thick, the total length of wire would
reach around the world 1.25 times!
• Wires were aerially spun across the
towers to build up the cables,
compacted and circumferentially
wrapped with 9 gauge wire followed by
lead paste and wrapping. Finally the
cables were painted.
Anchoring of wires
Installation of decks
Circumferential wires

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Inspections

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Inspection Process & Sampling

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Previous Inspections
• 2004 - Bodycote
1st
main cable inspection in Europe at cost of £2.75m following discovery of
excessively corroded wires in US
10 panel locations investigated
Estimated 8 – 10% loss of strength in cables
Estimated closure of bridge in 2014 - 2020
Subsequently acoustic monitoring and dehumidification system added (2006-
09)
• 2008 – Faber Maunsell
3 panel locations investigated (2 in previous inspection) – cost of £3.78m
Agreed with previous estimation of strength loss but concluded more
optimistic date of 2017 - 2021 more likely

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2012 Inspection
(Flint & Neill Ltd / ESR Technology)
• 8 panel locations investigated (4 East / 4 West)
• All locations not previously investigated
• Various ‘heights’ on cables
• Inspection carried out according to NCHRP 534
– Laboratory testing included chemical analyses, Preece testing,
galvanised coating weight loss, tensile testing, fatigue testing &
fracture analysis
• Results will allow 3rd
point to be plotted on strength loss & life
prediction and allow assessment of dehumidification system
effectiveness

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Panel Locations

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Laboratory Testing

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Galvanic Coating Tests
Results showed wires adequately coated

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Tensile Testing
Corrosion stage No. specimens Ave. UTS (MPa) <1544 MPa
Stage 1 65 1570 (51) 25 (38%)
Stage 2 271 1574 (51) 91 (34%)
Stage 3 467 1557 (64) 188 (40%)
Stage 4 280 1530 (±67) 173 (62%)
Total 1083 1555 (±63) 477 (44%)
Panel No. specimens Ave. UTS (MPa) <1544 MPa
(001) W94-92S 173 1572 (±67) 50
(002) W90-88N 183 1564 (±65) 62
(003) E94-92S 116 1589 (±56) 24
(004) W40-42S 69 1571 (±87) 26
(005) E100-98S 192 1544 (±51) 88
(006) E54-54S 61 1510 (±73) 45
(007) W52-50N 120 1535 (±44) 76
(008) E30-32S 169 1551 (±50) 106
Total East Cable 538 1549 (±59)
263
Total West Cable 545 1561 (±67)
214
Wedge location No. specimens Ave. UTS (MPa) <1544 MPa
12:00 94 1529 (±39) 58
01:30 124 1571 (±57) 44
03:00 143 1558 (±51) 52
04:30 135 1532 (±64) 75
06:00 121 1528 (±45) 78
07:30 134 1540 (±65) 69
09:00 184 1567 (±76) 67
10:30 148 1596 (±56) 34
• Performed to ASTM 586 and ASTM A370
• 254 mm gauge length
• 1083 tests
• Yield, UTS, elongation, RoA and E

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Fatigue Testing
• Test end point was failure, or, 2 million cycles – only S3 &
S4 specimens.
• The average number of cycles was calculated at 958,852.
• Failure of the wire was found to occur when a fatigue crack
reached between 1154 to 2830 µm.
• The average crack depth however was calculated at 1420
µm.
• Lowest number of cycles was 36,222 on specimens with
~2000 µm crack.
Cyclic fatigue test summary (ASTM E466). Stage 3 Stage 4
No. of specimens 23 42
No. completed 2x106
cycles 12 5
No. of erroneous* 3 4
Failed 8 33
Average number of cycles to failure excluding
erroneous values
617,927 547,095
Minimum number of cycles 131,517 36,222
Maximum number of cycles 1,686,881 1,952,165
*Test specimens which failed within the grips or region of stripped galvanising.
• Performed in accordance with ASTM E466
• Instron fatigue rig with 50 kN transducer
• Mean tensile stress: 618 N/mm2
±139 N/mm2
• Loading: 40 Hz sine waveform

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Fracture Mechanisms
In-situ Fatigue
Surface corrosion
Feathery propagation
Perpendicular ‘T’-crack

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Where we are today...
• The inspections have shown that over a period of almost 50 years the
wires on the Forth Road Bridge have generally seen a reduction in
tensile strength.
• The current findings revealed that the newly installed dehumidification
system appears to be slowing down the rate of corrosion of the wires.
• The Forth Road Bridge main cables will continue to require internal
inspections and strength evaluations for the remainder of the life of the
bridge.
• External condition of cable is not indicative of internal condition.
• Further work required to fully understand the wire fracture mechanism.
• Loading restrictions will be in effect ~2017.

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Forth Road Suspension Bridge
Questions?