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Assessment of Existing Steel Structures, Remaining Fatigue Life First edition 2008Keyword in: EnglishGermanFrenchMild steel (19 th century)Flussstahl (19. Jhd.)Acier doux (19. siécle)Mild steel (20 th century)Flussstahl (20. Jhd.)Acier doux (20. siécle)Assessment spectrumBeurteilungsspektrumSpectre d’évaluationFatigue strength curveErmüdungsfestigkeitskurveCourbe de résistance à lafatigueDetail categoryKerbdetail, Kerbklasse,KerbfallCatégorie de détail1.6 General assessment procedureDefinitionLow-carbon steel produced by blast process (Bessemer or Thomasprocess) or hearth process (Siemens-Martin process), see section 3.4.Excess carbon and other impurities were oxidised from pig iron toproduce steel, which was normally un-killed (rimmed). In theBessemer process only low-phosphorus pig iron could be treated byacid lining of the converter whereas in the Thomas process also pigiron with a high content of phosphorus could be converted because ofa basic of the converter. In the Siemens-Martin process pig iron couldbe melted together with steel scrap and/or iron ore. Metallurgicaltreatment was possible in the hearth furnace. Siemens-Martin steel hasa lower content of nitrogen than Bessemer or Thomas steel.Low-carbon low-alloyed steel produced in the end of the 19 th and thebeginning of the 20 th century using the Thomas or Siemens-Martinprocess. This kind of steel has properties close to those of modernmild steels (S235, S355), see section 3.4. Depending on the degree ofdeoxidation the steel can be un-killed (rimmed), semi-killed or killed.The total of all stress-range spectra in the remaining fatigue life of anassessed structure, which are relevant to the fatigue assessment.The quantitative relationship between the stress range and number ofstress cycles to fatigue failure, used for the fatigue assessment of aparticular category of structural detail. The fatigue strengths given inthese recommendations are lower bound values based on theevaluation of fatigue tests with large scale test specimens.The numerical designation given to a particular detail for a givendirection of stress fluctuation, in order to indicate which fatiguestrength curve is applicable for the fatigue assessment. The detailcategory number in these recommendations indicate the referencefatigue strength Δσ c in N/mm² related to a specific endurance (2·10 6load cycles) as it is defined in Eurocode 3, Part 1-9, too.(1) The assessment of an existing structure aims at producing evidence that it will function safelyover a specified residual service life. It is mainly based on the results of assessing hazards andload effects to be anticipated in the future, and of assessing material properties and geometrytaking into account the present state of the structure [Lit. 8]. Guidelines for existing structuresexist in a large number of countries. Thereby many countries have presented documents forparticular categories of structures. In Canada, Germany, the Netherlands, Switzerland, UKand USA such guidelines have been prepared at a detailed level. In any assessment, theproblem of fixing risk acceptance criteria is difficult since it must be compatible to codes fornew structures (limit state analysis, safety factor format, etc.), and with national determinedparameters (generally partial safety factor values). The engineer has to think about riskacceptance criteria and is referred to section 1.7 for more information.(2) The process of any structural assessment can be separated in the four phases, see Fig. 1-6 andfor example [Lit. 8, Lit. 9, Lit. 124].8
Assessment of Existing Steel Structures, Remaining Fatigue Life First edition 2008Phase I Preliminary Evaluation: the aim is to remove existing doubts about safety of thestructure using fairly simple methods and identify critical parts or members in thestructure. This is performed by gathering information on the structure from drawingsand design computations, carrying out a site visit, etc. The assessment should be carriedout by the engineer alone. Assessment is performed by using current codes and bymaking conservative assumptions where information is lacking or doubtful.Phase II Detailed investigation: the aim is to update information and to carry out refinedassessments only for those members where safety is not ensured. This is done by doingquantitative inspections (for example using easy to use, low tech NDT methods), the useof updated values for loads, resistance, as well as more accurate models (static system,structural behaviour). Here, in addition to the engineer, a specialised firm or agency orindividual experts are generally called in.Phase III Expert investigation: for problems with large consequences in terms of risks or of costsrelated to a decision, a team of experts should be called in order to check carefully theconclusions and proposals reached in Phase II. Discussions and further assessmentsusing specific tools (high tech NDT methods, probabilistic methods, fracture mechanics,etc.) can also be carried out to help in reaching decisions.Phase IV Remedial measures: the aim is to propose measures to have a fit for service structurewith sufficient safety. Different measures can be taken, among them one can mention:intensification of monitoring, reduction of loads or change in use, strengthening, repairor rehabilitation. The choice of the measures to be taken will be function of the structurestudied but in any case the proof of adequacy of the measures to insure safety must beshown.(3) These phases will be further developed - with a focus on existing steel bridges and fatigue - inchapter 2.(4) The problem of existing bridges and of their assessment have recently increased. Indeed, thecurrent low funding in the infrastructure sector of many European countries forces the ownersas well as the operators to postpone investments in new road and railway bridges andconsequently stretch the service life of their existing old structures. Therefore, the owner ofthe infrastructure nowadays sees itself facing two main challenges: need of a furthercontinuing safe operation of the ageing bridges and of cost effective maintenance. Methodsmust be provided that enable engineers to offer safe and cost effective assessment andmaintenance methods to their clients.9
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The mission of the JRC is to provid
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