While interest in structural health monitoring (SHM) has increased as related to both research and implementation, its basis and motivation can be traced to the very earliest endeavors of mankind to conceptualize, construct, worry about deterioration, and then attempt to repair (or otherwise prolong the life) of a structure. This is largely in response to the fact that over time all structures deteriorate and it is essential that the owner/operator has a good idea as to the extent of deterioration, its effect of remaining service-life and capacity, and has sufficient information to make a well-informed decision regarding optimality of repair. Thus it represents an attempt at deriving knowledge about the actual condition of a structure, or system, with the aim of not just knowing that its performance may have deteriorated, but rather to be able to assess remaining performance levels and life. This ability will, at some point in the near future, enable those associated with the operation of civil infrastructure systems to handle both the growing inventory of deteriorating and deficient systems and the need for the development of design methods that inherently prescribe risk to a system based on usage and hence differentiate between systems based on frequency of use and type of operating environment. Further such a system would enable decisions related to resource allocation to be made on a real time basis rather than years ahead thereby allowing for maintenance plans to be based on actual state of a structure and need rather than a time-based schedule. This would allow for real-time resource allocation thereby enabling a more optimal approach to maintenance and replacement of structural inventory. In its various forms, over the years, SHM has been represented as the process of conventional inspection, inspection through a combination of data acquisition and damage assessment, and more recently as the embodiment of an approach enabling a combination of non-destructive testing and structural characterization to detect changes in structural response. It has also often been considered as a complementary technology to systems identification and non-destructive damage detection methods. A decade ago Housner et al. (1997) defined it as “the use of in-situ, non-destructive sensing and analysis of structural characteristics, including the structural response, for detecting xv

changes that may indicate damage or degradation.” While this definition provides a basis for the development of a good data management system in that it enables the collection of incremental indicators of change, it falls short of the goal of considering the effect of deterioration on performance and thence on the estimation of remaining service life. These management systems thus focus on processing collected data, but are unable to measure or evaluate the rate of structural deterioration, and more importantly from an owner’s perspective, are unable to predict remaining service life and level of available functionality (e.g. the load levels that the structure can be subjected to within the pre-determined level of reliability and safety). In essence a true system should be capable of determining and evaluating the serviceability of the structure, the reliability of the structure, and the remaining functionality of the structure in terms of durability. This functionality has an analogy to the health management system used for humans wherein the patient undergoes a sequence of periodic physical examination, preventive intervention, surgery, and recovery (Aktan et al., 2000). Thus one would expect that a health monitoring system not only provides an indication of “illness” but also enables an assessment of its cause and extent, as well as the effect of that illness.