Tauranga Suspension Bridge Load Testing and Remedial Work

Trevor ButlerFrame Group Ltd

Introduction• Located in the Waioeka Gorge adjacent to SH 2

approx 25km south of Opotiki• The site is managed and interpreted by

Department of Conservation• Bridge consists of a 58m long suspension span

with short fixed spans at each end • The structure is a unique “multiple rope”

suspension bridge design (Similar to modern cable stayed designs)

Multiple Rope Suspension Design

• Developed by Public Works Dept as a low cost option for access to the back blocks

• Design was the subject of a paper presented to the Institute of Civil Engineers in London 1922

• 27 bridges of this design had been built by 1922 (very few remain in existence)

• Very efficient structural design (“design cannot be improved upon”)

• Deck has only a small amount of deflection under high point loads (vehicle loads)

Historic Significance• Built in 1922 to provide access for settlement of

marginal lands in the Tauranga valley for soldiers returning from World War One

• However farming was not sustainable on this land and after 10 yrs of back-breaking work, the settlers abandoned the farms penniless, allowing the land to revert back to forest

• This is another “Bridge to Nowhere” representing a failed attempt to farm unsuitable marginal lands

Historic Significance• Registered as Category 1 Heritage by the

Historic Places Trust• Recognised by IPENZ as a structure of

engineering significance• The site very accessible to the public for

interpretation (few minutes walk from SH2)• Provides pedestrian access to Tauranga valley

walkway (reverting farmland)

Life History• Built 1922• Abandoned in the 1930’s• Very little use or maintenance for 60yrs• Near collapse by 1990’s• Significant remedial work to towers and

cable terminations carried out in 1994• CNE timber preservative treatment applied

to tower timbers

Assessment in 2000

• Long term Maintenance Plan scheduling likely work & costs for next 30 yrs

• Critical components near failure were identified (tower transoms, cable grips, sway anchors)

• Other timbers sound but deteriorating slowly• Original cables still in reasonable condition• Decision made to implement maintenance plan • Critical components replaced in 2001• Further remedial work and treatment in 2002

Maintenance Plan Approach

• Replace key critical load members that are important for structural stability

• Instigate preservative treatment programme to prolong the life of all other original fabric that is accessible

• Replace or support components that in the future become deteriorated or threaten the overall structural integrity

Failure of beam in 2003• One of four beams between transoms C &

D broke at mid span causing the deck to sag on one side

• Previous visual inspections had failed to detect the degree of deterioration of this beam (Other beams appeared far worse)

• Highlighted the risk of partial failure of other deck sections if other beams have similar deterioration

Load Testing

• Necessary to reliably determine safe capacity to retain bridge open for pedestrian access

• To identify spans needing immediate strengthening (There were insufficient funds available strengthen all spans at this stage)

• Low risk progressive loading used, based on the DoC cable structure testing procedure

• Deflection carefully monitored during the loading process

Strengthening• Installation of glu-laminated pinus radiata

support beams hidden between existing hardwood beams

• Existing beam fabric CNE treated and is supported by the new beams

• Upgrade work funded by Environment BOP “Environment Enhancement Fund”

• Future strengthening work on other spans will be necessary to maintain safe public access

Questions

• Tauranga Bridge• Maintenance Planning• Load testing• Upgrade Solutions