Chris Hendy, Head of Bridge Design and Technology                             Atkins 

Design of Bearing Replacement Scheme on Forth Road Bridge

Forth Road BridgeBearing Replacement – Use ofEurocodesChris Hendy – Head of Bridge Design and Technology

Forth Road Bridge Bearing Replacement

• Need for bearing replacement

• Use of Eurocodes for assessment

• Bearing replacement scheme

• Use of Eurocodes for strengthening and jacking

Introduction and Bearing replacement

Forth Road Bridge Bearing Replacements

Existing structure:• Three span suspension bridge

with a central span of 1005 m and side spans of 408 m

• On both approaches to the bridge there are multi-span viaducts

• The south viaduct consists of 11 spans and the north viaduct consists of 6 spans. These spans vary between 33 m and 39 m.

South viaduct

North viaduct

Existing viaduct structure:• Box girders connected by transversely spanning cross girders at

approximately 3m spacings• Roller or rocker bearings on reinforced concrete portal piers

Typical cross section at pier

Forth Road Bridge Bearing Replacements

Bearing condition

Existing structure:

Typical Steel Roller Bearing Typical Steel Rocker Bearing

Existing structure - South Viaduct:• Roller Bearings at shared pier S3 and piers S4-S10• Rocker Bearings at south side tower / pier S0 and piers S1-S3

= rocker= roller

Bearing condition

Existing structure - North Viaduct• Roller Bearing at North side tower• Rocker Bearing at north abutment and piers N1-N5

= rocker= roller

Bearing condition

Roller Bearings:• Not free to roll – uneven wear due to

stress and/or corrosion• Higher stresses in pier than considered

in original design• Justified by use of cracked section

properties but box stresses too high• Bearings assessed according to

BS5400-9-1:1983 and BS EN 1337-4• Modern geometrical limits not met• Significant codified overstress

• Roller bearing at end of North viaduct near the side tower is near limit of movement range limit

Roller bearing at north side tower

Bearing condition

Rocker Bearings:• Bearing corrosion• Generally compliant with BS EN

1337-6:2004• Concrete delamination – (and at

rollers)

Bearing condition

Options for replacement schemes?

Bearing type for replacement:Roller replacement options• Pot bearings:- Insufficient room on piers without significant widening - Single pot overstresses diaphragm in hogging bending- Twin pots – insufficient room to get adequate lever arm between

them- Friction greater than rollers• Rollers:- No modifications to diaphragms- No change to designed articulation but concern over materials and space

• Sliding rockers:- No modifications to diaphragms- No change to designed articulation

Options for replacement schemes?

Roller/Slider Pot

As built assessment of box girder –Use of Eurocodes

As built assessment of box girder• Box girder assessed initially

before jacking design started• Significant overstresses

found to BS 5400 Part 3 but not to BS EN 1993-1-5

• New BD 100 requires strengthening to Eurocodes, but designers need to justify this

• Challenges are knowing assumptions in Eurocodes; for Forth, some relevant considerations were:

- Steel ductility- Torsional buckling of stiffeners

As built assessment of box girderBending - Eurocodes• Effective section used for all

components in section properties• Result is redistribution occurs

from heavily loaded parts

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

As built assessment of box girderBending – BS5400• Gross section used for all

components except compression flange

• Individual buckling checks on components – weakest governs

Check as strut

Check panels for buckling

Check as strut

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

As built assessment of box girderShear - Eurocodes• Same approach to design used as for

beams without longitudinal stiffeners• Slenderness however comes from

weaker of sub-panels or overall web buckling

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

Overall slenderness based on lowest crfrom:

or:

cr

yw

As built assessment of box girderShear – BS5400• Different and typically more

cautious approach used for beams with longitudinal stiffeners

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

Resistance based on lowest strength of:

As built assessment of box girderShear – Moment : Eurocodes• Beams with longitudinal stiffeners

treated same way as beams without i.e. interaction diagram

• Interaction weak in Eurocodes

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

As built assessment of box girderShear – Moment : BS5400• Immediate reduction in bending

with shear and vice versa• Eurocode benefit due to steel

ductility and recent testing – not all steel would comply

BS 5400

Usage factorCheck BS 5400 Part 3 Eurocode 3

Bending 0.95 0.90

Shear 0.85 0.75

Shear-moment 1.25 0.90

As built assessment of box girderTransverse stiffeners

• Transverse stiffener design to BS5400 Part 3 very conservative –based on Rockey’s theory

• True behaviour is that stiffeners attract negligible force – Hoglund’stheory

• EN 1993 somewhere in between• BS 5400 typical usage = 1.5• EN 1993-1-5 typical usage = 0.9• Benefit due to better understanding

of behaviour

As built assessment of box girderCaveats for use of Eurocodes –Torsional buckling

• BD 100 requires Eurocodes to be used for strengthening but requires designers to justify this

• Eurocode result and method not valid if torsional buckling can occur below fy

- Shape limits not met to BS 5400 Part 3, but allowable to work to a reduced stress

- Reassessed to EN 1993-1-5 but still fails so need to strengthen stiffeners

2

21LEC

GII

wT

pcr

As built assessment of piers• Pier assessment – central point load• Rebar is mild steel and lightly reinforced• Only works if Eurocode strut and tie invoked with tensile strength• Not long term solution due to deterioration

b

b

0.5b

0.25b to 0.30b

a

As built assessment of piers

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8

b/a

Bear

ing

pres

sure

(MPa

)

b

b

0.5b

0.25b to 0.30b

a

• Pier assessment – central point load• Rebar is mild steel and lightly reinforced• Only works if Eurocode strut and tie invoked with tensile strength• Not long term solution due to deterioration

As built assessment of piers

b

b

0.5b

~0.125b

• Two point loads at shared pier• Same conclusions

Replacement Solution

• Work part of FRB 15 year capital programme of works• No provision for replacement in the original design• Box girders need to be jacked up to allow removal of

bearings from the piers• Piers must be widened to allow correct positioning of

jacks and pier tops also need to be strengthened- Add corbels to edges of the piers- Reinforce pier tops and make necessary concrete/reinforcement

repairs• Box girders require strengthening for jacking

- Bolt external bearing stiffeners to outside of web directly above jacks

- Some web strengthening to allow holes to be drilled- Some box strengthening required independent of bearing

replacement scheme

Replacement SolutionDesign Concept

Replacement SolutionDesign Concept

• Strengthen/recast concrete pier tops and widen pier edges with addition of corbels to allow correct positioning of the jacks

• Bearing stiffeners installed on outside of box webs near diaphragm regions

• Jacks positioned directly below bearing stiffeners

Typical section through box girder at pier locations

• Four jacks per box at intermediate supports• Two jacks per box at end supports• Box section jacking stiffeners• Stiffener size and connection details varied according to girder

geometry

Replacement SolutionJacking Stiffener Details

• Four jacks per box at intermediate supports• Two jacks per box at end supports• Box section jacking stiffeners• Stiffener size and connection details varied according to girder

geometry

Replacement SolutionJacking Stiffener Details

Replacement SolutionCorbels

b 3b12 Ac1

Ac0

h

d1

b1

d 3d2 1

Dispersal at 1H : 2V max

line of action Limiting area for Ac1

Ac0

Load near an edge (plan)

0010 Af0.3A/AfAF ccdcccdcRdu • Bearing stresses used

Eurocode partially loaded area rules to increase resistance

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Construction sequence for corbels

• Drill in bars• Cast corbels• Install temporary

restraints• Jack up deck• Remove bearing• Break out concrete• Recast top of Pier• Replace bearing

Replacement Solution

Before corbelling + external stiffening:

Replacement Solution

After corbelling + external stiffening:

Replacement Solution

Replacement SolutionFeatures of Scheme

• Minimal work inside the box for H&S – external stiffeners

• Stiffeners bolted to minimise reduction to box strength during construction

• Only internal work is some additional bolted longitudinal stiffeners (to strengthen webs before drilling) and bolted flanges to existing stiffeners

Replacement SolutionFeatures of Scheme

• Minimal work inside the box for H&S – external stiffeners

• Stiffeners bolted to minimise reduction to box strength during construction

• Only internal work is some additional bolted longitudinal stiffeners (to strengthen webs before drilling) and bolted flanges to existing stiffeners

Role of Virtual Reality Model• To convey scheme to third parties• To visualise complex construction sequence

- Construction sequence drawings very complicated- DRA raised concerns over misunderstanding sequence on drgs

• For detailed visualisation of strengthening and planning• To monitor progress• To ease referencing of components between site and

design office• As a learning aid for site operatives• To allow rapid re-detailing when components don’t fit• As-built virtual reality record

Replacement Solution

• BD 100 invokes Eurocodes for modification• Use of Eurocodes significantly reduced strengthening

needed for in service• Use of Eurocodes significantly reduced strengthening

needed for bearing replacement• Eurocode rules will not always apply for assessment /

strengthening – care and understanding needed

Conclusion