Assessment of Existing Steel Structures: Recommendations for ...

More documents

Recommendations

Info

Assessment of Existing Steel Structures, Remaining Fatigue Life First edition 2008Cast ironWrought steelPuddled steelMild steel(19th century)Mild steel(20th century)Sulphurprint(Baumannprint)Content of sulphur isdepending on cokequalitySlag segregation linescontaining phosphorusand sulphurCore segregation containingphosphorus and sulphurLow content ofphosphorus and sulphurMicrostructure~1:400ChemicalanalysisTensiontestCast iron withlamellar graphiteC ≈ 2,0-4,0 %Mn ≈ 0,2-1,2 %Si ≈ 0,3-3,0 %S < ≈ 1,2 %P < ≈ 1,0 %Very brittle, almostno plasticityFerritic, Inhomogeneousgrain sizedistribution, Oxideinclusions, Slag linesVery variableC < ≈ 0,08 %Mn < ≈ 0,4 %S < ≈ 0,04 %P < ≈ 0,6 %Ferritic-pearlitic, Increasinggrain size from the edge tothe core, Oxide and sulphideinclusionsBessemer / Thomas steelC ≈ 0,02-0,1 %Mn ≈ 0,3-0,5 %S < ≈ 0,1 %P≈0,04-0,07%(B)/ -0,12%(T)Siemens-Martin steelC ≈ 0,05-0,15 %Mn ≈ 0,2-0,5 %S ≈ 0,02-0,15 %P ≈ 0,03-0,06 %Homogenous small grainLow-alloyed steel(T, SM)C ≈ 0,1-0,2 %Mn ≈ 0,4-0,5 %Si ≈ 0,01 %Blast Process: N>≈0,01%, Hearth Process: N≈0,08%, Thomas steel: Si ≈ 220 N/mm 2R m ≈ 370-440 N/mm 2ε > ≈ 20 %Local neckingLow-alloyed steelR e ≈ 240-280 N/mm 2R m ≈ 370-450 N/mm 2ε ≈ 15-25 %Table 3-2:Information on material characteristics of old iron and steels1) Literature [Lit. 96] gives also higher values up to 260 N/mm²; 2) elongation at rupture30
Assessment of Existing Steel Structures, Remaining Fatigue Life First edition 2008(9) Based on a statistical analysis (hypothesis testing) of the chemical and the microstructureproperties of specimens from riveted bridges found in literature [Lit. 11, Lit. 12, Lit. 13, Lit.124] it is concluded, that the obtained data for the strength and toughness of wrought steel canbe treated as a statistical homogenous population. The statistical distribution of the materialstrength has been derived from an amount of 205 tests at 0°C and 283 tests at -30°C.(10) The results of the statistical evaluation of old steels (except wrought steel) can be summarisedas follows:+10 °C R eL,5% = 229 N/mm² ≈ 230 N/mm²0 °C R eL,5% = 248 N/mm² ≈ 240 N/mm²J Mat, Modell, 5% = 30 N/mm-30 °C R eL,5% = 257 N/mm² ≈ 250 N/mm²J Mat, Modell, 5% = 17.5 N/mm ≈ 17 N/mm(11) Beside this, for wrought steel a tension strength of R eL,5% = 203 N/mm² ≈ 200 N/mm² at+10 °C was found.(12) If material testing on specimens from the bridge are to be avoided, a conservative safetyassessment for moderate climates may be carried out with the combination of 5% fractiles for-30 °C for J Mat -values and with the combination of 5% fractiles for 0 °C for R eL -values.(13) The modulus of elasticity (E) should be taken as 210.000 N/mm² for all kinds of steel.(14) If fracture mechanics approach on a riveted structure is applied in an expert investigation(Phase III), the following lower limit (5% fractile) for the threshold value of the cyclic stressintensity factor, ΔK th , can be used in crack propagation calculation:3 / 2Δ K = 63N / mm ≈ 2MPa m (constantly for all R-ratios).th(15) The comparison of the threshold values found at other research institutes with the lowerbound value mentioned above shows that a specific testing of the material characteristics ofthe examined bridge may also result in higher threshold values.(16) According to BS PD 6493 [Lit. 14] the threshold value ΔK th for modern steels can beestimated under consideration of a variable R-ratio using ( 3–1).− 1 < R ≤ 0.5 : ΔK0.5 < R ≤ 1 : ΔKthth( R)= 170 − 214 ⋅ R= 63N / mm3 / 2≈ 2MPain N/mmwhere R is the ratio of the minimum stress σ min,i to the maximum stress σ max,i .m3/ 2( 3–1)(17) It has to be noted that a consideration of the relationship of the threshold value to the R-ratio,especially with a R-ratio smaller than 0.5, can lead to noticeable more favourable resultsduring the crack growth calculation. However in most cases it is rather difficult or sometimeseven impossible to estimate the actual R-ratio of a structure [Lit. 63]. Referring to old steelsthe following quantitative assumptions can be made:• Wrought steel has higher ΔK th -values but the da/dN-ratio is also higher in comparison tomild or killed steel.• Thus, the slope of the Paris law, characterised with the below given material constants Cand m, is steeper for wrought steel than for mild steel, so at the end one should onlycarefully take profit from the higher ΔK th -values for wrought steel.31
Page 3: Assessment of Existing Steel Struct
Page 7 and 8: AcknowledgementsBoth ECCS and the J
Page 9 and 10: Assessment of Existing Steel Struct
Page 29: Assessment of Existing Steel Struct
Page 96 and 97:
Assessment of Existing Steel Struct
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108:
The mission of the JRC is to provid
show all

Assessment of Existing Steel Structures: Recommendations for ...

Create successful ePaper yourself

Delete template?

Save as template?