Saving Walton Bridge

John Hart, Tom Dowling, Michael Hughes

November 2015

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Walton Bridge

Overview

–Walton Bridge in context

– The situation in 2013

–Monitoring

– Buried arch behaviour

– Analysis and findings

–Repair, strengthening, and replacement options

– Selected approach and outcome

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Site Plan

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Walton Bridge from Downstream

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Walton Bridge

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Walton Bridge

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Walton Bridge

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Walton Bridge

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Walton Bridge

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Water and Efflorescence Stains

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Heritage Bridge

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Heritage Bridge

Situation in January 2013

– Previous assessment had shown the bridge to have

insufficient capacity for as-of-right trucks.

–Concern that multiple cracks in the arch indicated that the

bridge had been loaded close to ultimate capacity.

– Load limit imposed – 20 T, left lane only

– Speed limit imposed – 40 kph

– To be propped or strengthened within 6 months.

– To be replaced within 5 years.

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A Different Approach

– As part of a study of strengthening and replacement

options:

– Investigate post-cracking behaviour of the arch

–Could the cracks be from shrinkage rather than loads?

–Uncommon bridge form not covered by AS 5100

– Look to British practice

– Apply specialist software

– In the meantime, monitor the arch

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Crack Monitoring

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Displacement Monitoring

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Scant Information

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Key Dimensions

Dimension Measurement (mm)

Clear skew span 13800

Mid-span rise 4300

Quarter-span rise 3300

Arch thickness at crown 600

Arch thickness at abutment springing 1200

Arch thickness at pier springing 1800

Pier thickness 2500

Backfill depth at crown 600

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Geotechnical

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Buried Arch Basics

– An arch loves distributed loads which are carried mostly by

axial compression in a suitably shaped arch.

–Hence the feasibility of stone, brick, and mass concrete

arches since ancient times.

– These materials are also durable.

– Arches do not much like concentrated loads which cause

bending as well as thrust.

–More of an issue as truck and axle loads are now bigger.

– The fill over buried arches is advantageous.

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Arch Behaviour and Ductility

– An unreinforced concrete beam in flexure is brittle –

collapse is rapid once the flexural tensile capacity is

exceeded and the concrete cracks.

– Because an arch carries axial compression as well as

bending, it can carry the moment by the eccentricity of the

thrust rather than the tensile strength of the concrete.

– An arch has “geometric ductility” and does not collapse

when a load first causes cracking.

– Tests have shown that gross deformation occurs before

full collapse.

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Arch Line of Thrust and Hinges

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Arch Analysis

– Empirical – Modified MEXE

– Three Pin Arch – lower bound - ArchieM

– Four Pin Arch – upper bound - Limitstate RING

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ArchieM Typical Output

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LimitState RING

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Analysis FindingsRatio of Available Capacity to Vehicle Loading at ULS

Vehicle GVM (T) Governing

Axle

Archie M RING

Bus B7R 16 Single 2.6 3.9

Bus B21R 20 Dual 2.1 2.9

Semi-trailer 42.5 Triple 1.6 2.4

Rigid truck 22.5 Dual 1.6 2.4

SM1600 170 Triple 1.0 1.5

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Repairs

– Sealing of significant cracks

–Concrete patch repairs

–Drainage and waterproofing

– Anti-graffiti coating

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Strengthening Options

–Concrete saddle - casting a thickening on top of the arch

barrel – directly increases arch strength.

–Concrete slab at road surface – improve load distribution,

waterproofing, less traffic disruption

–Grouting the fill - to improve load distribution and decrease

permeability.

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Replacement Options

–Demolish and replace with precast concrete arches.

–Deck unit bridge – built on-line and above the existing

arches, which are retained but not traffic load bearing.

– Super-tee bridge – built off-line and the existing bridge

survives, unused.

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Refurbishment Choice

–Concrete slab at road surface level

–Waterproofing and drainage

–Concrete patch repairs

–Crack sealing

– Anti-graffiti coating

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Confirmation of Design Parameters

– Topographic survey

–Crack and defect mapping

–Geotechnical investigation of fill material

–Concrete cores for strength testing (19.5 Mpa)

–Drill to confirm arch thickness (600 mm)

–Ground penetrating radar

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GPR Output

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Outcomes

– A different approach to the analysis showed that the arch

does have sufficient strength

– Load and speed restrictions removed

–Repair and refurbishment completed

–Council avoided high cost of replacement

–Community avoided high impact of replacement

–One of the oldest bridges in Brisbane will continue to serve

a useful function far into the future.

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Questions?...Later

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