Electromagnetic actuators are an interesting option for inducing vibrations in structures. However, their performance when attached to a flexible structure is relatively unknown. Therefore, this thesis aims to investigate the coupled dynamic behaviour of an electromagnetic actuator and a flexible structure. The goal is to see if the actuator can induce large displacements in the structure, in a controllable manner, and using a small input power.
Physical experiments were performed with an actuator placed on top of a flexible beam. In addition, a computer model was made that could simulate the system and predict its response. In the experiments, multiple input settings for the actuator were tested using frequency sweeps. Two different control settings were compared: the open-loop setting, which controls the current that is sent through the actuator, and the closed-loop setting, which controls the motion of the moving cylinder in the actuator. For both settings, an input signal is sent to the system. Respectively the current or the cylinder motion, relative to the tip of the beam, has to follow that input signal. The amplitude and frequency of the signal can be adjusted.
The experiments showed that there is no perfect input setting. Each setting has its advantages and disadvantages. Therefore, the best input setting to use depends on the situation. Using the open-loop setting at the resonance frequency of the system resulted in large beam tip displacements, a high effectiveness. However, this coincided with a low predictability of the displacements. On the other hand, the closed-loop setting gave a high predictability with a low effectiveness.
Looking at the efficiency of the system, the beam tip displacements normalised by the electrical input power, also did not give an ideal input setting. This was a result of the dynamic behaviour of the beam and the actuator counteracting each other a little. The beam vibrated most efficiently at its resonance frequency. However, this coincided with large relative displacements of the moving cylinder in the actuator. This generated a large Back EMF, causing the actuator to use significantly more electrical power. Thereby, the Back EMF cancels out the efficiency of the resonance.
In the closed-loop setting, the relative displacement is being controlled, and therefore it cannot increase to large values. This kills the resonance in the system, preventing the beam tip displacement from increasing.
The computer model was reasonably capable of predicting the response of the system. Due to a few inaccuracies in the model, it often slightly overestimated the displacements of the beam. However, the model showed patterns comparable to the results of the experiments, with resonance peaks at the same frequencies.
The model was also used to simulate the system response to closed-loop settings where either the beam tip displacement or the absolute motion of the cylinder was controlled, instead of the relative motion. These control settings were not possible in the physical experiments, due to limitations in the test setup. However, the model results were promising. Both of these settings could give large beam tip displacements, a high effectiveness, in combination with a high predictability. More research with physical experiments on these settings is recommended.
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An analytical study of an energy-based approach to damping of a clamped timber column in free vibration under axial, lateral and torsional loading conditions was conducted. Backed by experiments it was confirmed that an energy-based approach, as proposed by Sánchez Gómez (2018), could describe the energy dissipation in a laterally vibrating timber column. With the help of an energy balance equation, the energy flow in a timber column was formulated including any energy dissipated during vibration. It was found that the energy in the system could be approximated by an exponential function for which a new dissipation constant E_D (s-1) was introduced. For lateral vibrating columns of Norway spruce, typical values of E_D were 0.4 - 0.6 s-1. Furthermore, a case study on a timber high-rise building demonstrated
that damping has a significant influence on reducing peak accelerations which, in turn, could potentially lead to lowered costs. This largely untapped design lever warrants significant research and design improvement for these timber high-rise structures and, hence, several recommendations for follow-up research are offered. ... Read more

The offshore monopile decommissioning demand will become definite in the coming years. Our responsibility is to ensure the rights and duties of other legitimate uses by completely removing the ageing monopile from the seabed to continuously redeveloping offshore wind farms within the same location. The growing number of past, present, and future monopile installations opens up the challenges and opportunities to be responsible and lead the decommissioning market. With the goal of complete removal, a novel GDP technique can be the win-win solution for offshore wind operators and contractors to extract the monopiles completely from the seabed using torsional and axial vibration
This thesis seeks to understand the torque and normal force to safely clamp a monopile during a torsional vibration so that the monopile continuously slips over the soil. Gradual soil failure along the pile-soil interface's full depth due to the monopile's torsional motion is a possible theory to explain the failure mechanism. When an upper part of the pile successfully moves relative to the soil, kinetic friction occurs until the soil resistance is larger than the shearing at one point. If more shearing is added by adding more torque, more layers below will be broken while the upper part keeps sliding due to lower friction than static friction. While the linear elastic theory of solid and thin shell bodies is used within a 3D FE modelling in Ansys to couple the soil and pile, the clamping force due to the GDP shaker is decoupled from the analysis. Failure criterion is defined outside the simulation so that the gradual soil failure is done through several simulations assuming discrete soil layers.
The FE model is constructed and verified by analytical calculation through the semi-infinite cavity-pile-soil, wave reflection, and finite cavity-pile-soil-spring-dashpot problems. Several cases of gradual soil failure are simulated and show that the torque amplitudes form a distribution. Firstly, a probabilistic sense is proposed to interpret the torque amplitude and search for the optimum depth of the soil failure. Secondly, a convergence check is made with the help of an analytical shell-spring by considering more soil elements by virtue of good correlation of the shear stress between the analytical and FE model. It eventually suggests that a convergence of the torque amplitude can be achieved, which reinforces the theory of gradual soil failure. The interpretation suggests that the current GDP shaker is one step closer for a monopile extraction test with typical monopile dimensions that correspond to a typical 1 m diameter. A first approximation of the required torque and clamping force is then proposed to benefit the analytical model for larger diameters up to 6 m. ... Read more

In the Netherlands, there is an increasing need to create residential spaces in its already crowded cities to accommodate the growing population. Constructing high-rise buildings is one way to try to solve the issue. However, high-rise buildings are sensitive to wind-induced vibrations. The accurate prediction of the dynamic response is important in high-rise building design. In building design practice, calculating the dynamic response of high-rise buildings under wind loading typically involves simplifying the structure as a single degree of freedom system, characterised by the first eigenmode properties and subjected to a white noise spectrum. The damping and natural frequency of the building substantially impact its dynamic response. These parameters are challenging to predict with accuracy. Additionally, in the presence of soft soils, soil-structure interaction can play an important role in energy dissipation and influence the structure's natural frequency. In practice, soil-structure interaction is either disregarded or simplified.
In order to assess the effect of soil-structure interaction on the dynamic response of the building, this study models in a robust manner: the foundation, the tower structure, and the wind load. This newly developed model is addressed as the HF model. The investigation consists of a case study and a parametric study. Additionally, this study compares the predictions by the newly developed model with those obtained with a model as specified in the Eurocode. The case study provides a comparison between the models and the measured dynamic response of the New Orleans Tower. The parametric study looks into the influence of soil stiffness and material damping on the dynamic response of high-rise buildings. Furthermore, it investigates how these parameters affect the dynamic response of structures with various slenderness ratios.
The case study was used to demonstrate that the developed model accurately predicts the measured dynamic response of the New Orleans Tower in the along-wind direction. For all the examined dynamic properties, the error compared with the measurements was lower than 7%. In the case of the Eurocode model, the results demonstrate that, when using the natural frequency estimation recommended by the Eurocode, it provides a 30%–35% underestimation of the peak acceleration compared to measurements. This Eurocode model overestimates the natural frequency and overall damping, which causes it to underestimate peak acceleration. However, the natural frequency determined in the design phase of the New Orleans Tower was significantly lower than the value obtained with the recommended estimation method in the Eurocode. With this lower natural frequency, the peak acceleration is overestimated by around 50%. This underestimation of the natural frequency and overestimation of the damping provided acceptable conservative results. Although this showcases the importance of accurate predictions of both natural frequency and damping, having poor predictions that cancel out each other’s effects is not desirable.
The results from the parametric study support the findings from the case study. This shows that the case study’s findings apply to the more extensive range of building configurations investigated in the parametric study. In general, the Eurocode model, for slender structures on soft soil, overestimates the natural frequency and overall damping, which causes it to underestimate peak acceleration.
The parametric study revealed that the building's first natural frequency and global damping ratio are significantly influenced by the soil stiffness and soil material damping, which can result in various effects on the peak acceleration. In the case of soil material damping, not considering it at all or considering a higher value could lead to 20% overestimation and 40% underestimation of peak acceleration, respectively. Soil stiffness had more intricate effects since it affected the natural frequency and the amount of energy dissipated by the soil radiation damping, the soil material damping and the structure material damping. For particular combinations of parameters, soil stiffness had up to a 40% increase in the peak accelerations compared to the fixed foundation.
The results of this research show that the soil stiffness and soil material damping, for the ranges of properties relevant to the Netherlands, significantly influence the accurate predictions of the wind-induced dynamic response of high-rise buildings. Especially, there was found a higher influence on the dynamic response for the extreme cases of high slenderness ratio structures on very soft soils. ... Read more

As the world's population keeps increasing and the rate of urbanizations increases, The rate at which high-rise structures are being built is skyrocketing. In the Netherlands, this is no different. It is expected that the amount of high-rise structures taller than 70 meters will more than double in the coming 20 years. As these structures become taller and more slender, they also become more susceptible to wind-induced excitations. Where once the design of a structure was predominantly governed by the Ultimate Limit State (ULS) design criteria, for tall structures, the Serviceability Limit State (SLS) design criteria becomes as, if not more, important. Therefore, the accurate prediction of the dynamic characteristics are becoming increasingly important. One of these dynamic characteristics is the natural frequency, also called the eigen frequency, of the structure.

The natural frequency is a parameter which is largely influenced by the mass and the stiffness of the structure. One would think that after the completion of structure, that the magnitudes of parameters can be determined with a high level of certainty, and that the natural frequency can be calculated accurately, but this is not the case. When comparing the measured natural frequencies of several high-rise structures in the Netherlands, to their natural frequencies determined in the design phases, an underestimation of between 20\% to 50\% was seen. Although the likelihood that these underestimations will lead to structural failure are small, it does lead to larger design forces and higher peak accelerations, which are used in determining occupant comfort in the structures. The aim of this research is to find the reasons for the discrepancies between the measured and the calculated natural frequencies.

A literature study was performed to determine what the most common methods of determining the natural frequency are during the design phase. There are three main methods which are used throughout different stages of the design phase to approximate the natural frequencies. At the start of the design phase, when structural parameters have not yet been determined, the natural frequency is approximated using empirical formulae. These formulae mostly only depend on 1 or 2 spatial parameters.
As the design progresses and the structural parameters are specified, dynamic beam theory can be used to determine the natural frequency. These calculations take the stiffness of the super- and substructure, and the mass of the structure, into account. As the design nears completion, the structure is modelled in a FE software package. The natural frequency can then be calculated by the software to give a final impression of the natural frequency.

The main parameters influencing the natural frequency of a system are the stiffness and the mass. This is no different for high-rise structures, but how do these parameters affect the natural frequency, and which of these parameters has the greatest effect on the natural frequency? A sensitivity study, looking at 5 existing high-rise structures in the Netherlands, was performed. Each structure was represented by 5 different beam models. One structural parameter was added to each subsequent beam model as to be able to quantify the influence of the added parameter. Lower and upper bounds were determined for each structural parameter. By varying these parameters and calculating the natural frequencies, the effect this variation has on the natural frequency can be determined. It was found that there are 3 parameters which have significant influence on the natural frequencies, namely, the superstructure stiffness, the superstructure density and the rotational stiffness of the foundation.
For all cases with a flexible foundation, the measured natural frequencies could not be reached, even after determining the natural frequencies using the extreme parameter combination, the natural frequencies were still underestimated. The analyses were done for both uniform beam models and multibeam models. The general trend was that the multibeam model produced higher frequencies. This is due to more of the overall stiffness and mass of the structure being situated in the bottom sections of the structure. Using a multibeam can lead to an increase in natural frequency of up to 15\%. Although the natural frequencies were increased, they were still nowhere near the measured natural frequencies.

The underestimation of the natural frequencies using the beams models, led to the question if there are other factors which are not yet taken into account when determining the natural frequencies. In the calculation of the stiffness of the new Erasmus Medical Centre (NEMC), it was assumed that the beams, columns, non-structural elements and the low-rise structure have a negligible influence on the stiffness of the structure. The underestimation in the natural frequencies, led to the conclusion that the stiffness of the structure is underestimated. A complete model of the NEMC was modelled using the SCIA Engineer software. All the structural systems were added to the model. Modal analyses, including different combinations of structural systems and parameter magnitudes, were performed. It was found that for the NEMC the assumption that the beams and columns have a negligible contribution to the natural frequency, was correct. The main contributors to the stiffness of the superstructure were the outer tube and the central cores. The partition walls were added to the model using low stiffness wall elements, by added the walls, the natural frequency was increased by 8.5\%. Assumptions were made to include the influence of the low-rise structure. The determined natural frequency was increased past the measured natural frequency, however, this result might not be realistic.

The final conclusion of the thesis is that the stiffness of the superstructure is underestimated. This leads to the conclusion that there are certain elements which provide the structure with extra stiffness, which is not yet taken into account. At the end of the thesis several recommendations are made as to determine where this extra stiffness comes from. ... Read more

The world faces an increasing energy problem, forcing people to search for sustainable energy sources. Offshore wind energy has shown great potential to financially compete with traditional energy sources. Recent developments like the slip-joint connection increase this potential. However, for further optimization of the design of a slip-joint, the location of the contact areas between the two cones must be known. Previous attempts to detect these contact areas based on techniques such as heat transfer or ultrasonic measurements have proven insufficient. A possible new way of detecting contact areas, is through the behaviour of Energy Flux. Energy Flux methods have shown great potential as a damping identification tool in other applications. Therefore, in this study the relation between Energy Flux behaviour and the presence of a contact point in the time-, frequency- and time-frequency-domain is studied. To this end, a numerical analysis of a vibrating simply supported Euler Bernoulli beam is conducted, simulating a contact area with a point load. The analysis in the frequency-domain showed the most promising results. Presence of a contact point (i) introduces peaks at twice the first and twice the second eigenfrequencies, and (ii) increases peak height at the location of the contact point. The pressure of the simulated contact point increased these effects. In the time-domain the presence of a contact point increased the amplitude of the cumulative energy flux. This change was most significant at the antinode of the first eigenmode. The location of the contact point was of little influence on this effect. These results show that the presence of a contact area influences the behaviour of the energy flux. The results are encouraging for a later implementation of the energy flux method for the detection of contact areas in a slip-joint. As a validation of these results, an experiment has been proposed. After execution of this experiment, further research is needed in (i) the behaviour of Energy Flux in a conical shape, and (ii) Energy Flux measurements of higher frequencies. ... Read more

Energy transfer between guides and bumpers duet to impulse loads

During reverse installation for executing platform decommissioning and removal projects, heavy modules such as the topside of an offshore platform are cut off their substructures and lifted back on the deck of crane vessels or onto cargo barges. To assure that the modules are placed on the intended location and to prevent the modules from moving during transportation, a guide and bumper system is used.

During set-down, it is possible that the module bumps into one of the guides. The relative motions between the vessel and the barge can cause significant impulse loads when the module and guide make contact. To prevent the module from being damaged, bumpers are welded onto the module to protect the module during the contact phase. Both the guide and bumpers are designed to only deform elastically and to cope with the expected impulse loads. the designs are based on internal standard criteria that state that the guide and bumper system will be designed for a maximum horizontal load that is 10% of the designed weight of the lifted module. During the design state, this load is statically applied on the weakest spot of the guide and bumper system. In reality, the loads are not applied statically but dynamically and it is still unknown how to correctly estimate the magnitude of the impact.

The estimated loads might differ from the actual loads due to unaccounted forms of energy transfer that occur during the impact, such as the rotation of the module, motions of the bumper and guides or deformations in the guides and bumpers. To analyse the energy transfer during impact and to say something about the magnitude of the impact an experiment was conducted with a scaled model. The scaled model exists of two standard steel guides clamped to steel plates and a squared module with a bumper that with the motions of a pendulum. The module is pulled back to a magnet, from where it is let go to hit both the guides once, after which the module is pulled back again.

The impact location on the guides and bumpers is enclosed with sensors that measure the potential energy in the form of strain and the kinetic energy in the form of accelerations. The sensors are situated so that they enclose the energy flow in every possible direction. For each of the enclosed segments, an energy balance was set-up. The guides were tested as a guide with an inclined brace and as a simple cantilever beam. A number of case studies were tested to analyse the energy transfer as a result of impulse loads and to estimate the magnitude of the loads; impact location on the guides, bumper height of the module (at the CoG of the module, above and below), weight of the module, deviation of the module and different damping materials around the guides. The energy balance consists of the external energy that enters the segment, which should be equal to the energy flux, the energy that exits or enters the segment through the cross-section of its boundaries, the energy rate of the segment and the energy that is dissipated.

Both the externally applied load which was needed to calculate the external energy that enters the system, as well as the rotational velocity that was needed to determine the energy flux, are computed with an analytical model.
This model compares the response of a unit load of 1 to the responses of the experiment. The difference between the two is computed as the applied load on the system. The computed energy balance shows the energy flow through the structure as a result of the impulse load. From the results, it is possible to conclude that the impulse loads in this experiment can be assumed to be linear elastic. Based on these experiments a more accurately description of impulse loads can be used as an input for models, for both duration and shape. The second one is that based on these experiments, at least 97% of the energy is transferred back into motions of the module, the effect of energy transfer in a linear elastic response has little to no effect on the magnitude of the loads.
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The trend of the slender and high-rise building has made the structure prone to dynamic loading. This thesis is focused on the dynamic behavior of the high-rise building subjected to wind action. The acceleration becomes a limiting criterion in designing such structure which can be categorized as the comfort criteria of the building. Nausea and motion sickness from the acceleration of the building has been studied by human experience, and numerous building code has included this in the design criteria [6]. The tuned mass damper (TMD) comes from the basic vibration absorber theory by Frahm in 1909; then the studied continued and applied in a building. At present, the TMD is a well-known technology to mitigate vibration, but it is not always applicable in every building case. Therefore a study of the interaction between the building properties also the soil structure interaction (SSI) is made in the application of the TMD.This thesis aims to study the dynamic behavior of a high rise building with the implementation of TMD and to take into account the SSI, also to indicate which type of building is preferable to apply a TMD. The model for the high-rise building is an analytical one-dimensional model which is validated by the finite element program (FEP). The analytical model can give a good fit for the building response but due to the model of the wind load is a random load, it is challenging to match precisely the TMD performance due to the comparison of different load phase. The physical characteristic and tendency of the TMD performance to different building parameter still can be studied in this analytical model. It is shown in this study that the damping plays an important role not only to reduce the acceleration of the building but also influence the effectiveness of TMD. The acceleration is drastically reduced in the lower damping ratio area, which makes theTMD more effective if the building has lower damping ratio contributed from the material, structural joints, and SSI. The reducement of the acceleration by increasing stiffness and mass is very limited compared to the application of TMD. There are two building data for the base of the analysis; the first is the European Patent Office EPO building which is designed by Zonneveld Ingenieurs and the new proposal of slender high rise building in Rotterdam. The EPO building has a unique geometry which the contribution of torsional vibration is high. The slender high rise building shows that the TMD is more effective in reducing the acceleration in this case. The reason is the slender high rise has higher acceleration compare to EPO building when the required building stiffness for the deformation limit is applied. ... Read more