The demand for offshore wind turbines (OWT) is rising due to the increasing need for renewable energy. This growth is reflected not only in amounts of OWTs but also in their power rating capacity and size. Currently, the L-flange is the most commonly used connection to join the tower to the foundation. However, this connection type has trouble to handle the unprecedentedly high overturning moments that must be transferred between OWT segments. The C1 Wedge Connection (C1-WC) offers a solution by using multiple fasteners to connect a fork-shaped upper flange (UF) to a lower flange (LF), which are both welded to 100+ mm thick tubular steel sections.
The UF is currently manufactured using submerged arc welding to join its three components: the crown (CR), inner web (IW), and outer web (OW). This research investigates the feasibility of electron beam welding (EBW) as an alternative technique. EBW offers increased production speed by utilizing higher power density, allowing the weld to be made in a single pass. However, its full-scale (long-term) performance is unproven. This study explores its feasibility through small-scale experimental tests. The thesis also examines an alternative weld location in the C1-WC UF, comparing the current and alternative weld positions in terms of fatigue and ultimate limit state (ULS) by using finite element analyses.
The main research question is:
To what extent is the welding technique 'Electron Beam Welding' feasible for application in the C1 Wedge Connection™ upper flange manufacturing?
Numerical analyses offer insights into fatigue performance and the use of increased strength in the net cross-section. Although the alternative design, with the weld located in the gross cross-section, demonstrates acceptable fatigue performance under preloaded conditions, it does not outperform the base-case design in service life. Moreover, the increase in net cross-sectional yield strength of the alternative design remains unutilized in the ULS. This suggests that the UF geometry could be designed more efficiently: by reducing the thickness of the inner and outer webs with 19%, an UF mass reduction of 13%, 3.3 tonnes of steel, is achieved.
Experimental tests presented challenges, particularly regarding the precision required for EBW. A slight misalignment in the weld led to incomplete fusion at the weld’s root. Hardness testing revealed a significant increase in hardness in the heat-affected zone (HAZ) and fusion zone (FZ). Charpy impact tests failed to meet the minimum requirement of 31 Joules at -40°C, while tensile tests showed that failure occurred outside the weld. This raised concerns about the weld’s ductility.
Overall, EBW is not yet a feasible option for full-scale production of the C1-WC, mainly due to the needed stringent tolerance requirements and thereby in achieving a consistent welds along the full OWT circumference. Submerged arc welding remains the most reliable method for assembling the UF. Future research should focus on improving EBW’s robustness and reliability, particularly in understanding its fatigue performance and how to account for geometric imperfections.

The aim of this project is to benchmark the outcomes of a detailed performance based analysis using CFD and LS-DYNA software against codified methods used more generally on projects, and in particular to demonstrate whether this leads to an increase in the predicted capacity of the bridge under fire loading. The effect and influence of different parameters on the fire resistivity have been investigated. These are the location of the fire, the influence of different live loads, the influence of imperfections, the effect of wind fire interplay, and the effect of internal radiation between surfaces within a structural section. This research shows that doing a detailed thermal structural analysis of fire on a steel bridge can indeed lead to material savings. Performance based analyses show that the bridge does not fail for the considered fire scenarios for at least 30 minutes. When applying temperatures based on Eurocode, failure is reached after 15 minutes. Using a detailed FEM model to determine critical temperature results in four times less protection necessary than when the conservative Eurocode limit of 350 °C limit is used, a critical temperature of approximately 600 °C is found for the case study.

Steel-concrete composite (SCC) slabs are extensively used in construction, particularly in multi-story buildings, due to their efficiency and structural performance. SCC slabs combine the strengths of steel and concrete, where steel provides tensile strength and concrete offers compressive resistance. This integration results in lightweight, robust structures that save costs and allow for taller buildings with larger spans.
New designs of steel-concrete composite slabs have to be extensively tested before the new designs are approved. It is therefore important that information is gathered during testing. Strain measurements are often done on composite slabs, for example, strain gauges have been applied to the steel sheeting and distributed optical fibers (DOFs) have been applied to the reinforcement rebars. However, there have never been distributed strain measurements on the steel sheeting of steel-concrete composite slabs. This research aims to gain new insights into the structural performance and behaviour of deep steel-concrete composite slabs using distributed strain measurements.
SCC slabs often have embossed regions of steel sheeting, these embossments or indentations transfer shear forces between the steel sheeting and the concrete. DOFs were applied on the embossed parts of the steel sheeting in the direction of the span in four composite slabs. The strain data showed oscillations in the strain values, and that the embossed/indented parts of the steel sheeting were in tension, while the flat areas were in compression. Local deformations, due to the geometry of the embossments, caused the oscillations.
The N.A. position was another critical focus of the study, which was determined using fiber data at five locations on the middle rib of three slabs. The N.A. was determined for three loading stages: initial, elastic, and plastic. The fibers captured the upward movement of the N.A. from the steel sheeting into the concrete during the initial and elastic stages, as expected according to theoretical calculations of the N.A. position. However, in the plastic stage, the N.A. shifted downward, back into the steel sheeting, instead of continuing its upward trajectory, as expected. This unexpected shift was attributed to partial shear interaction, where the steel sheeting and concrete began acting separately, leading to separate strain
profiles.
Buckling of the steel sheeting was observed in two slabs, Slabs 10 and 11, with fibers providing critical insights into the timing and extent of buckling. In Slab 11, where four fibers were located in the buckling zone, two fibers showed early signs of buckling in the top flange at 97% and 98.3% of the peak load. The strain patterns indicated buckling before the peak load was reached, offering valuable data on the onset of this failure mode. For Slab 10, buckling was detected after the peak load.
Lastly, the research explored the use of DOFs for crack detection. Fibers were applied to the bottom of the ribs on one slab to identify crack locations based on strain data. While cracks typically cause localized peaks in strain, the results revealed limitations in the accuracy of crack detection using DOFs on steel sheeting.
In conclusion, this research demonstrates the potential of DOFs for strain measurement in steel-concrete composite slabs, offering valuable insights into strain distribution, N.A. position, and buckling behaviour. However, the crack detection capability of DOFs, particularly when applied to steel sheeting, requires further refinement to improve accuracy and reliability.

In the construction industry, composite structures combining concrete and steel are commonly employed in floor systems due to their exceptional performance in compression and tension. With a growing emphasis on sustainability and Life Cycle Assessment (LCA), this thesis introduces a novel concept: a modularised floor system consisting of reusable concrete deck and composite beam. This research focuses on analysing the behaviour of an innovative demountable "concrete to concrete" by FEA. A connection between the composite beam and concrete deck connected by a shear connector (bolt). The goal is to understand the mechanical behaviour of such connections.
This design differs from cast-in-situ traditional composite floor systems by having a concrete deck divided into three separate parts that are connected by bolted shear connectors. The composite floor consists of composite girders and concrete decks. A key advantage of this system is the ability to extract the concrete deck from existing floor systems, offering economic benefits and reducing carbon emissions over its life cycle. The mechanical performance of the newly designed connection between two concrete segments (a composite beam and a concrete deck) is examined through a shear and bending model using Abaqus Software.
The shear model does not represent a real case loading, and it is introduced to gain confidence in the numerical analysis due to the absence of experimental data in this research. The specimen consists of two concrete blocks being pulled apart. These blocks are connected by a demountable shear connector (bolt) in the middle.
A three-point bending model presents the mechanical behaviour and realistic potential failure modes of the innovative demountable "concrete to concrete" connection. It consists of three connected concrete segments. This model reveals failure modes, including cracking at the “re-entrant corners” of the connection points, crushing of concrete at the mid-plane of the connection and under the bolt nut, and transverse concrete cracking originating from the bolt hole, refer to Figure 6.26.
To enhance the structural behavior of this modularised floor system, several methods are investigated. First, adding steel plates at the connections effectively mitigates concrete crushing at the mid-plane and prevents cracking at the re entrant corners. Second, relocating the connection to zero-bending moment positions results in a notable reduction in the three failure modes, improving loading capacity by about 10%. Furthermore, the environmental impact of this novel design is noteworthy. With the assumption given in this thesis, for an area of 5.67m*8m concrete deck, reusing the newly designed concrete deck can result in a savings of approximately 4.2 tonnes of CO2 emissions per subsequent life cycle. Similarly, reusing concrete decks from existing buildings can lead to a reduction of around 2.33 tonnes of CO2 emissions per life cycle with this size of the floor. Based on the assumptions made in this research, the results suggest that the newly designed concrete deck may have a lower loading capacity than traditional concrete slabs. However, its potential economic and environmental advantages make it a promising topic for future investigations.

Numerous traffic bridges have been constructed over the last 150 years. Because bridges are commonly built based on an expected lifetime of 75 to 100 years, a large quantity of bridges are reaching the end of their design lifetime, or even are far overdue. To ensure these bridges remain operational without experiencing catastrophic failures, they have to be recalculated, and if need be, repaired, strengthened or replaced. Particularly older bridges are commonly constructed utilizing rivets. Their often overly complex geometries, the fact that riveting has become largely obsolete as a construction process, and old bridges are commonly not design for fatigue loading, engineers regularly face significant challenges when reassessing such bridges.

While fatigue phenomena have been extensively investigated throughout the years, studies pertaining to the fatigue of riveted connections are relatively limited. The Eurocode on fatigue, EN-1993-1-9 includes only two detail categories. Additional guidelines, like RBK Steel expand upon these detail categories, but focus primarily on built-up beam cross-sections in riveted structures, rather than riveted connections. In order to attempt to more accurately assess the complex joints present in ancient steel bridges, this thesis attempts to answer the following question: What would be a suitable approach to model complex riveted joints and assess their fatigue life considering a balance between the level of complexity and applicability in design practice?

A literature study is performed to identify the different factors that affect the fatigue resistance of riveted connections, as well as to highlight several of the available methods to perform a fatigue assessment. Through the investigation of experimental studies complemented with Finite Element (FE) Analyses, a design methodology for a full scale riveted model is drawn up, and finally a FE model of a joint of the John S. Thompsonbridge is constructed. A critical location within the joint is identified. On this critical locations, several stress- and strain-based fatigue life analyses are performed, namely the use of Stress Concentration Factors (SCF), Smith-Watson-Topper’s (SWT) strain-life equation, and the multiaxial shear strain criterion (MSSC) method, to investigate the effects of incorporating mean stress effects and multiaxiality.

From these analyses, it is concluded that SCF appears to provide overly conservative fatigue life estimates, whereas SWT and MSSC provide more probable results. The increased life estimate through MSSC suggests a limited degree of multiaxiality present in the critical location. All three methods require a detailed FE model, complicating the fatigue assessment of the joint. While the SCF method is slightly simpler to use than SWT and MSSC, it does not weigh up to the conservativity of its life estimation. SWT is deemed the most suitable approach for the fatigue assessment of riveted joints, given that it is more widely applicable and relevant than MSSC.

Steel jacket support structures for offshore wind turbine towers made of circular hollow sections (CHS) are becoming a competitive solution compared to monopiles in deeper waters. The current limitation to improving the durability and cost-effectiveness of the CHS structures is the low fatigue endurance of their welded joints. Fatigue-driven design of such structures usually leads to thicker profiles of steel members as well as costly welding of joints. Due to excellent corrosion and fatigue endurance, high strength-to-weight ratio, easy and locally made fabrication, fibre-reinforced polymer (FRP) composite materials have been widely used to strengthen welded CHS joints. Although this strengthening technique can enhance the resistance and fatigue performance of the welded CHS joints, welding still serves as the main way for load transferring in the joints. Therefore, welds remain the source of stress concentration and brittle fatigue failure. Recently, an innovative joining technique, wrapped composite joint, has been proposed at TU Delft. In this joint, the CHS members are connected through the composite wrap. Loads are transferred at the bonded composite-to-steel interface, and welding is completely avoided. While monotonic tensile tests have shown the superior stiffness and resistance of the wrapped composite joint over its welded counterparts, the fatigue performance of the joint still needs to be investigated.

As the first investigation of the fatigue behaviour of the wrapped composite joints, the present study focuses on the most unfavourable failure mode, debonding at the composite-to-steel interface. A typical joint geometry in the jacket support structures, the K-K joint, is chosen as the research object, which is simplified to be the uniplanar X-joint. The general objective is to accomplish knowledge sufficient to predict the debonding behaviour of CHS-wrapped composite X-joints under tensile cyclic loads.

To achieve that goal, the present work starts from the interface level, where the fatigue crack growth (FCG) properties at the composite-to-steel interface are characterised through fracture mechanics experiments, i.e. 4-point bending end notched flexure (4ENF) tests. The steel surface of the specimens is prepared with different roughness levels, and its impact on FCG properties is investigated. The obtained FCG properties provide the basis for predicting crack growth at the joint level. In the wrapped composite joints, friction exists at the composite-to-steel interface due to the confinement by the composite wrap, which may retard the crack growth. This phenomenon is quantified in cyclic tests on joints with simple geometry, i.e. the axial splice joint (A-joint), where the debonding crack growth is monitored through the 3D digital image correlation (DIC) system. At the joint level, tensile cyclic tests are conducted on the wrapped composite X-joints with different surface roughness and at different scales. Post-fatigue static tests are conducted to check the influence of cyclic loads on the residual resistance of the joints. Using the finite element model, the methodology to predict the crack growth and stiffness degradation of the wrapped composite joints is established, which can consider the interaction between debonding on the chord and brace members. The prediction methodology is validated against the test results and used in a probabilistic analysis to explain and reproduce the scattering test results. Finally, the failure criterion of the joints under cyclic loads is proposed to establish the design S-N curves.

The present study found that the surface roughness of the steel tube plays an important role in the FCG properties of the composite-to-steel interface. A minor increase of the surface roughness can significantly improve the joint’s fatigue performance, with the parameter C of the Paris curve decreasing over magnitudes. At the joint level, the wrapped composite X-joints exhibited steady stiffness degradation during the tests due to debonding propagation at the composite-to-steel interface. Joints with reduced surface roughness show deteriorated fatigue performance but still have longer fatigue life over the welded ones. By including friction at the interface, the finite element model gives reduced strain energy release rates (SERR) at the crack front as the crack grows. Thus, the main source of the crack growth retardation is explained and can be quantified. The numerical results match well with test results of X-joints considering different surface roughness, different load levels and scales, and the relationship between debonding on the chord and braces is obtained. By studying the variability of surface roughness and FCG properties, the probabilistic analysis can reproduce scattering of the test results. Finally, the design S-N curves are obtained based on the experimental and numerical results, taking 5% resistance reduction as the failure criterion.

The present study provides a methodology for characterising and predicting fatigue debonding behaviour, not only for wrapped composite joints but also for other large-scale bonded joints with complex geometry, enhancing the application of bonded joints in engineering structures.
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The increase in demand for renewable energy has resulted in higher demand for wind energy. To meet this requirement, the wind turbine sizes are increasing rapidly, and this results in increasing load on the connections between MP-TP and the segments of the tower.
This research focuses on traditional L-flange connection and novel C1 wedge connection, former the most widely used in the offshore industry and other being new to the offshore industry. The objective of this thesis is to evaluate the limits of the L-flange and C1 wedge connection. The design of the L-flange connection carried out on the basis of Petersen’s theory[1]. It is designed to have higher ULS resistance with steel mass as low as possible. This analytical designed is then compared to finite element analysis (FEA). Previous study of Cheng[2] is used to validate the setup and methodology for FEM in Ansys.
For the same overturning moment, C1 wedge connection is designed using the design tool provided by C1 Connections. The design check for flanges is carried out. The design is then compared to finite element analysis (FEA). Fatigue limit state was verified for both the connections.
Based on the study, the following conclusions are made. Firstly, L-flange connection has around 30% higher ULS resistance compared to design overturning moment of 609MNm whereas C1 wedge connection has 55% higher ULS resistance. Secondly, L-flange connection has higher meridional deformation at the same elevation in the shell as compared to C1 wedge connection. The gap opening at the interface of the flanges is studied and it is observed that C1 wedge connection opens after the loss of contact force at the interface which is generated more efficiently through the pretension of stud whereas L-flange connection being an eccentric connection starts opening without full loss of contact force at the interface and the secondary path of load transfer is activated. This results in the lower meridional rigidity provided by the L-flange connection as compared to the C1 wedge connection. Lastly, C1 wedge connection provided an opportunity for several optimizations to have the same ULS resistance as that of L-flange connection. The mass of steel reduced for these optimizations. The L-flange connection weighed 53.72 tonnes for both ULS and FLS criterion whereas C1 wedge connection weights 20.8 tonnes for ULS criterion and 28.8 tonnes for FLS criterion.

The field of wind energy has experienced significant expansion and development driven by the need to reduce reliance on fossil fuels. Offshore wind turbines have become increasingly popular leading to larger turbines with greater energy output to match the growing demand. The most common foundation type of offshore wind turbines is the monopile foundation, consisting of two large diameter steel tubulars, the monopile (MP) and the transition piece (TP).
As wind turbine generators (WTGs) increase in size, the transportation and installation of MP foundations becomes more complex and expensive, requiring specialized vessels with adequate lifting capacity. This research investigates a new connection – the underwater wedge connection – as a method of connecting an MP to a TP underwater. By having an underwater connection, the size of the MP can be reduced as it no longer has to exceed the waterline.
The wedge connection consists of a dowel with an inclined plane and two flanges. A number of dowels will be placed around the circumference of the connection, fitted onto flanges on both the MP and TP. As the dowels are pushed into position, the inclined plane creates a vertical preload between the two flanges. This allows a tensile load on a segment of the connection (caused by the bending moment in the foundation) to be transferred to the foundation via two load paths: reduction of the preload, and by loading the dowel itself in shear.
First an analysis on the current state of art and relevant design codes was carried out. This analysis highlighted the requirements the connection needed to satisfy, in order to serve as starting points to make decisions regarding the design. The preliminary design of the connection is made using analytical calculations. The structural integrity of the flanges and the dowel at the Ultimate Limit State (ULS) was verified. A 3D model of the connection was then created to perform numerical analyses using ANSYS Static Structural. The behaviour of the connection under ULS and Fatigue Limit State (FLS) loading is studied. The opening and failure point of the connection along with the sensitivity of various parameters are also investigated.
The results show that connection was able to effectively transfer both tension and compression loads. The connection has a mechanical advantage of 1.95, meaning that it achieves the same preload as the ULS load by applying only 51.3% of the ULS segment load during installation. The connection opens gradually and only opens at a load higher than the ULS load. Even after opening, it continues to transmit loads effectively, with ultimate failure governed by the yielding of the lower flange. Additionally, the connection exhibits good fatigue resistance, with a low fatigue damage level of 3.8%.
It is recommended to conduct experiments to validate the numerical model employed in this study. Further studies should also be carried out to investigate the impact of structural imperfections on the behaviour of the connection.

This research aims to investigate the influence of Polyisocyanurate (PIR) insulation on small surface load distribution on corrugated steel roofing sheets. PIR foams are used for thermal insulation in industrial buildings. The distribution of surface loads induced by solar panels support structure might be altered by PIR foams. This is completely ignored in the design calculations according to current standard which might results in overconservative designs. In this study, the following main question is going to be investigated using experimental and numerical methods: ‘’What is the influence of PIR insulation on the transverse distribution of small surface loads on corrugated steel roofing sheets?’’.

The methodology involves conducting bending experiments to obtain strains and deflection data, which then are validated using numerical models. The experiments have been conducted on corrugated steel sheets supported by rollers. The PIR insulation is connected to the steel sheet by bolts, and a loading cylinder applies vertical force at the PIR insulation panel side. Two loading configurations are selected to simulate the representative loading conditions in real-life structures. Vertical deflections and strains in the bottom flanges of the corrugated sheet are measured during the experiments. The experimental data are used for the validation of the numerical models that are created by using ABAQUS FEA.

First, a linear F.E Model was developed and validated with experimental results up to the initiation of nonlinearities. The results of the linear models shows clearly that the PIR layer influence considerably the stress distribution in the adjacent ribs to the one of load application. A nonlinear analysis is conducted to determine the maximum load that the roof structure can resist. To capture geometrical nonlinearities more accurately, eigen buckling analysis was conducted to implement initial geometrical imperfections. Considering the imperfect initial geometry of the steel sheet and its plastic properties, maximum strength of the roof system could be estimated. As a note, the damage in the PIR insulation material was not considered as it is beyond the scope of this study, the material was assumed to be linear elastic.

The experiments demonstrate that the behaviour of the roof structure is primarily governed by the rupture of the PIR insulation, occurring prior to any yielding in the steel sheet. The linear numerical model predicts the behaviour of the roof structure within the elastic phase. The linear model can be used to study the influence of the several influencing parameters such as the dimension, PIR material properties and the loading conditions. Expanding the span from 2 to 6 meters with a single mid-span surface load increases the adjacent rib contribution by 20%, meaning that their normalized strain values relative to the load location's maximum strain grow by 20% in the elastic phase. The nonlinear model is employed to establish the force vs. displacement curve and identify the strength of the roof. The non-linear numerical models show that the ultimate load is found to be 8000 N when a single surface load is applied directly on the steel sheet. The experiments show yielding of the steel at 7300 N after the loading jack ruptures both the PIR surface and layer, then the jack reaches the profiled steel sheeting. For design purposes, it is recommended to exclude the influence of the PIR insulation beyond its elastic phase. Compression tests conducted on the specimen revealed damage within the PIR material at a load of 6250 N, which is considered the upper limit for the PIR's elastic phase.

Welding is a widely used process in various industries for joining materials, but it can introduce several complexities that affect the mechanical properties of the welded components. In particular, the presence of pores within the welded material can have significant implications for fatigue resistance, residual stresses, and overall structural integrity. The primary objective of this thesis is to develop and analyze thermal and mechanical models to understand how various parameters, including geometry, mesh, material properties, boundary conditions, and heat input, influence the behavior of a welded thick plate containing pores. The simulations are conducted with different pore configurations and multiple welding passes to comprehensively explore their impact. Two Finite Element (FE) models were designed. In the first model (Model 1), the objective was to replicate the experimental setup detailed by Qiang et al. in their work "Through-thickness welding residual stress and its effect on stress intensity factors for semi-elliptical surface cracks in a buttwelded steel plate" [41]. This endeavor was accomplished utilizing Abaqus as software. A Dflux subroutine, written in Fortran, was incorporated to effectively emulate the welding procedure. The model was constructed to account for multiple welding passes (up to 7). By extrapolating transverse and longitudinal stresses, a comparative analysis was conducted across five distinct models and the experimental data [41]. The intention was to elucidate variations and similarities in the results. Subsequently, the second model (Model 2) was conceived to exploit the more favorable boundary conditions present in Zhang’s model as described in "Effect of Welding Sequence and Constraint on the Residual Stress and Deformation of Thick Welded Butt Joint Made of Q345qD Steel" [58], when contrasted with the initial experiment. Following a similar procedure, the FE model was developed, with the added feature of an increased number of welding passes (up to 12). Distortions along three distinct paths and transverse and longitudinal residual stresses within two specific regions were extrapolated and subsequently compared. The investigation then progressed to the introduction of pores into Model 2, in the simulation characterized by the lowest number of passes. The aim was to assess the impact of these pores on temperatures, distortions, and residual stresses. By comparing the outcomes of the models both with and without defects, valuable insights were gained into the effect of pores during welding processes. The main findings of this research can be summarized as follows: • Thermal data extracted from a welding experiment conducted at four distinct positions within a 250mm x 200mm x 32mm plate, as documented in Zhang et al.’s publication [58], exhibited inconsistencies when compared to the results obtained through Abaqus simulations in this thesis. These disparities predominantly stem from the absence of accurately specified thermocouple placements in the reference paper [58]. Despite conscientious endeavors to pinpoint the precise thermocouple coordinates, discernible variations of 100°C and a maximum divergence of 25% between the outcomes of the simulations and the reference findings were identified. • Residual stresses obtained in Abaqus (Model 1) trying to replicate Qiang et al. paper [41] in a 700mm x 400mm x30 mm showed good agreement with the reference simulation but deviated from the experimental data. The obtained numerical values peaked around 150 MPa, while the experimental values reached more than double that magnitude (305 MPa), for specific simulations. The number of welding passes significantly influenced the compression at the sides of the plate, with an increasing trend as the number of passes increased. • The simulations (Model 2) created to replicate Zhang et al. experiments [58] demonstrated better agreement with the experimental data for both longitudinal and transverse residual stresses. The number of passes played a crucial role in the results, with the eight to twelve passes simulation showing the closest match to the experimental data. • The presence of the studied configuration of pores in the material did not affect significantly the shrinkage, distortions, and residual stresses in the simulations. Closer to the pores, higher values for residual stresses were found. Based on the findings of this thesis, several recommendations for future research can be made. Laboratory experiments should be conducted to validate the numerical results obtained in this study. Sensitivity analysis considering modifications in the dimensions, boundary conditions, and pores locations of the model can provide further insights. Additionally, incorporating thermally independent properties can reduce the computational time of the simulations. Overall, this thesis contributes to understanding the impact of pores on the thermal and mechanical behavior of thin and thick plates and provides a foundation for future research in this area.

The global focus on climate change and the transition away from fossil fuels has highlighted the importance of renewable energy sources. Offshore wind turbines are being optimized and are therefore growing in size and power.

This research focuses on bolted ring-flange connections, a connection type that plays a crucial role in the design of offshore wind turbines, as they transfer the external force between parts of a turbine. The objective of this thesis is to analyze how the increasing dimensions from current to future offshore wind turbines and geometric imperfections impact the reliability of analytical approaches for load transfer functions (LTFs) for these connections. Two components of this objective are considered: examining the influence of different dimensions of ring-flange connections and analyzing the impact of various gaps between flanges on LTFs for 'current generation' and 'next generation' turbines. Analytical calculations are compared to results obtained with finite element analyses, which are assumed to represent an actual connection.

Based on the research findings, the following conclusions are made. Firstly, the widely used tri-linear approach by Schmidt/Neuper [18] for obtaining the LTF in bolted ring-flange connections is found to be unreliable for current and future turbine sizes. This method highly underestimates the forces in the bolts when initial gaps are present between the flanges. Calculations performed with this approach could lead to an overestimation of the turbine's lifetime compared to reality by multiple years, possibly causing more maintenance or early failure. Alternative approaches, such as a very new and not yet approved polynomial approach, show reliable results, providing accurate estimations of bolt forces for large connection diameters. Additionally, currently verified tolerances for gaps between flanges (1 mm over 30° and 2 mm over entire circumference) are outdated, and larger gap heights or smaller gap lengths are expected in practice, especially for future turbines. These gaps lead to a larger bolt force in practice, decreasing the fatigue resistance and lifetime of the structure. Even though very small gaps are expected to occur often, lower bolt forces are obtained compared to larger gaps, both with an expected height to length ratio of 𝑢𝑔𝑎𝑝/𝑙𝑔𝑎𝑝= 0.53 ∗ 10^−3. In analytical design calculations, it therefore is recommended to consider larger sized gaps with a gap length of approximately 1600 mm with its expected gap height.

Offshore wind energy production is increasing rapidly and optimising the design of support structures has the potential to significantly reduce steel and costs. Jacket structures are commonly selected for deeper waters, and the design of these structures is primarily influenced by the fatigue performance of the tubular joints. The fatigue resistance of these joints can typically be evaluated using different stress assessment methods, each associated with a corresponding S-N curve. While the Hot Spot Stress (HSS) approach is commonly used for design purposes, the Effective Notch Stress (ENS) approach offers higher accuracy by taking into account the stress concentration originating from the weld profile. Consequently, the ENS approach has the potential to provide less conservative fatigue life estimations, leading to the reduction of steel and costs. However, the application to and research of the ENS approach for tubular joints is limited. Therefore, the objective of this thesis is to investigate the application of the ENS approach on a tubular joint. The research was divided into three parts, including an investigation on the required FE mesh, a study on sub-modeling of the ENS approach and a comparative analysis on the fatigue life predictions between the HSS and ENS approaches.

The FE mesh required for the ENS approach was evaluated on a cruciform joint according to the DNV-RP-C203 validation methodology and the Stress Concentration Factors (SCFs) values obtained for different mesh configurations were compared. Two mesh variants were developed, and different element sizes in the notch were evaluated. The results show that with a maximum element size in the notch equal to 0.39 mm, according to DNV, unacceptable SCF errors are obtained of -4% and 4%. Furthermore, with a maximum notch element size of 0.25 mm, according to IIW, SCF errors of -0.3% and -2.2% were obtained, indicating that accurate results can be obtained but are not guaranteed. Additionally, the accuracy of the mesh was found to be primarily influenced by the number of nodes surrounding the notch, rather than the size of the elements in the notch.

The accuracy of sub-modeling for the ENS approach on a tubular T-joint was evaluated by varying global modeling choices and sub-model sizes. The accuracy was measured by comparing sub-modeling stress results against those from a global T-joint FE model with the ENS approach applied. The results show that a global model composed of 3D elements with a weld profile that corresponds to the sub-model is recommended. Simplifying the global model by excluding the weld profile or using 2D elements has been found to provide inconsistent and inaccurate results. Moreover, a sub-model of the weld area with a size as small as 1.67 degrees of the brace radial angle has been found to provide consistent results. Thereby, sub-modeling reduces the required mesh size by a factor of 15, compared to the global modeling approach. Additionally, a rather surprising maximum principal stress error of approximately -6% was observed as a result of sub-modeling for all varied load types and sub-model sizes.

A comparative study between the HSS and the ENS approach was conducted for the computed fatigues of a tubular T-joint. Various FE models were composed for the T-joint subjected to different load cases, and equivalent SCFs were computed by introducing a correction factor based on the S-N curves of both approaches. The results show that the ENS approach provides a lower computed fatigue life over the HSS approach when applied to a tubular joint. This discrepancy is attributed to the FAT225 S-N curve, used for the ENS approach, which is not tailored for tubular joints but for welded straight plates. For the HSS approach, the choice of element type (2D vs 3D) and the inclusion of a weld profile have a significant effect on the stress distribution and the resulting fatigue life prediction. It was found that a model consisting of 3D elements with the inclusion of a weld profile yields the longest computed fatigue life for a tubular joint.

To conclude, the application of the ENS approach was investigated for a tubular joint, by evaluating the mesh sensitivity, examining the accuracy of sub-modeling and comparing the computed fatigue lives with those from the HSS approach. Further research is recommended to improve the computational efficiency of the ENS mesh and to achieve a deeper understanding of the -6% error found by sub-modeling. Additionally, the development of an S-N curve tailored for tubular joints is suggested to improve the accuracy of the ENS approach for this type of joint.

The building and construction industry is one of the main polluting sectors contributing to climate change, and is responsible for around 37% of global CO2 emissions. A circular approach is needed to address the global demand for construction materials and resources in a sustainable way. Steel is a commonly used material in construction and fully recyclable. Recycled steel has to be melted in a furnace, which is a polluting process. For reuse of steel only disassembly and transport is necessary. The general shift from recycling to reusing steel can offer environmental benefits.
A number of steel bridges in the Netherlands are being renovated or replaced by Rijkswaterstaat in the coming years. Rijkswaterstaat has been investigating the reuse of its bridges on object level, i.e. reusing the complete structure. This has proven to be difficult. Disassembling the bridge and reusing the structure on an element-level has not been thoroughly investigated. As reuse of construction elements in general is relatively novel practice, not much is known about how to determine the reuse potential of structural bridge elements. This thesis aim to identify current knowledge gaps and provide research results that encourages future reuse of steel bridges. As a case study the eastern Van Brienenoord bride is investigated. This bridge is scheduled to be replaced in 2026-2028 and there currently are no plans for reuse.
The goal of this thesis is to examine the reuse potential of steel bridges on element-level, focusing on two critical factors: fatigue and corrosion. Fatigue and corrosion are two of the most deteriorating processes for steel bridges. The reuse potential can be evaluated using the remaining service life of the elements. The remaining service life based on both fatigue and corrosion can be determined by implementing the corrosion assessment in the fatigue calculation. Fatigue assessments are based on stress ranges in structural members. Corrosion leads to a reduction in the cross sectional area, increasing the stress and thus stress ranges occurring in the critical structural details.
The design, decomposition, loading history and technical condition of the eastern Van Brienenoord are analysed. After critical review of the Van Brienenoord the structural elements of the bridge deck are selected to investigate further. Inspection reports by Rijkswaterstaat and Nebest are used to locate corrosion damage on the Van Brienenoord. The corrosion damage is determined using functions for uniform surface corrosion for steel. The critical structural details of the selected elements in the bridge deck are identified. Using the fatigue assessment procedure described in the Eurocode in combination with the reduced cross section the remaining service life of the structural elements is determined.
According to the results of the proposed assessment method in this research, all structural elements of the eastern Van Brienenoord bridge deck have significant service life left and are therefore suitable for reuse after disassembly. However, certain specific details that have a higher calculated fatigue damage may need to be repaired or removed.

Gates are an essential part of navigational locks, as they are responsible for creating a water barrier and can be repositioned to allow passage for maritime transit. Most lock gates are made of steel but a recent project in Sweden challenged the status quo by constructing the first stainless steel lock gate. The main question of this thesis aims to investigate the possibilities for the application of stainless steel in Dutch mitre gates and how optimisation through implementing a global arch shape can contribute to the material and weld reduction required to do this. To quantitatively assess the hypothesis of this thesis, Lock Veere was chosen as a case study. The finite element method was selected to simulate the operational conditions of a lock gate.
The first part of this thesis aims to define what needs to be taken into account when designing arch-shaped stainless steel mitre gates. The gate can be positioned in three different setups, which each provide different boundary conditions. Eight different load combinations were defined that could possibly be critical based on size, asymmetrical positioning or repetitive loading. In addition to the operational conditions, the requirements and limits of stainless steel in lock gates were considered.
The second part of this thesis investigates the feasibility and optimisation of the global shape. The effectiveness of shape is based on the loading system of a structure. For a lock, this is the water level difference that results in a uniform load over the width of the construction. Arches are relatively more resistant to this load, which allows for material reduction. Mitre gates have another dominant load situation; a point load with asymmetric support during an obstacle situation. Plates alone are not resistant to this load, so a frame around the plate is introduced. Based on the results found in this thesis, both components seem feasible and allow for further material reduction.
The third part of this thesis aims to prove the effectiveness of a complete gate design by simulating the construction with the operational conditions selected in the first part. The aim of the design was to be as simple as possible in order to meet the demands imposed by stainless steel. The final design allowed for 20\% weight reduction and 30\% weld length reduction compared to the traditional design.
Based on the results that were found, an answer to the main question posed by this thesis can be formulated. For the operational conditions found in Dutch mitre gates, an arch shape can be applied in combination with a surrounding frame. This reduces the weight, number of welds and weld length. For these reasons, manufacturing is expected to be less intensive and maintenance is more accessible due to component reduction. Global shape was thus able to contribute to the optimisation of Dutch mitre gate design. This optimisation through global shape adheres to the requirements for stainless steel. However, this result opens up interesting possibilities for further research on using global shape in combination with other materials as well.

The demand for offshore wind energy has never been higher, yet the number of ideal areas for wind farm installations is decreasing. As a result, larger wind turbines are being placed in deeper waters, which poses opportunities for floating foundations. However, due to the industry's inability to scale up the production of relatively expensive floaters, companies are exploring alternative foundation methods for offshore wind turbine generators in deep water, including the extension of the use of monopiles, which are now being considered for use in water depths up to 60 meters. However, higher loads in deep water require increased stiffness in the monopile structure, which is typically achieved by using larger thicknesses and diameters, resulting in XXL-monopiles. The steel used for these XXL-monopiles increases exponentially for greater water depths, and the designs cross the production limits. This study investigates the use of a guyed monopile as a more favorable alternative to a conventional monopile for deepwater applications with large wind turbine generators in the range of 60-120 meters of water depth and a rated power of 15 MW. The guyed monopile concept involves adding moorings to a conventional monopile to provide additional stiffness at a certain elevation, reducing the required material within the monopile. A numerical model is elaborated where the structure is represented by a one-direction FEM model having two degrees of freedom. The model is supported by springs that represent the soil and mooring system. The soil is modelled by a series of p-y reaction curves. For the mooring stiffness, a nonlinear stiffness approach is used where the axial stiffness and geometry of the mooring are considered. The method is used to determine the maximum stiffness that a mooring system can bring. After that, the mooring system is included in the FEM model. Static and dynamic analyses are performed to determine the response of the structure. The described methodology also implements requirements based on potential resonance (1P and 3P frequencies), ultimate limit state (yield strength, column buckling, and local buckling), and fatigue limit state (fatigue damage due to wind and waves). The static and dynamic responses are validated using Ansys. It is concluded that the moorings can provide enough stiffness to reduce the impact of the acting loads on the structure. A case study is conducted to compare the use of a conventional monopile and a guyed monopile for a foundation based on realistic wind and wave loads. Data of a reference wind turbine and a reference location is taken to find the advantages of a guyed monopile for a realistic case. The study iterates the process for different water depths and varying numbers of moorings. Greater water depths lead to more issues due to resonance and fatigue, which can only be overcome by increasing the diameter and thickness of conventional monopiles. This can also be overcome for guyed monopiles by increasing the mooring stiffness. The results show that the guyed monopile is favored over the conventional monopile in all cases, with up to a 45% reduction in steel required.
The case study's findings are used to identify key parameters in the design of a guyed monopile. The sensitivity of the mooring system stiffness is compared to other key design parameters, including monopile diameter, thickness, and embedded length. The study finds that the moorings have a favourable effect on the system's natural frequency. Also, bending and shear stress may be reduced when moorings are applied.

Rijkswaterstaat aims to build circularly from 2030 onward, with the purpose of having a fully circular economy by 2050. Therefore, the aim of this study is to design a reusable modular superstructure, with a small span, which can replace an existing so-called volstortligger superstructure. This is a structure that Dura Vermeer often uses and it regularly happens that these constructions are demolished after a lifespan of, for example, 10 years. This is a waste of materials and therefore an alternative will be investigated in this research. The following research question guides this study:

What is the optimal structure, in terms of shadow costs, mass and financial costs, for a modular superstructure?

In order to answer this question, a parametric study is performed using the software Grasshopper, using the Finite Element Model plug-in Karamba3D. In this way, quick structural analyses were performed, to explore multiple options. In total, more than 11.000 bridge design variants have been generated, divided over 12 different runs. The model allowed targeted optimization of shadow costs, weight and financial costs. Optimization is achieved by varying the following input parameters: number of steel girders, steel profile girders, outrigger deck, concrete deck thickness, concrete class, outrigger deck, FRP sandwich panel height, facing thickness, web thickness, and fibre volume.

In total, 6 different variants would be developed in this way, 3 with a concrete deck and 3 with an FRP deck. However, the optimization results in only 4 different variants, 3 with a concrete deck and just one with an FRP deck. The results show that the variant with the FRP deck is indicative for Shadow costs, Financial costs and weight.

After the four pre-designs had been developed, these variants were weighed against each other by means of a Trade-Off Matrix (TOM). Using this method, a distinction is made between the variants based on the pre-chosen performance indicators. All performance indicators have a weighting that adds up to a total of 100.

This study concludes that it can be advantageous to replace the traditional design with a demountable design, with the assumed boundary conditions of this study. All developed variants have an advantage with regard to environmental impact and financial costs compared to the traditional design, assuming a lifespan of 100 years in which it is moved 10 times.

The current understanding of auxetic metamaterials in structural engineering applications is limited. Auxetic materials exhibit lateral contraction under compressive loads, possessing a negative Poisson's ratio. The research addresses this gap, focusing on the use of auxetics for steel reinforcement in structural concrete elements to enhance sustainability by reducing material usage without compromising safety.

The research goals include:

Describing the state of the art in auxetic materials for concrete structural elements.
Investigating auxetic behavior under different loading conditions (compression, tension, shear, and bending).
Developing a geometrical design for auxetic steel reinforcement, considering practical aspects like manufacturability and mortar flow.
Performing numerical analyses on proposed beams under various loading conditions, comparing Young's modulus and Poisson's ratio.
Exploring upscaling solutions for architected metamaterials.

The research follows a systematic approach:

Literature Study:
Examines the use of auxetics in concrete structural elements.
Selects a bow-tie architecture for its adaptability and manufacturability using Laser Powder Bed Fusion (LPBF).

Geometrical Study:
Focuses on a cantilever beam subjected to rotation, ensuring pure bending with a linear geometry.
Chooses a range of unit cell angles for desired structural behavior.
Designs unit cells and beams, considering compatibility, continuous lattice, and manufacturability.

Numerical Study:
Utilizes FEM software (ABAQUS) to analyze effective Young's modulus and Poisson's ratio.
Validates models against literature using Euler-Bernoulli beam theory.
Studies the influence of boundary conditions on results.

Upscaling Technologies:
Explores technologies for upscaling auxetic lattices beyond current LBPF methods.
Proposes multi-robotic fabrication for automated assembly using conventional rebar.

Findings and conclusions include:

Selected Architecture:
Chooses bow-tie architecture for its modifiability and manufacturability.
Proposes "single cells" and "three angle" units for different loading conditions.

Design Expansion:
Develops a stacking sequence for a 3D lattice, considering dimensions and relative density.
Creates beams with auxetic, cubic, non-auxetic, combined, and gradual behaviors.

Numerical Model Conclusions:
Affirms constant moment and zero shear in designed beams.
Emphasizes the dominance of effective modulus in lattice geometry.
Validates Poisson's ratio behavior in different beams under pure bending.

Upscaling Proposal:
Recommends multi-robotic fabrication for full-scale lattice construction.
Uses software like Rhinoceros 7 and COMPAS FAB for design and control.

In summary, the research contributes to understanding auxetics in structural applications, providing insights into their behavior under various loads, proposing practical designs, and exploring upscaling possibilities. The proposed approach and findings aim to advance the use of auxetics in steel reinforcement for sustainable and safe construction practices.

Plated girders have been extensively used in steel bridge construction since the 19th century. Over the past century, the effective width method was developed to take the nonlinearities resulting from shear lag and plate buckling into account. This method leads to having a cross-section with a reduced area in which the stress can linear be considered. The section properties of the reduced cross-section can be used for determining the resistance and the longitudinal stress in the plated girder. The reduction of the cross-section area takes place in two steps. First, the section area is reduced in local scale for subpanels that have class 4, and local effective section properties will be determined. Second, the local effective cross-section is reduced to consider the overall buckling, which can be plate-like buckling, column-like buckling, or their interaction.
The longitudinal compressive stress that can be determined from effective section properties in a box girder with pure bending moment will be higher since the effective section includes a reduced second moment of inertia and an increased distance between the neutral line and the compression flange. This master thesis aims to assess whether effective section properties accurately determine the compression stress magnitude in box sections with pure bending moment that remain unaffected by local buckling and experience overall buckling in the form of pure column-like buckling in the compression flange.
The findings indicate that if the subpanels of the isolated compression plate are stocky, and the overall buckling failure has the form of pure column-like buckling, the stress, and behavior remain within the elastic region up to column-like buckling failure. There does not exist any sudden increase in compression stress magnitude in the plate. The analytical method that can address the maximum magnitude of the compression stress in the plate has contribution of the gross section area of the plate.
If such a stiffened plate with pure column-like buckling failure and stocky subpanels serves as the compression flange of a box girder, the same conclusion was reached regarding determining the first moment of inertia. However, in the presence of the slender web in the cross-section, the effective area of the web, in combination with the gross section area of compression flange should be considered for determining the section properties with which the maximum longitudinal stress can be determined.

Over the past few decades, the Netherlands has built numerous steel railway bridges to improve its infrastructure. The repeated loading from railway traffic makes steel bridges susceptible to fatigue damage. The stress ranges at fatigue-prone locations are generally higher in railway bridges than in road traffic bridges. Due to the limited space available due to road and rail alignment, the connection between the main girder and cross-girder is often identified as a critical fatigue location.

The main- to cross-girder connection can be designed for fatigue using different design principles. Designing the connection with a certain rotational stiffness leads to higher fatigue stresses at this location. Meeting the fatigue requirements at the connection can often be achieved by local adjustments, such as welding extra steel plates to the fatigue-induced location to evenly distribute stresses from the cross-girder to the main girder. This can be a costly solution. Designing a connection that is flexible could reduce stresses at the main- to cross-girder connection. Solving fatigue issues can be done by making global adaptations to the structure. The question arises: which aspects can be adjusted such that slight changes can notably improve fatigue resistance in a cost-effective manner?

A literature study and a finite element investigation of the main- to cross-girder connection are performed. A reference model is made after which parameters are altered to investigate their impact on the fatigue response of the connection. The reference model is based on the design of the bridges from project Oostertoegang. However, instead of considering a connection with a certain rotational stiffness, the design is made more flexible for this study. Ansys 2022 R2 is used to create a finite element model of the bridge with shell elements. The hot spot stress method is used to conduct the fatigue assessment. In total four critical fatigue locations are researched for six parameters. The parameters researched are: the center-to-center distance between the cross-girders, the height of the cross-girder and main girder, the thickness of the inner web plate of the main girder, the diaphragm, and the steel deck plate.

From the analysis, it can be concluded that for one detail (M3) local measures should be applied to meet fatigue criteria. The three other details can satisfy requirements within feasible limits. The most cost-effective and realistic way is to increase the thickness of the inner web of the main girder. Other cost-effective but less feasible solutions to optimize the flexible design for fatigue are: decreasing the thickness of the diaphragm and increasing the height of the cross-girder. These parameters show the best ratio between the costs needed to alter the parameter and the total fatigue damage change of the critical detail.

Furthermore, it is determined that the fatigue assessment using finite element analysis with shell elements can be optimized by using the hot spot stress method in combination with modeling the weld using an increased thickness.