Patologia estruturas

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The Identification of damage in Large Structures
By Alan P. Jeary D.Sc., Ph.D., CEng., FIStructE, FAIB, FRMetS, MAIBS, MIEE. University of Western Sydney, Australia. Introduction It is necessary, when considering the identification of damage in large structures, to be very precise about both what constitutes damage, and of the precise requirements for successful identification. Theseconsiderations are not trivial, our classical identification of damage is often unhelpful in an assessment of the state of health of a structure. Additionally, precisely what leads to successful identification of significant damage is a question that bears further investigation. Our appreciation of forensic engineering, as it applies to civil structures, is in its infancy. The tools available to us tend todeal with localized damage, and as such can be of limited used in an overall (holistic) assessment of the structure. Whilst the tools available for structural investigation are technologically good, information available for the assessment of complete structures has to be pieced together from the clues left by investigations using such devices as X-rays, ultrasound, holography, thermography, radar,and eddy currents. In each of these cases the techniques have the advantage of being non-destructive, but a disadvantage of only yielding local information. Tools available for the assessment of complete structures tend to involve destructive techniques. When disassembling a structure is possible to perform a detailed analysis of the state of health of joints within that structure. Clearly theproblems associated with destructive testing and do not allow an in-service assessment of the structure. Recently developed techniques, most of which have come from developments from space programs, and system identification of mechanical systems, have recently been used within the civil engineering arena to successfully identify significant damage within large structures. This paper addresses theseproblems, and analyses the requirement for the successful identification of significant damage within large structures. 1. Classical indicators of damage The first issue to consider is precisely what indicators of damage we would consider to be useful. Traditionally, structural investigators or surveyors have relied upon observation of the presence of cracking to give indications of structuraldistress. The careful noting of crack patterns has been followed by an analysis, possibly backed by computer modeling, of the likely extent of damage. Knowledge of yield line theory can be helpful in this analysis. In 1923 it was observed [Griffith, 1923] that materials do not achieve their theoretical maximum strength and imperfections within materials accounted for this loss of strength. Thescience of fracture mechanics quantified this effect as a result of Taylor’s early work, and much progress has subsequently been made. In practice, all materials contain a myriad of 1

micro-imperfections, each one of which can be made to work (to dissipate energy) when stresses appear on the surface of the imperfections. The larger the imperfection the smaller is the stress necessary to make theimperfection work. This working of the imperfection may appear as damage (e.g. an increase in a length of the imperfection), as a chemical change, or as a weakening of a process zone, which appears in front of a nascent crack. In each of these cases the working of the imperfection involves a sink for energy. In addition, investigators have relied on the retrieval of cored samples followed byassessment of the mechanical properties of the core, normally in some form of crushing machine. Such an approach leads to an assessment of the mechanical properties of small parts of the structure. The analyst is then faced with the task of piecing together this information, either intuitively or with the help of computer package, so as to form a picture of the health of the entire structure. 2....
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