Maintenance, Maintainability and Upkeep – a look.
ROBERTO ZANON
Engineering works, especially those serving transport infrastructure, are ideally designed to ensure their economic function throughout their useful life (or time of operation) in compliance with certain requirements and defined acceptance criteria, typically connected with user safety and environmental risks.
However, there will always be deterioration to some extent and in some way and, depending on the design strategy adopted, it will always reduce the capacity of the structural system in question to below what is considered acceptable in terms of accepted deterioration levels and economically adopted protective measures.
To ensure that the acceptance criteria are always respected throughout the useful life of the work, i.e. the ‘upkeep’ of the full operation of the work, it may be necessary to check the development of deterioration to assess the appropriateness of carrying out protective or repair work.
Considerations of this type have led to a change of perspective in the design strategy of new structures (for example, the shells for ships or aircraft, or off-shore structures) which has moved from the notion of duration of safety versus ‘collapse’ for the whole of the theoretical life to that of tolerance of a certain level of damage, when safety is ensured though appropriate inspection and control modes. A structure can be said to be ‘damage tolerant’ if that damage can have a reasonable evolution on it, so that it can be found during programmed inspections (routine) before breakage or, worse collapse, occurs.
The experience of the past also highlighted how economic limitations induced, or tend to induce, many managers, delegated to manage infrastructure assets, to use the structures they were responsible for beyond the terms of useful life initially considered in the designs of the works. Extending the useful life of structures in a stage of advanced ageing leads to the need to increasingly extend damage identification work and, as a result, repair, irrespective of the original safety criteria set out at the start of the project (which can effectively change in time, e.g. because of variations in legislation).
Therefore, it goes without saying that the conservation of very dated works should cause an intensification of inspections which, in turn, should or could lead to more frequent repairs, especially where the removal of the oldest structures is unacceptable for managers or owners of infrastructure. Similarly, it is even more obvious how inertia in detection/repair work may lead to dangerous situations, which can rapidly evolve towards collapse, in very old structures.
The safety-inspection (check) pairing should become the safety philosophy to adopt in design (maintenance plans) and, consequently the management of engineering works. Today, the frequency of checks can be significantly increased due to the installation of automatic monitoring devices supported by networks of increasingly high-performance sensors.
Such devices can certainly, and more easily, be envisaged right from the design stage of a new product; nevertheless, despite the above, it is even more useful to have, as happens, devices that can be easily installed on existing works to monitor.
In any case, whether they are visual inspections carried out in the field or assessments of large quantities of data collected via such devices, the ability to interpret is required, currently found in an adequately trained technician but it cannot be excluded that much of the interpretation process can be carried out by computer with artificial intelligence techniques. Anyway, to ensure safety through inspections, whether visual or monitoring, an indication is required of what has to be inspected, how and where and how often. Except for monitoring which undoubtedly may lead to some restrictions only during installation of the device, it should also be considered that inspections may create a disturbance, albeit limited, to the ordinary use of the infrastructure and should, therefore, be adequately planned.
However, even when there is continuous monitoring and, reliably, as a result of assessments of the data thus obtained, it is thought essential to consider some visual inspections appropriate during the life of the structure.
‘Maintenance’ can be defined as a series of economically balanced jobs involving the components of the structure with the aim of keeping it in a condition that makes it suitable for the performance of its function in a defined period of time with a sufficient level of reliability, availability, practicality, durability and aesthetics. The period of time is the interval between two inspections and, in theory, can be extended to the entire useful life of the structure.
“Maintenance is a series of economically balanced jobs intended to keep infrastructure suitable for the effective performance of its function”
With reference to the above definition, the concept of maintenance thus includes all the work that starts from the inspection to find any damage and its measurement, continues with the resulting assessments on safety and ends with the definition and fulfilment of the corrective actions, i.e. the repair. In addition, it can also be seen how this process in itself generally induces a correction of the ‘inspection programme’ for the foreseeable subsequent maintenance work.
‘Maintainability‘ is all the expedients introduced in a project to favour maintenance, whether during inspection (e.g. in a bridge by making access to certain sensitive details easy), repair (setting out easy dismantling of parts and specific procedures for their replacement, such as the replacement of cables on suspension or cable-stayed bridges).
Maintenance can foresee the creation of such expedients to favour subsequent maintenance where there are very few on structures to be worked on, naturally always within the framework of a balanced economic report. Although, as mentioned, maintenance focuses on elements or specific groups of elements, it can once again be seen how it should always also depend on the sensitivity of the structure to breakage of the elements, i.e. the consequences that such breakage may have on the functionality of the work. Focusing on the details of bridges, two main maintenance strategies can be seen – preventive and corrective maintenance.
The latter means that no action is taken until damage is seen. It can be adopted where breakage of the component is not critical and its replacement before breakage is not economic. Therefore, breakage of the element is awaited and this is then replaced or repaired. This type of maintenance generally leads to low short-term costs but these increase in time; further, indirect costs are produced, given the unpredictability of the acceptable collapse of the element in question, linked to the unplanned disturbance of the functionality of the work resulting from the repair.
Preventive maintenance sets out the regular performance of appropriate work according to a plan and maintenance programme. Depending on its intensity, such a strategy may lead to high costs but significantly reduces the risk of the structure being out of action; it can be ‘systematic’ or ‘conditional’.
Systematic preventive maintenance sets out a repair or replacement programme optimised to minimise the overall cost (direct + indirect costs) of maintenance during the conventional useful life of the structure; for example, the periodic cleaning of bridge draining systems falls within this category.
On the other hand, conditional preventive maintenance sets out a specific inspection programme (generally quite intense) following which the inspected component may or may not be subject to repair/replacement. This strategy lends itself to situations where the cost of a breakage is high and the inspection and repair/replacement costs put together are, in comparison, significantly lower. In this case, the definition of a correct, possibly optimised, inspection programme connected to the expected life of the component to inspect,
conditioned by the dispersion of data on the use of the article, the conditions triggering the damage, its evolution based on the use of the article, the inspections related to use over time, the probabilistic detection of the damage and the level of tolerable damage, becomes decisive.
A lot of research was carried out in the past on the definition of mathematical laws that connect the above elements so that the evolution of safety over the life of structures could be defined. Thus, it can be understood how continuous monitoring systems of different parameters like transiting loads, levels of stress and strain, temperature and others, and procedures allowing the collection and processing of such large amounts of data can give a considerable experimental contribution in that direction. It is hoped that such data, with the appropriate tools for its processing, will soon be made available to researchers into the phenomena in question.
Si auspica la messa a disposizione nel breve, agli studiosi dei fenomeni in questione, di tali dati con strumenti adeguati al loro trattamento.
Roberto Zanon – was born in Padua in 1962; he graduated from Padua University in 1987, specialising in Civil Engineering (Structures). After serving as an officer in the Engineering Corps, he was involved in many projects in the ambit of transport infrastructure, taking care of aspects connected to engineering works and their interaction with the context, alongside specific teaching in co-operation with the local university. He has been working with Net Engineering since 2002.