1、PD 6695-1-9:2008 Recommendations for the design of structures to BS EN 1993-1-9 ICS 91.010.30 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW PUBLISHED DOCUMENTPublishing and copyright information The BSI copyright notice displayed in this document indicates when the document
2、was last issued. BSI 2008 ISBN 978 0 580 57643 0 The following BSI references relate to the work on this standard: Committee reference B/525/10 and B/525/31 Publication history First published May 2008 Amendments issued since publication Amd. no. Date Text affected PD 6695-1-9:2008 BSI 2008 i PD 669
3、5-1-9:2008 Contents Foreword ii 1 Scope 1 2 Material and execution tolerances and inspection 1 3 Derivation of specific fatigue loading models 4 4 Determining fatigue strengths from tests 4 5 Assessment methods for fatigue design 6 6 Use of nominal, modified nominal and geometric stress ranges 8 7 S
4、tress concentration factors 16 8 Determination of fatigue load parameters and verification formats 17 Bibliography 20 List of figures Figure 1 Method of indicating quality requirements higher than Fat 56 on drawings when using BS EN 1090-2 for execution 3 Figure 2 Effect of stress concentrations on
5、nominal and modified nominal stresses 10 Figure 3 Example of hot spot stresses in a tubular lattice joint load applied in the brace 14 Figure 4 Typical stress concentration factors from radiused corners in flat plate (from BS 5400-10:1980) 16 Figure 5 Procedure on cycle counting using the reservoir
6、method 19 List of tables Table 1 Fatigue class requirement in BS 5400-6 corresponding to minimum required detail category in BS EN 1993-1-9 2 Table 2 Fatigue class requirement corresponding to minimum required detail category in BS EN 1993-1-9 2 Summary of pages This document comprises a front cover
7、, an inside front cover, pages i and ii, pages 1 to 20, an inside back cover and a back cover.PD 6695-1-9:2008 ii BSI 2008 Foreword Publishing information This Published Document is published by BSI and came into effect on 30 May 2008. It was prepared by Subcommittee B/525/10, Bridges, in consultati
8、on with B/525/31, Structural use of steel, under the authority of Technical Committee B/525, Building and civil engineering structures. A list of organizations represented on these committees can be obtained on request to their secretary. Relationship with other publications This Published Document
9、is a background paper that gives non-contradictory complementary information for use in the UK with part 1-9 of the Eurocode for the design of steel structures, BS EN 1993, and its National Annex. Presentational conventions The provisions in this Published Document are presented in roman (i.e. uprig
10、ht) type. Its recommendations are expressed in sentences in which the principal auxiliary verb is “should”. Commentary, explanation and general informative material is presented in smaller italic type, and does not constitute a normative element. The word “should” is used to express recommendations
11、of this Published Document. The word “may” is used in the text to express permissibility, e.g. as an alternative to the primary recommendation of the clause. The word “can” is used to express possibility, e.g. a consequence of an action or an event. Contractual and legal considerations This publicat
12、ion does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a Published Document cannot confer immunity from legal obligations. BSI 2008 1 PD 6695-1-9:2008 1 Scope This Published Document gives non-contradictory compl
13、ementary information for use in the UK with BS EN 1993-1-9 and its UK National Annex. 2 Material and execution tolerances and inspection BS EN 1993-1-9:2005, 1.1(2) 2.1 General The safety of structures subjected to fatigue loading is generally more dependent on deviations in materials and workmanshi
14、p than structures subject only to static loading. This is because fatigue life is very sensitive to local stress raisers such as joint misalignment or out-of-flatness, which cannot be discounted on the grounds of plastic redistribution (as is the case with ULS failure modes such as buckling and rupt
15、ure). Fatigue life is also particularly sensitive to pre-existing crack-like imperfections close to sites of potential fatigue initiation, as they in effect eliminate much of the early propagation life. For this reason workmanship and inspection requirements for execution are generally made more sen
16、sitive as the cyclic stress levels and loading frequency increase. 2.2 Implementation with materials and specification to BS 5400-6 In BS 5400-6, four levels of quality for welds have been specified depending on the magnitude and frequency of the cyclic stresses. For low levels of cyclic stressing w
17、hich would be acceptable for a curve conforming to BS 5400-10:1980 Class F2 SN ( %B c = (60) curve (see Table 1), the lowest level is acceptable. This is termed “unspecified” because the areas of the structure which would tolerate a fatigue Class F2 SN curve or lower (G or W) do not have to be ident
18、ified. Only if the degree of stressing is such that a higher fatigue SN curve than F2 is required does the minimum class requirement have to be identified on the drawings at that location in the structure. The method of doing this is by, using a “Fat” reference and an arrow on the drawings to denote
19、 the extent affected. The three minimum class requirements higher than “unspecified” are Fat F, Fat E and Fat D. In order to derive the minimum required Fat class from the relevant SN curve in BS EN 1993-1-9, which is denoted by the reference fatigue strength c , an approximate correlation has been
20、used in the National Annex. This is shown in Table 1.PD 6695-1-9:2008 2 BSI 2008 NOTE This correlation is nothing to do with the detail category at that point in the structure. A particular detail may be classified in BS EN 1993-1-9:2005, Table 8 as having a detail category of (say) 80. However, if
21、the fatigue stressing at that point is so moderate that detail category of (say) 36, i.e. c= 36 N/mm 2 , would still give an acceptable life, then the minimum class requirements would be “unspecified”. It is important not to over-specify or under-specify the minimum class requirement. The latter mig
22、ht lead to unsafe structures in later life. The above principle may be applied to structures other than bridges, where fatigue needs to be considered. Alternatively, ISO 10721-2 may be applied until such time as EN 1090-2 is published as BS EN 1090-2. 1) 2.3 Implementation with materials and workman
23、ship specification to BS EN 1090-2 1) The same principles as described in 2.2 should be applied except that the Fat quality designation system is numerical instead of alphabetical and the term “minimum class requirement” is replaced by “quality requirement”. The levels, which are numerically differe
24、nt from some of those in Table 1, are as shown in Table 2. The above quality designations are in alignment with those in ISO 10721-2 and assume that full static stressing will be applied as permitted by BS EN 1993-1-9. Table 1 Fatigue class requirement in BS 5400-6 corresponding to minimum required
25、detail category in BS EN 1993-1-9 Minimum class requirement (BS 5400-6:1999) r N curve (BS 5400-10:1980) rat N = 2 10 6 (BS 5400-10:1980) c (BS EN 1993-1-9:2005) N/mm 2 N/mm 2 Unspecified F2 60 56 Fat F F 68 63 Fat E E 80 80 Fat D D 91 90 1) At the time of publication of this part of PD 6695, EN 109
26、0-2 is still in preparation. The equivalent British Standards should be used until BS EN 1090-2 is published. Table 2 Fatigue class requirement corresponding to minimum required detail category in BS EN 1993-1-9 Quality requirement Required value of reference fatigue strength c Indicate on drawings
27、(see Figure 1) Fat 56 cu 56 N/mm 2 No Fat 71 56 c u 71 N/mm 2 Ye s Fat 90 71 c u 90 N/mm 2 Ye s Fat 112 90 c u 112 N/mm 2 Ye s Fat 140 112 c u 140 N/mm 2 Ye s BSI 2008 3 PD 6695-1-9:2008 Fat 56 is the new “unspecified” level. All zones of the structure requiring quality levels above Fat 56 should be
28、 indicated on the relevant drawings according to the method shown in Figure 1. The directions of the arrows are parallel to the direction of the relevant stress fluctuation. 2.4 Assurance of quality Fracture mechanics calculations confirm that when the cyclic stressing is sufficiently high that the
29、minimum class requirement exceeds 80 N/mm 2or thereabouts, the acceptable sizes of planar fabrication flaws such as cracks of lack of fusion, when orientated normal to stress direction, are generally no more than about 1 mm to 2 mm in height. For higher stress levels the acceptable sizes rapidly red
30、uce to fractions of a millimetre. Such sizes are not detectable by eye, even if surface breaking, and are close to the threshold of reliable detection and evaluation by normal commercial non-destructive testing techniques. They are also of a size which many welding processes operated under normal co
31、mmercial shop (or site) conditions cannot be relied upon not to leave in the weld. In joints with difficult access (both for welding and NDT) the sizes of flaws which are likely to occur and which might not be detected might be two or three times the above size. This leaves a problem of how the qual
32、ity required to sustain these high cyclic stress levels in a safe life design can be assured. The above problem can be overcome by restricting the detail category for certain details to a level where there is an improved probability of attainment of the required quality and a high probability of det
33、ection and correct evaluation. This applies not only to certain high category transverse and longitudinal welds, but also to flame cut edges and plain surfaces. The latter can be susceptible to corrosion pitting and very minor accidental damage. Where safe life design applies and fatigue stressing i
34、s very high, and where higher categories have to be used, it is recommended that the special acceptance criteria and methods of fabrication control and inspection are agreed with the relevant experts at the design stage. Alternatively the damage tolerant method might have to be applied. Figure 1 Met
35、hod of indicating quality requirements higher than Fat 56 on drawings when using BS EN 1090-2 for execution Fat 71 Fat 71 Fat 90 Fat 90 Fat 71PD 6695-1-9:2008 4 BSI 2008 3 Derivation of specific fatigue loading models BS EN 1993-1-9:2005, 2(2) Loading for fatigue should normally be described in term
36、s of a design load spectrum, which defines a range of intensities of a specific live load event, the method of application, and the number of times that each intensity level is applied during the structures design life. If two or more independent live load events are likely to occur, the sequence an
37、d phasing between them should be specified. Where no published data for live loading exist, load history data should be obtained from existing structures subjected to similar effects. Alternatively, loading data can be inferred by analysis of the response of continuous strain or deflection measureme
38、nts over a suitable sampling period. Dynamic magnification effects where loading frequencies are close to one of the natural frequencies of the structure should be taken into account. In this situation the partial safety factors for fatigue load intensity for safe life design should take into accoun
39、t the degree of confidence in the prediction of the design load spectrum from the available data. Recommended values of Ffare given in the National Annex to BS EN 1993-1-9. 4 Determining fatigue strengths from tests BS EN 1993-1-9:2005, 2(4) 4.1 General The guidance given in 4.2 to 4.7 is based on B
40、S EN 1999-1-3:2006, Annex C. 4.2 Test specimens Test specimens should represent the intended application detail as closely as practicable, with regard to material, dimensions, manufacturing procedures and workmanship quality limits allowed by BS EN 1090-2 and BS EN 1993-1-9. These features should be
41、 measured using the methods specified in BS EN 1090-2. 2) 4.3 Testing conditions The loading mode should represent the most likely loading mode expected to be applied to the detail in the structure. Any environmental conditions outside the scope of BS EN 1993-1-9 should be simulated. 2) At the time
42、of publication of this part of PD 6695, EN 1090-2 is still in preparation. The equivalent British Standards should be used until BS EN 1090-2 is published, and any other agreed alternative methods should be specified for the individual projects. BSI 2008 5 PD 6695-1-9:2008 4.4 Instrumentation The te
43、st specimen should be strain gauged in the region(s) of expected fatigue initiation in such a way that the nominal, modified nominal or hot spot stress at the initiation site can be determined. Load intensity should be measured continuously and turning points counted. Crack growth length (and in som
44、e cases depth) should be monitored using appropriate NDT methods when damage tolerant design data are sought. 4.5 Loading history Variable amplitude loading histories should be representative of the design load spectrum. Constant amplitude testing for the purposes of deriving an N design curve shoul
45、d employ a selection of load intensities to provide endurance data in the range of 10 5to (5 10 6 ) cycles. A minimum of ten finite endurance data points should be obtained (i.e. non-run-outs). The mean load level should be selected to ensure that tensile mean stress conditions at the initiation sit
46、e are representative of the upper bound likely to be experienced in the structure, taking into account lack of fit forces and residual stress effects not incorporated in the test specimen. 4.6 Monitoring of test The test specimen and loading conditions should be monitored at regular intervals to ver
47、ify that the data specified in the National Annex to BS EN 1993-1-9 are being correctly recorded. 4.7 Analysis of results The fatigue fracture face should be examined for evidence of material or manufacturing discontinuities particularly close to the initiation site. In the derivation of design data
48、 for general use, allowance should be made for the following effects, where they are not adequately represented in the test specimens: a) lack fit and residual stresses; b) dimensional tolerances and scale effects; c) manufacturing procedures; d) material and workmanship imperfections, taking into a
49、ccount the acceptance criteria and methods of inspection in the execution standard; e) environment.PD 6695-1-9:2008 6 BSI 2008 5 Assessment methods for fatigue design BS EN 1993-1-9:2005, 3(1) 5.1 General BS EN 1993-1-9 offers two main methods for fatigue assessment. These are “safe life” and “damage tolerant” methods. The essential difference between the two is that a prescribed inspection and maintenance regime for detecting and correcting fatigue damage is implemented through the design l
