IEC 61400-6:2020

IEC 61400-6 ®
Edition 1.1 2025-06
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
Wind energy generation systems –
Part 6: Tower and foundation design requirements
ICS 27.180 ISBN 978-2-8327-0510-0
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REDLINE VERSION – 2 – IEC 61400-6:2020+AMD1:2025 CSV
© IEC 2025
CONTENTS
FOREWORD . 9
INTRODUCTION . 11
INTRODUCTION to Amendment 1 . 11
1 Scope . 12
2 Normative references . 12
3 Terms and definitions . 13
4 Symbols and abbreviated terms . 18
4.1 Symbols . 18
4.2 Abbreviated terms . 25
5 Design basis including loading . 25
5.1 General . 25
5.2 Basis of design . 26
5.2.1 Basic principles . 26
5.2.2 Durability . 26
5.2.3 Principles of limit state design . 26
5.2.4 Structural analysis . 27
5.2.5 Assessments by tests . 27
5.3 Materials . 27
5.4 Loads. 28
5.4.1 Use of IEC 61400-1 or IEC 61400-2 load cases and partial safety
factors for loads . 28
5.4.2 Superseding of IEC 61400-1 or IEC 61400-2 partial safety factors for
materials . 28
5.4.3 Serviceability load levels . 28
5.4.4 Load combinations in ULS . 29
5.4.5 Structural damping values to be used in load calculations . 30
5.4.6 Definitions and methods for use of internal loads . 30
5.4.7 Definition of required load data for fatigue analysis . 31
5.4.8 Definition of required load data for extreme load level . 31
5.4.9 Vortex induced vibration . 31
5.4.10 Loads due to geometric tolerances and elastic deflections in tower
verticality . 32
5.5 Load data and interface reporting requirements . 32
5.5.1 Purpose . 32
5.5.2 Wind turbine specification . 32
5.5.3 Time history data . 33
5.5.4 Load origins . 33
5.5.5 Load components . 33
5.6 General structural design requirements . 34
5.6.1 Secondary structural influence . 34
5.6.2 Fatigue analysis . 34
5.7 Delivery documentation . 34
6 Steel towers . 34
6.1 General . 34
6.2 Basis of design . 34
6.3 Materials . 34
6.3.1 General . 34

© IEC 2025
6.3.2 Structural steels . 34
6.3.3 Bolts and anchors . 37
6.4 Ultimate strength analysis for towers and openings . 38
6.4.1 General . 38
6.4.2 Partial safety factors . 38
6.4.3 Verification of ultimate strength . 39
6.4.4 Tower assessment . 39
6.4.5 Detail assessments. 39
6.5 Stability. 40
6.5.1 General . 40
6.5.2 Partial safety factor. 40
6.5.3 Assessment . 40
6.5.4 Door frames/stiffeners . 41
6.6 Fatigue limit state . 41
6.6.1 General . 41
6.6.2 Partial safety factor for materials . 42
6.6.3 Assessment . 42
6.6.4 Details . 42
6.7 Ring flange connections . 43
6.7.1 General . 43
6.7.2 Design assumptions and requirements, execution of ring flanges . 43
6.7.3 Execution of ring flanges . 45
6.7.4 Fatigue limit state analysis of bolted connection .
6.7.4 Ultimate limit state analysis of flange and bolted connection . 51
6.7.5 Fatigue limit state analysis of flange bolts . 53
6.7.6 Fatigue limit state analysis of flange weld and fillet radius . 62
6.8 Bolted connections resisting shear through friction . 63
6.8.1 General requirements . 63
6.8.2 Test-assisted design . 64
6.8.3 Design without test . 65
7 Concrete towers and foundations. 66
7.1 General . 66
7.2 Basis of design . 66
7.2.1 Reference standard for concrete design . 66
7.2.2 Partial safety factors . 66
7.2.3 Basic variables . 67
7.3 Materials . 69
7.4 Durability . 69
7.4.1 Durability requirements . 69
7.4.2 Exposure classes . 69
7.4.3 Concrete cover . 69
7.5 Structural analysis . 69
7.5.1 Finite element analysis . 69
7.5.2 Foundation slabs . 70
7.5.3 Regions with discontinuity in geometry or loads . 70
7.5.4 Cast in anchor bolt arrangements . 71
7.6 Concrete to concrete joints . 71
7.7 Ultimate limit state . 71
7.7.1 General . 71

REDLINE VERSION – 4 – IEC 61400-6:2020+AMD1:2025 CSV
© IEC 2025
7.7.2 Shear and punching shear . 72
7.8 Fatigue limit state . 72
7.8.1 General . 72
7.8.2 Reinforcement and prestressing steel fatigue failure . 72
7.8.3 Concrete fatigue failure . 72
7.9 Serviceability limit state . 73
7.9.1 Load dependent stiffness reduction . 73
7.9.2 Stress limitation . 73
7.9.3 Crack control . 73
7.9.4 Deformations . 74
7.10 Execution . 74
7.10.1 General . 74
7.10.2 Requirements . 74
7.10.3 Inspection of materials and products. 74
7.10.4 Falsework and formwork . 74
7.10.5 Reinforcement and embedded steel . 74
7.10.6 Pre-stressing . 74
7.10.7 Precast concrete elements. 75
7.10.8 Geometrical tolerances . 75
8 Foundations – Geotechnical design . 75
8.1 General . 75
8.2 Basis of design . 75
8.2.1 General . 75
8.2.2 Geotechnical limit states . 76
8.3 Geotechnical data . 76
8.3.1 General . 76
8.3.2 Specific considerations . 78
8.4 Supervision, monitoring and maintenance of construction . 79
8.5 Gravity base foundations . 79
8.5.1 General . 79
8.5.2 Ultimate limit state (ULS) . 80
8.5.3 Serviceability limit state (SLS) . 83
8.6 Piled foundations . 85
8.6.1 General . 85
8.6.2 Pile loads . 85
8.6.3 Ultimate limit state . 86
8.6.4 Serviceability limit state . 87
8.7 Rock anchored foundations . 88
8.7.1 General . 88
8.7.2 Types of rock anchor foundation . 88
8.7.3 Geotechnical data . 88
8.7.4 Corrosion protection . 88
8.7.5 Anchor inspection and maintenance . 89
8.7.6 Post tension tolerances and losses . 89
8.7.7 Ultimate limit state . 89
8.7.8 Serviceability limit state . 90
8.7.9 Robustness check . 90
8.7.10 Rock anchor design . 91
9 Operation, service and maintenance requirements . 93

© IEC 2025
9.1 Operation, maintenance and monitoring . 93
9.2 Periodic structural inspections . 93
9.3 Embedded steel structural section inspections . 94
9.4 Bolt tension maintenance . 94
9.5 Structural health monitoring . 94
Annex A (informative) List of suitable design codes and guidelines for the calculation
basis . 95
A.1 General . 95
A.2 Reference documents . 95
Annex B (informative) List of material for structural steel . 96
B.1 General . 96
B.2 Structural steel . 96
Annex C (informative) Bolts . 97
C.1 General . 97
C.2 Reference documents . 98
C.3 Use of HV bolt assemblies . 98
C.4 Water immersion test . 99
Annex D (informative) Z-values for structural steel . 100
D.1 General . 100
D.2 Definition of Z-value according to Eurocode . 100
D.3 Reference documents . 100
Annex E (informative) Simplified buckling verification for openings in tubular steel
towers . 101
Annex F (informative) Fatigue verification . 104
F.1 General . 104
F.2 Specific details . 104
Annex G (informative) Methods for ring flange verification . 105
G.1 Method for ultimate strength analysis according to Petersen/Seidel . 105
G.1.1 Basics . 105
G.1.2 Calculation method . 105
G.1.3 Extension by Tobinaga and Ishihara . 109
G.2 Method for fatigue strength analysis according to Schmidt/Neuper Seidel . 110
G.2.1 Basics .
G.2.2 Formulas for the tri-linear approximation .
G.2.1 Conditions for calculation model . 112
G.2.2 Bolt force curve . 113
G.2.3 Bolt moment curve . 120
G.2.4 Calculation of force required to close inclination . 121
G.3 Reference documents . 122
Annex H (informative) Crack control – Guidance on 7.9.3 . 124
H.1 General . 124
H.2 Crack control based on Eurocode 2 . 124
H.3 Crack control based on Japanese standards . 124
H.4 Crack control based on ACI 318 . 125
H.5 Reference documents . 125
Annex I (informative) Finite element analysis for concrete . 126
I.1 General . 126
I.2 Order and type of elements . 126

REDLINE VERSION – 6 – IEC 61400-6:2020+AMD1:2025 CSV
© IEC 2025
I.3 Constitutive modelling . 127
I.4 Solution methods . 127
I.5 Implicit approach . 127
I.6 Steps in conducting of a finite element analysis . 128
I.7 Checking results . 128
I.8 Reference documents . 129
Annex J (informative) Tower-foundation anchorage . 130
J.1 General . 130
J.2 Embedded anchorages . 130
J.3 Bolted anchorages . 131
J.4 Grout . 131
J.5 Anchor bolts . 131
J.6 Embedded ring . 131
J.7 Anchorage load transfer . 132
Annex K (informative) Strut-and-tie section . 133
K.1 General . 133
K.2 Example of a rock anchor foundation . 134
K.3 Reference documents . 137
Annex L (informative) Guidance on selection of soil modulus and foundation rotational

stiffness . 139
L.1 General . 139
L.2 Soil model . 139
L.3 Dynamic rotational stiffness . 141
L.4 Static rotational stiffness . 142
L.5 Reference documents . 143
Annex M (informative) Guidance for rock anchored foundation design . 144
M.1 General . 144
M.2 Corrosion protection . 144
M.2.1 Standard anchors . 144
M.2.2 Corrosion protection of bar anchors . 145
M.3 Product approval . 146
M.4 Rock anchor design . 146
M.5 Grout design . 146
M.6 Testing and execution . 146
M.7 Suitability/performance test . 147
M.8 Acceptance/proof test . 147
M.9 Supplementary extended creep tests . 147
M.10 Reference documents . 147
Annex N (informative) Internal loads – Explanation of internal loads . 148
Annex O (informative) Seismic load estimation for wind turbine tower and foundation . 150
O.1 General . 150
O.2 Vertical ground motion . 150
O.3 Structure model . 150
O.4 Soil amplification . 151
O.5 Time domain simulation . 152
O.6 Reference documents . 152
Annex P (informative) Structural damping ratio for the tower of wind turbine . 153
P.1 General . 153

© IEC 2025
P.2 First mode structural damping ratio . 153
P.3 Second mode structural damping ratio . 154
P.4 Higher mode damping . 154
P.5 Reference documents . 155
Annex Q (informative) Guidance on partial safety factors for geotechnical limit states . 156
Q.1 General . 156
Q.2 Equilibrium . 156
Q.3 Bearing capacity . 156
Q.4 Sliding resistance . 157
Q.5 Overall stability . 157
Q.6 Reference documents . 158
Bibliography . 159

Figure 1 – Flange notations as an example of an L-flange . 36
Figure 2 – Door opening geometry . 41
Figure 3 – Flange gaps k in the area of the tower wall .
Figure 4 – Bolt force as a function of wall force .
Figure 5 – S-N curve for detail category 36 .
Figure 8 – Bolt force F as a function of external force Z (including dead weight) . 45
S
Figure 9 – Flange gaps with gap height k and gap length l at the tower wall and
k
flange surface inclination α . 45
S
Figure 10 – Illustration of parallel gaps and angular gaps . 46
Figure 11 – Example for flatness measurement evaluation (D = 6 000 mm) . 47
Figure 12 – Clarification of flatness values for the individual flange (u ) and resulting
tol
gap height after mating of two flanges (k) . 48
Figure 13 – Schematic representation of k and k . 51
limit,unloaded limit,loaded
Figure 14 – Schematic representation for the correct shimming of an unacceptable gap . 51
Figure 15 – Total settlement f as function of DFT . 56
Z,tot sbw
Figure 16 – Coating thickness reference points . 57
Figure 17 – Gap shape (L = 2 000 mm / k = 1,0 mm) . 60
gap design
Figure 18 – S-N-curve for bolts (examples M30 and M80 shown) . 62
Figure 19 – Distance requirements for the flange weld in case fillet radius is not
explicitly assessed . 63
Figure 6 – Thermal effects around tower cross-section . 67
Figure 7 – Illustration of rock anchor length . 93
Figure E.1 – Circumferentially edge-stiffened opening . 102
Figure E.2 – Definition of W and t according to JSCE . 103
s s
Figure G.1 – Simplification of system to segment model . 105
Figure G.2 – Locations of plastic hinges for different failure modes . 106
Figure G.3 – Geometric parameters . 107
Figure G.6 – Location of plastic hinges for T-flanges . 109
Figure G.4 – Modification factor 𝛌𝛌 for different 𝜶𝜶 [1] . 110
Figure G.7 – Illustration of bolt force model (L- and T-flanges) . 113
Figure K.1 – Example for the design of a deep beam using the strut-and-tie method . 133

REDLINE VERSION – 8 – IEC 61400-6:2020+AMD1:2025 CSV
© IEC 2025
Figure K.2 – Simple shapes of strut-and-tie models . 133
Figure K.3 – Three examples for carrying load in a deep beam . 134
Figure K.4 – Strut-and-tie models for a rock-anchor foundation . 136
Figure K.5 – Top tie reinforcement in a rock-anchor foundation. 137
Figure L.1 – Example stress-strain relationship for soil . 139
Figure L.2 – Loading and unloading behaviour of soil . 140
Figure L.3 – Variation of shear modulus with soil strain. 141
Figure L.4 – Reduction in rotational stiffness due to load eccentricity. 142
Figure L.5 – Illustrative example of reduction in foundation rotational stiffness due to
increasing load eccentricity . 143
Figure M.1 – Section through rock and anchor . 144
Figure M.2 – Typical anchor configuration with corrosion protection . 145
Figure N.1 – Representation of internal loads . 149
Figure O.1 – Structure model for response spectrum method . 151
Figure P.1 – First mode damping ratio for the steel tower of wind turbine . 154

Table 1 – Flange tolerances .
Table 3 – Flange tolerances . 47
Table 2 – Summary of geotechnical limit states . 76
Table B.1 – National and regional steel standards and types . 96
Table C.1 – Comparison of bolt material in ISO 898-1, JIS B1186 and ASTM A490M-12 . 97
Table C.2 – Mean preload after installation . 98
Table E.1 – Coefficients for Formula (E.3) . 102
Table G.1 – Data points for external forces and bolt forces for determination of
polynomial coefficients . 115
[1]
Table H.1 – Limit value of crack width based on Japanese standards . 125
Table P.1 – Damping coefficients . 153
Table Q.1 – Minimum partial safety factors for the equilibrium limit state (European
and North American practice) . 156
Table Q.2 – Minimum partial safety factors on for the equilibrium limit state (JSCE) . 156
Table Q.3 – Minimum partial material and resistance factors for the bearing resistance
limit state, ULS . 157
Table Q.4 – Minimum partial material and resistance factors for the sliding resistance

limit state, ULS . 157
Table Q.5 – Minimum partial material and resistance factors for the overall stability
limit state, ULS . 158

© IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 6: Tower and foundation design requirements

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IEC 61400-6 ®
Edition 1.0 2020-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –
Part 6: Tower and foundation design requirements

Systèmes de génération d'énergie éolienne –
Partie 6: Exigences en matière de conception du mât et de la fondation

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IEC 61400-6 ®
Edition 1.0 2020-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –

Part 6: Tower and foundation design requirements

Systèmes de génération d'énergie éolienne –

Partie 6: Exigences en matière de conception du mât et de la fondation

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-8803-0

– 2 – IEC 61400-6:2020 © IEC 2020
CONTENTS
FOREWORD . 9
INTRODUCTION . 11
1 Scope . 12
2 Normative references . 12
3 Terms and definitions . 13
4 Symbols and abbreviated terms . 17
4.1 Symbols . 17
4.2 Abbreviated terms . 19
5 Design basis including loading . 20
5.1 General . 20
5.2 Basis of design . 20
5.2.1 Basic principles . 20
5.2.2 Durability . 21
5.2.3 Principles of limit state design . 21
5.2.4 Structural analysis . 21
5.2.5 Assessments by tests . 22
5.3 Materials . 22
5.4 Loads. 22
5.4.1 Use of IEC 61400-1 or IEC 61400-2 load cases and partial safety
factors for loads . 22
5.4.2 Superseding of IEC 61400-1 or IEC 61400-2 partial safety factors for
materials . 22
5.4.3 Serviceability load levels . 23
5.4.4 Load combinations in ULS . 24
5.4.5 Structural damping values to be used in load calculations . 25
5.4.6 Definitions and methods for use of internal loads . 25
5.4.7 Definition of required load data for fatigue analysis . 25
5.4.8 Definition of required load data for extreme load level . 25
5.4.9 Vortex induced vibration . 26
5.4.10 Loads due to geometric tolerances and elastic deflections in tower
verticality . 26
5.5 Load data and interface reporting requirements . 27
5.5.1 Purpose . 27
5.5.2 Wind turbine specification . 27
5.5.3 Time history data . 28
5.5.4 Load origins . 28
5.5.5 Load components . 28
5.6 General structural design requirements . 28
5.6.1 Secondary structural influence . 28
5.6.2 Fatigue analysis . 28
5.7 Delivery documentation . 28
6 Steel towers . 29
6.1 General . 29
6.2 Basis of design . 29
6.3 Materials . 29
6.3.1 General . 29
6.3.2 Structural steels . 29

6.3.3 Bolts and anchors . 32
6.4 Ultimate strength analysis for towers and openings . 32
6.4.1 General . 32
6.4.2 Partial safety factors . 32
6.4.3 Verification of ultimate strength . 32
6.4.4 Tower assessment . 32
6.4.5 Detail assessments. 33
6.5 Stability. 33
6.5.1 General . 33
6.5.2 Partial safety factor. 34
6.5.3 Assessment . 34
6.5.4 Door frames/stiffeners . 34
6.6 Fatigue limit state . 35
6.6.1 General . 35
6.6.2 Partial safety factor for materials . 35
6.6.3 Assessment . 36
6.6.4 Details . 36
6.7 Ring flange connections . 36
6.7.1 General . 36
6.7.2 Design assumptions and requirements, execution of ring flanges . 36
6.7.3 Ultimate limit state analysis of flange and bolted connection . 38
6.7.4 Fatigue limit state analysis of bolted connection . 38
6.8 Bolted connections resisting shear through friction . 40
6.8.1 General requirements . 40
6.8.2 Test-assisted design . 41
6.8.3 Design without test . 42
7 Concrete towers and foundations. 42
7.1 General . 42
7.2 Basis of design . 42
7.2.1 Reference standard for concrete design . 42
7.2.2 Partial safety factors . 43
7.2.3 Basic variables . 43
7.3 Materials . 45
7.4 Durability . 46
7.4.1 Durability requirements . 46
7.4.2 Exposure classes . 46
7.4.3 Concrete cover . 46
7.5 Structural analysis . 46
7.5.1 Finite element analysis . 46
7.5.2 Foundation slabs . 47
7.5.3 Regions with discontinuity in geometry or loads . 47
7.5.4 Cast in anchor bolt arrangements . 48
7.6 Concrete to concrete joints . 48
7.7 Ultimate limit state . 48
7.7.1 General . 48
7.7.2 Shear and punching shear . 48
7.8 Fatigue limit state . 49
7.8.1 General . 49
7.8.2 Reinforcement and prestressing steel fatigue failure . 49

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7.8.3 Concrete fatigue failure . 49
7.9 Serviceability limit state . 50
7.9.1 Load dependent stiffness reduction . 50
7.9.2 Stress limitation . 50
7.9.3 Crack control . 50
7.9.4 Deformations . 51
7.10 Execution . 51
7.10.1 General . 51
7.10.2 Requirements . 51
7.10.3 Inspection of materials and products. 51
7.10.4 Falsework and formwork . 51
7.10.5 Reinforcement and embedded steel . 51
7.10.6 Pre-stressing . 51
7.10.7 Precast concrete elements. 52
7.10.8 Geometrical tolerances . 52
8 Foundations – Geotechnical design . 52
8.1 General . 52
8.2 Basis of design . 52
8.2.1 General . 52
8.2.2 Geotechnical limit states . 53
8.3 Geotechnical data . 53
8.3.1 General . 53
8.3.2 Specific considerations . 55
8.4 Supervision, monitoring and maintenance of construction . 56
8.5 Gravity base foundations . 56
8.5.1 General . 56
8.5.2 Ultimate limit state (ULS) . 57
8.5.3 Serviceability limit state (SLS) . 60
8.6 Piled foundations . 62
8.6.1 General . 62
8.6.2 Pile loads . 62
8.6.3 Ultimate limit state . 63
8.6.4 Serviceability limit state . 64
8.7 Rock anchored foundations . 65
8.7.1 General . 65
8.7.2 Types of rock anchor foundation . 65
8.7.3 Geotechnical data . 65
8.7.4 Corrosion protection . 65
8.7.5 Anchor inspection and maintenance . 66
8.7.6 Post tension tolerances and losses . 66
8.7.7 Ultimate limit state . 66
8.7.8 Serviceability limit state . 67
8.7.9 Robustness check . 67
8.7.10 Rock anchor design . 68
9 Operation, service and maintenance requirements . 70
9.1 Operation, maintenance and monitoring . 70
9.2 Periodic structural inspections . 70
9.3 Embedded steel structural section inspections . 71
9.4 Bolt tension maintenance . 71

9.5 Structural health monitoring . 71
Annex A (informative) List of suitable design codes and guidelines for the calculation
basis . 72
A.1 General . 72
A.2 Reference documents . 72
Annex B (informative) List of material for structural steel . 73
B.1 General . 73
B.2 Structural steel . 73
Annex C (informative) Bolts . 74
C.1 General . 74
C.2 Reference documents . 75
Annex D (informative) Z-values for structural steel . 76
D.1 General . 76
D.2 Definition of Z-value according to Eurocode . 76
D.3 Reference documents . 76
Annex E (informative) Simplified buckling verification for openings in tubular steel
towers . 77
Annex F (informative) Fatigue verification . 80
F.1 General . 80
F.2 Specific details . 80
Annex G (informative) Methods for ring flange verification . 81
G.1 Method for ultimate strength analysis according to Petersen/Seidel . 81
G.1.1 Basics . 81
G.1.2 Calculation method . 81
G.1.3 Extension by Tobinaga and Ishihara . 84
G.2 Method for fatigue strength analysis according to Schmidt/Neuper . 85
G.2.1 Basics . 85
G.2.2 Formulas for the tri-linear approximation . 86
G.3 Reference documents . 87
Annex H (informative) Crack control – Guidance on 7.9.3 . 88
H.1 General . 88
H.2 Crack control based on Eurocode 2 . 88
H.3 Crack control based on Japanese standards . 88
H.4 Crack control based on ACI 318 . 89
H.5 Reference documents . 89
Annex I (informative)  Finite element analysis for concrete. 90
I.1 General . 90
I.2 Order and type of elements . 90
I.3 Constitutive modelling . 91
I.4 Solution methods . 91
I.5 Implicit approach . 91
I.6 Steps in conducting of a finite element analysis . 92
I.7 Checking results . 92
I.8 Reference documents . 93
Annex J (informative) Tower-foundation anchorage . 94
J.1 General . 94
J.2 Embedded anchorages . 94
J.3 Bolted anchorages . 95

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J.4 Grout . 95
J.5 Anchor bolts . 95
J.6 Embedded ring . 95
J.7 Anchorage load transfer . 96
Annex K (informative)  Strut-and-tie section . 97
K.1 General . 97
K.2 Example of a rock anchor foundation . 98
K.3 Reference documents . 102
Annex L (informative) Guidance on selection of soil modulus and foundation rotational

stiffness . 103
L.1 General . 103
L.2 Soil model . 103
L.3 Dynamic rotational stiffness . 105
L.4 Static rotational stiffness . 106
L.5 Reference documents . 107
Annex M (informative) Guidance for rock anchored foundation design . 108
M.1 General . 108
M.2 Corrosion protection . 108
M.2.1 Standard anchors . 108
M.2.2 Corrosion protection of bar anchors . 109
M.3 Product approval . 110
M.4 Rock anchor design . 110
M.5 Grout design . 110
M.6 Testing and execution . 110
M.7 Suitability/performance test . 111
M.8 Acceptance/proof test . 111
M.9 Supplementary extended creep tests . 111
M.10 Reference documents . 111
Annex N (informative) Internal loads – Explanation of internal loads . 112
Annex O (informative) Seismic load estimation for wind turbine tower and foundation . 114
O.1 General . 114
O.2 Vertical ground motion . 114
O.3 Structure model . 114
O.4 Soil amplification . 115
O.5 Time domain simulation . 116
O.6 Reference documents . 116
Annex P (informative) Structural damping ratio for the tower of wind turbine . 117
P.1 General . 117
P.2 First mode structural damping ratio . 117
P.3 Second mode structural damping ratio . 118
P.4 Higher mode damping . 118
P.5 Reference documents . 119
Annex Q (informative) Guidance on partial safety factors for geotechnical limit states . 120
Q.1 General . 120
Q.2 Equilibrium . 120
Q.3 Bearing capacity . 120
Q.4 Sliding resistance . 121
Q.5 Overall stability . 121

Q.6 Reference documents . 122
Bibliography . 123

Figure 1 – Flange notations as an example of an L-flange . 31
Figure 2 – Door opening geometry . 35
Figure 3 – Flange gaps k in the area of the tower wall . 37
Figure 4 – Bolt force as a function of wall force . 39
Figure 5 – S-N curve for detail category 36 . 40
Figure 6 – Thermal effects around tower cross-section . 44
Figure 7 – Illustration of rock anchor length . 70
Figure E.1 – Circumferentially edge-stiffened opening . 78
Figure E.2 – Definition of W and t according to JSCE . 79
s s
Figure G.1 – Simplification of system to segment model . 81
Figure G.2 – Locations of plastic hinges for different failure modes . 82
Figure G.3 – Geometric parameters . 83
Figure G.4 – Modification factor λ for different α [1] . 85
Figure G.5 – Tri-linear approximation of the non-linear relation between bolt force and
tension force of the bolted connection . 86
Figure K.1 – Example for the design of a deep beam using the strut-and-tie method . 97
Figure K.2 – Simple shapes of strut-and-tie models . 97
Figure K.3 – Three examples for carrying load in a deep beam . 98
Figure K.4 – Strut-and-tie models for a rock-anchor foundation . 101
Figure K.5 – Top tie reinforcement in a rock-anchor foundation. 101
Figure L.1 – Example stress-strain relationship for soil . 103
Figure L.2 – Loading and unloading behaviour of soil . 104
Figure L.3 – Variation of shear modulus with soil strain. 105
Figure L.4 – Reduction in rotational stiffness due to load eccentricity. 106
Figure L.5 – Illustrative example of reduction in foundation rotational stiffness due to
increasing load eccentricity . 107
Figure M.1 – Section through rock and anchor . 108
Figure M.2 – Typical anchor configuration with corrosion protection . 109
Figure N.1 – Representation of internal loads . 113
Figure O.1 – Structure model for response spectrum method . 115
Figure P.1 – First mode damping ratio for the steel tower of wind turbine . 118

Table 1 – Flange tolerances . 37
Table 2 – Summary of geotechnical limit states . 53
Table B.1 – National and regional steel standards and types . 73
Table C.1 – Comparison of bolt material in ISO 898-1, JIS B1186 and ASTM A490M-12 . 74
Table E.1 – Coefficients for Formula (E.3) . 78
[1]
Table H.1 – Limit value of crack width based on Japanese standards . 89
Table P.1 – Damping coefficients . 117
Table Q.1 – Minimum partial safety factors for the equilibrium limit state (European
and North American practice) . 120

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Table Q.2 – Minimum partial safety factors on for the equilibrium limit state (JSCE) . 120
Table Q.3 – Minimum partial material and resistance factors for the bearing resistance
limit state, ULS . 121
Table Q.4 – Minimum partial material and resistance factors for the sliding resistance
limit state, ULS . 121
Table Q.5 – Minimum partial material and resistance factors for the overall stability
limit state, ULS . 122

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 6: Tower and foundation design requirements

FOREWORD
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International Standard IEC 61400-6 has been prepared by IEC technical committee TC 88: Wind
energy generation systems.
The text of this standard is based on the following documents:
FDIS Report on voting
88/751/FDIS 88/754/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the IS
...