CHAPTER 1 INTRODUCTION1.1 General

A component in a structure may be susceptible to one, two or more kinds of failure. For example, under given load conditions a roller bearing is most likely to fail through fatigue of its rollers after a certain number of rotations. We should thus know the different conditions that can cause the failure of a structural component. Some of the common causes of failure are yielding, deflection beyond a certain age, Buckling, Fatigue, Fracture, Creep, Environmental degradation, Resonance, Impact and Wear.1.2 Fatigue

Fatigue failures in metallic structures are a well-known technical problem. A single load application, far below the static strength of a structure, did not do any damage to the structure.Fracture due to fatigue and consequent failure is very common in steel structures. A repeated load applications can start a fatigue mechanism in the material leading to nucleation of a small crack, followed by crack growth, and ultimately to complete failure..Such failures occur predominantly at component connections. Connections refers to those locations in a structure where elements are joined to accommodate changes in geometry or fabrication or service requirements. Fatigue cracking in bridges, ships, offshore structures, pressure vessels and building occurs, almost without exception, at the welded or bolted connections.1.3 Welding and Welding Connections

Welding is the process of joining two pieces of metal by creating a strong metallurgical bond between them by heating or pressure or both. It is distinguished from other forms of mechanical connections, such as riveting or bolting, which are formed by friction or mechanical interlocking. It is one of the oldest and reliable methods of joining. Welding offers many advantages over bolting and riveting. Welding enables direct transfer of stress between members eliminating gusset and splice plates necessary for bolted structures. Hence, the weight of the joint is minimum. In the case of tension members, the absence of holes improves the efficiency of the section. It involves less fabrication cost compared to other methods due to handling of fewer parts and elimination of operations like drilling, punching etc. and consequently less labour leading to economy. Welding offers air tight and water tight joining and hence is ideal for oil storage tanks, ships etc. Welded structures also have a neat appearance and enable the connectionof complicated shapes. Welded structures are more rigid compared to structures with riveted and bolted connections. A truly continuous structure is formed by the process of fusing the members together. Generally welded joints are as strong or stronger than the base metal, thereby placing no restriction on the joints. Stress concentration effect is also considerably less in a welded connection.

1.4 Types of joints and welds

By means of welding, it is possible to make continuous, load bearing joints between the members of a structure. A variety of joints is used in structural steel work and they can be classified into four basic configurations namely, Lap joint, Tee joint, Butt joint and Corner joint.

For lap joints, the ends of two members are overlapped and for butt joints, the two members are placed end to end. The T- joints form a Tee and in Corner joints, the ends are joined like the letter L. Most common joints are made up of fillet weld or the butt (also calling groove) weld. Plug and slot welds are not generally used in structural steel work. Fig.1 Fillet welds are suitable for lap joints and Tee joints and groove welds for butt and corner joints. Butt welds can be of complete penetration or incomplete penetration depending upon whether the penetration is complete through the thickness or partial. Generally a description of welded joints requires an indication of the type of both the joint and the weld.Though fillet welds are weaker than butt welds, about 80% of the connections are made with fillet welds. The reason for the wider use of fillet welds is that in the case of fillet welds, when members are lapped over each other, large tolerances are allowed in erection. For butt welds, the members to be connected have to fit perfectly when they are lined up for welding. Further butt welding requires the shaping of the surfaces to be joined as shown in Fig1.2. To ensure full penetration and a sound weld, a backup plate is temporarily provided as shown in Fig1.31.4.1. Butt welds:

Full penetration butt welds are formed when the parts are connected together within the thickness of the parent metal. For thin parts, it is possible to achieve full penetration of the weld. For thicker parts, edge preparation may have to be done to achieve the welding. There are nine different types of butt joints: square, single V, double V, Single U, double U, single J, double J, single bevel and double bevel. They are shown in Fig1.1 In order to qualify for a full penetration weld; there are certain conditions to be satisfied while making the welds.

Welds are also classified according to their position into flat, horizontal, vertical and overhead. Flat welds are the most economical to make while overhead welds are the most difficult and expensive.

Figure 1.1 Different Types of Butt Welds

The main use of butt welds is to connect structural members, which are in the same plane. A few of the many different butt welds are shown in Fig1.4.There are many variations of butt welds and each is classified according to its particular shape. Each type of butt weld requires a specific edge preparation and is named accordingly. The proper selection of a particular type depends upon: Size of the plate to be joined; welding is by hand or automatic; type of welding equipment, whether both sides are accessible and the position of the weld.Butt welds have high strength, high resistance to impact and cyclic stress. They are most direct joints and introduce least eccentricity in the joint. But their major disadvantages are: high residual stresses, necessity of edge preparation and proper aligning of the members in the field. Figure 1.2 Common Types of Welds

Figure 1.3 Shaping of Surface and Backup Plate

Figure 1.4 Typical Connections with Butt Weld

To minimise weld distortions and residual stresses, the heat input is minimised and hence the welding volume is minimised. This reduction in the volume of weld also reduces cost. Hence for thicker plates, double Butt welds and U welds are generally used. For a butt weld, the root gap, R, is the separation of the pieces being joined and is provided for the electrode to access the base of a joint. The smaller the root gap the greater the angle of the bevel. The depth by which the arc melts into the plate is called the depth of penetration [Fig1.5 (a)]. Roughly, the penetration is about 1 mm per 100A and in manual welding the current is usually 150 200 A. Therefore, the mating edges of the plates must be cut back if through-thickness continuity is to be established. This groove is filled with the molten metal from the electrode. The first run that is deposited in the bottom of a groove is termed as the root run [Fig1.5 (c)]. For good penetration, the root faces must be melted. Simultaneously, the weld pool also must be controlled, preferably, by using a backing strip.

Figure 1.5 Butt Weld Details

1.4.2. Fillet Welds:Owing to their economy, ease of fabrication and adaptability, fillet welds are widely used. They require less precision in the fitting up because the plates being joined can be moved about more than the Butt welds. Another advantage of fillet welds is that special preparation of edges, as required by Butt welds, is not required. In a fillet weld the stress condition in the weld is quite different from that of the connected parts. A typical fillet weld is shown in Fig1.6

Design of SteFigure 1.6 Typical Fillet WeldThe root of the weld is the point where the faces of the metallic members meet. The theoretical throat of a weld is the shortest distance from the root to the hypotenuse of the triangle. The throat area equals the theoretical throat distance times the length of the weld.

The concave shape of free surface provides a smoother transition between the connected parts and hence causes less stress concentration than a convex surface. But it is more vulnerable to shrinkage and cracking than the convex surface and has a much reduced throat area to transfer stresses. On the other hand, convex shapes provide extra weld metal or reinforcement for the throat. For statically loaded structures, a slightly convex shape is preferable, while for fatigue prone structures, concave surface is desirable.

Large welds are invariably made up of a number of layers or passes. For reasons of economy, it is desirable to choose weld sizes that can be made in a single pass. Large welds scan be made in a single pass by an automatic machine, though manually, 8 mm fillet is the largest single-pass layer.1.5 Need for StudyFatigue cracking in steel structural connections such as bridges, ships, offshore structures, pressure vessels and building occurs almost at the welded and bolted connections. Due to unexpected loads such as earthquake, wind action, the connection in steel structures failed due to brittle fracture. Suchfailures occur predominantly at component connections. In this background, it was felt necessary to check, at least in selected cases, whether the steel structural connections fabricated In Indian conditions and made of structural steel available in open market in India, conforming to the IS (IS2062:2006) specification, would satisfy the IS codal provisions in terms of fatigue life, (number of cycles to failure). The steel used in the present studies was procured from open market in Chennai. Fabrication of the connections was carried out by a fabricator who is considered reasonably good and is regularly carrying out fabrication jobs related to structural engineering applications.

Some of the disadvantages of welding are that it requires skilled manpower for welding as well as inspection. Also, non-destructive evaluation may have to be carried out to detect defects in welds. Welding in the field may be difficult due to the location or environment. Welded joints are highly prone to cracking under fatigue loading. Large residual stresses and distortion are developed in welded connection.1.6 Objectives of the Study

To design a appropriate welded connection and Construction details as per Indian codal provisions. To determine the fatigue life evaluation of designed welded connection subjected to shear.

To evaluate fatigue strength of structural connections by S-N curve approach.CHAPTER 2 REVIEW OF LITERATURE

Abolhassan Astaneh et al (1990) discussed about a tee- framing shear connection consists of a steel tee section connected to a beam web and to supporting member such as column. It transforms end shear reactions to the supporting members. It was observed that considerable shear yielding occurred in the stem and flange of the tee prior to failure. The yielding caused a reduction of the rotational stiffness, which in turn caused redistribution of the end moments to the mid span of the beam.Shahram Sarkani et.al (1991) discussed about influence of small stress ranges on a recently proposed sequence dependent fatigue damage model was examined. The parameters needed to implement the sequence dependent damage model were obtained by combined experimental and analytical approach. It was observed that under constant amplitude testing results in variable amplitude fatigue life predictions that are in excellent agreement with experimental results.P.Dong (2001) discussed about the structural stress definition is consistent with elementary structural mechanics theory and provides an effective measure of a stress sate that pertains to fatigue behaviour of welded joints in the form of both membrane and bending components. The results strongly suggest that weld classification based S-N curve is determined by the relative composition of the membrane and bending components of the structural stress parameter.Timothy D.Righiniotis et al (2002) discussed about the application of a probabilistic fracture mechanics approach to predict the fatigue life of welded steel details in the presence of cracks under bridge spectrum loading. It was based on a recently proposed bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using simulation with Latin Hypercube Sampling, and emphasis is placed on a comparison between linear and bi-linear crack growth models. The latter is found to lead to higher fatigue life estimates and significantly different crack size distributions, both of which have implications on inspection schemes for steel bridge components.P.Johan Singh et al (2002) discussed about the influence of welding procedure on fatigue properties of gas tungsten arc welded (GTAW) AISI 304L load carrying cruciform joints, containing lack of penetration (LOP) has been studied using a crack initiationpropagation(I-P) method. The local stress-life approach is used to estimate the crack initiation life and a fracture mechanics approach for predicting crack propagation life of welded joints. Micro-measurements crack propagation gauges were used to find the crack initiation and propagation data during the fatigue process. The predicted lives were compared with the experimental values. It was found that the fatigue crack initiation lives of the joints fabricated by a double pass technique were relatively higher than the joints fabricated by a single pass technique.Elena Mele et al (2003) discussed about assessment of the cycle behaviour of beam to column welded connections which are made in European code. Three specimens groups, characterized by different values of the relative column beam panel zone strength, are designed and tested under different loading histories. The intermediate size specimens characterized by close values of beam and panel zone plastic capacity and, consequently, by occurrence of inelastic deformations both in the beam and in panel zone, it was observed that intermediate specimen were strong dependence of the cyclic behaviour, of the performance parameters values and of the failure mode on the applied history.S.J Maddox (2003) discussed about fatigue assessment of welded aluminium alloy structures and methods for the welded the fatigue evaluation of welded aluminium structures were assessed from the viewpoints of original design and estimation of the residual life of existing structures.Fidelis Rutendo Mashiri et al (2004) discussed about the fatigue tests were carried out on welded thin-walled (t< 4 mm) T-joints made of circular hollow section braces welded onto square hollow section chords, under the loading conditions of in-plane bending in the brace. Stress distributions were measured at different hot spot locations around the chordbrace junction, where cracks were observed to initiate and grow causing fatigue failure. An end of test failure criterion is chosen and shown to be are liable method for obtaining fatigue data that can be used for producing design SN curves. A through thickness crack is shown to occur when a surface crack has grown to a length equal to about 40% of the circumference of the weld toe in the chord.Ann Schumacher et al (2005) discussed about fatigue tests were carried out on welded circular hollow section K-joints typical to bridges. The tests specimens were large-scale (approximately 9 m long and 2 m high) trusses loaded in the plane of the truss. Measured member stresses showed that a significant proportion of the load in a truss member may be due to bending, underlining the importance of considering correctly this load case in the design of these structures. A comparison of fatigue SN results from smaller and larger welded circular hollow section (CHS) joints has shown the same trend indicated in design specifications: a thicker failed member results in a lower fatigue strength. In light of the size effect results presented in this paper and the major influence of this effect on the design of welded CHS joints in general, it is recommended that a soundly based solution with targeted SN curves and a representative size effect should be sought.M.K.Chrssanthopoulos et al (2006) discussed about the application of structural reliability techniques to fatigue related problems in welded steel structures has occupied intensively the engineering ct was observed that community for the past 20 years. It was observed that paramount importance to examine care fully for each application, be it an offshore node, a ship deck or a bridge girder, the factors that play an important role in fatigue and fracture behaviour. Procedures have been identified, tools exist, required data are available and case studies can be consulted, but the need for well thought out benchmarking of analytical results against experimental data bases, careful probabilistic modelling.T. Nykanen et al (2008) discussed about the influence of local geometrical weld variations on the fatigue strength of non-load-carrying cruciform fillet welded joints were systematically studied using plane strain linear elastic fracture mechanics (LEFM).The effects of weld toe radius, flank angle and weld size were considered.

Biehn Baik et al (2010) discussed about fatigue test have been conducted on specimens, which are flat plate with a notch, stud welded plate, T- shaped fillet welded joint and cruciform fillet welded joint, in a such a way that fatigue cracks could be initiated and propagated in bending load. Under this, surface crack formed flat semi-ellipse and propagated to about 80% of plate thickness before failure. Numerical results to the test results show that the proposed equation provides good accuracy of the numerical results.S.Vishnuvardhan et al (2011) discussed about fatigue life evaluation of structural connections welded and unwelded of IS 2062 Steel and these connections are compared with the characteristic S-N curves recommended in IS 800. Totally 27 specimens were tested for fatigue life evaluation. Five numbers of plain plate specimens, eight numbers of unwelded specimens and fourteen numbers of welded specimens conforming to constructional details of IS 800. It was observed that all welded specimens which were tested at a a maximum stress value equal to 60% of the yield strength of the material and above failed to satisfy the codal provisions, there was no significant variation in the predicted values of fatigue strength for different constructional details using EC 3 and IS 800 were almost the same.Yan-Bo et al (2012) discussed about the presence of residual stress in members can significantly compromise the stiffness and fatigue life of steel structural components. Nevertheless, due to the difference of stressstrain relations and material properties under ambient and high temperatures, the residual stress distribution in a high strength steel member is physically different from those fabricated from mild carbon steel. It was imperative to study the residual stress distribution for structural members fabricated from high strength steel. In this paper, the residual stresses of three welded flame-cut H-section columns with a nominal yield strength of 460 MPa but different cross-section dimensions were investigated. The magnitudes and distributions of the measured residual stresses are identical with those of carbon steel, however in relatively smaller residual stress ratios. It was observed that, based on the measurements, a simplified residual stress distribution for 460 MPa high strength steel members with welded flame-cut H-section is proposed.Y.Garbatov et al (2013) discussed about analyze of fatigue strength of small scale corroded steel specimens. The specimens were cut from a box girder, which was initially corroded in real sea water conditions. Fatigue assessment of crack propagation on a pit like crack flow based on a failure assessment diagram was performed. The admissible initial idealized flow defect has been defined, which matches the fatigue life achieved by the fatigue test for different load categories and corrosion degradation level.CHAPTER 3ANALYTICAL PROGRAM3.1 General

In this chapter discussed about the design of connection of particular type of connection with fatigue design provisions and evaluate fatigue life of connection respect to codal provisions. 3.2 Design of Connection

In this study, a welded steel connection have been chosen and it is shown in Fig 3.1, which is recommended by IS 800 : 2007. It describes Fillets welds transmitting shear. Stress range to be calculated on weld throat area

Figure 3.1 Construction details of welded connection (IS 800:2007)

3.3 Fatigue Design Provisions

IS 800 also includes a section on Fatigue where S-N curves have been given for different non welded and welded constructional details. Detail category is the designation (a number) given to particular constructional detail which represents the fatigue strength 5 x 106 cycles N/mm2 . The uncorrected fatigue strength of the standard detail category for the normal fatigue stress is given by equations. Fig.3.2 shows the standard S-N curves for each detail category for normal stress range in IS 800.When Nsc 5 x 106f = fn 5 5 x 106 / Nsc (3.1)Where ,

fn = Shear fatigue strength of the detail for 5 x 106 ,for detail category

f = Shear fatigue stress range of the detail, respectively, for life cycle of Nsc

Figure 3.2 IS 800:2007 fatigue strength (S-N) curves for Shear Stress.3.4 Fatigue Life Evaluation

Fatigue life evaluations have to be predicted for the fig 3.1. The shear fatigue strength(fn ), recommended as per IS 800 is 67 MPa for the fig 3.1. The various stress were choosen as per IS 2062 :1996. For the various stress range and shear fatigue strength, the number of cycles (Nsc) Were calculated as per equation (3.1) and is presented in table 3.1Table 3.1 Fatigue Life EvaluationS.No Shear fatigue strength

(fn ) MPaDesign shear fatigue stress range (f) MPaNumber of cycles(Nsc)

16722013099

2 672477343

3672349622

46718035726

56716653555

667138134880

76720717762

CHAPTER 4CONCLUSION

The Steel used in the present studies is regular structural steel used in India conforming to IS 2062. Chemical analysis and tension test have been carried out for further studies. With an objective of checking in selected cases whether the steel structural connections fabricated in Indian conditions and made of structural steel available in open market in India will satisfy the IS 800: 2007 codal provisions in terms of fatigue life evaluation welded specimens conforming to constructional details 39 have been carried out and compare with characteristic S-N curve in IS 800. REFERENCES1. IS 800: 2007 Code of practice for General construction in Steel, Bureau of Indian Standard, New Delhi.2. IS 2062:2006, Hot Rolled Low, Medium and High Tensile Structural steel, Bureau of Indian Standard, New Delhi.

3. IS 9595:1996, Metal- arc welding of carbon and carbon manganese steels- Recommendations , Bureau of Indian Standard, New Delhi.

4. S.Vishnuvardhan, G.Raghava ,P.Gandhi, M.Saravanan, Studies on Fatigue Life Evaluation of Structural Connections of IS 2062 Steel , IEI, Journal-cv,Vol.92, (2011).5. Elena Mele, Luis calado, Experimental Investigation on European welded connections, J.Structural engg, Vol.129 (2003),pp 1301-1311.6. Abolhassan & Nader (1990) Experimental studies and design of steel Tee Shear Connection, J.Structural engg, Vol.116 (10) , Pg.No.2882-2902.7. M.K.Chryssanthopoulos, T.D.Righiniotis Fatigue Reliability of welded Steel Structures, J.Construction Steel Research, Vol. 62 (2006), pp 1199-1209.

8. Shahram Sarkani, D.Lutes, J.Hughes Sequence Effects On Stochastic Fatigue Of Welded Joints, J.Structural engg, Vol.117 (1991), pp 1852- 1867

9. Yan-Bo Wang , Guo-Qiang Li , Su-Wen Chen, Residual stresses in welded flame-cut high strength steel H-sections Journal of Constructional Steel Research, Vol. 79 (2012), pp 159165.10.P.Dong A structural stress definition and numerical implementation for fatigue analysis of welded joints International Journal of Fatigue, Vol. 23(2001) , pp 865876.11. S.J. Maddox Review of fatigue assessment procedures for welded aluminium structures, International Journal of Fatigue, Vol. 25 (2003), pp 13591378.12. P. Johan Singh, D.R.G. Achar, B. Guha, Hans Nordberg, Fatigue life prediction of gas tungsten arc welded AISI 304L cruciform joints with different LOP sizes International Journal of Fatigue, Vol. 25 (2003) , pp 17.13. Timothy D.Righiniotis, Marios K. Chryssanthopoulos Fatigue and fracture simulation of welded bridge details through a bi-linear crack growth law Structural Safety, Vol. 26 (2004), pp 141158.

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