Eccs Cec m e k s

ECCS CEC M E K S

EUROPEAN CONVENTION FOR CONSTRUCTIONAL STEELWORK CONVENTION EUROPÉENNE DE LA CONSTRUCTION MÉTALLIQUE EUROPÄISCHE KONVENTION FÜR STAHLBAU

ECCS – Technical Committee 6 – Fatigue

Good Design Practice A Guideline for Fatigue Design

FIRST EDITION

2000

N° 105

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Good design practice

ISBN :92-9147-000-46 Copyright © 2000 by the European Convention for Constructional Steelwork All right reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Copyright owner : ECCS General Secretariat CECM Avenue des Ombrages, 32/36 bte 20 EKS B-1200 BRUSSELS (Belgium) Tel. 32/2-762 04 29 Fax 32/2- 762 09 35 http://www.steelconstruct.com ECCS assumes no liability with respect to the application of the material or information contained in this publication.

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SUMMARY This guideline has been designed for project managers in design offices, engineers in steel construction companies and construction survey engineers concerned with the manufacture of structures subjected to fatigue loads induced by frequently changing actions, traffic actions, wind induced oscillations or comparable actions. Contained herein is a review of the current knowledge in fatigue design and the fabrication of fatigue resistant structures. This document contains information about design that is in conformity with the currently available Eurocode 3 prestandards, which deals with the design of steel structures. Furthermore, it contains information about fabrication aspects not covered in the Eurocodes. The document should be viewed as a source of advice to be consulted before designing, fabricating, or repairing a structure subjected to fatigue. The document is organised as follows: • Chapter 2 : basic fatigue theory, modelling of fatigue actions and strength. The reader interested only in the design and fabrication aspects of fatigue resistant structures should go directly to the next chapters. • Chapter 3 : factors affecting fatigue controlled by the designer. • Chapters 4 and 6 : factors controlled by the fabricator/assembler. • Chapter 5 : existing weld improvement methods, that is methods for increasing the fatigue strength of selected details. • Chapter 7 : principles of the fitness-for-purpose approach, advice on methods for repairing structures during fabrication, erection, or repairing existing structures.

RESUME Ces recommandations ont étés rédigées pour les chefs de projets dans les bureaux d’étude, les ingénieurs travaillant dans des entreprises de constructions métalliques ainsi que ceux chargés du suivi de chantiers et traitent de la fabrication de structures soumises à des sollicitations de fatigue produites par des charges de trafic, des oscillations dues au vent, ou .d’autres actions variant fréquemment. Le lecteur trouvera dans ici un état des connaissances actuelles en matière de conception à la fatigue et de fabrication de structures résistantes à la fatigue. Ce document contient des informations en conformité avec les prénormes Européennes qui concernent les structures métalliques actuellement disponibles. De plus, des recommandations quant à la fabrication, sujet qui n’est pas traité dans les Eurocodes, sont également fournies. Ce document devrait être considéré comme une source de conseils à consulter avant de concevoir, fabriquer ou réparer une structure sujette à des sollicitations de fatigue. Il est organisé de la manière suivante : • Chapitre 2 : théorie de base en fatigue, modélisation des sollicitations et de la résistance en fatigue. Le lecteur uniquement intéressé par la conception et la fabrication de structures résistantes à la fatigue devrait passer directement aux chapitres suivants. • Chapitre 3 : facteurs influençant la résistance à la fatigue liés à la conception. • Chapitres 4 et 6 : facteurs influençant la résistance à la fatigue sous contrôle du fabricant ou du monteur. • Chapitre 5 : méthodes existantes de parachèvement des soudures, c’est-à-dire permettant d’accroître la résistance à la fatigue de certains détails choisis. • Chapitre 7 : principe de l’adéquation qualité-but (fitness-for-purpose), conseils sur les méthodes de réparation de structures durant leur fabrication, montage, ou de réparation de structures en service.

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ZUSAMMENFASSUNG Die vorliegenden Empfehlungen richten sich an Ingenieure in Planungsbüros und Stahlbaufirmen sowie an Ingenieure, die unmittelbar an der Bauausführung beteiligt sind. Sie beziehen sich auf die Herstellung von Tragwerken, die infolge Verkehrslasten, Wind oder anderen häufig wiederholten Einwirkungen auf Ermüdung beansprucht werden. Es wird ein Überblick über den aktuellen Stand der Kenntnisse bezüglich ermüdungsgerechtem Entwerfen und der Herstellung ermüdungssicherer Tragwerke gegeben. Die Hinweise in diesem Dokument stimmen mit den derzeitigen europäischen Eurocode 3 - Vornormen für Bemessung und Konstruktion von Stahlbauten überhein. Zudem werden Empfehlungen für die in den Eurocodes nicht behandelte Herstellung von Tragwerken gegeben. Dieses Dokument soll als Ratgeber für Entwurf, Herstellung oder Reparatur ermüdungsbeanspruchter Tragwerke dienen und ist wie folgt aufgebaut : • Kapitel 2: Grundlagen der Materialermüdung, Modellbildung von Ermüdungsbeanspruchungen und Ermüdungsfestigkeit. Leser, die sich nur für den Entwurf und die Herstellung ermüdungssicherer Tragwerke interessieren, können dieses Kapitel überspringen. • Kapitel 3: Einflussfaktoren bezüglich Ermüdungsfestigkeit beim Entwurf. • Kapitel 4 und 6: Einflussfaktoren bezüglich Ermüdungsfestigkeit bei Herstellung oder Montage. • Kapitel 5: Nachbehandlung von Schweissnähten zur Erhöhung der Ermüdungsfestigkeit ausgewählter Konstruktionsdetails. • Kapitel 7: Grundlagen des fitness-for-purpose Ansatzes, Empfehlungen bezüglich Verfahren zur Reparatur von Tragwerken während Herstellung und Montage oder im Gebrauchszustand.

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PREFACE In the 60 and 70’s, a great deal of research was focused on the effects of repetitive loading on steel structures such as bridges or towers. This work, as well as lessons learned from the poor performance of some structures, has led to a better understanding of fatigue behaviour and to substantial changes in fatigue provisions of steel structures design specifications. It is not, however, sufficient for the design engineer to choose a fatigue resistant detail to insure fatigue safety. The aspects related to the production of a structure are of great importance; for example, bad habits or last minutes changes in the shop can ruin a good fatigue design. In order to produce rules and guidelines for the general practitioner on good fatigue design practice, a working group was created in 1993 within the framework of Technical committee 6 “Fatigue”. The work of this working group has been co-ordinated by Mr. S. Piringer, Waagner Biro AG, Wien (A). Along with Dr. A. Nussbaumer, ICOM – Steel structures, EPFL, Lausanne (CH), Mr. S. Piringer is the author of the present document. Members of the Working Group were : M. A. Hirt Switzerland D. Kosteas Germany J. Krampen Germany H. P. Lieurade France A. Nussbaumer Switzerland S. Piringer Austria The document was reviewed by the Technical Committee 6 and it was also reviewed by a panel of experts in welding and fatigue. Their comments and suggestions where of great help to improve the quality of the document. Many thanks to all of them. Technical Committee 6 is at present composed of the following members : H. Agerskov Denmark B. Androic Croatia S. Böstrom Sweden J. Brozzetti France Ö. Bucak Germany C. A. Castiglioni Italy B. Chabrolin France P. J. Haagensen Norway M. A. Hirt Switzerland S. Juric Croatia A. Kähönen Finland K. Mach Austria A. Nussbaumer Switzerland E. Piraprez Belgium S. Piringer Austria T. Seeger Germany Corresponding Members are : B. Atzori Italy C. W. Brown Great Britain F. D. Fischer Austria S. Herion Germany H. Kolstein The Netherlands J. Krampen Germany S. J. Maddox Great Britain T. Rotter Czech Republic

(Chairman)

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G. Sedlacek D. R. Van Delft

Germany The Netherlands

Many thanks are also due to all the other persons, to numerous to mention here, who offered their continuous encouragement and suggestions. Finally, thanks are due to Mr. S. Piringer and the Waagner Biro AG, Wien (A) for the drafting of most of the figures and to Ms. Schumacher, ICOM – Steel structures, EPFL, Lausanne (CH), for re-reading and correcting the text.

Lausanne, Mai 2000

Dr. Alain Nussbaumer

Figures : Figures 3.1 and 3.2 have been graciously placed at our disposal by Prof. E. Niemi, Lappeenranta Univ. of Technology, Finland.

This publication has been prepared at ICOM – Steel Structures, Swiss Federal Institute of Technology, EPFL, Lausanne, Switzerland. ECCS N° 105

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CONTENTS 1

2

3

4

5

6

7

8

Motivation and Goals......................................................................................................................................8 1.1 General aims ................................................................................................................................................8 1.2 Applicability ................................................................................................................................................8 1.3 Definitions ...................................................................................................................................................8 1.4 Terminology ................................................................................................................................................9 1.5 Methods of assessment .............................................................................................................................11 Factors affecting Fatigue Life of Structures ................................................................................................13 2.1 Characteristics of fatigue process.............................................................................................................13 2.2 Fatigue loading ..........................................................................................................................................13 2.3 Stress and structure ...................................................................................................................................18 2.4 Material ......................................................................................................................................................21 2.5 Environment ..............................................................................................................................................23 Fatigue Resistant Structural Details .............................................................................................................25 3.1 General Design Strategies.........................................................................................................................25 3.2 Design of Details.......................................................................................................................................25 Factors Affecting Fabrication and Erection.................................................................................................33 4.1 Fabrication quality ....................................................................................................................................33 4.2 weld Execution ..........................................................................................................................................33 4.3 Control of welding ....................................................................................................................................39 Improvement Methods ..................................................................................................................................41 5.1 Introduction ...............................................................................................................................................41 5.2 Shape change methods..............................................................................................................................41 5.3 Residual stress methods ............................................................................................................................42 5.4 Overloading ...............................................................................................................................................42 5.5 Coatings .....................................................................................................................................................42 Quality Assurance .........................................................................................................................................44 6.1 General .......................................................................................................................................................44 6.2 Quality assurance testing methods ...........................................................................................................44 Methods for Repair........................................................................................................................................45 7.1 Fitness-for-purpose approach ...................................................................................................................45 7.2 Repair during fabrication or erection of new structures .........................................................................45 7.3 Existing structures .....................................................................................................................................46 Literature........................................................................................................................................................48 8.1 Fatigue literature .......................................................................................................................................48 8.2 Standards....................................................................................................................................................51 8.3 ECCS related publications........................................................................................................................53

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1

MOTIVATION AND GOALS

1.1

GENERAL AIMS

A good fatigue design is more than a design that follows design standards such as the Eurocodes. It must also include the structure’s production aspects. Indeed, a bad choice during design is likely to lead to unexpected costs during fabrication and assembly, and result in fatigue problems during service. Therefore, it is important to give hints to the general practitioner about good fatigue design that is in conformity with actual standards and experience. This guideline document covers rules for the design and assessment of structures subjected to fatigue loads induced by frequent changing actions, traffic actions, wind induced oscillations or comparable actions. The purpose of the guideline is to supplement the design of steel structures, which are currently documented in the following parts of Eurocode 3 (for definitions of abbreviations see Table 1.1) : • Part 1 : Steel structures in general and buildings (EC3-1-1 [S1]). • Part 2 : Steel bridges (EC3-2 [S2]). • Part 3 : Towers and masts (EC3-3-1), and chimneys (EC3-3-2) [S22]. • Part 6 : Crane supporting structures (EC3-6 [S23]). These recommendations can be applied to other types of steel structures assuming that they comply with the rules given in Section 1.2. During the transformation from ENV to EN, which started recently, it is the intent of CEN-TC250-SC3 to modify the organisation of Eurocode 3 parts. All rules concerning fatigue strength, except very specific ones, will be grouped in a new document entitled EN 1993, Part 1-9 : fatigue. The various fatigue chapters of EC3-1-1, EC3-2, EC3-3-1, EC3-3-2 and EC3-6 will consequently disappear. However, this reorganisation does not invalidate the guidelines given in this document. Apart from the Eurocodes, many existing design standards or guides do exist; only a few are cited in this document [L15, L16, S4, S8, S9].

1.2

APPLICABILITY

This guideline document is only applicable to steel structures. It covers the steel grades and connecting devices listed in EC3-1-1, Sections 3.2 and 3.3. In addition, it also covers structures made out of austenitic stainless steels. It may be used for other structural steels, provided that adequate data exist to justify the application. This document is not applicable to : • Low-cycle fatigue, that is when a few cycles cause fatigue fracture (e.g. earthquake) or, more generally, when nominal normal stress ranges exceed 1.5 fy or nominal shear stress ranges exceed 1.5 fy/√3 (e.g. pressure vessels, tanks or silos). • Structures subjected to temperatures exceeding 150°C (e.g. pressure vessels, pipework). • Structures in corrosive media (gases, liquids) other than normal atmospheric conditions. • Structures in sea water environment (e.g. offshore structures). • Structures subjected to single impact. • Concrete reinforcement.

1.3

DEFINITIONS

The symbols listed in Section 1.6 and 9.1.6 and definitions in Section 9.1.5 of EC3-1-1 are used [S1, S2]. The following tables (Table 1.1 and Table 1.2) as well as Section 1.4 summarise the definitions used in this document.

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Table 1.1 : Eurocode abbreviations Abbreviation:

Equivalent to :

Abbreviation:

Equivalent to :

EC1-1

ENV 1991-1 : 1994

EC3-1-1

ENV 1993-1-1 : 1992

EC1-3

ENV 1991-3 : 1995

EC3-2

ENV 1993-2 : 1997

EC1-2-4

ENV 1991-2-4 : 1994

EC3-3-1 and -2

prENV 1993-3-1 and -2 : 1997

EC1-5

prENV 1991-5 : 1997

EC3-6

prENV 1993-6 : 1998

Table 1.2 : Modal words English

Definition

German

French

shall

a strict demand; no deviation permitted

muß

doit

should

one of some possibilities is recommended

sollte

devrait

may

a certain solution need not be followed if other rules are also available

darf

peut

can

a physical capacity or possibility is existing

kann

peut

1.4

TERMINOLOGY

Longitudinal :

Direction of the main force in the structure or detail (Fig. 1.1).

Transverse :

Direction perpendicular to the direction of main force in the structure or detail (Fig. 1.1). FORCE

FORCE

LONGITUDINAL FORCE

TRANSVERSE FORCE

Fig.1.1: Directions of the main force Classified structural detail :

A structural element or structural detail containing a structural discontinuity (e.g. a weld) for which the nominal stress method is applied. The Eurocodes contain classification tables, which indicate strength curves for particular elements and details.

Detail category :

Classification of structural elements and details according to their fatigue strength. The designation of every detail category corresponds to its fatigue strength at two million cycles, ΔσC. Refer to the Eurocodes for a more detailed description.

S-N curve :

= Fatigue strength curve = Wöhler curve. A quantitative curve relating fatigue failure to the stress range and number of stress cycles. ECCS N° 105

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Stress range :

= Maximum stress minus minimum stress (Fig. 1.2).

Mean stress :

= 0,5 (maximum stress + minimum stress). See Fig. 1.2.

Defect/flaw :

An unintentional stress concentrator, e.g. slag inclusions, porosity, undercut, lack of penetration.

Crack :

A sharp defect for which the crack tip radius is close to zero.

Fatigue crack :

A sharp defect that has become larger due to the application of fluctuating loads.

Stress !

1 cycle

Maximum stress Stress amplitude Mean stress

Stress range

Stress amplitude Minimum stress

Time

Figure 1.2 : Stress-time history FEM :

Finite element method.

Crack initiation life : = Crack nucleation time. The portion of fatigue life consumed before a crack is produced. Crack propagation life :

Portion of fatigue life between crack initiation and failure (according to conventional failure criterion or actual member rupture).

Stress concentrator : Any change in geometry within component that causes a concentration of applied stresses, e.g. notch, bolt holes, welded connections, changes in sectional area. Stress concentrator severity :

= K = Stress concentration factor. The ratio of the concentrated stress to the nominal stress (Fig. 1.3).

nominal stress, !

concentrated stress,

!t = " . !

Figure 1.3 : Example of stress concentration factor ECCS N° 105

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Hot spot

A point in the structure where a fatigue crack is expected to initiate due to stress fluctuations in the component and one or a combination of stress concentrators.

Nominal stress

Stress in a component near the structural detail, resolved using simple elastic strength of material theory, i.e. beam theory (Fig. 1.4). Influence of shear lag, or effective widths of sections shall be taken into account. Stress concentrators and residual stresses effects are excluded.

Nominal stress

! nom

Figure 1.4 : Nominal stress distribution in I beam with flange attachment Modified nominal stress :

Nominal stress increased by an appropriate stress concentration factor to include the effect of an additional structural discontinuity that has not been taken into account in the classification of a particular detail such as misalignment, hole, cope, cut-out, etc.

Geometric stress :

= structural stress. Value of stress on the surface of a structural detail, which takes into account membrane stresses, bending stress components and all stress concentrations due to structural discontinuities, but ignoring any local notch effect due to small discontinuities such as weld toe geometry, defects, cracks, etc.

Hot spot stress :

Value of geometric stress at the weld toe used in fatigue verification. Note that the definition of the hot spot stress, and the related design fatigue curve, is not unique.

1.5

METHODS OF ASSESSMENT

If the structural detail corresponds to a standard structural detail, the nominal stress method is applicable. The structural detail category and the corresponding S-N curve are then found in the Eurocode relevant to the type of structure being assessed. If the structural detail resembles a standard structural detail, but contains an additional stress concentrator, the modified nominal stress method may be used. In order to do so, the stress concentration induced by the additional stress concentrator must be known, e.g. misalignment, effect of a hole in the vicinity of a weld, etc. If the structural detail cannot be found in the classification tables the geometric stress method, that is the determination of the hot spot stress, can be used in some cases. The fatigue resistance of the detail is then determined in terms of specific S-N curves that incorporate the hot spot stress range. In the Eurocodes the hot spot S-N curve to be used in a fatigue assessment depends on the parent material or the type and form of the weld.

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Table 1.3 summarises the assessment methods described above. Some other methods can be applied such as linear fracture mechanics, however these methods are more sophisticated and will not be considered in this document (see literature, e.g. [L15, L22, L23]). Finally, fatigue testing is recommended when no S-N curves or data are available for the structural detail to be assessed. Table 1.3 : Summary of assessment methods Type

Stress raiser

Figure (example)

Determined stress

Assessment method

A

Not considered. Problem solved using simple elastic strength of material theory (beam theory)

1.4

Nominal stress Nominal stress : range elementary theories of structural mechanics based on linear-elastic behaviour

B

A + Influence of a structural discontinuity not taken into account previously, but disregarding stress concentration effects resulting from the structural detail of the welded joint.

1.5

Modified Modified nominal nominal stress stress : range same as above, but nominal stress increased by concentration factor (from tables, graphs or formulae)

C

A + B + Influence of the structural discontinuity of the welded joint, including all stress raising effects of a structural detail in the vicinity of the joint, but excluding local stress concentrations due to the weld profile itself. Shall be determined at the surface (= hot spot) of the critical section where fatigue crack is expected to initiate.

1.6

Hot spot stress Geometric stress : range FEM analysis commonly applied, or parametric formulae if available; in general maximum principal stress is used

hole

A

!

A Stress distribution along A-A

!

Geometric stress

Modified nominal stress

Figure 1.5 : Modified nominal stress in detail combining butt weld and hole

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Figure 1.6 : Geometric stress at bar–plate connection

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2 2.1

13

FACTORS AFFECTING FATIGUE LIFE OF STRUCTURES CHARACTERISTICS OF FATIGUE PROCESS

The fatigue process can be divided into the following stages [L17, L23] : • Crack initiation or nucleation: Under cyclic loading, microcracks—without a preferential general direction—are initiated as a result of cyclic plastic deformation. This phenomena is accelerated by the presence of stress concentrators, where stresses above yield occur locally on a micro-scale even under low nominal stresses. • Stable crack growth: The microcracks propagate and join to form a dominant propagating crack, growing perpendicular to the principal tension stress in a stable manner. A characteristic feature of the crack surface is a flat, smooth region surrounding the initial defect, often exhibiting beach marks. Such beach marks are observable at a macroscopic level if the stress cycles are not of constant amplitude, but, for example, composed of blocks of stress cycles of constant amplitude. • Unstable crack growth: When approaching exhaustion of the load carrying capacity of the cross section, the crack propagation rate increases exponentially until ductile or brittle fracture of the component occurs. A rough crack surface is the characteristic feature of this stage. A large final fracture area indicates a high maximum load, whereas a small area indicates that fracture occurred under a lower load. When using the methods based on S-N curves described in Section 1.5, these three stages cannot be distinguished. In order to make them distinguishable, fracture mechanics methods are necessary.

2.2

FATIGUE LOADING

2.2.1

Number of load events

The designer must avoid the possibility of a decrease in the expected lifetime of the structure due to fatigue. To do this the designer should take into account the complete sequence of service loading events throughout the expected lifetime of the structure. Such loading events occur for example : • On bridges : commercial vehicles, goods trains. • On slender elements (chimneys, cables, etc.) : wind gusting. • On crane structures : lifting, rolling, inertial loads. The superposition of all non-permanent (fluctuating) actions, e.g. considering them as being in phase, is essential in order to find out the highest stress ranges : • Fluctuation in the magnitude of loads. • Movement of loads on the structure. • Changes in loading directions. • Structural vibrations due to loads and dynamic response. • Temperature fluctuations. The magnitude of the peak loads considered in static design is high and will never occur—or only a few times—during the life of the structure. For fatigue, this is of little concern as it only represents a few cycles in millions and it can be assumed that plastification induced by these peak loads can be neglected. Therefore, in the standards, the fatigue load models usually differ from the static design load models. To derive the fatigue load models for each type of structure, effective load histories and the damage produced by these loads have been used. The result is a load histogram, which defines a series of blocks of constant load levels and their corresponding number of cycles, as shown in Fig. 2.1. Thus only a limited number of different loads have to be used for fatigue design.

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Load

W1 W2 W3 W4 W5 n1

n2

n3

n4

n5

Number of occurrence

Figure 2.1 : Typical load histogram The specific load cases and required lifetime of the structure—or number of cycles—corresponding to each type of structure are given in the relevant parts of Eurocode 1 : • Part 1 : General information (EC1-1 [S18]). • Part 3 : Steel bridges (EC1-3 [S19]). • Part 2.4 : Slender elements (EC1-2-4 [S20]). • Part 5 : Crane supporting structures (EC1-5 [S21]). In order to produce the maximum load effect, it should not be forgotten that the loads have to be amplified often by an appropriate dynamic factor, as prescribed in the standards. For example, slender structures with natural frequencies low enough to react to the loading frequency, may suffer dynamic stress magnification [L15, L17].

2.2.2

Number of stress cycles

2.2.2.1 Cycles

The fluctuating loads considered in fatigue design cause stress events that differ in type, number and magnitude from component to component in the structure. For example, for a vehicle where the only loaded axle is the rear twin axle, the primary stresses induced by the vehicle while it is crossing the bridge will be : • In the main girder, one stress cycle per vehicle. • In a cross girder attached to the main girders and supporting longitudinal girders, “n” stress cycles of different magnitude per vehicle depending on the number of longitudinal girder spans acting as continuous girders. • In the splice of a longitudinal girder, two main stress cycles due to the twin axle and also additional smaller cycles due to the vehicle passing over other spans of the girder. The vehicle can also be a train, a lorry, a crane trolley, or an other moving load system with more loaded axles resulting in more complex stress histories than described in the above example. In this case, each component is subjected to a stress history that must be transformed into a stress histogram to perform a fatigue assessment. The methods used for this transformation are called cycle counting methods and are listed in the next section (Section 2.2.2.2). Since cycle counting is a complicated task, simplified load models calibrated so that only the maximum stress range produced by the model needs to be considered in fatigue verifications can be found in Eurocode 1. Apart from the primary induced stresses, the fluctuating loads can also induce : • secondary stresses, • impact stresses, • distortions, out-of-plane deformations, • vibrations. ECCS N° 105

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These effects - especially distortions and secondary stresses - are responsible for a large number of the fatigue cracks found in service [L24, L25]. However, these effects are often not anticipated by the designer and there is only little information in the standards. Experience has shown that most of these fatigue problems can be avoided by good detailing (for which hints will be given in Chapter 3). 2.2.2.2 Cycle counting methods

Several different methods of cycle counting exist ; each method being appropriate for a particular type of stress history. For the types of structures considered in this document, the most commonly used methods with their range of applicability - are listed below : A. Rainflow method: convenient for long stress histories, preferential method for use in computer programs. B. Reservoir method: easy to use by hand for short stress histories. When used correctly, both methods give the same result. More information about these methods as well as others can be found in the literature [L26, L17]. 2.2.2.3 Stress spectrum

The simplest stress spectrum form is the constant amplitude stress-time history with a constant mean load (Fig. 2.2). Such a stress spectrum is used on specimens tested in laboratories in order to produce consistent fatigue tests results. Stress !

1 cycle

Maximum stress Stress amplitude Mean stress

Stress range

Stress amplitude Minimum stress

Time

Figure 2.2: Constant amplitude stress-time history

The following parameters characterise a constant amplitude stress-time history : σmax = maximum stress σmin = minimum stress σm = mean stress = (σmax + σmin)/2 σa = stress amplitude = (σmax - σmin)/2 Δσ = stress range = σmax - σmin = 2⋅σa R = stress ratio = σmin /σmax A more complex stress spectrum, e.g. a variable amplitude stress time history, is represented in Fig. 2.3. Using a cycle counting method, such a spectrum can be converted to identifiable stress ranges using a suitable cycle counting method (see previous section) and represented as a distribution of stress ranges versus the number of cycles in the time period being considered. If this is further reduced to a histogram,

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any convenient number of stress interval can be used, but each block of stress cycles should be assumed, conservatively, to experience the maximum stress range in that block (Fig. 2.4) [L22]. Stress !

Equivalent constant amplitude stress range

"!E

Time

stress cycle

Figure 2.3: Variable amplitude stress-time history

Stress range !" Simplified histogram for design purposes

!"1 !"2 !"3

Actual spectrum

!"4

Block

!"5 n1

n2

n3

n4

n5 Number of cycles

Figure 2.4: Stress spectrum and corresponding histogram 2.2.2.4 Palmgren-Miner’s Rule

Test results from constant amplitude loading show that a normalised S-N curve for each standardised type of stress concentrator (detail category) can be drawn as a line in a log Δσ - log N diagram (see Fig. 2.5). For normal stresses, the slope of the line, m, is set equal to 3 in Eurocode 3 (for convenience, the negative sign is omitted). Under constant amplitude loading, fatigue life is infinite for stress ranges below the constant amplitude fatigue limit (CAFL). Under variable amplitude loading, fatigue life is also infinite as long as all stress ranges stay below the CAFL. If this is not the case, a modified S-N curve shall be used. This curve is identical to the previous one up to the CAFL. Below the CAFL, the slope of the S-N curve becomes 5, to simulate the damaging effects of smaller and smaller stress range cycles with increasing crack size ECCS N° 105

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[L17, L19]. The variable amplitude fatigue limit, e.g. cut-off limit, is then lower than the constant amplitude fatigue limit. log !" Fatigue strength curve 1

!"C !"1 !"2 !"3

m=3 Constant amplitude fatigue limit (CAFL) Cut-off limit m=5

6

2.10

N1 N2 5.10

6

N3

8

10

log N

Figure 2.5: Normalised S-N curve Under variable amplitude loading, the damage caused by each block of the spectrum can be defined as n/N, where n is the actual number of cycles in a particular block during the life time and N is the endurance (number of cycles to failure) under that stress range. For example, in Figure 2.5, under the stress range Δσ1, the endurance is equal to N1. Under the assumption that the loading sequence has no effect on the fatigue life, a linear damage accumulation rule, namely the Palmgren-Miner’s Rule, is then used to compute the total damage :

n1 n 2 n + +......+ n ! " N1 N 2 Nn The above equation indicates that, for any block of the spectrum, the expression n/N should be calculated. If the sum of damage due to all blocks is less than or equal to α, fatigue failure is prevented before the end of the design life. α is generally set equal to one in structural engineering fatigue regulations, but since for certain types of spectrums the linear damage accumulation rule has shown to be non-conservative, values inferior to one may be specified in some cases in order to insure safety. To account for the loading sequence, more complex cycle counting methods and accumulation rules can be used [L36], but these are out of the scope of this document. Alternatively for fatigue assessments, an equivalent constant amplitude stress range, ΔσΕ, having the same effect—in terms of damage—as the variable amplitude spectrum can be computed using the S-N curves and the Palmgren-Miner’s rule. For the simplified fatigue assessment procedures in the codes, the maximum stress range produced by the loading that is used in the verification is, in fact, already a calibrated equivalent constant amplitude stress range, and the sum of damage computations need not to be made.

2.2.3

Dynamic amplification factor

The correct dynamic factor shall be taken from the relevant standards if available; if not, a reasonable assumption shall be made or the factor shall be evaluated through testing. It should be noted that, for the same structure, the value of the dynamic amplification factor may differ from one limit state to another, e.g. ultimate limit state and fatigue limit state. It should be noted that in some standards the so-called « dynamic ECCS N° 105

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amplification factor » covers more than just dynamic amplification, but also correction factors that account for a difference between realistic loading and the loading applied to the structural model.

2.2.4

Loading rate

The loading frequency does not influence the fatigue life for frequencies up to 100 Hz, provided that steps are taken to ensure that the temperature of the detail does not rise significantly and there are no simultaneous corrosion effect [L26]. This is the case for the types of structures and applications considered in this document. However, if temperatures above 100°C occur, a reduction of the fatigue strength should be considered. It should be noted that the rate of loading can be of importance if brittle fracture can occur. In structures or parts of structures where high loading rates are expected, typically above 10 Hz, the best quality steels available must be used, refer to EC3-1-1 : Section 3.2.2.3.

2.3

STRESS AND STRUCTURE

Structures or components with high live/dead stress ratio or low category details, categories 45 to 36, that include high stress concentrators, are the most sensitive and should be checked first to indicate the most critical points. This check must cover any welded attachment to a member, and not just the main structural connections, as well as additional welding performed on the structure in service. If fatigue is the design limit state, simplicity of the details and smoothness of the stress path should be sought [L17]. Any change in the applied stress range, e.g. change of minimum or/and maximum stress, alters the probability of fatigue crack initiation and also the growth of existing cracks. Therefore, the lifetime can be greatly extended by reducing the stress range (by whatever means) affecting a detail. Taking for example the case of the ‘cruciform joint with full penetration butt weld’ detail, according to EC3 Fatigue strength curves, this is a detail category 71, which means it can sustain 2 x 106 cycles at a stress range of 71 N/mm2. A stress range reduction of 20 % to 57 N/mm2 on this detail will result in an allowable number of cycles of 3,9 x 106. This is nearly 2 times the original number of cycles or twice the characteristic lifetime !

2.3.1

Analysis

A detailed computation of the structure using simple elastic strength of material theory is sufficient in most cases. However, in cases where secondary stresses, distortions, etc., are anticipated to have a significant influence, finite element method (FEM) analyses should be carried out in order to account for these stresses in the fatigue assessment. Since significant experience is needed to interpret correctly the results of FEM analyses, one should be warned not to use FEM without sufficient experience or knowledge in this domain. Computations must be executed using an elastic model in order to get the stress ranges in the details. In fatigue design, structural details should not be analysed using complex numerical methods, such as the FEM, in order to deduce geometric stresses, which are afterwards classified into detail categories of Eurocode 3. This is because the detail categories in EC3 already include stress concentration factors as they use the nominal stress assessment method. When geometric stresses have been determined the “hot-spot stress”-based S-N curve must be applied.

2.3.2

Detail classification

Currently, classified structural details are described in different parts of Eurocode 3. The stress concentrations normally found in typical joints and details are included in the determination of the fatigue strength. The fatigue assessment of such classified details is based on the nominal stress method.

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If a constructional detail configuration from a type of structure can be found in the tables of the relevant part of Eurocode 3, and the description and requirements for this detail correspond, then the fatigue strength can be derived from the standard fatigue resistance S-N curves given in EC3-1-1. Each detail category corresponds to one S-N curve where the fatigue strength Δσ is a function of the number of cycles, N. These fatigue curves are based on representative experimental investigations. They include the effects of : • stress concentrations due to the detail geometry (detail severity), • local stress concentrations due to the size and shape of weld imperfections within certain limits, • stress direction, • expected crack location, • residual stresses, • metallurgical conditions, • welding and post-welding procedures. Additional stress concentrations not included in the constructional detail configuration, e.g. misalignment, large cut-out in the vicinity of the detail, have to be accounted for by the use of a stress concentration factor.

2.3.3

Determination of stresses

The value of the stress range to be considered in the design is the stress range at the location of the detail considered (see EC3-1-1). In order to avoid low-cycle fatigue behaviour, the stress range shall not exceed 1.5 fy for nominal normal stress or 1.5 fy/ √3 for nominal shear stress. Otherwise the stress range shall be reduced by appropriate means. The total stress, σ tot, in a section may be treated directly, or after resolution into the four parts 1) to 4) described below [L4, L22]. Typical schematic representations of these are given in Figure 2.6. Stresses resulting from residual stresses and local stress concentrations due to the weld profile itself are not included here.

!m

!t !b

0

!a

0

0

t Membrane stress

0

t

t

t

Bending stress

Additional stress due to structural detail

Other additional stress (misalignement, etc.)

Figure 2.6: Determination of stresses The four parts composing the total stress, σ tot, are : 1)

Membrane stress, σ m : The component of uniformly distributed stress which is equal to the average value of stress across the section thickness, σ m1 = σ m2.

2)

Bending stress, σ b : The component of stress due to imposed loading which varies linearly across the section thickness, σ b1 = - σ b2.

3)

Additional stress due to structural detail, σ t : The additional stress is given by the following formulas, depending on the definition of the stress concentration factor, Kt :

" t = K t ! remote stress ECCS N° 105

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or

# t = (K t " 1)! remote stress

This type of additional stress usually decays over distances greater than the section thickness. 4)

Other additional stresses (misalignments, holes, etc.), σ a : The misalignment stresses are usually predominantly bending stresses and their peak surface value can be expressed as :

# a = (K m " 1)! applied stress where Km is the stress magnification factor (see reference [L22] for formulas). This type of additional stress usually decays over distances greater than the section thickness. In the evaluation of the additional stress due to misalignment, only the membrane component of the applied stress has to be considered. If a misaligned joint is within the stress concentration field due to a structural detail, the membrane stress has to include the peak stress due to the structural detail. Formulas for the calculation of the secondary bending stresses due to misalignment can be found in [L22]. For holes, sharp corners, etc., the formula is the same as above except that the additional stress value can depend on both the membrane and bending stress components. The applied stress is the nominal stress in the gross section. This type of additional stress usually decays over distances less than about 20 % of the hole or corner radius, or 20 % of the thickness (see reference [L27] for formulas). If these type of peak stress effects are located within the zones of other additional stresses, the overall effects should be multiplied together. This procedure will always give conservative results, and a more precise evaluation requires FEM analyses of the detail.

2.3.4

Defects, flaws, imperfections

Any defect, flaw or imperfection causes a discontinuity in the stress flow [L14, L17]. For example, typical defects/flaws in welds are : slag inclusions, porosity, cavities, and lack of fusion or penetration. Typical imperfections are : undercut, misalignment and imperfect profiles. From the theory of notches it has been shown that the sharper the flaw (evaluated through its radius), the higher the local increase in deformations in the vicinity of the flaw [L20]. This theory, in combination with fatigue testing of machined specimens, has led to the derivation of strain-life curves. These curves, different from the S-N curves obtained by fatigue testing of specimens containing defects/flaws, can be used to evaluate the influence of a known defect/flaw on the fatigue life of a component using : • a fracture mechanics approach, • an elastic notch stress range approach, • a plastic notch stress range approach. These approaches are sophisticated. In this document the influence of defects/flaws are considered implicitly since the standard fatigue curves (S-N curves or hot-spot fatigue curves) depend upon both the material and defects/flaws present, even though these defects/flaws are undetectable. That is, stress concentrations or notch effects are not explicitly included in the described fatigue assessment methods (see Section 1.5).

2.3.5

Stress ratio

Under compressive stress, a crack is closed and therefore it cannot propagate. In real structures or full-scale test specimens, residual stresses and other built in stresses are often present in details due to welding, dressing, punching, lack of fit, support settlement, temperature gradients, etc. The presence of residual stresses often results in tensile stress cycles in details even under compressive nominal stresses. Conservatively, Eurocode 3 recommends use of a full range of stress cycles in fatigue assessments, except for non-welded and stress relieved details as specified in Section 9.7.1 of Part 1.1. For welded details, the influence of the stress ratio, R (defined in Section 2.2.2.3), on fatigue life is thus of no importance, apart from some specific details treated with an improvement method (see Chapter 5).

2.3.6

Problem of lightweight structures

ECCS N° 105

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21

Lightweight structures are structures for which the permanent loads are minimised and represent only a small portion of the variable design loads acting on the structure. In the case of lightweight structures subjected to predominantly fluctuating loads, design against fatigue is crucial. The stress ranges in all components are very high compared with the static stresses, and nearly all welded details, as well as other connection types, are ruled by fatigue design categories.

2.4

MATERIAL

All modern structural steels are iron alloys. The atomic structure of iron can either be ferritic (cubic spatial oriented atomic grid) or austenitic (cubic with centred faces atomic grid). At room temperature, the atomic structure of pure iron is always ferritic. The addition of certain elements to iron can favour the formation of an austenitic structure at room temperature. Such elements are, for example, nickel, manganese and copper. Other elements, on the contrary, favour the formation of a ferritic structure. This is the case with chromium, molybdenum, silicon, titanium, etc. Furthermore, when carbon is added and the temperature cycle during steel processing is controlled, a very hard microstructure, called martensitic, can be obtained at room temperature. Although there are differences in mechanical behaviour between ferritic and austenitic steel alloys, the general trend for fatigue design is that the rules for ferritic steels (with a ferritic and/or martensitic structure) can be applied to welded austenitic steels (excluding environmental considerations) [L33]. For non-welded steels, the S-N curves for ferritic steels show a limiting stress, or say fatigue limit, below which initiation of cracking does not occur ; this is not the case for austenitic steel. The fatigue limit of ferritic steels lies normally in the 106 to 107 cycle range. In the case of small test specimens, unnotched and polished, a correlation exists between the ultimate tensile strength of the steel, Su, and the high-cycle fatigue strength, So : So ≈ 0,5⋅Su This limit corresponds to the maximum stress range that the test specimen can sustain without failing after an unlimited number of cycles. The higher the steel grade, the better its fatigue limit and fatigue strength under higher stress ranges (finite lifetime). For real components, however, the effects of holes, notches, defects, and corrosion reduce the fatigue strength, by reducing the number of cycles needed to initiate a fatigue crack. This is particularly significant, because the rate of growth of a fatigue crack is largely independent of the tensile strength of the material. Thus, this reduction is proportional to the ultimate strength of the material, e.g. the higher the ultimate strength, the greater the reduction. As a consequence, the fatigue strength of high grade steel with severe notches is not higher than that of mild steel with the same type of notches. That is, the fatigue life of joints with welds or other defects, with the exception of machined joints, cannot be remarkably improved through the use of materials with better mechanical properties. The more economical solution for welded structures subjected to fatigue is thus to use normal structural steel (see following section). High strength steels are generally uneconomical except in structures with high, predominantly static loading or when the structure’s details are treated with some improvement method. In addition, it is emphasised that good workmanship is the most important parameter, since, irrespective of the steel grade, it results in higher fatigue strength of the details (refer to Chapter 4).

2.4.1

Normal structural steel (“Low strength steel”, “Mild steel”)

Normal structural steel ranges from a nominal yield stress equal to 235 N/mm2 to 355 N/mm2 [S17]. In the European material standards, these steels are described in EN 10025 [S10], EN 10113 [S11] and EN 10210 ECCS N° 105

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[S12]. For welded structures subjected to fatigue, normal structural steel is often the more economical solution, since the fatigue strength of joints with welds is independent of the static strength of the steel.

2.4.2

High strength steel

The criteria for denoting a steel grade as high strength are ambiguous. They are dependent on the country and the industry concerned. In structural applications and according to Eurocode 3, high strength steels must have a yield strength equal or greater than 420 N/mm² and are weldable. Satisfying these requirements are the S420 and S460 grades according to EN 10113 [S11] and EN 10210 [S12], and S500, S550, S690, S890, and S960 grades according to EN 10137 [S13]. These different steel grades obtain their mechanical properties through different means such as alloy content modification or special rolling or quenching/tempering processes. Other steels types such as stainless steels can also be considered as high strength steels, but these are treated in another section. In previous sections, it was said that the influence of the steel grade (ultimate strength) on the fatigue strength of a component is only significant if no defects or other starting points for cracks exist. Examples of components without significant defects are : plates without welds and cut with smooth edges, plates with drilled holes (only when carefully executed), machined parts, rods and cables. In these cases, the fatigue strength increases with increasing ultimate strength. But welded joints, especially hand welded joints, which always contain small crack-like defects (that is, cracks growing after a very short initiation period) have a fatigue life practically independent from the ultimate tensile strength of the parent material. In these cases, it is useless to use high strength steel except if the detail is treated by an improvement method. Improvement methods, described in Chapter 5, reduce the harmful effects of some types of defects on the fatigue strength of a detail. High strength steels are the steels that can benefit the most from these methods.

2.4.3

Thermo-mechanical steel

Thermo-mechanical steels (TM steels, often called “low alloy steels”) are characterised by their fine grain microstructure and their manufacturing process (see EN 10113, Part 3 [S11]). Both normal and high strength steels can be produced using thermo-mechanical processing. The superiority of the TM steels comes from their excellent toughness properties, with values higher than 50 J at -20°C (or even lower temperatures), which result in longer allowable crack sizes in components. This means a longer crack growth period (extended lifetime), less frequent inspection intervals and cracks that are easier to detect. Furthermore, because of lower carbon content, preheating before welding can be reduced or even omitted in the case of TM steels. All these aspects have a significant economic impact and make TM steels very competitive.

2.4.4

Weathering steels

Weathering Steels are steels containing 0.25 to 0.55 % of copper and 0.40 to 0.80 % of chromium (see EN 10155 [S14]). These steels form an oxide coating until it becomes a dense layer, which protects the underlying steel, thus giving it an improved atmospheric corrosion resistance as compared with ordinary steels. Corrosion however never completely stops—the corrosion resistance of these steels is not automatic, it depends upon the conditions of use. It should be noted that multipass welding of weathering steels must only be executed with special electrodes in order for the structure to keep its improved corrosion resistance. Rust pitting induced by weathering has been suspected to reduce the fatigue strength of components made out of weathering steel, especially for details of the highest categories (categories 160 and 140). For this reason, EC3-2 [S2] states conservatively that the highest detail category for weathering steels is 125. For lower category weld details, the stress concentration effect of the corroded surface is less significant than the weld itself, and hence the fatigue strength of weathering steel details is similar to that of uncorroded steel details. In bridges, the use of weathering steels is a good solution and should be promoted. Typical bridge details made out of weathering steels are as fatigue resistant as details made out of normal structural steel. Reference L28 provides more detailed information about the use of weathering steels in bridges. ECCS N° 105

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2.4.5

23

Stainless steels

Stainless steels are, in their simplest form, ordinary steels for which a minimum of 10.5 % of chromium has been added. In the case of structural stainless steels, the chromium content is between 17 to 19 %; moreover these steels also contain 8 to 11 % of nickel. The new European material standard for stainless steels is EN 10088 [S15]. The presence of chromium results in the formation of a layer of primarily chromium oxide on the surface of the steel when it is exposed to air. This layer gives the steel its ability to resist corrosion, as for example, atmospheric corrosion. However, the common interpretation that stainless steels are resistant to every conceivable corrosive environment is not correct. There are three major metallurgical families of stainless steels: martensitic, ferritic, and austenitic steels. In structural applications, the austenitic grades of stainless steels are used predominantly. For the fatigue design of welded structures, it has been shown [L32, L38] that the rules for ferritic steels can also be applied to welded austenitic steels (excluding environmental considerations). Thus, the recommendations contained in this guide are also valid for austenitic stainless steels. Austenitic stainless steels have up to 50 % higher thermal expansion coefficient and 50 % lower thermal conductivity as compared with carbon steels. Since higher distortions due to welding are to be expected with austenitic steels, the designer must design the structure accordingly.

2.5

ENVIRONMENT

2.5.1

Corrosion effect

Severe corrosion acts like sharp notches thus considerably reducing the lifetime of the structure under fatigue loading. Normal steel grades must therefore have adequate corrosion protection such as : • paint, • coating (which can further be used as a method of improvement of fatigue strength, see Section 5.4), • cathodic protection. Weathering steel grades, however, can be left unprotected in mild corrosive environments (acid rain is not considered an especially severe condition) such as structures exposed to rain washing and sun drying , free of salt, where details do not trap debris, do not stay wet for long periods of time, and are regularly maintained. In these cases, the details can be classified into standard detail categories since slight corrosion notches have less influence than the welding produced notches. Nevertheless, it should be emphasised that welding in addition to excessive corrosion notches reduce severely the fatigue strength of all types of steel. Special attention regarding corrosion protection should be given under the following circumstances : • steel structures in marine environments (250-500 m from the sea), or subjected to salt-laden fogs, • where run-off from de-icing salt reaches the structure and is not washed off by rain, • where there are highly corrosive chemicals or industrial fumes in the atmosphere.

2.5.2

Temperature

The effects of temperature on the fatigue strength of a detail should be checked. Generally speaking, it has been shown that there is no significant change in fatigue crack growth rates with low temperatures, down to –50°C, unless brittle fracture becomes the governing propagation mode. Thus, only high quality steels should be used in cases of exposure to low temperatures. In this document, temperatures above 150°C are not considered, but since a reduction in the fatigue strength can occur at temperatures exceeding 100°C, a conservative design approach is recommended. As a general rule, it can be said that the reduction of the fatigue strength is proportional to the ratio between the elastic modules at service and room temperature. If the elastic modulus at the service temperature is not known, the fatigue reduction factor given in [L15] can be used :

f ( temp) = 1,1955 " 1,06 ! 10 "3 ! DEG " 0,2 ! 10 "6 ! DEG 2 ECCS N° 105

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Good design practice

Where DEG is the temperature in Celsius. More guidance can be found in various codes and standards.

2.5.3

Aggressive media

Adequate corrosion protection as defined above may not always be sufficient for larger cracks. Once a crack has initiated and propagated to become a surface crack, it is in direct contact with the environment. In the presence of aggressive media, the crack can result in high reductions in the fatigue strength and life. A general guideline for this situation is not available. More guidance can be found in offshore structures codes, e.g. S25, S26, L22. As a last resort, tests should be carried out when media is thought to be aggressive.

ECCS N° 105

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3

FATIGUE RESISTANT STRUCTURAL DETAILS

3.1

GENERAL DESIGN STRATEGIES

3.1.1

Infinite Life Design

25

In this design approach, all relevant fatigue actions should be less than the fatigue endurance limit or, in a fracture mechanics assessment, the threshold stress intensity factor.. The requirements resulting from this design strategy are given in EC3-1-1, Paragraph 9.1.4 in terms of a stress range or stress cycles limit. A high survival probability can be expected from structures designed in this way and no regular fatigue monitoring needs to be specified.

3.1.1

Safe life design

This method is based on the calculation of damage during the structure’s design life using standard lower bound strength data and an upper bound estimate of fatigue loading. This will provide a conservative estimate of fatigue life, longer than the design life. Structures designed this way have a high survival probability, however, lower than those designed for infinite life design, and no regular fatigue monitoring needs to be specified.

3.1.2

Fail Safe Design

This type of design strategy can be applied only to redundant structural details, which means statically indeterminate structures (internally and/or externally). In case of a component failure, a redistribution of forces occurs resulting in a prolonged lifetime. The failure may then be detected and the structure repaired. In this design, there is a significant probability of failure of a component. The failure probability of the whole structure is, however, very low assuming that the structure is regularly inspected and has proper maintenance procedures.

3.1.3

Damage Tolerant Design

In damage tolerant design it is assumed that cracks that are big enough for detection with non-destructive testing method can be present in the structure. Since it is difficult to detect small cracks, the use of highly crack tolerant, i.e. tough materials, is recommended. The lifetime of the structure can be computed using probabilistic fracture mechanics methods, thus allowing for determination of inspection intervals. The inspection intervals are a function of the level of failure probability considered. Since the consequence of failure is included in the analysis, this design strategy can be applied to both redundant and non-redundant structures, which is not the case with a fail safe design. Once a crack is detected, a decision is taken. The influence of the decision on the failure probability can be modelled by updating the probabilistic fracture mechanics model used. Examples of decisions are (non-exhaustive list) : to take no action at all, to increase inspection intervals, to repair the crack, to change the component.

3.2

DESIGN OF DETAILS

3.2.1

General observations

Fatigue loaded structures should be designed with the aim of avoiding severe stress concentration details [L3, L21, L26]. Stress concentrations depend on the shape of the component and on the manufacturing process. They occur at corners, loading positions, abrupt section changes, etc. (refer to Fig. 3.1). In many cases, they can be avoided or their negative effects reduced through adequate design, as shown in Figs. 3.1 and 3.2. Moreover, designers should avoid structural discontinuities—such as welds—in highly stressed regions. Good fatigue resistant design includes the following precautions : • Change detail from a welded to a bolted shear connection. • Put details in zones near the neutral axis. ECCS N° 105

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• • • • •

•

Good design practice

Design details where bending moment is minimised, for instance by avoiding misalignment or offset, which causes secondary bending stresses (example : converging axes of truss diagonals and chords). Avoid the combination of several stress concentrations in the same region, like welds in zones affected by holes (Fig. 3.2), tapering, attachments, etc., as this increases further the stress concentration factor. Specify full penetration welds in all highly loaded joints. Put details in regions where the mean stress is compressive. Do not hesitate to avoid using a stiffener, except at supports, if the self-weight increase of the panel without stiffeners is only 10 to 15 % more than the weight of the original stiffened panel (web and flanges) ; this design will, in the end, be more economical and fatigue resistant. Ensure that support stiffeners are at the axes of the supports. Improved solution

1:4 to 1:5

1:4 to 1:5

1:4 to 1:5

Figure 3.1: Ways of improving the design by reducing the structural stress concentrations [L21]

Improved solution

Figure 3.2: Improving the design by moving the weld outside the stress concentration area [L21] When considering the local geometry of welds, it should be noted that high local stress peaks are essentially produced by non-smooth transitions between the plate surface and the weld flank (refer to Fig. 3.1). In the case of transverse joints, high local peak stresses can also result from large joint widths (see Fig 3.5). For longitudinal joints, the start-stop points due to the welding process are always sites of local stress peaks and therefore possible crack initiation sites. Local stress peaks also occur at notches in gas cut plates (drag lines). These local notches can remain even after a gas cut plate has been welded to another member, for example, in the gap between longitudinal fillet welds in a web to flange joint (refer to figure 4.1b).

3.2.2

Parts welded longitudinally or transverse to stress direction

3.2.2.1 Non-load-carrying parts

It can be seen in the classification tables in EC3-1-1 [S1] or in [S2, L15, L16] that (non-load-carrying) attachments should be connected by a weld transverse to the force flow rather than by a weld parallel to the force flow, as shown in Fig. 3.3. This is because the deflection of the stress lines is smaller if a short ECCS N° 105

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distance between start and end of the stress deflection exists. The length of a longitudinal attachment (the longest dimension parallel to the force flow) should be not exceed 50 mm to be classified in the best detail category [S1, S9, L15, L16].

Preferred!

Figure 3.3: Non-load carrying attachments For vertical T stiffener connections on the bottom flange, for example in bridge girders, the stiffener flange should be cut according to Fig. 3.4.

A-A

A

> 80

60°

t

mm

A a) Rolled T stiffener

B-B

B

60°

> 100

mm

t >

B

20 mm

b) Built-up T stiffener Figure 3.4: Connection of a vertical T stiffener on bottom flange For transverse joints, the overall joint width should be minimised as much as possible, for example, by using partial penetration welds instead of fillet welds when multi-pass welds are needed (refer to Fig. 3.5). ECCS N° 105

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For longitudinal attachments, significant improvement in the fatigue strength can be achieved by shaping the ends of the gusset and grinding properly the weld toe as well, see Section 3.2.6.

Preferred 1.7/2 > 1.2

t t

t

a) Fillet welds

b) Partial penetration welds Figure 3.6: Load carrying parts

ECCS N° 105

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3.2.3

Cover plates

Cover plates are generally used for the strengthening of flanges. To avoid corrosion problems and achieve better fatigue resistance, cover plates on beams and plate girders should be designed with end welds (see EC3-1-1 and EC3-2, Table 9.8.5). The width of the cover plate should be less than the width of the flange. If the thickness of the cover plate is more than the thickness of the Slope 1:4 flange, the ends of the cover plate should be tapered at a slope of 1/4 (Fig. 3.7). Cover plate ends can also be tapered in width for a smoother stress transition, but no increase in the detail category can be made for the improved shape. Since cover plates have a low fatigue strength [L13], it is recommended not to use cover Figure 3.7: Cover plate tapering plates if possible, but instead, plates of varying thickness. When strengthening a structure, the cover plates should be designed in the following way : The end of the cover plate should not be welded on its ends but connected with HSFG bolts (Fig. 3.8). The number of bolts is to be calculated according the actual force in the cover plate. The fillet welds along the sides of the cover plate should be executed with equal thickness and drawn up to the last bolt line, but not further. The end of the plate shall not be welded. Warning : This type of connection does not conform to [S2] and [S3].

Pretensionned bolts

Fig.3.8: Cover plate with HSFG bolts

3.2.4

Welding near plate edges

Free edges of plates should be kept free of welding if possible. Weld ends should be distanced at least 10 mm from an edge to avoid local stress concentrations (Fig. 3.9) and edge defects induced by welding. See EC3-1-1, Table 9.8.3 and 9.8.4 [S1]. Welds may, however, be placed not less closer to free edges as they have been shown not to reduce than 10 mm fatigue strength as long as they are well executed with, in particular, no undercut. These welds shall be controlled and any edge defects shall be ground to result in a smooth Figure 3.9: Welding near plate edges transition.

3.2.5

Lap joints

3.2.5.1 Single lap joints

Unsymmetric overlapping joints (one shear plane, without additional elements to stiffen the joint) should be avoided whenever possible. This type of joint is not permitted in components of bridge structures subject to fatigue (Fig. 3.10). The reason is the non-negligible bending stresses that occur in addition to the normal stresses. The fillet welds have the tendency to peel off from the plate due to stresses perpendicular to the plate surface. [S2, S9].

ECCS N° 105

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3.2.5.2 Double lap joints

Such joints should be avoided due to their low fatigue category and also because efficient protection against corrosion of these joints is very difficult or impossible to ensure (see Figure 3.10).

Avoid

Corrosion protection !

a) Single lap joints

b) Double lap joints Figure 3.10: Lap joints

3.2.6

Transition corners

Transition corners should be as smooth as possible and have the largest possible radius. The ends of gusset plates welded on or to the edge of a plate can be shaped so as to create transition corners thus reducing stress concentration and eliminating weld toe defects. Transition corners should be built up by welding prior to grinding and including the weld toe in the final radius (see Fig. 3.11). The minimum radius to which transition corners can be applied (in order to achieve significant fatigue strength improvement) is 15 mm [L33]. In the case of a gusset welded to the edge of a plate, the best detail category is obtained if the radius is larger than 1/3 of the width of the flange or plate. See EC3-1-1, Table 9.8.4 [S1]. Corner built up by welding prior to grinding including weld toe

K

Corner built up by welding prior to grinding including weld toe

Ensure smooth transition between full penetration and fillet welds Radius r

Grinding extension

K Plate width w

Weld type, see above

20 mm

Radius r

Plate thickness t

Figure 3.11 : Rounding of corners ECCS N° 105

Figure 3.12 : Notches in I-beams

Preferred

Good design practice

31

Notches in rolled I-beams should be rounded in such a way that the straight edges will be tangent to the rounding radius, but not radial running to the centre of the rounding circle (Fig.3.12) [L12]. Gas cut corners should be ground to eliminate surface defects (drag lines); grinding should be extended 20 mm beyond the ends of the transition radius.

3.2.7

Bolted connections

Bolted connections in fatigue loaded structures should be executed either with fitted bolts or with HSFG bolts (preloaded bolts). Bolted connections - especially if the bolts are in tension - should be designed to avoid the occurrence of prying forces. Bolts loaded in tension shall be fully preloaded [S1]. Compared to tension connections, shear connections are less susceptible to fatigue, especially when the forces are always acting in the same direction. Thus, in the case of shear connections with forces always acting in the same direction, normal bolts may also be used. Care should be taken with preloaded bolt connections where the total thickness of the assembled plates is less than 40 mm since a significant loss in pretension may occur with time. These connections should be regularly inspected. Moreover, the application of zinc paint as corrosion protection before connection of the plates is not advised, because it results in an increased loss in pretension.

3.2.8

Cables and anchoring of cables

Cables are understood in this document to be elements made out of cold drawn steel (high strength grade) and able to take tension forces only. Any compression force will reduce tension and thus result in increasing sag and slackening of the element. Since fluctuating loads on cables only produce tension stresses, the stress ratio R is positive in any case. As it is laid down in Section 2.4.2, any defect like a notch or crack has a major influence on the fatigue strength, especially in the case of high strength steels used for wires of cables Therefore, it is essential to avoid any factor that may cause such defects. Pressure on cables from sharp edges, tools or similar during erection also has to be prevented by means of constructional detailing (sufficient rounding of anchorage, sockets, saddles, clamps) or appropriate tools. Special care should be given when lifting and dragging or hauling cables not to demolish the smooth wire surface of the strands. Furthermore, special care is needed with respect to the socket filling process. In many cases the zinc coated wires are cleaned by aggressive acids before being placed into the socket and cast. If spots of such acid are not washed away, especially in the socket neck, this will cause rusting and corrosion notches. The socket outlet region of the strands should be therefore examined very carefully. Nevertheless, adequate corrosion protection must be applied and maintained over the total length of the cable. Defects of cable wires cannot be repaired in most cases. As one wire in a cable is a very small part of the whole cross section, rupture of one wire weakens the total strength of the cable by only a small percentage. However, it is postulated that no chain reaction of ruptures occurs, and that the force of the broken wire can be transmitted to the surrounding wires by friction. In the case where more than one wire is ruptured, the entire cable strand must be replaced [S2].

3.2.9

Tie rods and prestressing rods

Tie rods and prestressing rods are designed to carry tension forces only. If compression forces are acting, the total effective tension is reduced. Due to the stiffness of such elements, compression forces can be resisted as long as no buckling occurs. For fatigue design, the principles that were presented in the previous section on cables should be also followed for rods. For rods with threaded parts, the threads shall be milled and not cut-out. Prying effects should be avoided in rods as bending causes additional stress variations especially in fatigue critical zones. Rods should be loaded axially and not eccentrically. ECCS N° 105

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Good design practice

3.2.10 Orthotropic decks The plates used in orthotropic decks should not be thin nor too slender to avoid fatigue problems. Stiffness criteria to avoid fatigue cracking are given in EC3-2, Annex G [S2]. More information can also be found in a recent guide for detailing orthotropic decks written for the German Ministry of Transport [L39].

3.2.11 Hollow Section Structures For structures or parts of structures made from hollow sections, for example trusses, special literature exists. CIDECT (Comité International pour le Développement et l’Etude des Constructions Tubulaires) has produced a series of design guides. A new guide for designing hollow sections joints under fatigue loading (Guide N° 8) is in preparation, see reference [L40] for more information. A summary of the most classical hollow section joints and their detail categories can be found in the tables of EC3-1-1. Special care should be taken to the welding procedures and sequence of welding, to preheating and fitting tolerances. In these joints, the stress concentration factor is mainly due to geometry, not to weld notch effects. Full penetration welds should be executed preferably, however these can result in fabrication and inspection difficulties. For example, cutting an elliptical opening in a hollow section in order to allow access to the inside to facilitate welding and inspection of the but welds, and closing this opening with a weld after fabrication is complete, can result in poor fatigue resistance. In cases where fillet welds are used, the weld size shall be equal to the wall size of the attached elements.

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4

FACTORS AFFECTING FABRICATION AND ERECTION

4.1

FABRICATION QUALITY

Welds should meet the requirements of the different parts of Eurocode 3 (the chapters concerning fatigue, information found in the detail category tables) or of existing guides on quality levels for various features (defects, flaws, imperfections), for example EN 25817 for arc-welded joints in steel [S24]. Any fault in workmanship may potentially reduce the fatigue strength of a detail. Good workmanship, on the contrary, will result in an increase in the fatigue strength, often above the characteristic S-N curves given in the codes. These curves correspond to lower bound test results obtained from average fabrication quality details. Even though good workmanship cannot be quantified in order to be used in fatigue assessments—S-N curves refer to failure from undetectable defects/flaws⎯it can be considered a welcome supplementary safety margin. The good workmanship criteria, however, on which the weld quality specifications of the codes and standards are based are sometimes not directly related to the effect of the feature specified on fatigue strength (or any other strength criteria) [L33]. Faults in workmanship proven to be detrimental to fatigue strength include the following [L21, L26] : • Weld spatter. • Accidental arc strikes. • Unauthorised attachments. • Corrosion pitting. • Weld flaws, particularly in transverse butt welds. • Poor fit-up. • Notches, sharp edges. • Eccentricity and misalignment. • Distortion. These workmanship faults should be eliminated through continual education of the welders, their superiors and adequate inspection. As some of the weld requirements may be irrelevant to fatigue, or indeed insufficiently stringent to meet the fatigue strength represented by the relevant fatigue design S-N curves, an approach for quantifying the consequences of not meeting the requirements from the codes exists. The approach is called fitness-forpurpose and is described in section 7.1.

4.2

WELD EXECUTION

4.2.1

General observations

The execution of welding should follow Eurocode 3 and ENV 1090 - execution of steel structures - rules. Part 1 of ENV 1090 [S16] covers both general rules and rules for buildings, but does not cover steel structures susceptible to fatigue. Hence, execution requirements for welding of runway beams and elements of buildings that support cranes are not covered. For welds subjected to fatigue, Part 5 of ENV 1090 covers requirements for the weld execution of bridges and, by extension, all steel structures susceptible to fatigue. In all cases the following principles are deemed as minimum standards [L16] : • Butt welds with partial penetration (as opposed to full penetration welds) shall be treated as fillet welds. • In butt welds, welds smaller than the plate thickness are not advised. • Leg size of fillet welds should not vary by more than 10 % along the weld length. • Transverse butt welds may have concavity of weld surface if: - length of concavity in weld direction is not longer than plate thickness, t,

ECCS N° 105

34

•

•

• • •

Good design practice

- depth of concavity is not more than 0,1 t, - remaining weld thickness is not less than t. Undercut: - for transverse welds : visible undercut not permitted for detail categories higher than 56. For detail categories not more than 56, depth of undercut shall not exceed 0,05 t or 0,5 mm. - for longitudinal welds : undercut depth shall not exceed 0,1 t or 1 mm. Depth of slag inclusions appearing at weld surface should be treated like undercut. Size of hidden slag inclusions should not exceed double size of permitted undercut depth. Clear distance between inclusions should be not less than nine times the size of the longest inclusion. Cracks detected by non destructive testing (NDT) methods are not advised and must either be shown to be harmless by a fitness-for-purpose assessment or be repaired (refer to Chapter 7). Lack of fusion in full penetration butt welds is not advised. Small, distributed gas pores can be left without assessment or reparation, providing that the maximum diameter of the largest pore does not exceed 0,25 t or 3 mm, and that the conditions in Table 4.1 are observed [L16]. Table 4.1: Limitations in pore sizes Detail category

Max. % of projected surface area

below 71

5%

71 – 90

3%

above 90

Gas inclusions not permitted

Weld execution is verified using NDT methods. Different methods exist; these methods are described briefly in Section 4.3.

4.2.2

Drag lines in gas cut material

As drag lines in gas cut edges with depth exceeding 0,3 mm reduce the fatigue strength, they shall be ground to result in a smooth transition (Fig. 4.1a). Drag line flaws shall not be filled up with weld material without reconsidering a new detail category [S9]. Drag lines can remain even after a gas cut plate has been welded to another member. In case of severe drag lines, a reduction of the original fatigue strength of the detail is possible. A typical example is the web to flange joint made with longitudinal fillet welds (Fig 4.1b).

Drag lines Remaining drag lines

a) Plate

b) Web to flange joint

Figure 4.1:Drag lines in gas cut material

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4.2.3

Stop/start positions of welds

Start-stop positions of continuous welds are only permitted if such points are melted and sealed correctly at new start (Fig. 4.2) [S9]. Start-stop positions are to be avoided in high stress concentration zones, for example ends of longitudinal attachments. Stop-StartPositions to be melted and sealed

Fig.4.2: Stop/start positions of welds

4.2.4

Cope holes

Cope holes, also called mouse holes, are often used in web to flange joints. This detail has been shown to result in a reduction in fatigue strength when the loading produces significant shear as well as bending stress [L33]. Therefore, this detail can be used in slender beams (L/h > 12) subjected to bending, but should be avoided in beams with significant shear. Also, the cope hole radius should be made as small as possible. Refer to [L15] for more information. Generally speaking, cope holes in highly stressed regions such as longitudinal fillet or butt welds should be avoided if possible, since start-stop positions are needed and often become points of crack initiation. Moreover, good quality corrosion protection cannot be achieved in cope holes. Current knowledge shows that weld crossings resulting from the suppression of copes holes may contain welding flaws, but that such joints are relatively tolerant to embedded flaws and that adequate welding quality can be produced without cope holes.

4.2.5

Backing strips

Backing strips are often used for butt welds, especially in hollow section joints, and can be classified into two groups according to their direction : 1. Transverse running butt welds with backing strips: where possible, removable backing strips should be used, i.e. made out of ceramics or a similar material (Fig. 4.3a). Copper backing strips can reduce the fatigue strength, because during a long lasting welding procedure the copper may intrude into the base material. Alternatively, permanent steel backing strips should be used. 2. Longitudinal running backing strips: removable or permanent (steel) backing strips can be used. With permanent backing strips, problems can be encountered when the length of the weld is greater than the length of one backing strip. Indeed, two backing strips should be joined with full penetration butt welds in order to avoid a lack of penetration and the risk of cracking when welding over the space between backing strips [L33]. When properly used, permanent strips have a negligible effect on fatigue strength. Even though some codes require a tight fit-up between backing bars and the plates joined, it has been shown that this is not necessary [L33]. The positioning tack welds and fillet weld between backing strip and base plate should lie within the butt weld (see Fig. 4.3b). Sealing welds executed on the backing bar are to be avoided and sealing should be done using coatings.

tack weld executed before welding a) Removable backing strip

b) Steel backing strip Figure 4.3: Backing strips ECCS N° 105

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4.2.6

Changes in width or thickness

Plates with changes in width or thickness (Fig. 4.4) should be tapered with a slope not greater than 1:4 as indicated in EC3-1-1, Table 9.8.3 [S1]. 1:4 1:4

1:4

1:4

Figure 4.4 : Changes in plate width or thickness

4.2.7

Crossing of welds

Crossing of welds is allowed and even recommended for suppressing cope holes in highly stressed regions (refer to Section 4.2.4). Start-stop positions at weld crossings is not advised. In the case of an attachment onto a beam, the attached part should be either fitted tightly to the first weld (Fig. 4.5a and 4.5b) or a circular cut-out should be made (Fig. 4.5c). The radius of the cut-out should be as large as possible, minimum 35 mm, (the thicker the attached plate, the larger the radius) in order to enable continuous welding. Fitting between the plates is recommended for thick attachments.

tst

a) View of attachment

R

fitted

b) 1st solution

c) 2nd solution

Figure 4.5: Crossing of welds for attachments

4.2.8

Fit of stiffeners

In order to have an economical detail and a better detail category for details attached to girder flanges, the welds between the stiffeners and the flange (Fig. 4.5a) can sometimes be avoided if the end of the stiffener is fitted to the flange (Fig. 4.6a). Another method uses a fitting plate between the stiffener and flange, fixed to the stiffener, but not to the flange (Fig. 4.6b). These solutions are only valid for double-sided stiffeners.

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Good design practice

fitted, no weld

fitted, no weld

a) Directly to the flange

b) With intermediate fitting plate Figure 4.6: Fit of stiffeners

For one-sided stiffeners and where the contact between flange and stiffener is not needed, for example if the stiffener is only provided to avoid web buckling, a gap length of 4 times the web thickness or 60 mm, the lesser of the two, should be left between the stiffener end and the flange (Fig. 4.7) [S9, L29].

<

t

4t

Figure 4.7: Stiffeners with gap

4.2.9

Welds made from one side only

Fillet welds made from one side should be executed in such a way to minimise lack of fusion—a minimum of 80% fusion should be ensured (Fig. 4.8a). Partial penetration should be avoided whenever possible (Fig. 4.8b). On the other hand, sagging of the root must be also avoided (Fig. 4.8c). If possible, a removable or permanent backing strip should be used [S9], see Section 4.2.5. The same goes for single side butt welds without backing strips; they should be avoided due to the asymmetry of the stress flow and presence of a sharp notch (Fig. 4.9) [S9]. Refer also to guidelines in [L39].

a)

b)

recommended

avoid

avoid

c)

Figure 4.8: Example of fillet welds welded from one side due to trapezoidal stiffener ECCS N° 105

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Good design practice

Notch

Notch

Figure 4.9: Example of single side butt welds between two plates or hollow sections

4.2.10 Minimisation of residual stresses Constraints in fixed parts of the structure cause residual stresses. It is one of the most important tasks of the designer to minimise as much as possible all shrinkage obstacles. Guidelines are given for example in reference [S9], Section 5.2. In this respect, structures should be designed as flexible as possible, with a low degree of indeterminacy. The welding procedure should be planned carefully regarding sequence and direction of runs, preheating, tack welds, counter-curving and pre-setting. Only the minimum thickness of the welds necessary for ultimate limit state or serviceability should be executed to avoid excessive shrinkage. However, it must be noted that a minimum weld size dependent on the thickness of the parts to be welded is required due to the thermal flow [S9]. Special care should be taken when austenitic steels are used.

4.2.11 Weld pre-preparation by machining Weld edge pre-preparation by grinding or machining brings no increase in the detail category as this zone is remelted during welding. Simple gas cutting by hand or machine is sufficient [S9].

4.2.12 Weld run-on/off pieces The use of run-on and run-off pieces is required if the static design relies on the full strength of the weld over the full thickness and length of the elements being joined together. This should be extended to all elements susceptible to fatigue since these pieces secure smooth endings of weld. The removal of run-on and run-off pieces is to be executed carefully. These are to be cut off after welding and the remaining plate edges ground flush.

4.2.13 Mechanical damages Special attention should be given to damage caused by careless handling during fabrication, transport and erection. All notches caused by transport chains, by marking, hammer blows, etc. shall be removed by grinding in fatigue critical zones. As general rule, no ignition prints or spatters of electrodes are permitted on components (Fig. 4.10) [S9].

Ignition prints not permitted Notch by chain not permitted

Figure 4.10: Examples of mechanical damages

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Good design practice

4.2.14 Erection devices Erection devices and details like ears, holes, Temporary lifting ear welded attachments, and tack welds should be treated as important as the permanent structural details (Fig. 4.11). With every case the engineer shall be informed of the proposed devices and shall approve them before fabrication. Such devices shall be categorised according to Eurocode 3 if they are not removed after erection. If they have to be removed after erection, due care shall be used. Whenever Figure 4.11: Example of erection device possible, they are to be cut off after erection of the element and the remaining plate edges ground flush. The removed device locations shall be examined for cracks and any other surface defects. Those found should be repaired keeping in mind the guidelines in Chapter 7. Cases where part of the device remains shall be investigated according to the detail categories of Eurocode 3. All the same, holes drilled in elements during erection as well as misplaced holes can cause problems if they are filled with weld material (Fig. 4.12). Recommendations given in Section 7.2, repair during fabrication or erection of new structures, should be followed.

No welding of holes if not re-categorised!

Figure 4.12: Fill up of holes

4.3

CONTROL OF WELDING

Control of welding is performed with various techniques using the properties of magnetic materials, penetrating dyes, radiography or ultrasounds. Each technique is described below. It should be noted that the technique using Eddy currents cannot be applied to steel structures, as it is only valid on non-ferrous conducting materials (aluminum, titanium). The techniques, generally more than one, to be used depend upon the type of welded joint. For fillet welds, a complete control should include: a visual test, dimensional measurements, a magnetic test and a ultrasonic test.

4.3.1

Visual Test (VT)

Visual testing implies careful inspection of the welded surface and surrounding zones in order to detect all visible flaws, discontinuities, corrosion marks, big cracks, surface porosity, weld splatter etc. For better results, good lighting and a magnifying glass (from 2 - 10 x magnification) should be used. The results of inspection should be recorded by writing and, in some cases, by micro-photography.

4.3.2

Magnetic Test (MT)

Magnetic particle testing requires application of a magnetic sensitive media on the detail, then subjecting the detail to a magnetic field and looking for field anomalies. This method can only be applied to ferrous materials. With this method, cracks and pores at the weld surface (with AC or DC current) or up to 2 mm in ECCS N° 105

40

Good design practice

depth (only with an AC current) can be detected but not the ones located in the depth of the material. The minimum detectable size of flaw depends markedly on the surface conditions. The results of testing should be recorded by writing.

4.3.3

Dye penetration Test (PT)

Dye penetration testing involves applying a dye to the detail that penetrates into the flaws, carefully wiping off surplus dye from the examination zone surface, then either applying developer powder that soaks dye from flaws, or using a UV lamp to detect fluorescent penetrated locations. Only surface defects can be seen with this method, but it is applicable to all non-porous materials, such as ferrous and non-ferrous metals. Detectable flaw sizes are similar to the MT method and also depend markedly on the surface conditions. The results of testing should be recorded by writing. As surface discontinuities are decisive for fatigue lifetime, VT, MT and PT are very important NDT methods.

4.3.4

Radiographic Test (RT)

Radiographic methods involve placing a source of X-rays or gamma-rays on one side of the detail and a photographic film on the other side, thereby getting a 2D picture of the internal structure. One important difference between these two methods is that the X-rays source is an electrical machine whereas the gammarays source is a radioactive substance. They both are used to detect volumetric defects inside the material. The limitations are their poor ability to detect tight cracks, small cracks and other planar defects lying at an angle to the radiation beam. As the contrast between parent material and defects diminishes with thickness, these methods cannot be used for thick plates. Considering cost, portability, reliability, they should not be used on steel welds thicker than about 30 mm. Some difficulties, especially on site work, arise from the use of ionising radiation sources. This requires special safety precautions such as : lead shielding, warning signs, barrier around the working area, radiation monitoring devices, etc. After testing, the results recorded on a photographic film can be compared to reference records. The film must be placed directly on the plate or weld surface whereas the radiation source can be located at some distance from the detail.

4.3.5

Ultrasonic Test (UT)

Ultrasonic testing is done by sending a beam of ultrasound in the detail using a small probe (the transducer) coupled to the surface by a layer of liquid. The pulse of the ultrasound reflected back from flaws or surfaces is picked up by the same probe and displayed on an oscilloscope screen. Different probes exist, but not all types of weld geometry can be examined with UT. Ultrasonic testing is used to detect planar defects inside the material ; it is less efficient in detecting volumetric defects because of the dispersion in the reflected ultrasound. For thin plates, thickness below ca. 10 mm, this test gives poor results. Application of UT requires experience in interpreting the oscillogram screen display. The surface of the plate or weld where the transducer is to be placed has to be prepared (cleaned or ground). Ultrasonic inspection of austenitic steels welds is more difficult compared to other steel welds, because of the microstructure of such welds. In most case, the results are documented by writing.

ECCS N° 105

Good design practice

5

IMPROVEMENT METHODS

5.1

INTRODUCTION

41

An improvement method is a procedure that extends the fatigue life of a welded joint without changing the applied stresses and without changing the overall joint geometry or shape. Apart from some other very special methods, the improvement methods for welded structures can be separated into two main groups : A) Methods that smoothen the weld bead-base plate transition and eliminate surface defects, either by removal (shape change methods) or addition (coatings) of material. The primary goal of these methods is to reduce the local stress concentrations in the detail. B) Methods that change tensile residual stresses into compressive stresses in the superficial layer and, to a limited extent, change the shape of the weld bead-base plate transition. These methods are most efficient in modifying the effective stress ratio to which the detail is subjected. The beneficial influence of the improvement methods is strongly dependent on the method of application, quality control and fatigue loads applied to the welded detail. The nominal stress spectrum must not contain stress ranges nor peak stresses in tension or compression exceeding the nominal yield stress of the steel (Δσmax < fy and σmax < fy). Improvement methods are the most efficient for details with high local stress concentrations and details in the low stress range / high cycle region. The most efficient method is, however, dependent on the type of welded joint. In general, such methods are only effective on surface notches, not for internal defects, e.g. only the improvement of weld toes is possible. Considering the major effects affecting the fatigue life of welded joints, as listed in Chapters 3 and 4, details may be categorised as follows: • Load-carrying attachments. • Non load-carrying attachments : - Attached in the longitudinal direction. - Attached in the transverse direction. Generally it can be said that fatigue failure starts : - From the toe of a non-load carrying weld. - From the root of a load carrying weld. Another influence factor is the weld size. A detail will most likely suffer from : - Toe cracks with large weld dimensions including full penetration welds. - Root failures with small weld dimensions. Therefore, improvement methods will primarily be applied to medium to large non-load carrying attachments. A general rule is that improvement methods shall not be used to compensate for bad design or poor workmanship. Improving the fatigue strength of welded joints may be of economical interest in special cases. Improvement methods should be considered in cases where a large number of similar or equal fabrication details or methods are to be used and where adequate quality control can be insured. An improvement is especially effective in the case of details made out of high strength steels.

5.2

SHAPE CHANGE METHODS

Shape changing methods are [L1] : • Grinding. Machining process which removes material (depth 0.5 to 0.8 mm) at the weld toe using a disk grinder or a rotary burr grinder. • TIG or plasma dressing. Process of remelting weld toe using a tungsten inert gas (TIG) or a plasma torch. Shape changing methods alter the initial defect characteristics and the stress concentration by removal of material. They are best for welds where the majority of the fatigue life is expended for growing cracks from ECCS N° 105

42

Good design practice

their initial size up to 1 mm deep, and are suitable for all stress range levels. These methods require good access to the detail with hand-operated machining devices. the methods may expose internal defects, thus making them more severe, and may cause tensile surface stresses. Quality control of the methods can be done by visual inspection to insure that the original weld toe has been removed and by comparison of the new toe with reference mouldings of weld shape. In case of grinding only traces in direction of stress flow are permitted. For grinding and TIG dressing, a method of application is given in [L37].

5.3

RESIDUAL STRESS METHODS

Methods to relieve the residual stresses, and which sometimes even produce compressive stresses at the hot spot are : • Prior overloading (see next section) • Post welding heat treatment, stress relief of welds (especially for TM-steel) • Shot peening. Cold-working process which consist of striking the surface of the component, usually with a high velocity stream of metal or glass particles. • Hammering (peening). Cold-working process which consist of striking the surface of the component with a tool which can be a pneumatic or an ultrasonic hammer. Residual stress methods lower the effective stress ratio R (minimum stress / maximum stress). These methods are more efficient on high yield strength steels. An uncontrolled application of one of these methods can cause cracking. These methods are only appropriate to high cycle fatigue (which is equivalent to low stress range), because they loose their efficiency under high stress ranges and can even become nonfavourable. Quality control of the methods is more difficult than for shape change methods. It involves visual inspection to check the uniformity of the treatment and the coverage rate (full removal of marking media deposited before treatment) [L1, L37]. For hammer peening, a method of application is given in [L37].

5.4

OVERLOADING

Overloading prior to in-service conditions can be considered an improvement method for structures made of tough material, since it may produce an increase in fatigue strength. However, this method should be used with caution as it can initiate cracks. The principle of the method is to load the structure (in compression or tension) until yielding occurs in certain zones, the yielding level being influenced by the residual stresses in the details for welded structures. Therefore, this method can be efficient to bring down the tensile residual stresses, or even create local compressive zones, at defects in welded details when applying tensile stresses. Due attention should be given to the buckling and static stability of the structure. The overloading method should not be used for bolted structures, instead, the method of cold expansion of bolt holes can be used to create compressive residual stress zones around the holes. This method consists of introducing an oversized hard tool in the hole, thus inducing plastic deformations in the radial and circumferential directions. Upon removal of the tool, the elastic material surrounding the hole attempts to force the plastically deformed material to return to its original position with the results that compressive residual stresses will be created in the band of material around the hole [L30].

5.5

COATINGS

Most coatings are anti-corrosion coatings. These coatings prevent damage due to rust notches (see Section 2.5.1). They are very efficient for very short crack growth in corrosive environment (marine, chemical plants, automotive), but ineffective on embedded defects. Existing surface defects may be too large for any possible improvement. Corrosion protection is required on welded joints improved by shape change methods, because corrosion effects may completely eliminate the benefit of the improvement

ECCS N° 105

Good design practice

43

[L1, L26]. In addition, some coatings are believed to reduce the stress concentration by straining with the joint under load, but actual knowledge and experience in this domain is too limited.

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6

QUALITY ASSURANCE

6.1

GENERAL

Requirements should be incorporated in the quality assurance documents to ensure that structural details comply with the relevant quality requirements for fatigue. Such requirements should be stated explicitly on the fabrication drawings and the erection diagrams for each detail [L16]. In order to be on the safe side in terms of weld quality, a proper system of quality assurance should be applied. A possible well suited quality management system is provided by the ISO 9000 system. Weld quality should fulfil the requisites stipulated for quality class B according to EN 25817 [S24], which is equivalent to ISO 5817. Weld irregularities should be estimated according to EN 26520 (ISO 6520) [S27]. In all cases the rules of good workmanship should be obeyed. If defects exceeding the acceptance levels are detected in material or welding, the rules given in Chapter 7 : Methods for Repair, should be considered.

6.2

QUALITY ASSURANCE TESTING METHODS

The common quality assurance is performed by non-destructive testing (NDT) of the details using of one or more of the methods described in Section 4.3. More information on the various NDT methods can be found in the litterature [L31]. Before choosing a method the definition of what the NDT shall detect is necessary. The NDT detection capabilities also depend on the weldment type. Whenever possible, the design of the weldment should be made to facilitate NDT (extension of free surface for scanning, local geometry). The extend of testing is prescribed by competent standards or by the design engineer. Requirements should never be less than the requirements given for structures designed against the static limit state. NDT methods involve complementary processes. For most high quality welded fabrications in ferritic steels, the combination usually chosen is magnetic particle inspection (MT) and ultrasonic testing (UT), whilst for austenitic steels the combination is penetrant testing (PT) and radiography (RT). The human factor is very important in NDT, and NDT requires well trained certified personnel.

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Good design practice

7

METHODS FOR REPAIR

7.1

FITNESS-FOR-PURPOSE APPROACH

NDT controls during fabrication and erection cannot be properly implemented without the establishment of acceptance levels. A distinction must be made between acceptance based on quality control and acceptance based on fitness-for-purpose. Quality control levels are of considerable value in the monitoring of weld quality during production. These levels are, out of necessity, both arbitrary and conservative and are based on good workmanship criteria. If flaws more severe than the quality control levels are revealed, rejection is not necessarily automatic, but may be based on fitness-for-purpose [L22, L33]. By this principle a weld in a particular fabrication is considered to be adequate for its purpose provided the conditions to cause failure are not reached during service (including a safety margin). A second reason for the use of the fitness-forpurpose principles is that the repair of a weld is always a difficult task and if not carefully planned and executed it will result, in terms of fatigue, in a lower strength than the strength of the detail prior to the removal of the flaw. Recommendations on the use of fitness-for-purpose can be found in documents such as the IIW [L18] and the BS PD 6493 [L22] guides. In these guides formulas for assessing the stress range magnification resulting, for example, from axial and angular misalignment are given. This approach, however, should not be used on a regular basis to evaluate the adverse effect of a weld flaw on the fatigue resistance. It requires expertise to be used properly and leads sometimes to the use of rather sophisticated analysis tools⎯FEM analyses, fracture mechanics methods⎯that can be expensive and time consuming.

7.2

REPAIR DURING FABRICATION OR ERECTION OF NEW STRUCTURES

If flaws or defects are found in a component during fabrication or erection, the engineer shall be informed and shall decide upon the action to be taken by answering the following questions : • Does the defect found reduce the fatigue strength of the detail in an unacceptable manner ? Use either quality standards or fitness-for-purpose evaluation (refer to references [L18, L22, L33] for guidelines). • Can the defect be removed without major changes in the component, e.g. by careful grinding ? • Should the defective area be removed and replaced ? In cases where some type of repair is needed, a description of the repair procedure shall be written prior to execution and prior to approval by the engineer. As a general rule, a non-welding repair should preferably be considered prior to any welding repair, because welding causes shrinkage and additional residual and secondary stresses, which in many cases favour fatigue crack development. Examples of non-welding repairs are grinding, peening, and hole drilling. Filling misplaced holes, cut-outs, etc. with weld material is not recommended. It can be done, but requires reclassification of the detail [S9] into a category that is often lower than the unwelded detail category. In the case of holes, it is possible to leave them open where possible, or to fill them with rivets, tightened bolts or eventually injection bolts (see Fig. 7.1). Another option is to fill the hole with epoxy resin. The least recommended method is to fill the hole with a round piece of steel held in place with fillet welds ; if executed, the filled welds should be of quality, as small as possible (only to avoid corrosion) and, in no way, fully penetrated.

Warning: notches! Preferred : Open

Pretensionned bolts

Rivet

If the repair method contains welding, a qualification of the welding procedure (WPS) is needed. The usual procedure is Figure 7.1: Filled up of holes in plates to remove the defect or crack by gouging, then rewelding ECCS N° 105

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and grinding to improve shape and removing surface defects. For minor repairs at weld toes, the possibility of using TIG [L35] or plasma remelting should not be forgotten (see Chapter 5 and next section).

7.3

EXISTING STRUCTURES

7.3.1

Assessment and control

Assessment and control of steel structures is regulated in a large number of standards and guidelines. For bridges, the reader can refer for example to [S5-S7]. The following points only gives examples of how to undertake such investigations [L2, L5-L9]. • Plan regular inspections of the structure, with special attention to spots where excessive stresses from fatigue relevant loads occur. The inspection interval depends mainly on the number and aggressiveness of the load cycles. • Obtain information from drawings and static calculations. Perform an on-site inspection to validate this information. • Using the information gathered, determine the areas that : a) are under extreme stress, b) have the worst detail categories, c) do not perform as expected. • Inspection methods : a) Always perform a visual examination. Check for bad workmanship signs. If some areas show irregular signs—visible cracks and excessive corrosion, deformations or cracks in the paint coating—complete inspection with method b) for such areas. b) Magnetic particle testing of welds in areas of high stress or with poor detail categories, but also where method a) causes suspicion. If there are still doubts, go to method c). c) More sophisticated investigation methods such as ultrasonic testing or radiography are useful where other methods are deemed too inaccurate. Such extensive methods need costly preparation and can be performed only by very skilled specialists. • Results of inspections should be collected over the lifetime of the structure and be compared carefully. Sometimes the results can give indications of inadequate behaviour and the subsequent action to be taken, e.g. sudden or progressive increase of deflections.

7.3.2

Repair and strengthening

In cases where strengthening of a structure is being considered or repair is needed, the following rules must be taken into account [L10, L11, L34]: • Old structures are in many cases built of material with very poor welding qualities even if they have withstood a long fatigue life. Based upon erection date, an approximate classification of the construction material can be made : − before 1860 : cast iron. Low ductility, low tensile strength material, not weldable. − in the period 1860 - 1900 : wrought steel. Flaky metallographic structure. Anisotropic mechanical characteristics, not weldable. − in the period 1900-1950 : mild steel (obtained using Thomas converter). Homogeneous material with low toughness. Not good weldability. − after 1950 : modern steel. Refer to weldability of steel alloy used. Note that the dates given in the above classification are only indicative for industrialised countries and are strongly dependent upon the country in which the structure has been erected. Old structures need special care. To obtain more information on the material, tests on either easily replaceable components or on sub-sized specimens removed from the structure should be made (Fig. 7.2).

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Tensile test specimen Chemical analysis specimen Loading direction

45 mm

? CT sample for fracture mechanic test

Figure 7.2: Sub-sized plate samples • Every member in a structure is subjected to a certain magnitude of stress (dead load, permanent loads, thermal loads, prestressing, etc.). On the contrary, any reinforcing or replacement member (plate, profile) can be assumed as free of stress. This results in very different stress levels between the members under live load : the original members in the structure may eventually reach their yield stress, while the added members have a relatively low level of stress. In such cases the effects of fatigue loading on the structure should be checked very carefully. • Residual stresses due to shrinkage of welds (especially when oversized) induce a very different stress distribution in the structure from the general calculated stresses. • Strengthening and stiffening of a structure often displaces the problem from one location to another. In case of repair of fatigue cracks, the repair procedure depends on the following factors [L34]: • Size, number of cracks, • Type of stress causing the cracking, • Remaining service life of structure, • Frequency and thoroughness of the inspection program, e.g. design strategy (refer to Chapter 3), • Anticipated quality of repairs (accessibility, welding position, ...), • Service loading controls (stop or reduced traffic, etc.) available during repairs. In most cases, a bolted repair is better than a welded repair. A welded repair requires prior knowledge of the base metal characteristics. Bolted repairs also have the advantage of adding redundancy to a structure. In welded structures, surface cracks with limited depth can be repaired either by grinding, TIG or plasma remelting [L35]. Grinding should not expose the root of a weld or a previously embedded flaw. In the case of through-thickness cracks, the first step is to determine the location of the cracks ends as precisely as possible. The simplest method of repair once the cracks have been found, is to drill a hole at each crack tip. The hole should be positioned so that its centre is at the crack tip in order to reduce the overall length of the crack and to minimise section loss. In most applications, a diameter between 18 and 30 mm (the longer the crack, the larger the hole) is sufficient to retard significantly or even prevent crack reinitiation. The following formula can also be used to determine the required radius, ρ, so that no initiation occurs [L29] :

$>

#" 2 ! L 70 ! f y

(for fy in N/mm2)

Where Δσ is the stress range at the cracked detail (near the hole to be drilled) and L is the crack length. All holes should be ground and surface polished. In cases where no adequate corrosion protection is available, high strength bolts can be inserted in the holes (Fig. 7.1). In cases where the holes are sufficient to stop the crack, such holes can be left as is if the remaining cross section is sufficient to withstand the service loads. If not, splice plates should be bolted over the defective area. ECCS N° 105

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8

LITERATURE

8.1

FATIGUE LITERATURE

L1

Methods of Improving the Fatigue Strength I. F. C. SMITH, M. A. HIRT; Publication ICOM of Welded Joints N° 114, April 1983.

L2

Fatigue and Fracture of Riveted Bridge E. BRÜHWILER, I. F. C. SMITH, M. A. HIRT; Members Journal of Structural Engineering Vol. 116, No. 1, Jan., 1990

L3

Fatigue-Resistant Steel Bridges

L4

Recommendations Concerning Stress International Institute of Welding, IIW Determination for Fatigue Analysis of Commissions XIII and XV, Chairman E. Welded Components NIEMI, IIW Doc. XIII-1458-92 and XV-797-92, 1993.

L5

Bewertung der Spontanbruchgefahr E. BRÜHWILER, M. A. HIRT, U. MORF, R. angerissener Brückenbauteile aus HUWILER; Publication ICOM N° 198, Schweisseisen February 1989.

L6

Probabilistisches Verfahren zur Beurteilung P. KUNZ; Ecole Polytechnique Fédérale de der Ermüdungssicherheit bestehender Lausanne, Thesis EPFL N° 1023, 1992. Brücken aus Stahl

L7

Bridge Management 2 - Inspection, Edited by J. E. HARDING, G. A. R. PARKE maintenance, assessment and repair and M. J. Ryall: Department of Civil Engineering, University of Surrey, UK; Thomas Telford, London, 1993.

L8

Untersuchungen zur Betriebsfestigkeit von E. HAIBACH, I. PLASIL; Der Stahlbau 9/1983. Stahlleichtfahrbahnen mit Trapezhohlsteifen im Eisenbahnbrückenbau

L9

Ermüdungsgerechte Konstruktion einer M. HERZOG, Aaarau; Der Stahlbau 9/1981. geschweißten Eisenbahnfachwerkbrücke

L10

Methoden zur Sanierung von C. MIKI, H. TAKENOUCHI, T. MORI; Ermüdungsschäden in Anschlüssen von berichtet in Stahlbau 59, 1990. Querverbänden stählerner Balkenbrücken

L11

Ermüdungsrisse Stahlbrücken

L12

Zum Verhalten ausgeklinkter Träger unter J. SCHEER, H.-J. SCHEIBE, D. KUCK; zyklischer Beanspruchung Bauingenieur 65, 1990, p. 463.

L13

Schwingfestigkeitsversuche für den E. HOFFMANN, R. OLIVIER; Der Stahlbau Stumpfstoß in übereinanderliegenden 9/1977, p. 263. Gurtplatten

ECCS N° 105

in

I. F. C. SMITH, M. A. HIRT; Journal of Constructional Steel Research,, Elsevier Science Publishers Ltd. England, 1989.

amerikanischen F. NATHER, Bauingenieur 64, 1989, p. 217.

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L14

Größeneinfluß von Fehlern auf G. SCHULZE, Berlin; Gebrauchseigenschaften geschweißter Schneiden 34, N° 5, 1982. Bauteile - Beitrag zur Sicherung der Güte von Schweißarbeiten nach DIN 8563 Teil 3

Schweißen

und

L15

Recommendations on Fatigue of Welded International Institute of Welding, IIW/IIS, Joint Components Working Group XIII-XV, Chairman A. Hobbacher, IIW Doc. XIII-1539-96 / XV-84596, Published by Abington Publishing, Cambridge, UK, 1996.

L16

Recommendations for the fatigue design of –European Convention for Constructional steel structures Steelwork, ECCS, Publication N° 43, Brussels, 1987.

L17

Fatigue I and II

L18

IIW guidance on assessment of the fitness- International Institute of Welding, IIW/IIS, IIW for-purpose of welded structures Doc. SST-1157-90, 1990.

L19

Construction Métallique / Notions M.A. HIRT, R. BEZ; Traité de Génie Civil de fondamentales et méthodes de l’Ecole polytechnique fédérale de Lausanne dimensionnement (EPFL), Volume 10, Presses Polytechniques et Universitaires Romandes, Lausanne, Switzerland, 1994.

L20

Kerbspannungslehre

L21

Aspects of Good Design Practice for E. NIEMI, ESIS 16, 1993, Mechanical Fatigue-Loaded Welded Components Engineering Publications, London, UK, pp. 333351.

L22

Guidance on methods for assessing the BS PD 6493: 1991, British Standards Institution, acceptability of flaws in fusion welded London, 1991. structures

L23

Elementary Fracture Mechanics, Fourth BROEK, D., Kluwer Academic Publishers, revised edition Dordrecht, The Nederlands, 1986.

L24

A Survey of Fatigue Cracking Experience in MIKI, C., SAKANO, M., International Institute Steel Bridges of Welding, IIW/IIS Doc. XIII-1383-90, 1990.

L25

Fatigue and Fracture of Steel Bridges - Case FISHER, J. W., Wiley Interscience, New York, studies ISBNO-471-80469-X, 1984.

L26

Fatigue of Welded Structures, 2nd Edition

GURNEY, T.R., Cambridge, UK, Cambridge University Press, 1979.

L27

Roark's Formulas for Stress and Strain

ROARK, R.J., and YOUNG, W.C., Sixth Edition, McGraw-Hill Book Company, 1989.

Working group 12, European Steel Design Education Programme (ESDEP) courses, Vol. 18 and 19, Published by the Steel Construction Institute, Ascot, UK, 1995.

H. NEUBER, Springer, Berlin, 1958.

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L28

Weathering Steel in Bridges

L29

Distortion-Induced Steel Bridges

L30

Improving the fatigue performance of bolt CANNON, D.F., SINCLAIR, J., SHARPE, holes in railway rails by cold expansion K.A., Conference Proceedings, Fatigue life analysis and prediction, ASM, 1986.

L31

Introduction to the non-destructive testing of R. HALMSHAW, Second Edition, Abington welded joints Publishing, in association with The Welding Institute, Cambridge, 1996

L32

Fatigue strength of welded joints in three KOSKIMAKI, M., and NIEMI, E., International types of stainless steel Institute of Welding, IIW/IIS, IIW Document No. XIII-1603-95, 1995.

L33

Developments in fatigue design codes and MADDOX, S.J., The Welding Institute, fitness-for-service assessment methods Abington Hall, Abington, Cambridge,CB1 6AL, U.K., 1996.

L34

Focusing on Fatigue, guidelines for crack KEATING, P.B., Civil Engineering, Vol. 64, repair No. 11, Nov. 1994, pp.54-57.

L35

Fatigue performances of repairing welds TAKENOUCHI, H., MIKI, C., SATO, S., with TIG-dressing for fatigue damaged International Institute of Welding, IIW/IIS, Doc. highway bridges XIII-1509-93, 1993.

L36

An alternative to Miner’s rule cumulative damage calculations ?

L37

IIW Recommendations for Weld Toe HAAGENSEN, P.J., and MADDOX, S.J., Improvement by Burr Grinding, TIG International Institute of Welding, IIW/IIS, dressing and Hammer Peening for Steel and Commission XIII, Working group 2, July 1999 . Aluminium Structures

L38

Fatigue design data for welded stainless MADDOX, S.J., BRANCO, C.M., SONSINO, steels C.M., and al., International Institute of Welding, IIW/IIS, Doc. XIII-1768-99, 1999.

L39

Ermüdungssicheres Konstruiren von Lehrstuhl fuer Stahlbau/RWTH orthotropen Platten für Strassenbrücken Germany, Feb. 1998.

L40

Design guide for circular and rectangular CIDECT Research Project 7M, Final Report, hollow section joints under fatigue loading July 1998.

ECCS N° 105

Fatigue

FISHER, M., Structural Engineering International, Vol. 5, No. 1, February 1995, pp. 51-54 Cracking

in FISHER, J.W., YEN B.T., JIAN, J., and WAGNER D.C., NCHRP Project 12-86(6), National Cooperative Highway Research Program (NCHRP) Report 336, Highway Research Board, Washington, D.C., 1990.

for GURNEY, T., MADDOX, S.J., IABSE Workshop Lausanne 1990, Report N° 59, IABSE, Zurich, 1990, pp. 189-198.

Aachen,

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8.2

STANDARDS

S1

ENV 1993-1-1 : 1992, EUROCODE 3: Design of steel structures – Part 1-1: General rules and rules for buildings, European Committee for Standardisation (CEN), Brussels, April 1992.

S2

ENV 1993-2 : 1997, EUROCODE 3: Design of steel structures - Part 2 Bridges, European Committee for Standardisation (CEN), Brussels, Oct. 1997.

S3

DS 804 - Vorschrift für Eisenbahnbrücken und sonstige Ingenieurbauwerke (VEI), Deutsche Bundesbahn, 1983.

S4

BS 5400 : Part 10 Steel, concrete and composite bridges - Code of practice for fatigue, British Standard Institution (BSI), London, 1980 (revised edition 1999).

S5

Richtlinie 805, Tragsicherheit bestehender Eisenbahnbrücken, Deutsche Bahn AG, Berlin, Deutschland, Jan. 1997.

S6

Richtlinie 803, Inspection von Ingenieurbauwerken, Deutsche Bahn AG, Berlin, Deutschland, Jan. 1997

S7

AASHTO, Guide specification for fatigue evaluation of existing steel bridges, Am. Assoc. of State and Highway Transp. Officials, 444 North Capital St. N.W., Washington, D.C., 1990.

S8

SIA 161: Steel structures, Swiss Society of Engineers and Architects, Zürich, Switzerland, 1990.

S9

ÖNORM B 4300-5, Stahlbau – Ermüdungsfestigkeit, Wien, Austria, 1994.

S10

EN 10025 : 1993, Non-alloyed Steels, European Committee for Standardisation (CEN), Brussels, Dec. 1993.

S11

EN 10113 : 1993 (in three parts), Weldable fine grain steels, European Committee for Standardisation (CEN), Brussels, June 1993.

S12

EN 10210-1 : 1994 and EN 10210-2 : 1997, Hot finished structural hollow sections of nonalloy and fine grain structural steel, European Committee for Standardisation (CEN), Brussels.

S13

EN 10137 : 1995 (in three parts), Plate and wide flats made of high yield strength structural steel in the quenched and tempered or precipitation hardened condition, European Committee for Standardisation (CEN), Brussels, Dec. 1995.

S14

EN 10155 : 1993, Structural steels with improved atmospheric corrosion resistance, European Committee for Standardisation (CEN), Brussels, 1993.

S15

EN 10088 : 1995, Stainless steels, European Committee for Standardisation (CEN), Brussels, 1995.

S16

ENV 1090 : 1996 (in five parts), Execution of steel structures, Part 1 : General rules and rules for buildings and Part 5 : Supplementary rules for bridges, European Committee for Standardisation (CEN), Brussels, 1996.

S17

ISO 10721-1:1997, Steel structures - Part 1: Materials and design, International Organisation for Standardisation (ISO), Geneva, Switzerland, 1997.

S18

ENV 1991-1:1996, Basis of design and actions on structures, Part 1 : basis of design, European Committee for Standardisation (CEN), Brussels, 1996.

S19

prENV 1991-3:1997, Basis of design and actions on structures, Part 3 : traffic loads on bridges,

ECCS N° 105

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European Committee for Standardisation (CEN), Brussels, 1997. S20

prENV 1991-2-4:1997, Basis of design and actions on structures, Part 2-4 : wind actions, European Committee for Standardisation (CEN), Brussels, 1997.

S21

prENV 1991-5 : 1997, Basis of design and actions on structures, Part 5 : actions induced by cranes and other machinery, European Committee for Standardisation (CEN), Brussels, 1997.

S22

prENV 1993-3:1997, Towers, masts and chimneys, European Committee for Standardisation (CEN), Brussels, 1997.

S23

prENV 1993-6:1997, Crane supporting structures, European Committee for Standardisation (CEN), Brussels, 1997.

S24

EN 25817 (ISO 5817) : Arc-welded joints in steel – Guidance on quality levels for imperfections, European Committee for Standardisation (CEN), Brussels, Nov. 1992

S25

American Petroleum Institute, Recommended practice for planning, designing and constructing fixed offshore platforms, API RP2A-LRFD, Dallas, 1993.

S26

DEn, Offshore installation : guidance on design and construction, Departement of Energy, London, UK, 1990.

S27

EN 26520 : Classification of imperfections in metallic fusion welds, with explanations, European Committee for Standardisation (CEN), Brussels, 1992.

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8.3

53

ECCS RELATED PUBLICATIONS

N° 43

Recommendations for the fatigue design of steel structures, 1987.

N° 68

E.R. for aluminium alloy structures fatigue design, 1992.

N° 81

The use of weathering steel, 1995.

N° 93

Executing Steel Structures to Eurocodes 3 and 4 : Guide to ENV 1090 – 1, 1997.

ECCS N° 105