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Concrete Structures for Wind Turbines

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Description

The wind energy industry in Germany has an excellent global standing when it comes to the development and construction of wind turbines. Germany currently represents the world's largest market for wind energy. The ongoing development of ever more powerful wind turbines plus additional requirements for the design and construction of their offshore foundation structures exceeds the actual experiences gained so far in the various disciplines concerned.
This book gives a comprehensive overview for planning and structural design analysis of reinforced concrete and pre-stressed concrete wind turbine towers for both, onshore and offshore wind turbines. Wind turbines represent structures subjected to highly dynamic loading patterns. Therefore, for the design of loadbearing structures, fatigue effects - and not just maximum loads - are extremely important, in particular in the connections and joints of concrete and hybrid structures. There multi-axial stress conditions occure which so far are not covered by the design codes. The specific actions, the nonlinear behaviour and modeling for the structural analysis are explained. Design and verification with a focus on fatigue are adressed. The chapter Manufacturing includes hybrid structures, segmental construction of pre-stressed concrete towers and offshore wind turbine foundations.

Selected chapters from the German concrete yearbook are now being published in the new English ""Beton-Kalender Series"" for the benefit of an international audience.
Since it was founded in 1906, the Ernst & Sohn ""Beton-Kalender"" has been supporting developments in reinforced and prestressed concrete. The aim was to publish a yearbook to reflect progress in ""ferro-concrete"" structures until - as the book's first editor, Fritz von Emperger (1862-1942), expressed it - the ""tempestuous development"" in this form of construction came to an end. However, the ""Beton-Kalender"" quickly became the chosen work of reference for civil and structural engineers, and apart from the years 1945-1950 has been published annually ever since.
Editorial IX

1 Introduction 1

2 Actions on wind turbines  5

2.1 Permanent actions 5

2.2 Turbine operation (rotor and nacelle) 5

2.3 Wind loads 5

2.3.1 Wind loads for onshore wind turbines 5

2.3.1.1 Wind loads according to the DIBt guideline 6

2.3.1.2 Checking the susceptibility to vibration 8

2.3.1.3 Example of application 9

2.3.2 Wind loads for offshore wind turbines 21

2.3.2.1 Classification of wind turbines 21

2.3.2.2 Determining the wind conditions (wind climate) 22

2.3.2.3 Normal wind conditions 23

2.3.2.4 Extreme wind conditions 25

2.3.2.5 Wind farm influence 30

2.4 Height of sea level 31

2.5 Hydrodynamic environmental conditions 32

2.5.1 Sea currents 32

2.5.2 Natural sea state 34

2.5.3 Harmonic primary wave 35

2.5.4 Waves of finite steepness 38

2.5.5 Statistical description of the sea state 40

2.5.6 Short-term statistics for the sea state 41

2.5.7 Long-term statistics for the sea state 47

2.5.8 Extreme sea state values 50

2.5.9 Breaking waves 52

2.6 Hydrodynamic analysis 53

2.6.1 General 53

2.6.2 Morison formula 54

2.6.3 Potential theory method -- linear motion behaviour 60

2.6.4 Integral equation method (singularity method) 62

2.6.5 Vertical cylinders (MacCamy and Fuchs) 66

2.6.6 Higher-order potential theory 70

2.6.7 Wave loads on large-volume offshore structures 72

2.7 Thermal actions 77

2.8 Sea ice 78

2.9 Icing-up of structural members 83

3 Non-linear material behaviour 85

3.1 General 85

3.2 Material laws for reinforced and prestressed concrete 86

3.2.1 Non-linear stress-strain curve for concrete 86

3.2.2 Non-linear stress-strain curve for reinforcing steel 87

3.2.3 Non-linear stress-strain curve for prestressing steel 90

3.3 Bending moment-curvature relationships 91

3.3.1 Reinforced concrete cross-sections in general 91

3.3.2 Prestressed concrete cross-sections in general 92

3.3.3 Annular reinforced concrete cross-sections 94

3.4 Deformations and bending moments according to second-order theory 97

3.5 Design of cross-section for ultimate limit state 98

3.5.1 Material resistance of concrete 98

3.5.2 Material resistance of reinforcement 99

3.6 Three-dimensional mechanical models for concrete 100

3.6.1 Failure envelopes and stress invariants 101

3.6.2 Common failure models for concrete 102

3.6.3 Three-phase model 103

3.6.4 Constitutive models 105

4 Loadbearing structures and detailed design 107

4.1 Basis for design 107

4.2 Structural model for tower shaft 108

4.2.1 Rotation of the foundation 108

4.2.2 Stability of towers on soft subsoils 110

4.3 Investigating vibrations 111

4.3.1 Mass-spring systems with single/multiple degrees of freedom 111

4.3.2 The energy method 113

4.3.2.1 Practical vibration analysis 114

4.3.2.2 Example of application 114

4.3.3 Natural frequency analysis of loadbearing structure 116

4.4 Prestressing 118

4.4.1.1 Prestressing with grouted post-tensioned tendons 119

4.4.1.2 External prestressing with unbonded tendons 119

4.5 Design of onshore wind turbine support structures 120

4.5.1 Total dynamic analysis 120

4.5.2 Simplified analysis 121

4.5.2.1 Sensitivity to vibration 121

4.5.2.2 Vibration damping 122

4.5.3 Design load cases according to DIBt guideline (onshore) 122

4.5.3.1 Critical design load cases 122

4.5.4 Partial safety factors according to DIBt guideline 124

4.6 Design of offshore wind turbine structures 125

4.6.1 Control and safety systems 125

4.6.2 Design situations and load cases 127

4.6.3 Fundamental considerations regarding the safety concept 128

4.6.3.1 Safety analysis 128

4.6.3.2 Combined sea state and wind 130

4.6.4 Design load cases according to GL guideline 131

4.6.4.1 Commentary to Table 4.4 135

4.6.5 Partial safety factors according to GL guideline 138

4.7 Ultimate limit state 138

4.7.1 Deformation calculations according to second-order theory 138

4.7.2 Linear analysis of internal forces 143

4.7.3 Analysis of stresses in tower shaft 144

4.7.4 Special characteristics of prefabricated construction 145

4.7.4.1 Terminology 145

4.7.4.2 Shear force transfer across opening joints 146

4.7.4.3 Detailed design 149

4.7.4.4 Transferring prestressing forces 149

4.7.4.5 Erecting and prestressing precast concrete elements 149

4.7.4.6 Design of openings 151

4.8 Analysis of serviceability limit state 153

4.8.1 Action effects in tower shaft due to external actions 153

4.8.1.1 Limiting the deformations 153

4.8.1.2 Limiting the stresses 153

4.8.1.3 Limiting crack widths and decompression limit state 153

4.8.2 Restraint stresses acting on shaft wall 154

4.8.3 Special aspects of construction with precast concrete elements 155

4.9 Fatigue limit state 156

4.9.1 Fatigue-inducing actions on wind turbine support structures 157

4.9.1.1 Actions due to wind and turbine operation 157

4.9.1.2 Actions due to waves and sea state 158

4.9.2 Fatigue analyses according to DIBt wind turbine guideline 159

4.9.2.1 Simplified analyses for concrete 159

4.9.2.2 Direct analysis according to DIBt guideline 161

4.9.3 Multi-stage fatigue loads 164

4.9.4 Numbers of fatigue cycles to failure for multi-axial fatigue loads 164

4.9.4.1 Procedure 164

4.9.4.2 Derivation of damage variables kfat c and kfat t 166

4.9.4.3 Failure envelope for fatigue load 170

4.9.4.4 Failure curves for biaxial fatigue loads 171

4.9.5 Design proposal for multi-axial fatigue 174

4.9.5.1 Procedure for designing on the basis of the linear accumulation hypothesis 174

4.9.5.2 Derivation of modification factor lc3 (N, r) for fatigue loads on compression meridian 175

4.9.5.3 Derivation of modification factors lc2 (N, a) for biaxial fatigue loads 177

4.10 Design of construction nodes 180

4.10.1 Loads on nodes 180

4.10.2 Composition of forces at the ultimate limit state 180

4.10.3 Characteristic values for loads 181

4.10.4 Example of calculation 184

4.10.5 Load on circular ring beam at ultimate limit state 187

4.10.6 Design of circular ring beam 188

4.11 Foundation design 188

4.11.1 Calculating the internal forces 188

5 Construction of prestressed concrete towers 191

5.1 Introduction 191

5.2 Hybrid structures of steel and prestressed concrete 191

5.3 Prestressed concrete towers with precast concrete segments 193

5.3.1 Examples of design and construction 193

5.3.2 Further developments in precast concrete construction 197

5.4 Offshore substructures in concrete 198

5.4.1 Compact substructures with ice cones 199

5.4.1.1 Middelgrunden offshore wind farm 199

5.4.1.2 Sequence of operations on site  202

5.4.2 Design, construction, transport and erection of concrete substructures 202

5.4.2.1 Special design criteria 203

5.4.2.2 Construction 203

5.4.2.3 Transport and erection 204

5.4.2.4 Spread and deep foundations 205

5.4.2.5 Innovations 207

References 209

Index 217