SIST EN IEC 61400-6:2020/A1:2025

SLOVENSKI STANDARD
01-oktober-2025
Sistemi za proizvodnjo energije na veter - 6. del: Stolp in obravnava temeljnih
zahtev - Dopolnilo A1 (IEC 61400-6:2020/AMD1:2025)
Wind energy generation systems - Part 6: Tower and foundation design requirements
(IEC 61400-6:2020/AMD1:2025)
Windenergieanlagen - Teil 6: Auslegungsanforderungen an Türme und Fundamente
(IEC 61400-6:2020/AMD1:2025)
Systèmes de génération d'énergie éolienne - Partie 6: Exigences en matière de
conception du mât et de la fondation (IEC 61400-6:2020/AMD1:2025)
Ta slovenski standard je istoveten z: EN IEC 61400-6:2020/A1:2025
ICS:
27.180 Vetrne elektrarne Wind turbine energy systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61400-6:2020/A1

NORME EUROPÉENNE
EUROPÄISCHE NORM August 2025
ICS 27.180
English Version
Wind energy generation systems - Part 6: Tower and foundation
design requirements
(IEC 61400-6:2020/AMD1:2025)
Systèmes de génération d'énergie éolienne - Partie 6: Windenergieanlagen - Teil 6: Auslegungsanforderungen an
Exigences en matière de conception du mât et de la Türme und Fundamente
fondation (IEC 61400-6:2020/AMD1:2025)
(IEC 61400-6:2020/AMD1:2025)
This amendment A1 modifies the European Standard EN IEC 61400-6:2020; it was approved by CENELEC on 2025-07-18. CENELEC
members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this amendment the
status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This amendment exists in three official versions (English, French, German). A version in any other language made by translation under the
responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as
the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61400-6:2020/A1:2025 E

European foreword
The text of document 88/1088/FDIS, future edition 1 of IEC 61400-6/AMD1, prepared by TC 88 "Wind
energy generation systems" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 61400-6:2020/A1:2025.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-08-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-08-31
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61400-6:2020/AMD1:2025 was approved by CENELEC as
a European Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Add the following references:
Publication Year Title EN/HD Year
IEC 61400-1 2019 Wind energy generation systems - Part 1: EN IEC 61400-1 2019
Design requirements
+A1 2024 +A1 —
ISO 898-1 - Mechanical properties of fasteners made of EN ISO 898-1 -
carbon steel and alloy steel - Part 1: Bolts,
screws and studs with specified property
classes - Coarse thread and fine pitch
thread
ISO 898-2 - Fasteners - Mechanical properties of EN ISO 898-2 -
fasteners made of carbon steel and alloy
steel - Part 2: Nuts with specified property
classes
ISO 898-3 - Mechanical properties of fasteners made of EN ISO 898-3 -
carbon steel and alloy steel - Part 3: Flat
washers with specified property classes
ISO 965-2 - ISO general purpose metric screw threads - -
- Tolerances – Part 2: Limits of sizes for
general purpose external and internal
screw threads - Medium quality
ISO 965-5 - ISO general purpose metric screw threads - -
- Tolerances - Part 5: Limits of sizes for
internal screw threads to mate with hot-dip
galvanized external screw threads with
maximum size of tolerance position h
before galvanizing
ISO 4759-1 - Tolerances for fasteners - Part 1: Bolts, EN ISO 4759-1 -
screws, studs and nuts - Product grades A,
B and C
ISO 4759-3 - Tolerances for fasteners - Part 3: Washers EN ISO 4759-3 -
for bolts, screws and nuts - Product grades
A, C and F
Under preparation. Stage at the time of publication: EN IEC 61400-1/prA1:2023.
IEC 61400-6 ®
Edition 1.0 2025-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
AMENDMENT 1
AMENDEMENT 1
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
ICS 27.180  ISBN 978-2-8327-0435-6

IEC 61400-6:2020-04/AMD1:2025-06(en-fr)

– 2 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 6: Tower and foundation design requirements

AMENDMENT 1
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
Amendment 1 to IEC 61400-6:2020 has been prepared by IEC technical committee TC 88: Wind
energy generation systems.
The text of this Amendment is based on the following documents:
Draft Report on voting
88/1088/FDIS 88/1096/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Amendment is English.

IEC 61400-6:2020/AMD1:2025 – 3 –
© IEC 2025
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications/.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
___________
INTRODUCTION
Clauses and subclauses as given in this document are replacing or amending the respective
clauses and subclauses of IEC 61400-6:2020. The main part of this amendment concerns
updated knowledge for the design of L-flanges and modifications required due to changes to
IEC 61400-1.
The previous method of fatigue assessment using the Schmidt/Neuper trilinear bolt force curve
approximation has been removed as the default method from the document. It has been
replaced with a physically more accurate method.
The updated methodology for fatigue assessment of L-flanges has been calibrated so that the
target failure probability defined in IEC 61400-1 is achieved. Where existing flange designs are
checked with the updated method, over-utilization can be found, which in some cases can show
an order of magnitude higher than nominally acceptable damage.
This does not impose an immediate risk for the turbines affected, though, due to the following
factors:
a) in most cases, such designs have significant conservatism in the fatigue loads assumed,
e.g. due to the assumption of uni-directional wind combined with type class turbulence
conditions,
b) experience shows that broken bolts are almost always found and replaced before a turbine
collapses.
It is not necessary to re-assess existing flange designs using the new method. In cases where
broken bolts are found in operating turbines, the affected flange should be checked with the
new methodology. Based on the assessment results and the root causes analysis for the failure,
further measures should be defined (e.g. shorter inspection intervals).
2 Normative references
Add the following new references to the existing list:
IEC 61400-1:2019/AMD1:2024
ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel – Part 1:
Bolts, screws and studs with specified property classes – Coarse thread and fine pitch thread

– 4 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
ISO 898-2, Fasteners – Mechanical properties of fasteners made of carbon steel and alloy steel
– Part 2: Nuts with specified property classes
ISO 898-3, Mechanical properties of fasteners made of carbon steel and alloy steel – Part 3:
Flat washers with specified property classes
ISO 965-2, ISO general purpose metric screw threads – Tolerances – Part 2: Limits of sizes for
general purpose external and internal threads – Medium tolerance quality
ISO 965-5, ISO general purpose metric screw threads –Tolerances – Part 5: Limits of sizes for
internal screw threads to mate with hot-dip galvanized external screw threads with maximum
size of tolerance position h before galvanizing
ISO 4759-1, Tolerances for fasteners – Part 1: Bolts, screws, studs and nuts – Product grades
A, B and C
ISO 4759-3, Tolerances for fasteners – Part 3: Washers for bolts, screws and nuts – Product
grades A, C and F
3 Terms and definitions
Add, after 3.42, the following new terms and definitions:
3.43
bolt assembly
fastener, nut(s), optionally washer(s), preloading method and lubrication system
EXAMPLE A stud assembly for tension-tightening can comprise a stud and one roundnut on each side, without
additional washers.
3.44
design gap height
k
design
95 % fractile value of the log-normal distribution defined by k and COV
mean k
Note 1 to entry: See 6.7.5.2.
3.45
unloaded gap height limit
k
limit,unloaded
allowable maximum gap height after mating of flanges, without influence of loading by dead
weight of tower section(s) above the flange or preload of bolts, determined such that the
calculated damage for the bolts does not exceed the allowable damage as given in 6.7.5.4
3.46
loaded gap height limit
k
limit,loaded
allowable maximum gap height after mating of flanges, and after application of for example
dead weight of tower section(s) above the flange and/or partial preload of bolts, determined
such that the calculated damage for the bolts does not exceed the allowable damage as given
in 6.7.5.4
3.47
flatness deviation of individual flange
u
tol
allowable flatness deviation as defined in 6.7.3.1 for the individual flange

IEC 61400-6:2020/AMD1:2025 – 5 –
© IEC 2025
4 Symbols and abbreviated terms
4.1 Symbols
Add the following symbols to the existing list:
a flange dimension (nominal distance from inside of flange to bolt circle diameter)
A nominal area of the bolt shaft with diameter d
a* auxiliary value to compute bolt bending moment
a' reduced effective flange dimension according to Tobinaga/Ishihara
A flange cross section area in circumferential direction
cf
A nominal stress area of the bolt in thread
S
b weld neck thickness (normally equal to the thickness of the connected tower
shell) (in 6.3.2.3 only)
b flange dimension (nominal distance from bolt circle diameter to middle surface
of connected tower shell)
b' distance in between plastic hinges for failure modes B, D, E
{B,D,E}
c flank height of the weld preparation (in 6.3.2.3 only)
c segment width measured at the middle surface of the shell (tower wall)
C stiffness of the compression spring (representing the compressed parts)
D
COV coefficient of variation
COV coefficient of variation of gap height
k
COV coefficient of variation of preload force
p
C stiffness of the tension spring (representing the bolts)
S
d nominal diameter of the bolt
D outer diameter of the tower shell
D auxiliary values to determine coefficients for bolt force polynomial
{1,2}
d diameter of the bolt hole
b
DFT dry film thickness (DFT) of coatings applied to the flange surface beneath
sbw
washers (sbw), i.e in the contact area between washers and flange
D outside diameter of the washer
w
E Young's modulus of steel
F * preload bolt force used for modified torque method
p,C
F ' preload bolt force used in the design calculations (design preload)
p,C
F mean preload force after installation
p,inst.,mean
F design preload averaged over 5 bolts after installation
p,mean
F bolt force
S
F (Z) bolt force as a function of external force Z applied on flange segment
S
F bolt forces for determination of polynomial bolt force model
S,{0,1,2,3}
F minimum (constant) bolt force for theoretically fully closed connection under
S,min
compression
F bolt force used to verify preload loss criterion
S,loss
– 6 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
F '(Z) slope (derivative) of bolt force curve as a function of external force Z on flange
S
segment
F design value of tension resistance of bolt
t,R
F limit tension resistance for failure mode A
u,A
F limit tension resistance for failure mode B
u,B
F limit tension resistance for failure mode D
u,D
F limit tension resistance for failure mode E
u,E
f ultimate tensile strength of bolt
ub
F preload
V
F preload force at which edge contact occurs
V,c
f nominal yield limit of the bolt material
yb
f total amount of settlement in the connection
Z,tot
f multiplication factor used to calculate total gap stiffness
tot
G shear modulus of steel
G dead weight of the RNA
RNA
G dead weight of tower above flange connection considered
twr
h flange neck height
n
h distance from flange surface to weld preparation
wp
h distance from flange surface to weld toe
wt
I flange moment of inertia for a bending moment vector pointing in radial direction
cf
(bending in circumferential direction)
I flange moment of inertia for a bending moment vector pointing in tangential
tg
direction (bending in tangential direction)
k flange gap height
k(l) gap height at position l of total gap length L
gap
k design gap height
design
k stiffness factor to calculate meridional shell stiffness
fac
k bending stiffness of the flange
fl
k total gap stiffness
gap,tot
k gap height after application of a defined load
limit,loaded
k gap height after mating of flanges without any load
limit,unloaded
k mean gap height
mean
k measured gap height
measured
k segment stiffness
seg
k meridional stiffness of the shell / initial shell stiffness
shell,ini
K shell parameter
l distance from transition radius to weld preparation (in 6.3.2.3 only)
l length of the bolt between the bolt head and the nut
L circumferential length measured at mid surface of shell over 30° sector
30°
IEC 61400-6:2020/AMD1:2025 – 7 –
© IEC 2025
l = L spanning length of the gap, measured on the outside diameter of the tower wall
k gap
connected to the flange
M external bending moment
m slope parameter of a fatigue resistance curve
M bending moment at ZZ=Δ
0 dw
M mean value of entry i in the Markov matrix
mean,i
M plastic limit bending moment for flange or shell
pl,3
M plastic limit bending moment for shell
pl,Bl
M plastic limit bending moment for flange
pl,Fl
M plastic limit bending moment for shell, including interaction with external tension
pl,N,Bl
force N
M plastic limit bending moment for flange, including interaction with shear force V
pl,V,Fl
M bending moment used to calculate bolt force for preload loss check
loss
M bolt moment
S
M (Z) bolt moment curve as function of external force Z
S
M minimum bending moment for theoretically fully closed connection under
S,min
compression
N number of cycles
n shell parameter
n number of bolts (in case of an L-flange) or bolt pairs (in case of a T-flange)
bolts
N plastic limit normal force for shell
pl,Bl
p load factor of the tension springs
p 95 % quantile of the log-normal distribution
R mean radius of flange body
Fl
R outer radius of shell (tower wall)
shell
s shell (tower wall) thickness
t flange thickness
t thickness of the washer
w
u auxiliary displacement value for computation of flange segment stiffness
u flatness tolerance for individual flange
tol
u flatness tolerance per flange over a circumferential length of 1 000 mm
tol,1m
u flatness tolerance per flange over 30° sector
tol,30°
u flatness tolerance per flange around the entire circumference
tol,360°
V shear force in flange
V plastic limit shear force
pl,Fl
w flange width
W section modulus of the tower with outer diameter D and wall thickness s
twr
Z tower shell force (external force on the segment)
Z force values to construct tower bolt force model
{0,1,2,3}
Z force at which the connection is theoretically fully closed
close
– 8 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
Z maximum segment force from the Markov matrix
max,FLS
Z total segment force
tot
Z (M) total segment force as a function of external bending moment M
tot
α auxiliary values to determine polynomial coefficients
{0,1,2}
α circumferential angle of the gap
gap
α stiffness correction factor
k
α flange surface inclination (taper)
S
bending resilience of bolt according to VDI 2230 [5]
β
S
δ resilience of the clamped parts
P
δ resilience of the bolt
S
∆F expected reduction of preload force due plastic strain development
pl
∆F expected reduction of preload force due to settlements
Z
∆Z range of external force applied to flange segment
∆Z segment force resulting from the dead weight
dw
∆Z force for theoretical closure of flange gap due to shell and flange stiffness
gap
∆Z total force for theoretical closure of flange gap, including contributions from
gap,tot
flange inclination and reduced efficiency due to edge contact
∆Z reduced efficiency of the preload due to early prying at the edges (for T-flanges
gap,c
only)
∆Z force required to close initial flange inclination (taper)
gap,inclination
α relative inclination of the outer flange surfaces after preloading
∆
∆σ combined stress range
∆σ stress range from axial forces in the bolt
axial
∆σ stress range from bending moments in the bolt
bending
∆σ reference stress range of resistance S-N-curve
c
λ ' lever arm ratio taking the action point correction into account
κ reduction factor for bolt bending stresses
b
µ mean value of log-normal distribution for gap heights
k
v Poisson's ratio
σ standard deviation of log-normal distribution for gap heights
k
χ slope of bending moment function
ini,M
χ modified initial slope of the polynomial approximation
ini,mod
χ true slope of the polynomial approximation
ini,true
γ
M1 partial safety factor (PSF) for stability

IEC 61400-6:2020/AMD1:2025 – 9 –
© IEC 2025
4.2 Abbreviated terms
Add the following abbreviated terms to the existing list:
2K-PUR 2 Component Polyurethane
BTQP bolt tightening qualification procedure
COV coefficient of variation
DFT dry film thickness
EP epoxy
FEA finite element analysis
FLS fatigue limit state
HV high-strength bolts intended for preloading (German designation for "Hochfest
vorgespannt")
PUR polyurethane
RNA rotor nacelle assembly
SCF stress concentration factor
TSM thermal spray metallizing
ULS ultimate limit state
6.3.3 Bolts and anchors
Replace subclause 6.3.3 with the following:
In this document, the term "bolts" is used for the fastener elements. Instead of (head) bolts,
also partially or fully threaded studs with nuts on both ends may be used.
Standardized bolt assemblies should be used.
Bolt assemblies consisting of either modified components (fastener, nut, washer(s)), preloading
method and/or lubrication system may alternatively be specified. It shall be evaluated whether
the design principles and methodologies presented in this standard can still be applied to the
chosen bolt assembly.
Fasteners as specified in [15] may be used.
Design codes can require selection of bolt assemblies compliant to local standards defining the
product characteristics of the bolt assembly. A comparison of local design codes and industry
design guideline practice can be found in Annex A and Annex C.
The material property class for bolt assemblies and anchors shall comply with the requirements
stated in ISO 898-1, ISO 898-2 and ISO 898-3. Material properties for bolt sets with metric sizes
larger than M39 shall account for the influence of size effects and manufacturing methods. For
large bolt sizes the test method may be adapted.
NOTE 1 Different materials and manufacturing methods are used for large diameters. ISO 898 testing is obtained
directly on the fastener or on machined test pieces with maximum diameter reduction 25 %. For large bolt sizes,
further adaption is suggested as the test specimen can be too large to be conveniently tested when following the
standard ISO procedures.
For preloaded connections, only bolt sets with either property class 8.8 or 10.9 should be
selected. Where higher grades are considered, fatigue performance, tensile strength and strain
and sensitivity to hydrogen embrittlement shall be evaluated by testing.

– 10 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
NOTE 2 Risk of hydrogen embrittlement is significantly increased above grade 10.9, and currently no standardized
test methods are available to reliably mitigate this risk.
The product characteristics of bolt assemblies shall be obtained through type testing on at least
5 samples for each required characteristic under responsibility of the bolt manufacturer.
The type testing program shall include at least:
a) material properties as per ISO 898-1, ISO 898-2 and ISO 898-3 as applicable,
b) product grade as per ISO 4759-1, ISO 4759-3, ISO 965-2 and ISO 965-5,
Type testing should be repeated in case of different nominal diameters, manufacturing methods,
material property class, coating type, type and source of material.
Bolting assemblies shall comply with the requirements set for the serial production for each
applied bolting assembly lot (e.g. analogous to the procedures according to EN 14399-1).
Suitability for preloading according to 6.7.3.2 shall be documented.
6.5.2 Partial safety factor
Replace subclause 6.5.2, including footnote 8, with the following:
Partial safety factors shall be chosen based on the applied verification standard and method.
When using EN 1993-1-6, γ = 1,1 should be used.
M1
Only when using the modified expressions for meridional buckling (D.1.2.2) in accordance with
EN 1993-1-6:2007/AMD1:2017, γ = 1,2 shall be used.
M1
6.7 Ring flange connections
Replace subclause 6.7 with the following:
6.7.1 General
The regulations stated in 6.7 are valid for both L- and T-steel-to-steel flange connections,
excluding the tower top flange. For the tower top flange, see 6.4.5.2.
6.7.2 Design assumptions and requirements
Ring flange connections shall be tightened in a controlled manner in several steps in
accordance with the requirements given in 6.7.3.2.
The fatigue assessment shall be based on the non-linear bolt force function F = f(Z) and bolt
S
moment function M = f(Z) from which the fatigue ranges of the bolt force F and bolt moment
S S
M can be read off for a given range of the tower shell force Z (see Figure 8 with illustration for
S
the bolt force).
NOTE Figure 8 shows two different examples of bolt force curves for flanges with imperfections. Different
imperfections (e.g. different gap height or length) result in different bolt force curves. Hence, there is no unique bolt
force curve with imperfections.

IEC 61400-6:2020/AMD1:2025 – 11 –
© IEC 2025
Figure 8 – Bolt force F as a function of external force Z (including dead weight)
S
The stiffness of the flange connection may be assumed as equivalent to the stiffness of the
tower shell, and it is not necessary to consider the negative effect from flange opening, e.g. for
load simulations or calculation of natural frequencies.
6.7.3 Execution of ring flanges
6.7.3.1 Flange tolerances
The tolerances for flange flatness and (if applicable) allowable gaps during installation shall be
stated in the drawings or manufacturing specifications and/or installation manuals.
NOTE 1 Flange gaps k in the tower wall region are causing increased fatigue loading of the bolts. Gaps are identified
as "parallel gaps" when they occur around part of the circumference with the mating surfaces of the upper and lower
flange being parallel to each other. Gaps are identified as "angular gaps" when the mating surfaces are not parallel
to each other. Both types can be combined, i.e., a parallel gap can occur on top of an angular gap, see Figure 10.
The damage influence of parallel gaps grows with decreasing spanning length l over the circumference of the flange
k
for a constant gap height k. Parallel gaps with circumferential lengths within ~30° to 120° of the circumference have
the largest damaging effect, as the (design) gap height k increases with gap length l . To ensure fatigue strength of
k
the connection, it is important to limit the size of the gaps in terms of height and length, so that gaps are sufficiently
closed and the pressure body in the flange is created in accordance with requirements for fatigue assessment.

1 2 3 4
Figure 9 – Flange gaps with gap height k and gap length l at the tower wall and
k
flange surface inclination α
S
– 12 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
Figure 10 – Illustration of parallel gaps and angular gaps
Following completion of the production of the individual tower sections, the taper to the inside
of the connecting surface of each flange (see Figure 9b) and Figure 9c)) shall be checked and
should be within the limits specified in Table 3.
For L-flanges, a positive taper (opening to the inside, see Figure 9b)) should be specified.
Negative taper (Figure 9a) and Figure 9d)) is not permitted.
If required in accordance with 6.7.3.3, gap limits as specified by the designer shall be checked
during installation, noting that the region near to the tower wall is decisive.
Following completion of the production, flatness measurements should be performed with the
following procedure.
a) Measurement points in the region of the tower wall, i.e. on the outside of the flange (close
to point II as shown in Figure 16) for L-flanges and in the centre of the flange for T-flanges,
shall be measured with a maximum distance of 500 mm, evaluated on a circumferential
path.
b) A best-fit plane shall be determined, resulting in the smallest mean square error for the
distance of the individual measurement points to the best-fit plane.
c) For determining the taper, additional points on the inside (for L-flanges) and on inside and
outside (for T-flanges) shall be measured.
d) The tolerances in accordance with Table 3 shall be evaluated.
The starting point of measurements and the direction shall be defined by the manufacturer and
documented in the as-built documentation.
NOTE 2 It is useful that measurements are done clockwise starting from a defined starting position ("zero mark").
This ensures that analytical mating of flanges from different manufacturers can be done without uncertainty about
the measurement sequence.
NOTE 3 The global flatness u is the largest flatness difference between two measurement points along the
tol,360°
entire circumference and local flatness u is the largest flatness difference between two measurement points
tol,1m
within a maximum distance of 1 000 mm.
NOTE 4 When using a distance between measurement points of 500 mm, three points at 0 mm, 500 mm, 1 000 mm
are evaluated and the distance between any two of those three points is decisive.
NOTE 5 An example is given in Figure 11, where the evaluation returns a global flatness value of 2,2 mm and a
local flatness value of 1,16 mm.
NOTE 6 Different terminology is used in the industry for flatness tolerances. Instead of "flatness", the term
"waviness" is also used. This tolerance definition can differ from tolerance definitions in other standards using the
same terminology. The definition described in detail in 6.7.3.1 is governing for wind turbine structures and other
definitions do not apply.
IEC 61400-6:2020/AMD1:2025 – 13 –
© IEC 2025
NOTE 7 The relation between flatness values of the individual flange, as specified in Table 3, and the resulting gap
height k between two flanges is illustrated in Figure 12 for clarity. Flatness values u are evaluated based on the
tol
individual flanges, the upper left chart in Figure 12 shows flatness measurements for two flanges of a connection.
NOTE 8 The resulting gap height k between two flanges in the unloaded condition is determined by a mating
analysis, where the contact points of the flanges are established. The contact between the two flanges is found by
both translations and rotations, so that the resulting gap heights k cannot be determined directly from the flatness
protocols. Further information on the required steps is provided in [10]. Acceptable gaps are not directly linked to the
flatness tolerances due to the statistical effect when mating two flange surfaces. It is incorrect to assume that
allowable gaps are twice the flatness tolerance values [10].
Table 3 – Flange tolerances
Characteristic Limiting value
Flatness deviation u over a circumferential length of 1 000 mm, using a minimum See 6.7.5.2
tol,1m
of 3 measurement points with a maximum distance of 500 mm
Flatness deviation u over a circumferential angle of 30° See 6.7.5.2
tol,30°
Flatness deviation u around the entire circumference See 6.7.5.2
tol,360°
Taper α to the inside of the connecting surface of each flange From 0,0° to 0,7°
s
During factory acceptance testing of the tower sections, u and u shall be evaluated
tol,1m tol,360°
for all diameters. u should be evaluated in addition for D ≥ 6,0 m.
tol,30°
Where shimming is part of the design, the minimum taper requirement should be taken as 0,2°.
NOTE 9 This is to ensure that the inside gap is not closing at the preload level applied when checking for gaps.
Additionally, an inside taper positively influences the bolt force curve.

The x-axis in the middle of the plot shows angle in [°] and the axis shown in the bottom part shows lengths in [mm]
around the circumference.
Figure 11 – Example for flatness measurement evaluation (D = 6 000 mm)

– 14 – IEC 61400-6:2020/AMD1:2025
© IEC 2025
Figure 12 – Clarification of flatness values for the individual flange (u ) and resulting
tol
gap height after mating of two flanges (k)
6.7.3.2 Preloading
For preloading of tower flange bolts, torque-tightening or tension-tightening procedures may be
applied.
A bolt tightening qualification procedure (BTQP) for a specific configuration of bolting assembly
shall be developed to determine tightening parameters and the corresponding mean value as
well as the COV (see 6.7.5.1). In the BTQP, a specific configuration is defined as:
a) one single bolting assembly type (defined by fastener, nut and washer),
b) one type of lubricant (if applicable),
c) one tightening method.
An example of a BTQP may be found in e.g. ISO 17607-6:2023, Annex N [19].
The effect of bolt size and clamp length to the bolt diameter ratio should be appropriately
considered in the development of the BTQP.
Clause C.3 may be used for HV bolts.
The bolt manufacturer shall establish procedures to ensure that the characteristics of the
products considered by the BTQP are maintained during serial production for each applied
bolting assembly lot (e.g. analogous to the procedures according to EN 14399-1 [20]).
Verification of friction during serial production for each applied bolting assembly lot is not
required for tension tightening.

IEC 61400-6:2020/AMD1:2025 – 15 –
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Alternatively, on-site or laboratory test methods may be defined to verify that the bolting
assemblies and installation procedures will perform as required. Herein, testing (e.g. according
to EN 1090-2:2018 [22], Annex H) may be carried out. In such case, at least five representative
samples from each applied bolting assembly lot shall be tested.
NOTE 1 Where tension tightening tools are used that automatically check and record process parameters such as
pressure and nut rotation angle or direct length measurement, this can be demonstrated as being sufficient to verify
correct preloading.
When tension-tightening is used, nuts with proven suitability shall be used on the tensioned
side (e.g. flange nuts, also known as roundnuts).
At least two-stage tightening shall be performed.
NOTE 2 Two-stage tightening means that it is important to have two complete rounds of tightening to compensate
for the loss of preload induced by sequential tightening. Each stage is understood as a complete sequential tightening
of all bolts on the 360° circle, where using multiple tools at different positions of the circumference (e.g. spaced 180°
apart) at the same time is allowed. Typically, bolts are initially torqued with a low value of ~10 % of nominal torque.
This is not counted as an additional stage.
The following minimum requirements apply for torque-tightened bolts:
a) Minimum 50 % and maximum 75 % of nominal torque should be applied for the first stage
and 100 % of the nominal torque for the second (final) stage.
b) Torque-tools should undergo suitability testing demonstrating that the tightening process
reliably achieves target preload levels.
NOTE 3 A lower torque value is suggested for first stage to ensure that the break-away torque is overcome in the
second stage.
NOTE 4 Suitability testing for torque tools is important because achieving the target torque value alone is not
sufficient. This is for example the case when tools are applying the torque very fast and shut down immediately after
reaching nominal torque. In that case, time is not sufficient to develop the nut rotation required for the target preload
to be achieved.
The following minimum requirements apply for tension-tightened bolts:
a) 100 % of the installation preload should be applied on each stage.
b) For the second stage, multiple pulls should be executed until the residual nut rotation angle
is small; unless proven otherwise, a differential angle of 5° may be used as the criterion to
end the process. Alternatively, differential direct length measurements may be used.
c) Free rotation of the nuts under the installation tension should be ensured and the turn-down
torque shall be high enough to overcome potential resistance (e.g. from impurities in hot-
dip galvanisation).
For both torque-tightened and tension-tightened bolts, procedures should be used to document
nut rotation angles and/or direct length measurements including acceptance levels for the
second stage. Acceptance levels should be defined in the BTQP.
For all cases, re-tightening of bolts after installation has been completed should be done.
Where tension-tightening is used and the reduction of calculated settlement values by 50 % as
per 6.7.5.1.2 is applied, re-tightening shall be done after minimum 240 power production hours
but, in any case, not later than six months after commissioning.
Otherwise, or where torque-tightening is used, re-tightening may also be done earlier but, in
any case, not earlier than 72 h after installation.

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6.7.3.3 Gap height verification during installation
Assessment of the gap heights and lengths during the assembly phase may be omitted if the
design gap height according to 6.7.5.2 is used.
Design gap heights may also be derived from a database of relevant flange geometries, based
on the same principles as used in [17].
If a mating analysis, based on flatness measurement protocols, demonstrates that gap heights
expected when mating actual flanges are smaller than the unloaded gap height limit
, shimming may generally be omitted. The evaluation methodology for determining
k
limit,unloaded
the resulting gap profile should follow the approach specified in [17].
Alternatively, methods shall be used during execution which ensure that the pressure body is
created in the flange in accordance with requirements for fatigue assessment.
Shimming may be used as a proven method. Where shimming is required to achieve target
fatigue life, the following conditions apply:
a) The unloaded gap height limit k shall be specified by the designer of the
limit,unloaded
connection as a function of gap angle or gap length.
b) For the assessment of the gaps during the assembly phase, the additional influence of self-
weight and preload shall be considered to derive the loaded gap height limit k , see
limit,loaded
Figure 13. The number of preloaded bolts and initial preload at this stage shall be specified
by the designer.
c) The gap heights and lengths shall be checked using suitable tools, e.g. feeler gauges.
Considering the requirement for a positive flange taper (see 6.7.3.1) the gap on the outside
(below or close to the tower wall) shall be measured and evaluated with respect to
k .
limit,loaded
d) Where the measured gap height is k > k , the gap shall be filled with the
measured limit,loaded
largest possible shim plate thickness at each position, see Figure 14.
e) Thickness increments for the shim plates should not exceed 0,5 mm.
f) Multiple shim plates may be stacked, but the maximum number of shim plates at each
position should not exceed 3.
g) Inside gaps shall not be shimmed with higher shim plate thickness as used in the area below
the tower wall, meaning that shimming shall not be done in a way that earlier inside contact
is provoked.
h) The shims or filler material should have sufficient modulus of elasticity and compressive
strength (yield point under compression) to replicate the effect of the parent flange material.
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