An increasing trend is occurring in the use of high strength steel (HSS) in tubular joints. Reason for this trend is the high yield strength of HSS which leads to reduced weights and costs. Characteristic for HSS is the reduced ductility in comparison to normal steels. Therefore, Eurocode 3 part 1-8 (2020) recommends material reduction factors (Cf) for the design of joints made of steel with a yield strength higher than 355 MPa. The standard includes steel grades up to S700. The material reduction factors need to be validated to ensure efficient design of tubular HSS joints. This research intents to investigate the behaviour of X joints made from different kind of steel grades. The goal is to investigate if the material reduction factors for HSS, proposed in the new version of the Eurocode, are justified. Coupon experiments are conducted to determine the material properties of the X joints. Two geometrically identical X joints with varying steel grades of S355 and S500 are experimentally tested. The conducted X joint experiments, which are evaluated by digital image correlation (DIC), provide the necessary evidence to understand the behaviour of the failure mode and the fracture strain. Finite element analysis confirms the experimental evidence. The results from both the experiments and finite element analysis are used to validate the Eurocode design resistance. Furthermore, a parametric study is executed to investigate the influence of the material properties and the brace width over chord width ratio on the numerical resistance of X joints. This results in a total of 39 different X joint models that are included in this research. Lastly, the obtained numerical resistances are compared to the predicted design resistance according to Eurocode.

In the course of this thesis, a research has been conducted to the application of fracture mechanics on an othotropic steel deck (OSD). Obtaining accurate predictions of crack path and fatigue life assessment were the main objectives for this thesis. As 3D crack propagation modelling in orthotropic steel decks is an arduous task, simpler studies were examined first in order to obtain confidence for the task at hand. These studies are comprised of a through thickness crack in an asymmetrical four point bending specimen and a semi-elliptical crack in a centre cracked plate. Furthermore, studies on computational fracture mechanics and the integration of Python scripting language with Abaqus were conducted for numerical analysis of crack propagation in complex structures. The extended finite element method was used for evaluation of contour integrals of a crack with a constant mesh throughout the propagation analysis. Post-treatment of the results was applied by smoothing stress intensity factors and the shape of the crack front. Moreover, inaccurate contour integral results were excluded from analysis through extrapolation. A Python code was developed for updating the crack front for a given number of loading cycles integrated in Paris' law. The investigated crack was located at the rib-to-crossbeam joint of an OSD. Here, crack propagation occurs along the lower weld toe and in the longitudinal direction of the rib. Validation of results was obtained through experimental studies conducted at the TU Delft laboratory. The weld geometry was incorporated in the model from measurements in order to identify local stresses. Finally, results of strain gauges were used to validate the model. Measurements of the experimental crack growth were obtained at the surface of the crack only. Consequently, assumptions were made of both the depth and shape of the crack in the thickness direction of the rib. A semi-elliptical crack shape was selected with an initial crack depth of 1 mm. Initially, Paris law parameters C and m were selected as recommended by BS7910. The numerical results of crack path complied with those of experimental results, with deviations of no more than 1 mm. Both data sets show that the propagation rate remained somewhat constant. On the other hand, fatigue life assessment results showed stronger deviation. A parametric study was performed to investigate the effect of the initial crack depth. Three different cases were examined for which the initial crack depths were selected as 1, 2 and 3 mm. Varying the initial crack depth did not lead to a significant change in crack path at the surface of the rib, while the depth in the thickness direction was affected. Regarding the fatigue life assessment, the results with respect to an initial crack depth of 1 mm, complied best with experimental results. Given the strong correlation between experimental and numerical results, the methodology followed in this thesis has proven valuable regarding fatigue crack modelling. As the methodology is based on fracture mechanics, an initial crack is required prior to evaluation. Consequently, the application in engineering practices lies in re-evaluation of structures. Moreover, as the parameters in crack propagation relations have a significant influence on the fatigue life assessment, these should be carefully chosen. Therefore, more insights must be obtained regarding the selection of such parameters.

Orthotropic steel decks (OSDs) are susceptible to fatigue failure due to cyclic loading by heavy traffic passing the deck. These cyclic loads can cause fatigue cracks in the rib-to-deck joint. In and around the joint residual stresses are present due to the welding process. In this thesis, the effects of residual stresses on the fatigue crack propagation rate is evaluated. This research is based on finite element analysis and verified against experimental data. First, a thermomechanical weld simulation analysis is made in order to define and verify the residual stress field of the OSD-specimen. This analysis consists of two parts starting with a thermal model in which the temperature field during the welding process is analysed. This temperature field output is introduced into the mechanical model in which the residual stress field is modelled and verified. This data is subsequently used in a crack propagation model based on the extended finite element method (XFEM) in which the effect of the residual stresses on fatigue crack propagation is quantified. Next to that the residual stress component in each direction is analysed separately to obtain the effect of the residual stress per stress direction. The fatigue crack simulation including residual stress field shows good correlation (R2=99%) compared to the experimental data. The simulation without residual stress field shows less correlation (R2=95%), which shows that including the residual stresses improves the results significantly. Introducing only one residual stress component does not significantly improve the results compared to the model without the residual stresses. The effects of the residual stresses are relatively large as the tensile transversal residual stresses increase the crack propagation, while the tensile longitudinal residual stresses decrease the crack propagation rate.
In the case study the obtained knowledge from the research will be applied on the Suurhoffbridge of Rijkswaterstaat. On this highway bridge fatigue cracks occurred and these are measured using Time Of Flight Diffraction (TOFD) measurements. Firstly, the geometry and boundary conditions of FEM-model are verified using literature data based on hot-spot stresses. Secondly, the critical load position of the truck load is determined by obtaining influence lines based on the stress intensity factor. These lines show that critical position is reached when the wheel load is positioned between the webs of the stiffener. Also, from the influence lines can be concluded that the residual stresses in the transverse direction increase the stress intensity factors, while the influence of the residual stresses in longitudinal direction is negligible. At last, a fatigue crack propagation simulation with and without residual stresses is performed. The transversal tensile residual stresses are essential to initiate crack propagation, as at the weld root, compressive transversal stresses are present due to the traffic load which prevent crack propagation. Overall, the fatigue crack simulation including the transverse residual stresses corresponds relatively well compared to the measured data as the difference is approximately 14% also considering that two wheel load that have limited effect have been ignored in the analysis. If these two wheel loads would be included the results will improve. However, there are many assumptions made that can effect the result, such as the Paris law properties and load parameters. A slight change in these parameters would effect the results greatly. In order to verify the results, the assumptions made in the case study, should be verified using measurement data of the structure.

Wind turbines constitute a sustainable and effective solution to produce energy using wind power. Offshore wind turbines are becoming of special interest and more demanding with continuously increased diameters. However, their design poses great challenges, since the installation and fabrication tolerances in combination with high cyclic loading play an important role on the fatigue assessment of the wind turbine structure.One part of specific interest for the fatigue assessment of a wind turbine is the way that the transition piece is connected to monopile. Since now, grouted and bolted connections are commonly used, but they have many drawbacks. These disadvantages regarding their performance inspired by C1 Connections BV to invent a new type of connection to connect large diameter tubulars, the C1 Wedge Connection. This innovation allows to increase the fatigue life of the wind turbine structures and decrease simultaneously their construction and installation cost. The work that will be presented here deals with the fatigue assessment of the C1 Wedge Connection, and how the presence of imperfections may affect the fatigue life of it.After analysis of the load path and the innovative preload mechanism, a reference wind turbine has been selected and the 3D CAD software of Solid Edge is used to design all the components of the wedge connection and assemble them to their final position. The stiffness of the main components of the wedge connection (TP-flange, TP-shell and MP) is calculated analytically, based on the method proposed by Seidel and adapted for the C1 Wedge Connection. An analytical model using Timoshenko beam theory has been developed for TP-flange stiffness calculation. This model and all analytical calculations were verified against FEM results built on ANSYS Workbench and mechanical for that purpose.Once the components’ stiffnesses have been calculated based on analytical formulas, an approximation of the maximum closeable gap is plotted for different gap sizes for this specific geometry of the connection and friction coefficient μ=0.15 which was proved being conservative after calibration using results from tests taken place at TU Delft lab.Secondly different scenarios have been investigated using FE models regarding the position of the gap among the different components of the connection. From this analysis, in terms of magnitude of the remaining gap and the reaction force at the interface of both flanges, was concluded that the worst scenario is the presence of imperfection only at MP side.This worst scenario (only imperfect MP flange) is selected and examined for compression to tension under ULS overturning moment. The fatigue damage of the imperfect structure is calculated and compared with a perfect one. For the purpose of this thesis, preloading has been applied by once at all connections of the ring. Peak stresses are calculated and based on selected S-N curve from DNV-GL codes and by making use of the Miner’s rule, the fatigue damage at the critical positions of both perfect and imperfect connection is calculated. For the same imperfect structure, the fatigue damage is calculated again based on nominal stresses measured underneath each hole of imperfect MP segment. In that case the specific for the C1 Wedge Connection S-N curve is used, based on fatigue single segment test results taken place at TU Delft lab, in 2018.From this research, it is concluded that based on peak stress method the selected imperfect structure is not able to withstand the fatigue damage loads. On the other hand, the same cannot be stated using nominal stress method which is more realistic based on test results and not sensitive to mesh details. In that case the fatigue damage for every hole is <<1.

Injection Bolts have been preferred for several reasons over other alternatives for the rehabilitation of old riveted steel bridges since 1970’s. More recently they have been used to obtain slip critical shear connections in new structures such as new bridges, large roofs, storm barriers and other structures exposed to cyclic loading. In addition, they are tested as a feasible alternative to traditional shear connectors in steel-concrete composite flooring systems, in combination with oversized holes in the flange of steel girder to allow reusability. On the grounds of this research a novel injection material was developed at T.U Delft, namely steel reinforced resin as an alternative to conventional resin allowed in the Netherlands for structural applications without compromising stiffness of connection. Due to limited research on fatigue of resins used in structural applications, in this study an initial attempt to investigate the cyclic behaviour of conventional and steel reinforced resin subjected to uniaxial compressive cyclic bearing stress, in confined conditions is performed. First, for this objective an experimental program using a tailor-made test setup was carried out, and the fatigue response in terms of slip over number of cycles for resin and steel reinforced resin in oversized holes of 6mm and 10mm was obtained. With this specimen configuration, is intended to reproduce as possible the stress state of double lap shear connection without endanger the fatigue cracking of steel parts. Frequency, stress ratio and other testing parameters that can influence the results have been chosen based on relative literature and assembly of setup. Three stress ranges used, with the maximum related to the recommended long term bearing stress for conventional resin, while the failure criterion was set at 0.3mm slip which is related to this long term bearing strength at the end of working life. Secondly, a numerical model of test set-up to investigate the static and quasi-static behaviour of specimens was developed and validated by the quasi static experimental results of initial stiffness of connection. Finally, through statistical treatment of test results fitted curves obtained, and characteristic curves with 95% probability of survival are proposed for both resin types and hole clearances based on probabilistic model of ASTM standard and probabilistic model proposed by Schneider and Maddox, using Student’s t distribution. On the basis of experimental results, large scattering was observed for both type of resins, and more prominent for the larger hole clearance and higher stress range. The reasons for this scatter are discussed and recommendations for future improvement of set-up are proposed. Due to scatter the results could not fully support the expectations that fatigue behaviour of steel reinforced resin is superior to that of conventional, as it is the case in static and creep behaviour. From a selection of specimens with limited scatter in 6mm oversized holes, a trend that connections with steel reinforced resin lead to less slip due to cyclic loads after 500.000 cycles was observed. From the numerical research on static models of specimens it was verified that the bearing stress state of resin layer is almost identical to that of the double lap shear connection of EN 1090-2 indicating a similar stress transfer mechanism. From the statistical evaluation of test data with ASTM probabilistic model the use of steel reinforced resin leads to higher fatigue endurance related to conventional resin in 10mm oversized hole, while the fatigue life of specimens with conventional resin decreased with larger hole clearance namely from 6 to 10mm.

Fatigue is an important issue for orthotropic steel deck (OSD) bridges and therefore a design criterium. Cracks will initiate at several locations of the bridge deck. It is likely that a crack at the trough web to deck plate joint at the crossbeam will initiate first. This crack initiates at the weld root and propagates through the deck plate. A crack at this location is harmful because it is not optically visible and therefore hard to detect. The first part of the report focuses on the fatigue assessment of the trough web to deck plate joint at the crossbeam. Fatigue assessment of this joint is possible with different assessment methods which are the nominal stress method and the hot spot stress method as given in the Eurocode, and the effective notch stress method, described by the IIW. A hand calculation using a simple 2D model is made and a finite element (FE) model is used for the assessment. Both a shell and solid element model are build. The fatigue life calculation of the joint for a deck segment is made. Experimental results for the same deck segment are available and used for validation of the FE models. The fatigue life of each method is compared to the determined load cycles to failure from the experiment. There is concluded that the hot spot stress method results in the most accurate fatigue life prediction and a solid element model gives a slightly longer fatigue life than using shell elements. The effective notch stress method results in a conservative fatigue life for this specific joint. Stresses in the zone near the weld root of the rib-to-deck plate joint at the crossbeam are in compression, while fatigue cracks will initiate due to tensile stresses. During welding, residual stresses appear in the joint. Therefore, a fatigue life prediction is made which includes these residual stresses. An approximation of the residual stresses is included in the initial state of the finite element model. Using the Smith, Watson and Topper (swt) parameter, which is a local critical plane based fatigue assessment, the amount of load cycles to failure is determined. The result is comparable to the experimental result. The second part of the report is focused on the geometry of the deck segment. A shell element model and the hot spot stress method are used to analyse different parameters. When the width between the trough webs is larger, a thicker deck plate thickness is required. Four alternatives for a bridge deck are made. The alternatives have a different amount of troughs over the width, which means that the width between the trough webs differs per alternative. The deck plate thickness is chosen so that each alternative results in a damage value just below 1 for traffic category 2 and a design life of 50 years. Fatigue load model 4 is considered. The weight of the alternatives is determined. There can be concluded that reducing the amount of troughs over the width results in a heavier deck. This is because the weight of the deck plate is a large part of the total weight of the OSD and if more troughs are used, a thicker deck plate is applied. There was assumed that the stress in the joint only is affected by deformation of the deck plate. However, the in-plane deformation of the crossbeam has a negative effect on the stress range in the trough web-to-deck plate joint. Besides the geometry, the effect of some assumptions for modeling are investigated. Including a load dispersal through the surface layer by applying a larger wheel contact area results in a reduced fatigue damage value of about 50% and is therefore beneficial to take into account in the design calculations. The stress range in the joint is highest if a wheel load is placed in the center of the trough. A transverse shift of the wheel results in a large decrease in stress. Therefore it is important to take into account the design load cycles per transverse wheel position.

Orthotropic bridges are being built for more than half a century. These bridges are prone to high cycle fatigue due to traffic loading. Early onset of fatigue cracking is a serious issue in both old and recently built bridges. To prevent this a number of different cross section have been suggested. Sandwich Panel is one of the suggested alternative that has become more feasible to produce due to advancement in Hybrid Laser Arc Welding (HLAW) technology. One type of sandwich panel with corrugated core was investigated in this thesis. Ease of manufacturing, stronger welds, reduced complexity of welding and improved transverse shear stiffness are some advantages of sandwich panel over conventional orthotropic decks. The aim of this thesis is to apply J-Integral based local approach for fatigue assessment of sandwich panels subjected to lateral loads. J-Integral represents a way to calculate the strain energy release rate and is a measure of crack tip elastic-plastic field. It has already been used successfully to quantify fatigue performance of web core sandwich panels and is therefore used in this thesis. A second aim of this thesis is to justify the use of foam filling in sandwich panels by comparing fatigue performance of empty and foam filled sandwich panels. For validation, bending behaviour of panel was first investigated using Finite Element Analysis (FEA). The model developed for dual stake laser welded panel accurately estimated response parameters (compared to experimental results of Tan et al. [33]) like maximum deflection (2.2% difference), critical stresses in transverse direction (9% difference) for top and bottom face plates etc. For verification of Contour Integral technique of Abaqus/CAE, J-Integral at notch tip of ASTM CT specimen was calculated. Compared with empirical solution the FE estimate showed 16% difference with FE estimate being more conservative. Seven different types of foam were considered as filling in sandwich panels. The foams were of polymeric type - H45, H60, H80, H100, H130, H180, H200, and H250 (Unit weight of foams - 48 to 250 kg/mኽ and Young’s Modulus - 45 to 250 MPa). Based on two stage 3D FEA on empty and foam filled sandwich panels, it was observed that J-Integral value at critical tensile notch decreased consistently as stronger foam fillings were used in the panel (83.8% reduction observed between empty panel and H250 foam filled panel). Cycles to failure for panels were calculated using regression equation derived from existing research on web-core sandwich panel. Between empty and H100 foam filled panel an increase of 171 times was observed in cycles to failure. This increase justifies the use of foam filling.

Orthotropic steel decks are widely used in bridge construction nowadays, especially in case of long span bridges. One of the main problems of bridges that utilize this technology is the susceptibility to fatigue failure. Cyclic loading due to wheel loads, in combination with the complex network of welded details, make such types of decks prone to fatigue cracking, which poses a threat to the structure's integrity.
One of the factors that contribute to crack initiation and propagation under cyclic loading are welding-induced residual stresses. The localized heat input, due to welding, and the subsequent cooling, force the adjacent area to undergo expansion and shrinkage, which are, however, restricted by the surrounding material. As a result, residual stresses are formed, which can reach the yield strength of the material in the region near the weld. When these are tensile, they can accelerate the crack propagation rate, thus severely reducing the fatigue life and endangering the structure's safety. Knowing the residual stress distribution in welded joints of orthotropic steel decks can prove to be useful in the evaluation of resistance against fatigue.
In this research project, a finite element model which is able to predict residual stresses in normal strength steel weldments, has been developed. In the first stage of the research, a small-scale welding experiment has been conducted, during which temperature and distortion measurements were derived. The welding procedure was then modelled in the finite element software ABAQUS. Experimental data have been used to validate the accuracy of the adopted heat source model and parametric studies have been carried out, in order to investigate the influence of the heat source parameters and the heat losses on the temperature distribution.
The validated methodology was then applied on a trough-to-deck plate welded connection of an orthotropic steel deck. Longitudinal and transverse residual stresses have been calculated at various positions of the deck and stiffeners. The influence of the welding sequence on the residual stresses has been investigated, by considering two welding scenarios; parallel and sequential welding. The effect of the adopted stress-strain relationship under elevated temperatures on the residual stress field has been examined, by considering an ideal plastic material model and one that includes strain hardening. In addition to these, the impact of the boundary conditions has also been studied. Finally, the effect of welding on the appearance of imperfections is discussed, by considering the deflection of the deck plate, after the process. As a result, valuable information is obtained about the shape of residual stresses, the associated deformations, and the factors that affect them, in a welded connection of an orthotropic steel deck.

In steel structures, a lot of attention is paid to lightweight structure, i.e. reduction of dead load without compromising structural safety, integrity and performance as well as cost-effectiveness. Thanks to modern steel aluminium sandwich panel manufacturing technology a new possibility became available for lightweight structural design.The objective of this thesis is to evaluate the application of sandwich panels in the construction of steel structures with the aim of weight reduction without affecting other parameters like safety, performance, cost, etc. In this thesis, both column and plate buckling theories are considered and applied to the sandwich panel to evaluate its behaviour under in-plane compressive load. Effects of various material models and imperfections on buckling strength of sandwich panel are evaluated. Stiffened plate and sandwich panel is compared in terms of buckling resistance and self-weight. Three different sandwich panels made from faceplates of steel grade, S355, S690 & S1100, are used for replacement of S355 stiffened plate. Efforts made to understand the effect of various physical parameters on buckling resistance of sandwich panel in both column and plate buckling theories.Finally, as a case study, sandwich panel technology is used to redesign the Huisman structure. The objective is to investigate whether applying sandwich panels in redesign makes it possible to obtain a sufficient weight reduction without losing its performance. For this case study, sandwich panels with faceplates of steel grade S355, S690 and S1100 are used. Static and buckling strength of the new design is evaluated. Also, the cost of new design and original design is evaluated and results are compared. Cost analysis is done to evaluate whether a sandwich panel is an economical solution.Findings of this thesis are that in future it is possible to use sandwich panels in offshore structures to save a significant amount of weight while taking considerations into account. Sandwich panels can be successfully used to replace stiffened plates. Sandwich panels with faceplates made from extra high strength steel can give significant weight reduction. But the use of sandwich panels also results in an increase in the overall cost of the structure. So in terms of costs, it is questioned whether or not using sandwich panels is economically beneficial for offshore equipment.

Wind turbines constitute a sustainable and effective solution for the production of energy using wind power. Offshore wind turbines are becoming of special interest. However, their design poses great challenges, since an offshore structure is subjected to combined wind and wave dynamic loading that is characteristic of the installation site and play an important role on the fatigue failure of the wind turbine structure.

One part of specific interest for the fatigue assessment of a wind turbine is the way that the transition piece is connected to monopile. Since now, grouted and bolted connections are commonly used, but their many drawbacks regarding their performance inspired Fistuca BV to invent a new type of connection, the wedge connection, in order to increase the fatigue life of the wind turbine structures and decrease simultaneously their construction and installation cost. More specifically, a basic flange with inclined planes is attached on the top of the
monopile and a more complex one (fork-shaped flange) is fitted to the bottom of the transition piece. Inclined dowels are used to connect the two flanges and secure the structure. The final positioning of the dowels is achieved by applying external pressure on the back side of them, which is converted to a vertical reaction force between the two flanges through the inclined planes, acting as preload.

The work that will be presented here deals with the fatigue assessment of the wedge connection, and how the presence of the preloading force may affect the fatigue life of it.

A reference wind turbine and a reference location have been selected first, in order to calculate all the fatigue loads resulted on the structure due to wind and wave at the level where the wedge connection has been selected to be located (+4m above the MSL). The concept of damage equivalent loads has been applied. Damage loads due to waves have been calculated in the frequency domain, while damage loads due to wind have been handled in the time domain due to strongest non-linearities in the rotor aerodynamics. The process of damage
loads’ calculation was carried out using MATLAB.

Once the loads have been calculated, the 3D CAD design software of SOLIDWORKS is used to design all the components of the wedge connection and assemble them to their final position. To facilitate the design process and reduce the computational time, the flange connection is reduced to a single wedge segment, on which the highest stresses are developed. Two load cases are examined for each one of the above resulted damage loads: one with preload and another one without preload. For the purpose of this thesis, xx MPa external pressure has been applied for the final positioning of dowels in case of preload. ANSYS Workbench has been used for the finite element analysis. Nominal stresses are calculated and based on selected S-N curves for both DNV-GL codes and Eurocode 3 and by making use of the Miner’s rule, the damage and the fatigue life of the connection is calculated. The selection of the appropriate
S-N curve for each load case and for each component of the connection, has been based on results from test specimens.

A parametric study has been carried out as well, in order to check several parameters that may affect both the accuracy of the finite element analysis and the stress development on the connection, and as a result the damage and the fatigue life of it.