ISO/FDIS 4354

INTERNATIONAL STANDARD
ISO/DISFDIS 4354:2025(en)
Revised Version of
2026-02-17
ISO/TC 98/SC 3
Secretariat: JISC
Date: 2026-09-22
Wind actions on structures
Actions du vent sur les structures
FDIS stage
ISO/DISFDIS 4354:20252026(en)
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ii
ISO/DISFDIS 4354:20252026(en)
Contents Page
Foreword . v
Introduction . vii
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Symbols . 1
4 Wind actions . 2
5 Wind pressure . 3
6 Wind force . 3
7 Site peak dynamic pressure . 7
8 Exposure factor . 8
9 Pressure and force coefficients . 8
10 Dynamic response factor . 8
11 Criterion for aeroelastic instability . 9
12 Methods of determination of wind loads . 9
Annex A (informative) Determination of reference wind speed . 10
Annex B (informative) Determination of exposure factors . 12
Annex C (informative) Aerodynamic pressure and force coefficients . 24
Annex D (informative) Dynamic response factors . 37
Annex E (informative) Structures subject to critical excitation vortex resonance and
aeroelastic instability . 55
Annex F (informative) Wind load effect combinations . 58
Annex G (informative) Wind tunnel testing . 60
Annex H (informative) Computation-based methods . 61
Annex I (informative) Reliability considerations . 62
Bibliography . 64

iii
ISO/DISFDIS 4354:20252026(en)
Foreword
iv
ISO/DISFDIS 4354:20252026(en)
Foreword
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documentdocuments should be noted. This document was drafted in accordance
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 98, Bases for design of structures,
Subcommittee SC 3, Loads, forces and other actions.
This third edition cancels and replaces the second edition (ISO 4354:2009), which has been technically
revised.
The main changes are as follows:
— flowcharts were introduced to clarify the procedures for evaluating wind actions on cladding/local
areas and on the whole structure;
— the document was revised to focus primarily on the peak wind speed methodology while maintaining
procedures for conversion between different averaging times;
— provisions related to the mean wind speed methodology were removed to improve clarity and avoid
duplication;
— Annex A (Determination of reference wind speed), Annex B (Determination of exposure factors),
Annex C (Aerodynamic pressure and force coefficients) and Annex D (Dynamic response factors)
were substantially revised;
v
ISO/DISFDIS 4354:20252026(en)
— Annex H (Computation-based methods) was fully revised to reflect current wind engineering
practice;
— editorial revisions were implemented throughout the document to improve readability and
consistency.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
vi
ISO/DISFDIS 4354:20252026(en)
Introduction
This document is intended for use by countries lacking an adequate wind loading standard and as a
reference to bridge between existing International Standards related to wind actions on structures.
This document is based primarily on the peak wind speed methodology using a 3-second gust wind speed
measured at a height of 10 m over open country terrain. Procedures for conversion between different
wind-speed averaging times are also provided. The analytical procedures can be applied to other
averaging times with appropriate adjustment of the relevant parameters, as described in Annex A.
Parameters without a specified averaging time are considered to correspond to a peak value with T = 3 s.
Analytical methods are provided for the determination of design wind pressures, forces and associated
structural effects, including along-wind response, across-wind response and wind directionality effects.
For structures with complex geometry, unusual size, significant dynamic sensitivity, or complex
aerodynamic behaviour, specialized engineering assessment can be necessary. Such assessment can
include wind tunnel testing or computational fluid dynamics.
The data provided in the annexes are formally informative and limited to the most common applications
but are intended to be used within the definitions of this document. Additional data will be periodically
provided in ISO Technical Reports for use on the same basis.
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FINAL DRAFT International Standard ISO/DISFDIS 4354:20252026(en)

Wind actions on structures
1 Scope
This document describes the actions of wind on structures and specifies methods for calculating the
characteristic values of wind loads for use in the design of buildings, towers, chimneys, bridges and other
structures, as well as their components and appendages.
The provisions of this document are compatible with ISO 2394 and other International Standards related
to wind loads.
The document provides analytical methods for determining wind loading, including the evaluation of
design pressures, along-wind and across-wind forces, moments and wind directionality effects for
structures of simple shapes and surroundings.
For structures with complex shapes, unusual dynamic behaviour, or significant aerodynamic sensitivity
(e.g. tall buildings, long span bridges, large span roofs, guyed masts, offshore and moving structures), or
complex wind directionality effects, wind tunnel tests, computation-based methods, or other specialized
studies can be used.
This document covers the three main storm types: synoptic winds, thunderstorms and tropical cyclones
(hurricanes and typhoons).
2 Normative references
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.
ISO 2394, General principles on reliability for structures
There are no normative references in this document.
3 Terms, definitions and symbols
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1 Symbols
For the purposes of this document, the following symbols apply.
Symbol Term Unit
A tributary or local area (area of application of pressure coefficient Cp) m
Aref reference area for force on overall structure or part of structure m
ISO/DISFDIS 4354:20252026(en)
Cdyn peak dynamic response factor 1
C peak exposure factor 1
exp
C force coefficient 1
F
Cp pressure coefficient (time and spatially averaged) 1
CσF standard deviation force coefficient 1
F peak force N
F peak force on a tributary or local area N
loc
g peak factor 1
gv wind speed peak factor 1
h mean roof height m
I wind speed turbulence intensity 1
v
−2
p pressure Nm
−2
qsite site peak dynamic pressure Nm
−1
V peak wind speed ms
−1
V critical wind speed at the top of the structure ms
hcr
−1
V mean wind speed ms
m
−1
Vref regional peak reference wind speed (with return period) ms
−1
Vsite site peak velocity ms
ρ air density kgm
σ standard deviation of force N
F
4 Wind actions
Wind actions considered in the design of structures can produce the following effects:
a) excessive forces or instability in the structure or its structural members or elements;
b) excessive deflection or distortion of the structure or its elements;
c) repeated dynamic loading causing fatigue of structural elements;
d) aeroelastic instability, where the motion of the structure in the wind produces aerodynamic forces
that augment the motion;
e) excessive dynamic movements causing concern or discomfort to occupants or onlookers;
Additionally, interference effects from existing and potential future buildings shall be considered, as
these can amplify the aforementioned impacts.
NOTE Wind pressures and forces provided in this document represent equivalent static wind loads, not pure
external excitations. These equivalent static wind loads are based on the assumption of linear elastic structural
behaviour and can require special consideration when applied to designs in the plastic region.
ISO/DISFDIS 4354:20252026(en)
5 Wind pressure
For the actions referred to in Clause 4 a), b), c) and e), the effective wind pressure, p, shall be determined
using a relationship that incorporates the site dynamic pressure, q , as defined in Clause 7 and Clause 8,
site
a pressure coefficient, C , and a dynamic response factor, C . This relationship is represented by
p dyn
Formula (1):
p = q × C × C (1)
site p dyn
The wind pressure is assumed to act statically in a direction normal to the surface of the structure or
element, unless tangential frictional forces are specifically identified. Both internal and external
pressures shall be considered. Integration of these pressures shall be undertaken to determine net forces
or forces for defined tributary areas. Wind effects from all directions shall be considered.
6 Wind force
For some structures, it maycan be appropriate to represent the wind forces, F, by their resultants. These
resultants shall include along-wind (drag), across-wind (lift), torsional and overturning actions. Different
magnitudes and distributions of the wind force maycan be necessary to evaluate the actions described in
Clause 4 a), b), c) and e).
The derivation of effective wind forces on an element, or resultant forces and moments, shall be
determined using the peak reference dynamic pressure method. This method assumes that dynamic
effects can be represented by a maximum or peak loading effect based on a peak reference pressure
combined with a mean pressure coefficient (or mean pressure coefficient modified for local effects
related to the area of application and statistical characteristics), and a peak dynamic response factor, C .
dyn
The general relationships are given in Formulae (2) and (3):
F = q × C × C × A (2)
loc site p dyn
F = q × C × C × A (3)
site F dyn ref
Formula (2) is used for the force on a tributary or local area, A. Formula (3) is used for the total force on
the whole structure or part of the structure. A and A are the area of application and projected area
ref
normal to the wind direction of the building at height z above the ground, respectively. The value of C
dyn
may be taken as 1,0, except where the structure is dynamically wind-sensitive, as described in Annex D.
In many cases, the total loads on the whole structure will be determined from loads on various
components, or facades, or from along-wind and across-wind components. These forces contribute
simultaneously but are not usually well correlated. Methods for determining load combinations are given
in Annex F.
When using the dynamic method, the distribution of equivalent static wind forces should include not only
the mean and fluctuating (background) wind forces acting on the structure’s exterior but also the inertial
forces due to motions of the structure’s mass.
Figures 1 and 2 illustrates the procedures for determining wind actions on local areas and the whole
structure, as discussed in this clause.
ISO/DISFDIS 4354:20252026(en)
ISO/DISFDIS 4354:20252026(en)
Figure 1 — Flow chart of wind actions on cladding or local areas throughout the structure
ISO/DISFDIS 4354:20252026(en)
ISO/DISFDIS 4354:20252026(en)
Figure 2 — Flow chart of wind actions on the whole structure
7 Site peak dynamic pressure
The site peak dynamic pressure, q , shall be determined from the regionally derived reference wind
site
speed, V , along with appropriate exposure factors, C , related to wind speed for the site, as defined by
ref exp
Formula (4):
q = 0,5 × (ρ(V ) (4)
site site
where ρ is the air density.
In Formula (4), theThe peak design wind speed at the site, V , adjusted for local exposure conditions, is
site
given by Formula (5):
V = V C (5)
site ref exp
where the exposure factor is determined as described in Clause 8.
The reference wind speed, typically specified for large geographical areas, serves as a baseline for
adjusting to local conditions at the site where the structure is located. It refers to a standard exposure
(i.e. roughness, height and topography), averaging time and the probability of exceedance over one year
(which can be approximated by an average return period for design application as required from
serviceability to ultimate limit state determinations). In some situations, the reference wind speed can
vary with direction.
ISO/DISFDIS 4354:20252026(en)
In the annexes of this document, the standard exposure is defined at 10 m height in open country terrain,
with q isbeing based on a maximum 3-second mean gust wind speed, V . Analysis procedures and
site ref
values are provided in Annex A and Annex B.
In certain cases, critical loading can occur at wind speeds differing from, and potentially lower than, those
specified above (e.g. due to vortex shedding). These critical wind speeds, denoted V (with reference to
hcr
height h) shall be substituted for V . These cases are discussed in Annex D.
site
8 Exposure factor
The exposure factor, C , related to wind speed, accounts for the variability of the wind speed at the site
exp
of the structure for each storm type. It considers the following factors:
a) height above ground level,;
b) roughness of the terrain (including change of roughness), and);
c) topography.
Values of the exposure factor are provided in Annex B and may vary with wind direction. Further
guidance and requirements on the application of directional design wind speeds isare also given in
Annex B.
9 Pressure and force coefficients
A pressure coefficient, C , is an aerodynamic wind-induced pressure expressed as a fraction of the
p
reference wind pressure. A force coefficient, C , is an aerodynamic wind-induced force expressed as a
F
ratio of the aerodynamic force exerted on a structure or its parts to the reference wind pressure
multiplied by a reference area.
Both pressure and force coefficients are influenced by the shape of the structure, exposure conditions,
relative wind direction, Reynolds number and averaging time. Values for pressure and force coefficients
are provided in Annex C. These values are presented in tables as non-simultaneous values for the design
of cladding or parts of the structure, and in figures as simultaneous distributions for the design of the
load-bearing structure.
Enclosed structures are subjected to internal pressures, which depend on the size and distribution of the
openings in the building envelope and any pressurization, whether mechanical or otherwise. These
internal pressures shall be considered by combining pressure coefficients for the external pressures with
those for the internal pressures.
Pressure and force coefficients can be determined from one of the following sources:
a) Annex C;
b) appropriate wind tunnel tests, as described in Annex G;
c) appropriate computationally based data, as described in Annex H;
d) other codes or standards, provided that any discrepancies (e.g. in averaging time and exposure
conditions) are properly adjusted and adequate provision is made for a dynamic response factor.
10 Dynamic response factor
The dynamic response factor, C , accounts for the following actions of the wind:
dyn
ISO/DISFDIS 4354:20252026(en)
a) fluctuating pressures due to random wind gusts acting over part or all of the surface area of the
structure;
b) fluctuating pressures in the wake of the structure (vortex shedding), which produce resultant forces
acting across-wind, along-wind and torsionally;
c) fluctuating pressures induced by the motion of the structure itself due to wind.
Information on these effects and the appropriate values for the dynamic response factor can be found in
Annex D.
11 Criterion for aeroelastic instability
For structures affected by the wind actions, as specified in Clause 4 d), which can cause aeroelastic
instability, it shall be demonstrated that the performance of the structure, without the application of
additional load factors, is acceptable up to a wind speed exceeding the design peak wind speed.
Unless alternative rational procedures are available, this wind speed shall be at least the ultimate limit
state site peak velocity, V , or 1,5 times the ultimate limit state 10-min mean design wind speed for the
site
site location and height of the structure.
12 Methods of determination of wind loads
This document provides the method for determining design wind loads based on the peak velocity
pressure. This method is intended for detailed application where dynamic response effects are
significant, utilizing an appropriate peak dynamic response factor, as outlined in Annex D. When dynamic
response effects are negligible, such as in the design of cladding for most typical structures or the main
structural systems of small-sized to medium-sized structures with low dynamic response, the peak
velocity pressure method may be simplified by considering the peak dynamic response factor, C , to be
dyn
unity.
For certain wind-sensitive structures, supplementary specialist studies are recommended. These include
structures that are particularly flexible, slender, tall or lightweight, or located within complex
surroundings. Unusual geometries can also result in unexpectedly large wind responses. In these cases,
expert studies, typically including wind tunnel testing, are advised. These tests assist to establish the
overall structural loads and distribution of external local pressures. Suitable testing procedures are
described in Annex G.
This method of analysis is intended for use with performance-based and limit state design methods, with
requirements based on an appropriate probability of exceedance for the regional reference wind speed,
as defined in Clause 7.
This document can be used to interpret between national and regional wind loading standards using the
relationships provided in Annex A and Annex B. When interpreting between national standards or using
this document, the most applicable storm type to both ultimate limit state and serviceability design
should be considered.
Alternative methods of analysis to those recommended in this document may be permitted if the level of
safety achieved is generally equivalent to that of this document. Guidance on the level of safety is provided
in Annex I.
ISO/DISFDIS 4354:20252026(en)
Annex A
(informative)
Determination of reference wind speed
A.1 General
Clause 7 defines site peak dynamic pressures in terms of reference wind speeds and exposure factors.
The reference wind speeds for any probability of exceedance in one year (average return period) shall be
determined from regionally derived reference wind speeds. For use in this document, the reference wind
speed will be referenced to standard averaging time and exposure, defined as follows:
— V is the maximum wind speed averaged over 3 s, referenced to a height of 10 m over flat open
ref
country terrain.
Many national standards are based on annual extremes, which can be appropriate when a single storm
type causes the extremes. However, in general, yearly extremes do not form an appropriate basis for the
extreme value analysis of wind speeds. This is particularly true when the respective storm phenomenon
tends to occur in families or clusters. In such cases, more than one event corresponding to the analysed
storm type might occur in a given year, while in another year no event of the analysed storm type might
occur. The ensemble of yearly extremes, therefore, can contain irrelevant data and neglects other relevant
data. The ensemble shall consist of independent extremes above an appropriate threshold for each storm
type.
A.2 Analysis procedures
The defined reference wind speeds shall be obtained from regional wind speeds. If these wind speeds are
not already referenced as outlined in Clause A.1, they shall be converted using the following procedures:
a) Conversion for wind speeds referenced to different terrain roughness or height, V , should use the
tr, z
exposure factors, C , as given in Annex B for the appropriate storm type, as per Formula (A.1):
exp
V
tr,z
V = (A.1)
ref
C
exp
b) Conversion to wind speeds, V , referenced for different averaging times, T, shall use an averaging
T
time factor, k , which is based on a peak factor, g , and the turbulence intensity, I , as per
T v v
Formulae (A.2) and (A.3):
V = kV (A.2)
T TT= 3 600 s
k 1+ gI
(A.3)
T vv
The factor k relates to the maximum wind speed averaged over a given period of time (T in seconds)
T
within the hourly mean wind speed for a given hour. The peak factor, g primarily depends on the time
v,
over which the maximum wind speed is averaged, T, and is weakly dependent on height. A full derivation
methodology is provided in References [2], [3], [4] and [45], with a summary available in Reference
[5 [6]. For this document, average values of g for heights between 3 m and 300 m are presented in
v
=
ISO/DISFDIS 4354:20252026(en)
Table A.1, along with an evaluation of k for a range of values of T for specific reference conditions of
T
z = 10 m and z = 0,03 m.
The turbulence intensity, I , is defined as the standard deviation of wind speed divided by the hourly
v
mean wind speed, using Formula (A.4):
σ
v
I = (A.4)
v
V
m, T=3 600 s
Table A.1 — Averaging peak factors, g , and evaluation of reference averaging time factor, k
v T
Averaging time Average peak factor Reference averaging time factor
T (s) g k
v T
1 3,90 1,62
3 3,00 1,53
10 2,40 1,42
30 1,65 1,27
100 0,90 1,15
600 0,28 1,05
3 600 0 1,00
The most common conversions needed for reference conditions in relation to averaging time, T, are:
— V = 1,53 V
ref ref, T = 3 600 s
— V = 1,46 V
ref ref, T = 600 s
Anemometer measurements might not have been made in the open country reference conditions of
z = 0,03 m, and more detailed correction maycan be needed to obtain the appropriate peak factor as well
as the conversions provided by Formula (A.1), such as using Formula (B.2) with Figure B.1.
The hourly mean wind speed is more relevant for synoptic storms. However, for tropical cyclone storms
and thunderstorms there are no statistically stationary values of the hourly mean wind speed. In these
cases, an equivalent 10-minute mean wind speed can be used in this document to facilitate the
determination of the dynamic response, although the use of the 3-second wind speed is recommended in
thunderstorm climates. Additionally, historical wind speed data in thunderstorm climates shall be
referenced to averaging times of 3-second or less to determine relevant extreme value design wind
speeds.
ISO/DISFDIS 4354:20252026(en)
Annex B
(informative)
Determination of exposure factors
B.1 General
Clause 8 defines the exposure factors in terms of the variations in reference wind speeds, and
consequently, site peak velocity pressures. These variations are influenced by height, terrain roughness,
changes in terrain roughness and topography for various storm types. The exposure factor, which is
composed of factor for each of these phenomena, is expressed by Formula (B.1):
C = k × k × k (B.1)
exp tr,z trchange topog
where
k is the peak terrain roughness and height exposure factors;
tr,z
k is the peak terrain roughness change exposure factors;
trchange
k is the peak topography exposure factors;
topog
z is the height above ground level.
B.2 Wind profiles over flat terrain
B.2.1 General
Values of the terrain roughness and height exposure factors are provided for three storm types, along
with values of turbulence intensity.
B.2.2 Synoptic storm profiles
For four terrain roughness categories, synoptic storm hourly mean wind speed and turbulence intensity
profiles are defined using logarithmic law relations (after Reference [2 [3]). These relations apply to
roughness lengths of z = 0,003, 0,03, 0,3 and 3,0 m, which are defined as terrain roughness categories 1,
2, 3 and 4, respectively. Each category is associated with a gradient height (z ), where the hourly mean
G
−1
wind speed is standardized at 50 ms . Refer to Formulae (B.2) to (B.8) for detailed formulation.
23 4

u      
z z z z z

*
V = ln +−5,75 1, 88 −1,33 + 0, 25 (B.2)
     

tr, z , T= 3 600 s
     
0,4 z z z z z
     
0G G G G

u
*
z = (B.3)
G
6 f
ISO/DISFDIS 4354:20252026(en)
η
  
z
7,5ηu 0,538+ 0, 09ln

*

z

  0
σ = (B.4)
v
 u 
*
1+ 0,156ln
 
 
fz
 
6 fz
η 1− (B.5)
u
*
f 2Ω sinφ𝑓𝑓    = 2𝛺𝛺sin𝜙𝜙 (B.6)
where
η is a non-dimensional scaling coordinate,;
f 𝑓𝑓 is the Coriolis parameter,;
ϕ is the latitude,;
−6 −1
Ω is the angular velocity of the Earth’s rotation (= 72,9 × 10 rad s ),);
 
surface friction shear stress
u is the friction velocity .;
 
*
 
atmospheric air density
 
σ
vz,
I = (B.7)
vz,
V
z ,T= 3 600 s
where
I I is the turbulence intensity based on hourly mean wind speed at a specific location of height
V v,z
and terrain roughness.
VV 1+ 30, I (B.8)
( )
tr,z z ,T= 3 600 s vz,
Values for z for various terrain roughness conditions are provided in Figure B.1.
Some evaluations of the terrain roughness, height exposure factors and turbulence intensities for
synoptic storm profiles are given in Table B.1 for a latitude of 40°. The Deaves and Harris formulae are
not valid for latitudes approaching zero degrees and would not normally be used for values of ϕ < 20 °.
The evaluations in Table B.1 differ slightly from those defined in AnnexClause B.1, as the Deaves and
Harris formulae are based on a mean averaging time of T = 3 600 s, used in some countries. The mean
wind speed profiles have been evaluated for T = 3 600 s and then converted to the peak reference time of
T = 3 s used in this document. The terrain roughness and height exposure factors in Table B.1 are
presented as ratios relative to peak reference wind speed, V , for T = 3 s at z = 10 m in open country
ref
terrain, as expressed in Formula (B.9):
V
𝑉𝑉
tr,z
tr, z
k = 𝑘𝑘 = (B.9)
tr, z
tr,z
𝑉𝑉
𝑟𝑟𝑟𝑟𝑟𝑟
V
ref
=
=
=
ISO/DISFDIS 4354:20252026(en)
where
𝑘𝑘 is the terrain roughness and height exposure factor at height z for the specified terrain
tr, z
category.
is the terrain roughness and height exposure factor at height z for the specified terrain
k
tr,z
category.
The logarithmic law profiles can be approximated by power law profiles, where the only variable for
terrain roughness category is the power law exponent, β, as given in Formula (B.10):
β
𝛽𝛽
 z
𝑧𝑧
V = V 𝑉𝑉 =𝑉𝑉 � � (B.10)
tr, z tr, z=10 m
 
tr,zztr,=10 m
 
Values for β have been fitted between 10 m and 200 m and are given in Table B.1.

ISO/DISFDIS 4354:20252026(en)
Key
z0 roughness length (in metres)
Regarding urban terrain category: Due to the complex aerodynamic effects often presented in urban environments, standard
values might not fully capture the unique wind interactions. It is recommended to utilize wind tunnel tests or computation-
based methods for detailed analysis in these settings, accounting for the varying geometries and densities of urban
structures.
Figure B.1 — Description of terrain roughness lengths, z
ISO/DISFDIS 4354:20252026(en)
Table B.1 — Terrain roughness and height exposure factors, k , and
tr, z
turbulence intensity profiles for four terrain roughness categories over flat terrain
for synoptic storms at latitude ϕ = 40°
Characteristics
Terrain roughness
Height ktr,z Turbulence intensity
category
z Iv,z,T = 3 600 s
(T = 3 s)
m
1. Open sea flat surface 3 0,97 0,148
z = 0,003 m
5 1,03 0,142
10 1,11 0,135
20 1,19 0,127
50 1,28 0,112
100 1,33 0,095
200 1,39 0,076
500 1,49 0,052
1 000 1,58 0,032
β — β = 0,074 —
2. Open country/ 3 0,83 0,203
open sea in ultimate limit
5 0,90 0,191
state conditions
z = 0,03 m
0 10 1,00 0,178
20 1,10 0,165
50 1,21 0,147
100 1,29 0,128
200 1,36 0,106
500 1,48 0,074
1 000 1,58 0,048
β — β = 0,103 —
3. Suburban 3 0,84 —
z0 = 0,3 m
5 0,84 0,311
10 0,84 0,269
20 0,96 0,239
50 1,12 0,208
100 1,23 0,184
200 1,33 0,156
500 1,47 0,111
1 000 1,58 0,075
ISO/DISFDIS 4354:20252026(en)
β — β = 0,152 —
4. Urban 3 0,59 —
z = 3,0 m
5 0,59 —
10 0,59 0,677
20 0,74 0,473
50 0,95 0,355
100 1,12 0,302
200 1,27 0,254
500 1,46 0,184
1 000 1,59 0,126
β — β = 0,256 —
NOTE Values in terrain roughness categories 3 and 4 below 5 m and 10 m have been left conservatively constant as these
heights are close to or below the actual roughness elements in these categories. The effects of shielding can be determined
from wind tunnel measurements of relevant references.
β   power law exponent used to approximate the logarithmic wind profile.
B.2.3 Tropical cyclone storm profiles
Research on wind speed profiles for tropical cyclones (typhoons or hurricanes) has shown a wide scatter
of results. A recent review, Reference [9 [10], has recommends using logarithmic-law (or power-law)
profiles near the ground (up to 500 to 1 000 m). Hence, the terrain roughness category 2, given in
Table B.1, is used for tropical cyclones.
Measurements from References [6] and [7] and [8] and studies by Reference [8 [9] have shown episodes
of relatively high turbulence intensity in tropical cyclone records. In particular, the vertical components
relative to the longitudinal components are significantly higher than for synoptic wind flow. During these
episodes there is evidence that the increased energy occurs in the inertial subrange, hence the averaging
time factor given in Table A.1 is not applicable.
B.2.4 Thunderstorm profiles
While the general characteristics of thunderstorms and downbursts are understood, data on their wind
profiles data are limited. Existing measurements and some unpublished data have led to the development
of a preliminary envelope of peak wind speeds for this document. Ongoing data collection, such as the
efforts at Texas Tech University , is important for continued validation. An enveloping profile of peak
wind speeds for thunderstorms is provided using Formula (B.11):
−4 −6 2 −8 3 −12 4
k = 0,821 + 7,55 × 10 z − 6,75 × 10 z + 1,06 × 10 z − 4,97 × 10 z
tr,z,peak
+ 0,079 ln(z − 1,4) (B.11)
Information on effective turbulence intensity in severe thunderstorm downdrafts is given in Reference
[10 [11].
Some evaluation of this enveloping profile is given in Table B.2. No guidance can be given with respect to
turbulence intensities in thunderstorms; only the synoptic storm turbulence intensities for the
appropriate terrain are available at present.

Wind Science and Engineering Research Centre, Texas Tech University, Lubbock, Texas, USA.
ISO/DISFDIS 4354:20252026(en)
Table B.2 — Terrain roughness and height exposure factors for peak wind speeds, k , for three
tr, z
terrain roughness categories for thunderstorms
Terrain roughness category Height ktr,z
Z
m
3 0,86
5 0,93
10 1,00
20 1,06
1, 2 and 3
50 1,15
Open sea, country and suburban
100 1,20
200 1,20
500 1,02
1 000 1,00
For thunderstorm, only the peak wind speeds are relevant for determining design wind speeds. The use
of peak wind speeds for design loads is recommended, although an artificial mean wind speed approach
could be used for dynamic analysis. It is noted that in many cases for tall structures, the thunderstorm
data might only control the ultimate limit state design loads and at the serviceability levels the synoptic
storm winds are likely to control. In all mixed storm environments, the extreme wind speed analysis
should be carried out for data separated into each storm type.
B.3 Turbulence spectrum and length scale
B.3.1 Turbulence spectrum
The most commonly used expression for the longitudinal spectrum of turbulence is due to von Karman,
which is given in Formula (B.12):
fL
v
4

fS V
v m
= (B.12)
σ
v

fL
v
1+70, 8


V

m

where
f is the frequency, in hertz (Hz);
2 -‒1
S is the longitudinal spectrum of turbulence in terms of wind speed, in m s ;
v
L is the longitudinal integral length scale, in metres (m);
v
-‒1
V is mean wind speed, in metres per second (ms );
m
-−1
σ is the standard deviation of wind speed, in metres per second (ms ).
v
ISO/DISFDIS 4354:20252026(en)
B.3.2 Integral length scale of turbulence
The determination of the integral length scale of turbulence is complex, but for wind engineering
calculations, it can be conveniently approximated using Formula (B.13):
05,
 z
L = 100 (B.13)
 
v
 
where
L is the integral length scale, in metres (m);
v
z is the height above ground, in metres (m).
B.4 Change in terrain roughness effects
Adjustments to height exposure factors and terrain roughness due to changes in terrain roughness in
immediate approach fetches can be approximated by a linear weighting of the height exposure factors
and terrain roughness over a defined averaging distance for a given height. These adjustments are given
in Table B.3 and Table B.4 and are illustrated in Figure B.2. For heights over 10 m, a lag distance as a
function of height needs to be applied using Formula (B.14):
1,25
 
z
xz=   (B.14)
lag 0,tr max
 
03, z
 0,tr max
where
x is the distance downwind from the start of a new terrain roughness to the position where the
lag
developed height of the inner layer equals z (as shown in Figure B.2, expressed in metres);
z is the larger of the two roughness lengths at the boundary between different terrains;
0,tr max
z is the reference height of the structure above the average local ground level.
Table B.3 — Averaging distance for structure height
Structure height Averaging distance upwind of structure
m m
h < 50 1 000
50 ≤ h < 100 2 000
100 ≤ h < 200 3 000
Table B.4 — Roughness lengths for terrain roughness categories
Terrain roughness category Roughness length
m
1 0,003
2 0,03
3 0,3
ISO/DISFDIS 4354:20252026(en)
4 3,0
a)  Notation for changes in terrain roughness category

k x ++k x kx
tr 2 tr 2 tr 4 tr 4 tr 33tr
k = for the case illustrated
tr z
Averaging distance
b)  Example of changes in terrain roughness category
ISO/DISFDIS 4354:20252026(en)
Key
1 start of new terrain roughness
2 structure
3 actual surface
a
Wind direction.
b
Developed height of inner layer.
c
New terrain roughness category.
d
Upstream terrain roughness category.
e
Averaging distance.
f
Terrain roughness category 3.
g
Terrain roughness category 4.
h
Terrain roughness category 2.
i
Lag distance (tr3 to tr4).
j
Lag distance (tr4 to tr2).
k
Lagged response at height, z.
Figure B.2 — Changes in terrain category
B.5 Topographic and orographic effects
For structures situated in hilly or undulating terrain, the speed-up of the wind velocity over hills and
escarpments is a significant effect to consider. The "“topographical multiplier"”, k , for the peak
topog
velocity speed-up over small-scale features (i.e. peak topographic exposure factor) can beis defined as
follows:the ratio of the peak wind speed at height z above the feature to the peak wind speed at the same
height z above the upwind flat ground.
k =
(peak wind speed at height z above the feature) /
topog
(peak wind speed at height z above the upwind flat ground)
For a particular site, the value of this topographic multiplier can be obtained through appropriately
conducted wind tunnel model tests or computation-based methods. If such tests or calculations are not
possible, the topographic multiplier can be obtained using Formula (B.15):
k 1+ 0,,69kψ s xz (B.15)
( )
topog 1
where
ψ
is the slope of the hill (calculated as H/ 2L , where H is the slope height of the feature and
H
L is the slope length, as shown in Figure B.3); and
H
k is given in Table B.5.
The function s(x,z) is calculated using Formula (B.16):
=
ISO/DISFDIS 4354:20252026(en)
z
−k
x
L
H
s xz, 1− e (B.16)
( )
kL
2H
where
𝑧𝑧
| | −𝑘𝑘
𝑥𝑥
𝐿𝐿
H
𝑠𝑠(𝑥𝑥,𝑧𝑧) = 1− 𝑒𝑒 (B.16)
𝑘𝑘 𝐿𝐿
2 H
k and k are also given in Table B.5; and
2 3
L is the horizontal distance upwind from the crest of the hill or escarpment to a level half the height
H
below the crest, as shown in Figure B.3.
Table B.5 — Parameters for the calculation of topographic multipliers
k2
k k
1 3
x < 0 x > 0
Two-dimensional ridge 4,4 0,75 0,75 1,5
Two-dimensional escarpment 3,6 0,75 2 1,25
Three-dimensional axisymmetric hills 3,2 0,75 0,75 0,8
NOTE 1 For slopes less than 0,05, the effects of topography can be ignored
...