rkgoel fatigue

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FATIGUE DESIGN OF WELDED CONNECTIONS FORRAILWAYBRIDGES

Goel Ravindra Kumar

(Presented in International Seminar organized by Indian Institute of Welding, Mumbai,

Feb. 2005)ABSTRACT

The design of the connections forms an important part of the overall designof a bridge structure. The bridge designer while designing the connectionshas to consider the factors such as optimum location of the joint, the type ofconnections bolted or welded, shop connection or field connection, sizes offabricated members and their transportation requirement to site besides thestrength considerations. Careful attention is required to be given to thestrength and fatigue behaviour of the chosen connection. Different types ofwelding techniques are used in fabrication of structural members depending

upon their strength requirements and criticality from fatigue consideration.The philosophy of fatigue design has been briefly discussed in this paperand the Indian Railway practice for design and manufacture of weldedcomponents for bridges has been presented. Shop welding has beenadopted by Indian Railways using submerged arc welding technique toreduce the number of rivets drastically. However, the field connections havestill been kept riveted. The performance of the welded connections providedhas been reported satisfactory. The different types of welded and nonwelded connections adopted have been listed and the need of research inassessing fatigue strength of welded connections in Indian conditions hasbeen identified.

KEY WORDS

Welded connections, fatigue behaviour, bridge structure, weldingtechniques, stress concentration, welded components, fatigue strength,submerged arc welding technique.

AUTHOR DETAILS

Ravindra Kumar Goel is Director/Steel Bridge in Bridges & StructuresDirectorate at Research Designs & Standards Organisation, Ministry ofRailways, Lucknow-226011 (U.P.), India. He has earlier worked as Dy.Chief Engineer/Bridges and In-charge of Bridge Workshop, NorthernRailway, Jalandhar Cantt. He has good experience of fabrication of steelstructures and bridges using riveted as well as welded connections.Fabrication of welded bridge girders was started at Bridge Workshop,Northern Railway, Jalandhar Cantt. under his guidance. He has alsodeveloped and implemented quality system ISO-9002 for fabrication of steelstructures and bridges at the Fabrication Workshop.


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1.0 INTRODUCTION

On Indian Railways, majority of bridge superstructures are of steel.Till 1985, only riveted connections were being used, as provenwelding technology and requisite infrastructural facilities for fabrication

of bridge girders was not available. The riveted connections tend tomake the structure heavy and thus uneconomical besides posingmaintenance problems. During the last two decades proven weldingtechnology has been established and use of welded connections hasbeen started on Indian Railways. Railway bridges are subjected toheavy dynamic loads and fluctuations of stresses may cause fatiguefailure of members or connections at lower stresses than those atwhich they would fail under static load. Such failures are primarilydue to stress concentrations introduced by constructional details. Alldetails are therefore to be designed to avoid as far as possible stressconcentration likely to result in excessive reductions of the fatigue

strength of connections. In view of above the welded connections aredesigned for non-critical locations only.

2.0 PHILOSOPHY OF FATIGUE DESIGN

2.1 Design Approaches & Design Input Requirement

Two major approaches are extensively followed in engineeringapplications, they are safe-life design and fail-safe design approach.The safe-life design approach aims at determination of life of thestructure, before the end of which the structure can be repaired,

replaced, or retired. At the same time, it has also been recognizedthat some structural damage are inevitable and failure would occurand that the catastrophic failure is rarely tolerable. Fail-safe designrecognizes that fatigue crack may occur and arranges the structure sothat cracks will not lead to failure of the structure before they aredetected and repaired. Multiple load paths and crack stopper built atintervals into the structure are some of the means to achieve fail-safedesign.

There are two primary groups of information that are necessary as aninput for a comprehensive fatigue analysis. One group of informationis the data related to the material behavior when subjected to cyclicloading, such as laboratory tests for constructing S-N curve, and otherfactors that would help to evaluate life of the structure. The laboratorytests must simulate the stress environment that the structuralcomponent will experience. The second group of information is thedetermination of the total number of cycles that the structure willundergo throughout its life (Load Spectrum). With these two groups ofinformation available, complete fatigue analysis can be done.


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The number of cycles to failure, N, obtained by S-N curve (under loador stress condition) is related to total life of the member up to failure.In reality fatigue cycles throughout the life of the structural partconsists of two phases crack initiation and propagation. Propagationmeans stable crack growth up to crack instability. The S-N curveapproach of assessment of fatigue damage does not separate the

crack initiation phase from the propagation phase. Thus it is assumedthat the crack is already initiated in the member and the total numberof cycle associated with crack propagation to failure is determined.

3.0 IRS METHODOLOGY OF DESIGN

3.1 Methodology

IRS methodology of fatigue design is based on stress ratio ofminimum and maximum principal stresses to be transmitted by theconnection. The allowable stress P depends on the ratio of minimum

stress fmin to maximum stress fmax, number of repetitions of stresscycle N, the method of fabrication and the type of connection. Indetermining the ratio fmin/ fmax gross area is used. To allow for theeffect of fatigue the allowable working stresses are determined from

Appendix G of IRS Steel Bridge Code. This appendix covers mild andhigh tensile steel fabricated for connected by welding, riveting orbolting. The allowable stresses given in the Appendix are theprincipal stresses at the point under consideration depending uponthe weld details. Thus in the design of girder web, the combinedeffect of bending and co-existent shear stresses is taken.

There are seven classes of details from A to G where A is the mostsuperior details and G is the most inferior detail (Figure-2)

The permissible fatigue stress, P is a function of s tress ratio (fmin/fmaxon gross area, No. of cycle N method of fabrication and type ofconnections where,

fmin = minimum force/stress primarily function of dead load.

(Figure 2) Detail class of connection

A B C D E F G

(Inferior most)(Inferior most)


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fmax = maximum force/stress primarily function of DL+LL+impact.

N = No. of cycles for the specified route/section.Permissible fatigue stress values are available for 0.6,2, 4 & 10 million of cycles for mild and high tensile steel.

A-G = Details of joints depends upon method of fabrication,type of connections, direction of force on the weld,location of weld in the member etc.

Note : For intermediate values of N, log interpolation ispermissible.

3.2 Design Steps

(1) For sections, depending upon the number of locomotives runperday the number of cycles of loading N is decided.

(2) The class of welding and method of fabrication to be compliedwith is already decided.

(3) The minimum stress or loads for different members for deadload is calculated by any acceptable method of structuralanalysis.

(4) The maximum stress or loads for different members for

DL+LL+Impact is calculated. The ratio of fmin/fmax is obtained.This may be (+) ve or (-) ve depending upon the nature ofstresses/force.

(5) The permissible stress in fatigue is picked-up from relevantclass of details depending upon N and ratio fmin/fmax.

The actual stress in the members must be less than thepermissible stress in the members.

4.0 PERMISSIBLE STRESSES IN WELDS

Since fatigue strength of welded structures depends upon theconstructional details, this is to be decided in consultant with thefabrication agency. It is apparent that any improvement in weldingtechnology adopted in the fabrication would be taken in to account atthe stage of deciding the construction detail. Accordingly, thepermissible stresses and the size of members and weld sizes aredetermined.


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Stresses due to dead load, live load and impact, stresses resultingfrom curvature and eccentricity of track, and secondary stresses asdefined in clause 3.3.2 (a) of IRS Steel Bridge Code, are consideredfor effects due to fatigue. All other items mentioned in clause 3.1 ofIRS Steel Bridge Code and secondary stresses as defined in clause

3.3.2 (b) thereof, are ignored when considering fatigue.

Permissible Stress in butt welds are not to exceed the permissiblestresses of the parent metal as specified in IRS Steel Bridge Code.However, all the butt welds are to be examined radiographically orany equally effective non-destructive test method

The basic permissible stress in fillet welds based on a thickness equalto the throat thickness is limited to 100 N/mm

2(10.2 kg/mm

2) where a

fillet weld is subjected to shear stress in two directions, the actualstress shall be taken as the vector sum of the separate shear stresses

and not to exceed 100 N/mm2

(10.2 kg/mm2

).

Load carrying fillet welds are designed such that the stress on thetotal effective area of fillet welds does not exceed the relevant valuesspecified in Table for Class G Constructional details, Appendix G toIRS Steel Bridge Code, subject to a maximum of 100 N/mm2 (10.2kg/mm2). These welds are also designed so that secondary bendingstresses are not developed (e.g. single lap joints shall not be used).

5.0 REDUCTION IN PERMISSIBLE STRESSES

5.1 The permissible stresses for field welds of structural members arereduced to 80%. As per existing policy field welds are not permittedfor bridges carrying road/railway loading. Thus all the welding inrailway bridges is limited to shop connections and all the fieldconnections are still riveted.

5.2 If over-head welds are unavoidable, the stresses permitted arereduced to 80% and further reduced to another 80% if field welding isinvolved.

5.3 In structures subjected to dynamic loading, tensile or shear stressesin butt welds is not permitted to exceed 66 2/3% of the permissiblestresses unless the welds are examined radiographically,ultrasonically or other non-destructive testing methods which areequally effective and present satisfactory evidence to the Engineerthat welds are meeting the quality requirement. These permissiblestresses are reduced to 80% for over-head welding and furtherreduced to 80% for field welding.


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6.0 WELDED & NON-WELDED CONNECTIONS ADOPTED

From different considerations all the connections in fabrication of steelbridges cannot be welded. Direction of welding with respect to thedirection of principal stresses plays an important role in determining

the class of connection. The basic permissible stresses aredetermined accordingly and the decision to adopt the connection aswelded is taken on the basis of relative advantages and economy.Sometimes, the classification of connection so determined, forces thedesigner to increase the complete cross-sectional area and theadvantages sought by adopting welded connections are nullified. Asall field connections are to be riveted one, the choice also depends onthe transportation facilities likely to be available from the shop to thesite. The different types of welded and non-welded connectionsadopted on IRS bridges are listed as under

Table 1

IRS Type of Connections

Welded Non-welded

1. Web to flange connectionof stringer/plate girder.

1. Intermediate stiffener to web inplate girders.

2. Connection of end stiffenerto web in plate girders

2. Lateral bracings (top & bottom)

3. Connection of horizontalstiffener to web not used.

3. All diaphragm connections togirders

4. Shear connector to topflange

4. Lateral connections such asbatten and lacings in built upmembers.

5. Longitudinal connections infabricating built upmembers of open webgirder bridges

6. Butt welds in web andflange

Out of these welded connections web to flange connection is the mainconnection involving major quantum of welding work. This connectionis designed to transmit the horizontal shear force combined with anyvertical loads which are directly applied to the flange. Where a load isdirectly applied to a flange, it is considered as dispersed uniformlythrough the flange to the web at a slope of two horizontal to onevertical. Butt welding has also been successfully adopted in place of


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spliced joints in plate girder bridges. The typical cross sections of buttwelded joints and the members built up by longitudinal fillet weldingusing submerged arc welding are shown in Figure 3.

(a) Typical cross section of butt weld

(b) Typical cross section of a welded plate girder


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(c) Typical cross sections of the built up members of a truss girder


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7.0 MATERIAL & CONSUMABLES USED

7.1 Raw Steel

Mild steel conforming to IS:2062 Grade B fully killed and fullynormalized/controlled cooled is permitted for use in dynamically

loaded structures. However, plates less than 12mm thick need not benormalized/controlled cooled. Wherever, the service temperaturesare likely to go below zero degree census steel conforming to IS:2062grade C is recommended ensuring impact properties at (-)20

0C to

(-)400C. There is a general difficulty in getting rolled sections like

angles, channels, I-sections etc conforming to IS:2062 in Grade B orC apparently due to a small requirement of construction industry.Therefore, rolled section in Grade A are permitted till such time theyare readily available in grade B/C.

High tensile steel complying the requirement of IS:8500 grade 540

(copper bearing quality) is prescribed for welded work. All finishedsteel is to be well and cleanly rolled to the dimensions, sections andmasses specified. The finished material is ensured to be reasonablyfree from surface flaws; laminations; rough/jagged and imperfectedges; and all other harmful defects.

(d) Typical cross sections of other members of a truss girder


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7.2 Welding Electrodes

All welding electrodes used for fabrication of welded components areto be strictly as per IRS M-28 & IRS M-39 for metal arc welding andsubmerged arc welding wire flux combination respectively. Welded

construction work is to be carried out generally in accordance with theprovisions of Indian Railway Standard Welded Bridge Code. Theelectrodes have been classified into 20 different classes. Thepurpose for which, each class of electrode is to be used, together withthe range of codings is given in of IRS M:28-1976. Filler wire for CO2welding should be as per RDSO specification for CO2 welding fillerwire (Tentative). RDSO issues periodical list of approved suppliersof electrodes for metal arc welding.

8.0 WELDING TECHNIQUES

8.1 All welds are done by submerged-arc welding process either fullyautomatic or semi-automatic. Carbon dioxide welding or manualmetal-arc welding may be done only for welds of very short runs or ofminor importance or where access of the locations of weld do notpermit automatic or semi-automatic welding.

8.2 Except for special types of edge preparation, such as single anddouble U the plates which are to be joined by welding may beprepared by using mechanically controlled automatic flame cuttingequipment and then ground to a smooth finish. Special edgepreparation is made by machining or gouging.

9.0 WELDING PROCEDURES

9.1 The welding procedure is to be such as to avoid distortion andminimize residual shrinkage stresses. Properly designed jigs areused for assembly. The welding techniques and sequence, quality,size of electrodes, voltage and current required are monitored asprescribed by manufacturers of the material and welding equipment.

9.2 Site welding is not to be undertaken except in special circumstanceswith the approval of the Engineer. Site welding should be confined toconnections having low stresses, secondary members, bracings etc.

9.3 Manual metal arc welding is permitted with adequate precautions asper IS:9595 and under strict supervision of competent supervisor.


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10.0 SEQUENCE OF WELDING AND WELD PASS

Distortions may occur due to heat emission during welding process.To avoid such distortions proper sequence of welding is followed.The correct sequence is quite often developed with experience forwhich frequent interaction of designer with the fabrication is

necessary. Some of the cases are illustrated as under:

10.1 For fabrication of welded composite girders, channel shearconnectors shall be welded on top flange plate prior to assembly of I-section. This facilities correction of any distortion of flange platedeveloped during the welding of channel shear connectors.

10.2 In making of a typical I-section four fillet welds are to be made. Thewelding sequence to be followed is indicated by number 1 to 4 asshown in the Figure 4.

10.3 Whenever a square butt weld in a 10 or 12mm thick plate is requiredto be made, the sequence to be adopted is shown in Figure 5.

2

1

10mm

or 12mm

(Figure 4) Sketch showing sequence of square butt welding

(Figure 5) Sketch showing the sequence of fillet welding for fabricating the I-section

42

3 1


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11.0 QUALITY CONTROL

To ensure required quality of welded connections, a proper qualityassurance programme is to be decided before starting the fabrication.To ensure that quality of fabrication does not suffer in any way,inspection of bridge girders, has been entrusted to Research, Design

and Standards Organisation, Ministry of Railways, Lucknow. RDSOlooks after in detail the various aspects involved in the fabrication andinspection of these girders (such as specification of steel, weldingprocess, fabrication techniques, stage inspection etc.) to maintain thequality of not only the end product but quality of the process as awhole.

12.0 CONCLUSION:

Indian Railways is in the process of adopting more and more weldedconnections for design of railway bridges. So far the welding has been

used to make shop connections in fabricating individual bridgemembers. The connections have been made using submerged arcwelding technique and besides reducing the dead weight, these arefound quite convenient from the maintenance point of view. However,the use has been restricted to shop welding only, that too for noncritical locations because of proneness of welded connections tofatigue failure. So far, the welded connections have been usedextensively at the location of web to flange connections of stringersand plate girder bridges. Longitudinal fillet welding has also beenused extensively to fabricate built up members of open web girders.

Further research is required to correctly assess the fatigue strength ofdifferent types of welded connections under different type of loadingconditions. There is also a need to improve the reliability of weldedconnections for increasing its use in Railway bridges which aresubjected to dynamic loading. Any improvement in the weldingtechnology adopted and its reliability from fatigue consideration willgreatly help the designers in adopting welded connections for othercritical locations also.

13.0 ACKNOWLEDGEMENT

The author gratefully acknowledges the encouragement and supportprovided by Executive Director (B&S), RDSO in preparation of thispaper. The assistance provided by Shri A.K. Pandey, SectionEngineer and Smt. Suman Verma, Steno Grade-1 of B&S Dte.,RDSO/Lucknow is also thankfully acknowledged.

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