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To enable further growth of wind turbine dimensions and rated power, it is essential to decrease structural loads that wind turbines experience. Therefore a great portion of research is focused on control algorithms for reduction of wind turbine structural loads, but typically wind turbine rotor is considered to be perfectly symmetrical, and therefore such control algorithms cannot reduce structural loads caused by rotor asymmetries. Furthermore, typical approach in the literature is to use blade load measurements, especially when higher harmonics of structural loads are being reduced. In this paper, improvements to standard approach for reduction of structural loads are proposed. First, control algorithm capable of reducing structural loads caused by rotor asymmetries is developed, and then appropriate load transformations are introduced that enable presented control algorithms to use load measurements from various wind turbine components. Simulation results show that proposed control algorithm is capable of reducing structural loads caused by rotor asymmetries.

Steel monopiles are support structures mostly applied for offshore wind turbines. Their installation is straightforward, in particular, in shallow and medium waters. While the wind turbine tower is primarily affected by wind, the wave loads are dominant for the monopile, as it is submerged to a large extent. This study deals with the influence of uncertainties in material and load parameters on the behaviour of those structures. It is investigated how the scattering of material properties (namely Young's modulus of elasticity) affect the structural response. In addition, loads with different characteristics are applied, and it is examined how the changes in loads influence the structural response. The analysed output data of interest are the extreme stresses leading to the accumulation of fatigue damage. In order for a realistic modelling, wave loads are considered with irregular sea states with different wave characteristics (significant wave heights and wave peak periods). The final aim of the analysis is to classify the effects of specific wave characteristics on the stresses by means of a sensitivity analysis. The analysis shows that variations in the wave peak period have the strongest influence on stress outputs. This effect results from the strong sensitivity of the structural dynamical response to the decrease of the difference between the values of the wave peak frequency and the natural frequencies of the structure. © 2017 The Authors. Published by Elsevier Ltd.

Seakeeping analysis has progressed from the linear frequency-domain 2D strip method to the nonlinear time-domain 3D panel method. Nevertheless, the violent free surface flows such as slamming and green water on deck are beyond the scope of traditional panel methods based on potential theory. Recently, Computational Fluid Dynamics (CFD) has become an attractive numerical tool that can effectively deal with the violent free surface flows. ABS, as a classification society, is putting forth a significant amount of effort to implement the CFD technology to the advanced strength assessment of modern commercial ships and high-speed naval craft. The main objective of this study is to validate the CFD technology as a seakeeping tool for ship design considering fully nonlinear three-dimensional slamming and green water on deck. The structural loads on a large container carrier were successfully calculated from the CFD analysis and validated with segmented model test measurements.

Stone columns (or granular piles) are increasingly being used for ground improvement, particularly for flexible structures such as road embankments, oil storage tanks, etc. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate to form the stone column. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. In such soils, the required lateral confinement can be induced by encasing the stone columns with a suitable geosynthetic. The encasement, besides increasing the strength and stiffness of the stone column, prevents the lateral squeezing of stones when the column is installed even in extremely soft soils, thus enabling quicker and more economical installation. This paper investigates the qualitative and quantitative improvement in load capacity of the stone column by encasement through a comprehensive parametric study using the finite element analysis. It is found from the analyses that the encased stone columns have much higher load carrying capacities and undergo lesser compressions and lesser lateral bulging as compared to conventional stone columns. The results have shown that the lateral confining stresses developed in the stone columns are higher with encasement. The encasement at the top portion of the stone column up to twice the diameter of the column is found to be adequate in improving its load carrying capacity. As the stiffness of the encasement increases, the lateral stresses transferred to the surrounding soil are found to decrease. This phenomenon makes the load capacity of encased columns less dependent on the strength of the surrounding soil as compared to the ordinary stone columns. � 2006 Elsevier Ltd. All rights reserved.

This work concerns with experimental studies on buckling of thin-walled circular cylindrical shells under transverse shear. The buckling loads are also obtained from finite element models, empirical formulae and codes and are compared. Experiments are conducted on 12 models made of stainless steel by rolling and longitudinal seam welding. In situ initial geometric imperfection surveys are carried out. The tests are conducted with and without axial constraint at the point diametrically opposite the loading. Theoretical analyses are carried out using ABAQUS finite element code. Two finite element models considered are: (i) geometry with real imperfection (FES-I) and (ii) critical mode imperfect geometry (FES-II). In the former, the imperfections are imposed at all nodes and in the latter, the imperfection is imposed by renormalizing the eigen mode, using the maximum measured imperfection. General nonlinear option is employed in both the cases for estimating the buckling load. Galletly and Blachut's expressions, design guidelines of Japan for LMFBR main vessel expressions (empirical formulae), ASME and aerospace structural design codes are used for comparing with experimental loads. The comparisons of experimental, numerical and analytical buckling loads reveal the following. The numerical results are always higher than the experimental values; the percentage difference depends on the wall thickness. FES-II predicts somewhat a lower load than that of the FES-I. The Japanese guidelines predict the lowest load, which is conservative. Experimental loads are lower than that predicted by both ASME and aerospace structural design codes. ? 2004 Elsevier Ltd. All rights reserved.

This paper presents the results of three-dimensional non-linear finite element analysis of raft and piled raft foundations that have been loaded until failure. The raft, pile and soil have been discretised as eight-noded brick elements. The soil has been modelled as a Drucker–Prager elasto-plastic medium. The analyses have been done for a raft, a piled raft, a group of piles and an individual pile. The load–settlement behaviour of all these foundations is presented. The effect of raft thickness and soil modulus on the load–settlement behaviour of rafts and piled rafts is also presented. The axial load distribution for piles in a piled raft foundation is shown, the development of contact stress with increase in loading intensity is presented for rafts, and curves of contact stress against settlement are shown for rafts and piled rafts. The addition of even a small number of piles has been found to increase the load-carrying capacity of a raft foundation. The axial load distribution shows that the piles in a piled raft foundation reach their ultimate capacity earlier than the raft does. The contact stress is found to be minimum at the centre of the raft and maximum at the corner. When the raft reaches its ultimate load-carrying capacity, uniform contact stress is found below the raft except for a very small zone at the corner of the raft. This uniform contact stress is found to be the maximum value that can develop below a raft and a piled raft. The finite element results compare well with the reported literature.

Recent results are presented on the modelling, control and identification of helicopters carried out under the Helicopter Active Control (HELI-ACT) project. Designing high-bandwidth control systems pushes the existing flight mechanics models to their limits, especially if structural load prediction and alleviation are to be investigated. We describe how engine dynamics have been added to the simulations we have developed and how time-domain system identification has been used to obtain an updated linearized model. This has been done in an effort to address deficiencies that were observed in the original models. The use of nonlinear elements is discussed in the context of optimizing handling quality metrics. Results are presented from the flight-test of control laws incorporating nonlinear elements, designed to optimize attitude quickness, and it is shown how extremely accurate predictions of bandwidth and quickness were made using the new model., 2007 9th European Control Conference, ECC 2007, Kos, Greece, July 2-5, 2007, available, unclassified, unlimited

Given that most applications of shape memory alloys (SMA) are in the wire form, a reduced order model and analysis has been attempted in this paper. It takes into account the fact that the predominant actions are axial (bending inclusive) and torsional. A thermodynamic framework is first developed to simulate the behaviour of the SMA material under thermo-mechanical loading that is a combination of axial and shear stresses arising at a point in a wire due to axial and torsional loads applied to the wire. Since only a few variants relevant to axial-torsion are going to be active in transformation under this kind of loading, a reduced order model that tracks the evolution of four martensite variants and an austenite variant is proposed. It is shown through simulations that these five model parameters amply form a minimal set of model parameters sufficient for simulating response under tension-torsion loading excursions. The model is further applied to the structural member, in this case, a wire of circular cross-section subject to a twist and an axial extension and the capability of the model to simulate the kind of response expected in wires. Incorporation of this model into a large deformation space frame nonlinear analysis will help in the design and analysis of several applications where SMA wire forms are used. � 2014 IOP Publishing Ltd.

To enable further growth of wind turbine dimensions and rated power, it is essential to decrease structural loads that wind turbines experience. Therefore a great portion of research is focused on control algorithms for reduction of structural loads, but typically wind turbine rotor is considered to be perfectly symmetrical, and therefore such control algorithms cannot reduce structural loads caused by rotor asymmetries. In this paper, the reduction of wind turbine structural loads caused by rotor asymmetries is discussed. To this aim, suitable transformation of structural loads and control algorithm based on such transformation are proposed. Simulation results show that proposed control algorithm is capable of reducing structural loads caused by rotor asymmetries.

This paper presents a novel type of variable speed wind turbine with a new drive train different from the variable speed wind turbine commonly used nowadays. In this concept, a synchronous generator is directly coupled with the grid, therefore, the wind turbine transient overload capability and grid voltage support capability can be significantly improved. An electromagnetic coupling speed regulating device (EMCD) is used to connect the gearbox high speed shaft and synchronous generator rotor shaft, transmitting torque to the synchronous generator, while decoupling the gearbox side and the synchronous generator, so the synchronous generator torque oscillations during a grid fault are not transmitted to the gearbox. The EMCD is composed of an electromagnetic coupler and a one quadrant operation converter with reduced capability and low cost. A control strategy for the new wind turbine is proposed and a 2 MW wind turbine model is built to study the wind turbine fault ride-through capability. An integrated simulation environment based on the aeroelastic code HAWC2 and software Matlab/Simulink is used to study its fault ride-through capability and the impact on the structural loads during grid three phase and two phase short circuit faults.

The use of wind energy for generating electricity has been constantly and rapidly increasing over last few decades and this growth is expected to continue. In order to enable even greater role of wind energy in power production wind turbinespsila sizes and rated powers must increase. As wind turbines grow in size they are subjected to extreme loads and fatigue caused by uniform turbulent winds. This prevents further growth of wind turbines. Therefore control methods that assure loads reduction become a necessity. In this paper methods for wind turbine loads reduction based on individual pitch control are explored. To overcome problems related to needed measurements of blade loads a method for their estimation based on loads measured on the fixed parts of the structure is proposed.

Stone columns (or granular piles) have proven to be ideal ground reinforcement for supporting flexible structures such as embankments and storage tanks. Stone columns installed in very soft soils will have very low lateral confinement: hence they undergo excessive bulging, leading to undue settlement and limited load-carrying capacity. In these situations, the strength and stiffness of the stone column can be enhanced by encasing the individual stone columns with a suitable geosynthetic. The encasement improves load transfer to deeper depths of soil. This paper investigates the qualitative and quantitative improvement of load capacity of individual encased stone columns through laboratory model tests. These tests were performed in a rigid unit cell that represents the stone column and the soil within the contributary area around the stone column. The load tests indicated a clear improvement in the load capacity of the stone column due to encasement. Encasement with geosynthetics having higher modulus resulted in stiffer response. The effect of encasement was found to decrease with increase in the diameter of the stone column. The improvement in the performance of stone columns was found to be significant, even with partial encasement. � 2007 Thomas Telford Ltd.

The energy dissipation capacity of conventional concentric braced frames in multi-storeyed buildings degrades drastically under earthquake loading, due to buckling of bracings under the compression regime. Non-Buckling bracing is an innovative patented system, in which the buckling of the bracing is prevented and the energy is dissipated by plastic deformation under tension-compression cycles, with stable hysteresis. The strength and stiffness of Non-Buckling bracing can be tailored to obtain the desired level of storey drift limit and energy absorption. A single-bay ten-storey concentric braced frame with diagonal Non-Buckling bracing is chosen for studying their effect during earthquake loadings. Non-linear time history analysis was carried out using SAP2000 to study the variation of maximum base shear, roof displacement, inter-storey drift, Damage Index and stresses in various members due to the El Centro earthquake accelerogram. It is observed that Non-Buckling bracings are effective in dissipating energy and controlling inter-storey drift. � 2006 Elsevier Ltd. All rights reserved.

Safety assessment and rational design of bridge structures requires the uncertainty associated with vehicle loads to be modeled as accurately as possible. This modeling is rendered difficult by the presence of vehicle axle weights that involve different combinations of unimodal and multimodal probability distributions with different dependence structures. In this paper, a transformation invariant approach using copula functions is proposed for the multivariate simulation of dependent axle weights of different vehicle classes. Copula based dependence modeling, which is widely used in the financial risk analysis, is applied to model and simulate three different vehicle cases with different combinations of marginal probability distributions for axle weights. The database of observed vehicle weights is based on the data collected at five locations on national highways in India. The dependence between multimodal distributions of axle weights is accurately considered and simulations are carried out. The simulated axle weights are found to be in very good agreement with the observed data. This type of simulation is useful in carrying out simulation-based reliability analysis of bridges and pavements. � 2006 ASCE.

The behaviour of single piles under lateral dynamic loading is critical, and has been an important field of research since the 1950s. Many analytical or semi-analytical linear and non-linear models are available to estimate the dynamic lateral stiffness, but it is essential to determine the dynamic characteristics of the soil–pile system through full-scale lateral dynamic pile load tests for important structures and for validation of existing models. This paper presents the results of field lateral dynamic load tests conducted on 33 piles of varying types–driven precast concrete, driven cast-in-situ concrete and bored cast-in-situ concrete–at different petrochemical complexes in India. The results indicate that driven precast concrete piles have stiffnesses that are four to five times higher than those of driven cast in situ piles. The lateral stiffness was also estimated using the computer program PILAY for all piles and compared with the stiffness determined from the field tests. The estimated stiffness shows good agreement with the field values for stiff clay sites, but greatly overestimates the values for soft clay sites.

The phases involved in the project for the construction of the longest bridge made of newspaper and cotton rope are discussed. A cable-stayed system was decided to use in the construction because of the efficiency in load transfer offered by the system. The project involved the process of designing, treating the constituent materials, fabricating, and erecting a unique structure made of unconventional materials. Finite-element modeling and analysis of the bridge was carried out to calculate the pretension in all the stay cables to avoid excessive deformation of the deck under load, and to arrive at the length of all the ropes and coordinates of the attachment points before pretensioning.

An experimental study of the effect of plate slenderness ratio and column slenderness ratio on the collapse load of simply supported stiffened plates with initial imperfections, loaded in compression, is presented. A generalized computer program for the semiempirical solutions based on the strut approach and the orthotropic plate approach is developed. A finite element analysis program based on the orthotropic plate approach is developed and a new collapse criterion is introduced. The analytical calculations are compared with the experimental results and uncertainty parameters are calculated. The effect of initial geometric imperfections, plate slenderness ratios, and column slenderness ratios on the collapse load of stiffened plates is studied. A set of conclusions is drawn based on the experimental and analytical studies carried out.

Lattice microwave towers and transmission towers are frequently made of angles bolted together directly or through gussets. Such towers are normally analysed to obtain design forces using the linear static methods, assuming the members to be subjected to only axial loads and the deformations to be small. The effects of the end restraints, eccentricity of connections and secondary bracings (redundants) on the strength of the compression members are usually accounted for in the codal recommendations by modifying the effective length of the members and thus the design compressive strength. Hence, forces in the redundants are not known from the analysis and their design is empirical. In this study, non-linear analysis of angle compression members and the single panel of angle planar as well as three-dimensional lattice frames, as in typical lattice towers, are carried out using MSC-NASTRAN software. Account is taken of member eccentricity, local deformation as well as rotational rigidity of joints, beam-column effects and material non-linearity. The analytical models are calibrated with test results. Using this calibrated model, parametric studies are carried out to evaluate the forces in the redundants. The results are compared with codal provisions and recommendations for the design of redundants are presented. ? 2001 Elsevier Science Ltd. All rights reserved.

Experimental investigations are carried out up to collapse on eighteen stiffened steel plates having initial imperfections under uniaxial compression with simply supported boundary conditions on both loading and unloading edges. The thickness of the flange plates is varied as 4 mm and 5 mm respectively. Three types of commercially available open section rectangular flats are used as stiffeners. Six panels without cutout, six panels with square cutout which extends the full width in between stiffeners (d/b=1.0), four panels with rectangular cutout (d/b=1.5) and two panels with reinforced rectangular cutout are fabricated. The initial geometric imperfections such as plate imperfection ?(x), overall imperfection of the whole panel ?(sx) and torsional imperfection in stiffener ?(sy) are measured for all panels fabricated. The axial deformation of the whole panel, out-of-plane deflections and strains along the midsection of the panels measured during the tests is discussed. The reduction in strength of the panels due to the presence of square cutout, rectangular cutout and increase in strength due to reinforcement around rectangular cutout are calculated based on the experimental observations.Experimental investigations are carried out up to collapse on eighteen stiffened steel plates having initial imperfections under uniaxial compression with simply supported boundary conditions on both loading and unloading edges. The thickness of the flange plates is varied as 4 mm and 5 mm respectively. Three types of commercially available open section rectangular flats are used as stiffeners. Six panels without cutout, six panels with square cutout which extends the full width in between stiffeners (d/b=1.0), four panels with rectangular cutout (d/b=1.5) and two panels with reinforced rectangular cutout are fabricated. The initial geometric imperfections such as plate imperfection ?x, overall imperfection of the whole panel ?sx and torsional imperfection in stiffener ?sy are measured for all panels fabricated. The axial deformation of the whole panel, out-of-plane deflections and strains along the midsection of the panels measured during the tests is discussed. The reduction in strength of the panels due to the presence of square cutout, rectangular cutout and increase in strength due to reinforcement around rectangular cutout are calculated based on the experimental observations.

Long hollow rectangular composite box beam configurations are used quite commonly in engineering structures especially in aerospace, ship building and civil engineering construction. In the present study a finite element method is presented for the determination of transverse displacement and layer-wise stress distribution of thin-walled plate structure assembled from flat plates. The finite element formulation is based on Mindlin plate theory which takes shear deformation of the beam into consideration. Effect of material and lay-up sequence on the structural behavior of rectangular cross-section thin-walled composite box beam under uniformly distributed load on the top face with end supports is studied. Copyright ? 1996 Elsevier Science Ltd.

A numerical method for the nonlinear and instability analysis of thin-walled members (NISAT) subjected to biaxial bending has been developed which includes the warping of degree freedom at each mode. The analysis includes the effects of geometric nonlinearity, out-of-plane bending and the consequent torsion, secondary effects of load applied away from the shear centre, large displacements and rotations in the three-dimensional space and initial imperfections. The effect of large displacements is included in the nonlinear analysis following the updated Lagrangian method where the nodal coordinates are updated before every load increment. The analysis does not consider distortion of the cross-section. The nonlinear biaxial bending behaviour of thin-walled Z- and sigma sections subjected to transverse load is studied analytically using the NISAT program and the results are compared with experimental values. The failure load, midspan displacements and the normal stresses at critical points of the midspan section are also compared.

The Ocean Brick System (OBS) is a modular system consisting of hollow concrete precast blocs (10m x 10m x 10m) piled up like cubes and interconnected to create a stiff, light and strong structure which can be used for artificial islands, artificial reefs, elevation of vulnerable low lands, deep water ports, breakwaters and foundation of offshore wind turbines. The paper focuses on the experimental results on the wave loading and the stability of the OBS used as a foundation of the support structure of offshore wind turbines. Diagrams for the prediction of total horizontal forces, vertical forces and overturning moments induced by irregular waves on the OB-structure are derived and verified through additional stability tests and stability analysis.

In the field of ocean engineering, pile foundations are extensively used in supporting several structures. In many cases, piles are subjected to significant lateral loads. The environment prevalent in the ocean necessitates the piles to be designed for cyclic wave loading. In this investigation, the behaviour of rigid piles under cyclic lateral loading has been studied through an experimental programme carried out on model piles embedded in a soft marine clay. Static tests were also conducted on piles embedded in a clay bed prepared at different consistencies suitable to field situations. Cyclic load was applied by using a specially designed pneumatic controlled loading system. Tests were conducted on model piles made of mild steel (MS), aluminium and PVC with wide variation in pile soil relative stiffness. For cyclic load levels less than 50% of static lateral capacity, the deflections are observed to increase with number of cycles and cyclic load level and stabilise after a certain number of cycles. For cyclic load levels greater than 50% of static lateral capacity, the deflections are observed to increase enormously with number of cycles. The results of post-cyclic load tests indicate that the behaviour under static load can improve for cyclic load levels less than 40% of the static lateral capacity. The variations in the load capacity due to cyclic loading are explained in terms of the changes in strength behaviour of soil.

The paper presents a displacement incrementation procedure to handle multiple loadings in post-buckling analysis of structures by Dynamic Relaxation (DR). This procedure is generalized and a ‘variable-arc-length’ method is proposed to automate the tracing of load-deflection path. The resulting algorithm exactly traces limit points and can handle ‘snap-through’ or ‘snap-back’ problems. The efficiency of the proposed method is demonstrated by typical examples of truss, beam and shell structures.

The stresses arising in a railway wheel due to an impact loaded are presented in this paper. The wheel is modelled using a string of conical shell finite elements. The response of the wheel to an impact loaded is analysed using the modal superposition technique. The stresses at various locations on the wheel are determined and compared with those arising due to a static load.

This article presents a new mathematical model for helicopter comprehensive analysis with the features of flexibility and mathematical simplicity. The model synthesizes the rigid fuselage motion model with 6 degrees of freedom, coupled flap-lag-torsion elastic rotor blade motion model, unsteady aerodynamics model with dynamic stall and high order generalized dynamic wake model. A new blade structural operator with implicit form is formulated, and the components of the blade structure model are independent of each other so that it is convenient to change or handle any component of blade structure without changing the others. What is more, the entire model is developed in a strict state-space form to simplify the comprehensive analysis. Finally, the UH-60 helicopter is taken as an example to predict the blade natural characteristics, the trim characteristics including controls and fuselage attitudes as well as the airloads at blade section under the flight conditions of high speed with moderate thrust and high thrust with moderate speed. The results are compared with UH-60 flight test data and those predicted by two well-known comprehensive codes. The validity of the model presented in this article is verified.