2.4.4 Other inputs to the bridge management process
Context for use of structure information
The gathering and assembly of information for use in the structure management process cannot be done in isolation from an understanding of the way in which the information will be used.
The following is intended to provide the context for the relationship of the information ‘inputs’ to the management ‘outcomes’, and discuss other important inputs to the management process.
Other inputs comprise:
- deterioration prediction
- management inputs
- engineering inputs
- the analytical process.
The analytical process compares the value of various maintenance options. Estimates for those repairs can be based upon historical cost information, provided the analyst exercises caution.
Circumstances for making comparisons may not be exactly equivalent, and the limitations of adjusting values of past repairs to their present day worth must be realised.
When estimating costs on the basis of historical actual costs, it is necessary to make specific allowances for the effects of changed circumstances such as new regulations and legislation.
Estimated costs used in the evaluation of alternative maintenance proposals should include agency costs and road user costs in order to indicate the most favourable treatments from a community perspective. Estimates of road user costs are sensitive to the estimation methodologies used. Consistent methods for estimating user costs are essential to achieve credible results.
At the project level, accuracy in the estimation of costs for maintenance works is critical to the process of selecting appropriate treatments for individual projects and in the development of a viable works program. It is also critical at the works procurement stage when contracts are awarded and required to be within allocated budgets. Methods that may be used to improve costs include a 95% confidence limit using a Monte‑Carlo approach to the associated risks. It is noted that at the higher level of asset management the estimation of ‘costs’ is fairly broad.
A predictive process for assessment of the likely deterioration profiles of structure elements may be considered as part of an overall structures management system. Prediction subsystems can be probabilistic or deterministic.
Analysis of a time series of inspection condition data (e.g. by regression) may enable development of algorithms for material/element deterioration where no action has been taken, or test the effect of repairs on subsequent deterioration rates. With nearly 20 years of condition data (level 2 inspection), a reliable algorithm for predicting the deterioration profile of structural elements has yet to be developed that can be consistently applied to structures at a network wide level.
The accuracy of the process and the predictability of the results are highly dependent on the quantity and quality of data available for analysis. This requires consistency in inspection practice and rating of element condition over a number of inspection cycles which has implications on the resourcing and training of inspection personnel.
Furthermore, given the complexity of highway structures, particularly bridges, for a reliable result, deterioration profiles need to be based on data representing structures with similar operating conditions such as:
- construction quality
- maintenance history.
In addition, visual inspection alone is inadequate for many of the construction materials/components used in highway structures. There is a need to develop reliable and cost-effective sampling and testing methods to determine condition deterioration details and material properties. This may be difficult to justify at a network level and may therefore be better suited for structure specific management (i.e. iconic, complex structures).
Management inputs to the analytical process of the bridge management system reflect:
- Standards: Definition of target levels of service based on road function and expected usage (e.g. load capacity, route clearances, safety requirements, availability), over-weight vehicle management, tolerable bridge conditions, aesthetic and cultural (e.g. heritage) requirements, etc.
- Budgets: Size and spread of funds available for works programs.
Standards and budgets may be used in combination to determine a feasible strategy, where projects are developed to meet the scope of that strategy: the ‘top-down’ philosophy. Alternatively, projects to restore desirable standards may be proposed but their acceptance is limited by the size of the available budget: the ‘bottom‑up’ philosophy.
Engineering inputs to the structure management analytical process include:
- Select repair options: assess the various repair options to restore particular defects. This may involve structural analysis and testing. Standard repair methods may be available in existing maintenance manuals.
- Develop projects: analyse the available feasible actions and develop a preferred project that satisfies management objectives. One option could be to do nothing.
- Prepare estimates: submit budget estimates for the preferred repair option. Estimates should include all costs, direct and indirect.
The value of engineering input into the structure management process cannot be emphasised strongly enough. Inputs based upon engineering decisions will ensure that control is retained of what could easily become a ‘black-box’ process, and that the interpretation of management criteria into practical solutions is achieved.
The analytical process
The approach to whatever analytical process is adopted for the management of highway structure may be either top‑down or bottom-up.
The analytical process combines all the system inputs to provide a selection of feasible projects developed to satisfy or move towards achieving desirable or target standards, within any budget constraints. The process is iterative, and is based on an optimisation algorithm that ranks the projects in accordance with predetermined management criteria, such as minimising whole-of-life costs.
Ranking project priority would be by an index, and selection of projects is determined by those projects that return the highest benefit. Types of indices include:
- Condition – an index developed from the element condition states reported at the time of inspection. Higher values for indices indicate structures in worse condition.
- Reliability – an index developed by assessing the capability of a structure to safely provide a continued service.
- Remaining life – the estimated number of years with continued routine maintenance and projected loading, until the structure is expected to require rehabilitation, strengthening or other upgrading, or replacement.
- Risk – an index developed by assessing the probability and the consequence of failure of a structural element at a specific structure considering factors related to location, element condition, element criticality, environment, loading and design.
- Socio-economic – an index developed to include economic and relevant social issues, such as environmental, cultural (e.g. heritage), and aesthetic issues.
- Sufficiency indicators – example sufficiency indicators include:
- vertical clearance
- load carrying capacity
- barrier standard
- waterway/flood capacity.