Bridges

New Zealand has many historic and significant steel truss road and rail bridges, with the majority of these constructed between early 1900s to mid-1960s. Steel truss bridges are able to span long distances, and bridge members commonly include rivetted sections built up from plate, angles and channels, with lacing or battening between components.

Currently, New Zealand design standards do not adequately cover the assessment of built-up steel members for truss bridges for a number of member types. The authors experience with various New Zealand and international standards, such as AASHTO Manual for Bridge Evaluation, have found that outcomes can be greatly impacted by the assessment loading, load factors and load combinations. Further, the outcomes can be impacted by choice of technical standards and structural analysis procedures used in the assessment to determine load effects and member capacities. Particular to built-up steel members, assessment outcomes are impacted by assumptions in member restraint, effective lengths, condition factors and strength reduction factors which are covered differently in design and assessment standards. Local refinements in modelling of members with realistic boundary conditions and nonlinear analysis can be applied to reduce conservatism in initial assessments.

These considerations and the assessment methods for three significant bridges in New Zealand are presented in this paper. These bridges being:

• Victoria Bridge in Cambridge, a grade 1 listed trussed arch bridge constructed in 1907;
• the Waikato River Bridge in Ngaruawahia, a steel truss bridge constructed in 1953;
• and the Auckland Harbour Bridge steel truss structures, the longest clear span bridge in New Zealand completed in 1959.

The assessment of historic steel truss bridges is complex and a staged assessment approach is proposed, with level of assessment complexity increasing when required to optimise the load capacity of structure and provide the most value to asset owners.