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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.
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 occur 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.
Contents
1 Introduction
2 Actions on wind turbines
2.1 Permanent actions
2.2 Turbine operation (rotor and nacelle)
2.3 Wind loads
2.3.1 Wind loads for onshore wind turbines
2.3.1.1 Wind loads according to the DIBt guideline
2.3.1.2 Checking the susceptibility to vibration
2.3.1.3 Example of application
2.3.2 Wind loads for offshore wind turbines
2.3.2.1 Classification of wind turbines
2.3.2.2 Determining the wind conditions (wind climate)
2.3.2.3 Normal wind conditions
2.3.2.4 Extreme wind conditions
2.3.2.5 Wind farm influence
2.4 Height of sea level
2.5 Hydrodynamic environmental conditions
2.5.1 Sea currents
2.5.2 Natural sea state
2.5.3 Harmonic primary wave
2.5.4 Waves of finite steepness
2.5.5 Statistical description of the sea state
2.5.6 Short-term statistics for the sea state
2.5.7 Long-term statistics for the sea state
2.5.8 Extreme sea state values
2.5.9 Breaking waves
2.6 Hydrodynamic analysis
2.6.1 General
2.6.2 Morison formula
2.6.3 Potential theory method -- linear motion behaviour
2.6.4 Integral equation method (singularity method)
2.6.5 Vertical cylinders (MacCamy and Fuchs)
2.6.6 Higher-order potential theory
2.6.7 Wave loads on large-volume offshore structures
2.7 Thermal actions
2.8 Sea ice
2.9 Icing-up of structural members 83
3 Non-linear material behaviour
3.1 General
3.2 Material laws for reinforced and prestressed concrete 8
3.2.1 Non-linear stress-strain curve for concrete
3.2.2 Non-linear stress-strain curve for reinforcing steel
3.2.3 Non-linear stress-strain curve for prestressing steel
3.3 Bending moment-curvature relationships
3.3.1 Reinforced concrete cross-sections in general
3.3.2 Prestressed concrete cross-sections in general
3.3.3 Annular reinforced concrete cross-sections
3.4 Deformations and bending moments according to second-order theory
3.5 Design of cross-section for ultimate limit state
3.5.1 Material resistance of concrete
3.5.2 Material resistance of reinforcement
3.6 Three-dimensional mechanical models for concrete
3.6.1 Failure envelopes and stress invariants
3.6.2 Common failure models for concrete
3.6.3 Three-phase model
3.6.4 Constitutive models
4 Loadbearing structures and detailed design
4.1 Basis for design
4.2 Structural model for tower shaft
4.2.1 Rotation of the foundation
4.2.2 Stability of towers on soft subsoils
4.3 Investigating vibrations
4.3.1 Mass-spring systems with single/multiple degrees of freedom
4.3.2 The energy method
4.3.2.1 Practical vibration analysis
4.3.2.2 Example of application
4.3.3 Natural frequency analysis of loadbearing structure
4.4 Prestressing
4.4.1.1 Prestressing with grouted post-tensioned tendons
4.4.1.2 External prestressing with unbonded tendons
4.5 Design of onshore wind turbine support structures
4.5.1 Total dynamic analysis
4.5.2 Simplified analysis
4.5.2.1 Sensitivity to vibration
4.5.2.2 Vibration damping
4.5.3 Design load cases according to DIBt guideline (onshore)
4.5.3.1 Critical design load cases
4.5.4 Partial safety factors according to DIBt guideline
4.6 Design of offshore wind turbine structures
4.6.1 Control and safety systems
4.6.2 Design situations and load cases
4.6.3 Fundamental considerations regarding the safety concept
4.6.3.1 Safety analysis
4.6.3.2 Combined sea state and wind
4.6.4 Design load cases according to GL guideline
4.6.4.1 Commentary to Table 4.4
4.6.5 Partial safety factors according to GL guideline
4.7 Ultimate limit state
4.7.1 Deformation calculations according to second-order theory
4.7.2 Linear analysis of internal forces
4.7.3 Analysis of stresses in tower shaft
4.7.4 Special characteristics of prefabricated construction
4.7.4.1 Terminology
4.7.4.2 Shear force transfer across opening joints
4.7.4.3 Detailed design
4.7.4.4 Transferring prestressing forces
4.7.4.5 Erecting and prestressing precast concrete elements
4.7.4.6 Design of openings
4.8 Analysis of serviceability limit state
4.8.1 Action effects in tower shaft due to external actions
4.8.1.1 Limiting the deformations
4.8.1.2 Limiting the stresses
4.8.1.3 Limiting crack widths and decompression limit state
4.8.2 Restraint stresses acting on shaft wall
4.8.3 Special aspects of construction with precast concrete elements
4.9 Fatigue limit state
4.9.1 Fatigue-inducing actions on wind turbine support structures
4.9.1.1 Actions due to wind and turbine operation
4.9.1.2 Actions due to waves and sea state
4.9.2 Fatigue analyses according to DIBt wind turbine guideline
4.9.2.1 Simplified analyses for concrete
4.9.2.2 Direct analysis according to DIBt guideline
4.9.3 Multi-stage fatigue loads
4.9.4 Numbers of fatigue cycles to failure for multi-axial fatigue loads
4.9.4.1 Procedure
4.9.4.2 Derivation of damage variables kfat c and kfat t
4.9.4.3 Failure envelope for fatigue load 170
4.9.4.4 Failure curves for biaxial fatigue loads
4.9.5 Design proposal for multi-axial fatigue
4.9.5.1 Procedure for designing on the basis of the linear accumulation hypothesis
4.9.5.2 Derivation of modification factor lc3 (N, r) for fatigue loads on compression meridian
4.9.5.3 Derivation of modification factors lc2 (N, a) for biaxial fatigue loads
4.10 Design of construction nodes
4.10.1 Loads on nodes
4.10.2 Composition of forces at the ultimate limit state
4.10.3 Characteristic values for loads
4.10.4 Example of calculation
4.10.5 Load on circular ring beam at ultimate limit state
4.10.6 Design of circular ring beam
4.11 Foundation design
4.11.1 Calculating the internal forces
5 Construction of prestressed concrete towers
5.1 Introduction
5.2 Hybrid structures of steel and prestressed concrete
5.3 Prestressed concrete towers with precast concrete segments
5.3.1 Examples of design and construction
5.3.2 Further developments in precast concrete construction
5.4 Offshore substructures in concrete
5.4.1 Compact substructures with ice cones
5.4.1.1 Middelgrunden offshore wind farm
5.4.1.2 Sequence of operations on site
5.4.2 Design, construction, transport and erection of concrete substructures
5.4.2.1 Special design criteria
5.4.2.2 Construction
5.4.2.3 Transport and erection
5.4.2.4 Spread and deep foundations
5.4.2.5 Innovations
References
Index
