Asset management of tourist and heritage structures

29 June 2018

Asset management applies to all activities involved in stewarding structural assets including bridges, tunnels, stations and retaining walls. Many tourist and heritage (T&H) structures are more than a century old and must be kept safe and fit for purpose.

A safe, reliable, efficient and sustainable approach to managing these structures requires a well-developed capability in asset management. This capability will be reflected in the asset performing optimally for the level of commercial viability.

Registered heritage bridges are usually open to vehicular traffic. They are maintained to the same performance standards as equivalent non-heritage bridges and in a manner that ensures the heritage characteristics of each are preserved.

A maintenance program is a cost-effective way to preserve the value of an asset and ensure its sustainable future use. This should identify:

  • works necessary to bring the structure to an acceptable standard
  • works to prevent damage
  • and emergency works that may result in rapid deterioration and catastrophic failure of the structure.

Risk assessment

One of the most important requirements in any bridge risk assessment is the evaluation of the hazards to which a bridge might be exposed.

An important part of a risk assessment is to set up the hazard inventory by listing all hazards that might conceivably endanger the existence of the bridge.

For existing bridges, the risk of collapse should be assessed taking into account influences of nature and human acts. Their structural resistance will decrease with time as the materials used in their construction deteriorate. To minimise the likelihood of collapse, risk assessments must be undertaken on a regular basis as well as when the function of a bridge has changed.

Bridges are subjected to natural hazards, such as extreme heat and freezing temperatures, dry and wet season, earthquakes and flooding, which may damage or destroy them. Bridges are vulnerable not only to forces of nature, but also to modern methods of transportation (e.g. train/ship/truck impact or overloading). To prevent bridge failures, a rational and comprehensive method for assessing the risk of structural collapse is required.

There has been extensive research on risk management for bridges that have already been commissioned and are in service. The following have been identified as the major causes of collapse for bridges during their service lifetime:

  1. natural hazard – especially flooding/scour
  2. impact – from boats, vehicles and trains
  3. overloading – due to change of use, change of loads, illegal loading
  4. deterioration – corrosion of steel, concrete deterioration, fatigue.

An existing bridge is deemed to have failed when one or more of the following occurs:

  • failure of an individual element of the structure
  • partial or complete collapse of the structure
  • poor serviceability and poor in-service behavior of the bridge manifested through cracks, corrosion and excessive deformation.

Inspection regime

The asset management should include routine inspections of the assets to note any deterioration and urgent maintenance and repair works. Physical, ocular and/or full-scale engineering inspections should be undertaken periodically and the condition report benchmarked against the previous report. Some defects may require supplementary evaluation. Leaning walls, widespread fracturing of brickwork, or significant movement of structural elements under load are examples of defects that should be monitored until they are repaired.

The effectiveness of the asset management regime for a heritage structure should be reviewed regularly. An important part of the maintenance program is a review of previous actions to gauge the success of the works applied. This can guide the expenditure of future funds to achieve targeted results. Issues to consider when reviewing the work include:

  • whether the maintenance action was necessary or appropriate
  • the timing and standard of works undertaken
  • the timeframe of the planned maintenance work.

A standard checklist or inspection form, completed during the inspection, should be used to promote completeness and consistency between inspections. The checklist, or form, also creates records to demonstrate that inspections have been completed in accordance with the rail infrastructure manager’s safety management system (SMS).

The major safety relevant elements of a bridge that are inspected include:

  • foundations (concrete/steel piles, footings)
  • substructure (piers, abutments, wingwalls)
  • superstructure (parapets, slabs {top, bottom, cantilevers}, beams, webs).

The inspectors determine what type of damage an inspected element is subject to and the extent of the damage. In general, they state:

  • the type of damage/deterioration process (for example, corrosion, cracks, spalling,).
  • the severity with respect to the considered type of damage
  • the extent of the damage within an element.

Condition ratings are used to describe the current in-situ status of the components of a bridge and not its original state. They are commonly assigned by evaluating the severity of deterioration and the extent to which damage is present throughout the component being rated.

The severity of the deteriorations is recorded and, preferably, supported by photos showing easily recognisable damage/condition. An element condition rating does not translate directly into an overall rating of a bridge’s condition, but it is a good indicator of the quality of specific elements. Overall condition ratings may be obtained by combining the element ratings, taking into account their importance within the bridge system.

It is important to understand how critical the condition problems are to the risk of collapse. The inspections are intended not only for the safety assessment of a bridge, but also for the planning of maintenance actions and non-structural repairs such as replacement of bearings. Also, it is paramount to quantify the influence of the condition on the risk of collapse, and how the condition rating of a bridge is related to the level of safety, and hence to the risk.

The overall condition of a bridge asset should be graded, including an assessment of the effectiveness of the asset management regime. Knowing your assets and implementing a regular maintenance program is crucial to ensuring their sustainable use into the future.

Engineering management for structures

As T&H structures are continuously ageing and deteriorating, rail infrastructure managers need to understand the structures’ structural capacity to ensure their ongoing safety. In particular, they need to account for the current condition of the bridges and the demands from the current vehicle loads using those assets.

Currently, there is a range of engineering techniques for evaluating structures that assist the asset owners to understand their adequacy. There are cost implications so it is important to be aware of the complexity and benefits so that value for money is obtained without inadvertently compromising safety.

The following levels of assessment, in order of complexity, are outlined by AS5100:

  1. Theoretical analysis
  2. Analysis using the results of field investigation
  3. Test loading in the field or in the laboratory.

Theoretical analysis is the most cost effective, most commonly used technique and can be undertaken by either a generic assessment or a load rating assessment.

A generic assessment compares a rating vehicle’s design action with what is believed to be the original design actions. The capacity of the structure is assumed to be consistent with the original design vehicle. The structure may have been overdesigned in the first instance or, since construction, it may have deteriorated and lost capacity. Generic assessments are a good starting point to understand what may be critical on their network if the asset owner does not have complete load rating information for their assets.

AS 5100 provides a load rating equation that can be used to assess structures and determine a structure’s ‘as new’ and ‘as is’ rating factor for a nominated rating vehicle. The importance of load rating existing structures led to the addition of a specific section (Part 7) to the bridge design code AS 5100. It introduced a tiered approach to load rating acknowledging that there is a cost-benefit assessment required to determine the appropriate approach.

With additional cost, field investigation and detailed assessment in accordance with the standards can determine both a structure’s ‘as new’ and ‘as is’ condition. An initial assessment may follow a visual inspection but ultimately an inspecting engineer needs to quantify the extent of deterioration as it relates to the structural capacity. For instance, a visual inspection may only pick up that rusting is occurring, while a full engineering inspection will quantify its extent and the effect it has on the strength of the structure. Once quantified, the information can be used later for structural analysis. Rust on steel structures is only one example of how a visual inspection might lead to a more detailed assessment. Other examples of visual observations include concrete cracks, spalling or sagging of structural elements.

Further monitoring and test loading in the field may be necessary to produce more accurate results and to avoid unnecessary costly repair works. Load capacity or load rating assessments should be undertaken in a structured manner by an appropriately qualified and experienced bridge engineer.