ABSTRACT
New Zealand has introduced High Productivity Motor Vehicles (HPMV’s) to improve freight efficiency. The implementation of this increased vehicle loading (maximum vehicle gross mass increase from 44 tonnes to about 60 tonnes) has been constrained by the load capacity of older bridges. A national bridge evaluation and strengthening programme has been undertaken to facilitate the introduction of HPMV’s.
4,000 State Highway bridges and about 15% of the 14,000 Local Authority bridges were screened. The initial conventional screening was based largely on bridge age, design loading, configuration and structural data.
Bridges on higher volume routes which failed the initial evaluation then underwent detailed evaluation, involving:
Detailed inspection and condition assessment
Review of As-Built data
Grillage analysis
Maximising load capacity through:
– A ‘risk-based’ approach to evaluation
– Reduced load factors
– Material testing
– Compromise of serviceability criteria
– Focus on structural safety rather than asset protection
– ‘Whole of life’ cost analysis
– Relevant use of international standards
– Increased surveillance
– Structural response monitoring
The ‘risk based’ detailed assessment approach reduced the ‘initially restrictive’ bridges by 60% with associated very large cost savings.
Bridge strengthening was undertaken on specific routes (HPMV Investment Routes) where justified by the economic benefits of the improved freight efficiency. The predominant strengthening comprised post-tensioning of older reinforced concrete bridges.
The bridge evaluation programme resulted in various beneficial outcomes:
A comprehensive live load evaluation of the national bridge stock was overdue. The program provided a thorough knowledge of the bridge stock live load capacity
Some bridges were identified to be highly overstressed under current loading
Observation of historic performance of identified weak bridges, confirmed that many older bridges, particularly well designed and detailed reinforced concrete structures, are very resilient to increased loading.
A ‘risk-based’ approach to evaluation can be an extremely cost effective means of maximising load capacity and reducing the need for strengthening, along with the potential adverse impact on the community, stakeholders and the environment
Evaluation techniques involving thorough investigation and analysis and a risk-based approach augmented as necessary by strengthening, can support retention of older bridges for increased loading
‘Whole of life’ cost analysis shows that compromise of serviceability criteria (focus on road user safety rather than asset protection), resulting in potential asset consumption (earlier bridge replacement and/or increased maintenance) can be very cost effective for older bridges
Bridge monitoring through Weigh-in-Motion (with number plate recognition) and structural monitoring systems can assure appropriate bridge risk management performance
Many older reinforced concrete bridges can be relatively easily strengthened to meet modern loading demands using conventional construction techniques
The initial relatively high cost of detailed evaluation and analysis using enhanced methods has been well rewarded by the resulting large cost savings of reduced strengthening
Analysis of the entire State Highway bridge stock showed that a significant increase in loading (to a maximum gross mass of 50 tonnes) could be achieved for most routes without any strengthening
25% of all freight is now carried on HPMV’s and this continues to increase rapidly
