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CHAPTER 6 WIND LOADS C6-1
CHAPTER 6 WIND LOADS
Outline
6.1 General
6.1.1 Scope of application
6.1.2 Estimation principle
6.1.3 Buildings for which particular wind load or wind induced vibration is taken into account
6.2 Horizontal Wind Loads on Structural Frames
6.2.1 Scope of application
6.2.2 Equation
6.3 Roof Wind Load on Structural Frames6.3.1 Scope of application
6.3.2 Procedure for estimating wind loads
6.4 Wind Loads on Components/Cladding
6.4.1 Scope of application
6.4.2 Procedure for estimating wind loads
A6.1 Wind Speed and Velocity Pressure
A6.1.1 Velocity pressure
A6.1.2 Design wind speed
A6.1.3 Basic wind speed
A6.1.4 Wind directionality factor
A6.1.5 Wind speed profile factor
A6.1.6 Turbulence intensity and turbulence scale
A6.1.7 Return period conversion factor
A6.2 Wind force coefficients and wind pressure coefficients
A6.2.1 Procedure for estimating wind force coefficients
A6.2.2 External pressure coefficients for structural frames
A6.2.3 Internal pressure coefficients for structural frames
A6.2.4 Wind force coefficients for design of structural frames
A6.2.5 Peak external pressure coefficients for components/cladding
A6.2.6 Factor for effect of fluctuating internal pressures
A6.2.7 Peak wind force coefficients for components/cladding
A6.3 Gust Effect Factors
A6.3.1 Gust effect factor for along-wind loads on structural frames
A6.3.2 Gust effect factor for roof wind loads on structural frames
A6.4 Across-wind Vibration and Resulting Wind Load
A6.4.1 Scope of application
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A6.4.2 Procedure
A6.5 Torsional Vibration and Resulting Wind Load
A6.5.1 Scope of application
A6.5.2 Procedure
A6.6 Horizontal Wind Loads on Lattice Structural Frames
A6.6.1 Scope of application
A6.6.2 Procedure for estimating wind loads
A6.6.3 Gust effect factor
A6.7 Vortex Induced Vibration
A6.7.1 Scope of application
A6.7.2 Vortex induced vibration and resulting wind load on buildings with circular sections
A6.7.3 Vortex induced vibration and resulting wind load on building components with circularsections
A6.8 Combination of Wind Loads
A6.8.1 Scope of application
A6.8.2 Combination of horizontal wind loads for buildings not satisfying the conditions of
Eq.(6.1)
A6.8.3 Combination of horizontal wind loads for buildings satisfying the conditions of Eq.(6.1)
A6.8.4 Combination of horizontal wind loads and roof wind loads
A6.9 Mode Shape Correction Factor
A6.9.1 Scope of application
A6.9.2 Procedure
A6.10 Response Acceleration
A6.10.1 Scope of application
A6.10.2 Maximum response acceleration in along-wind direction
A6.10.3 Maximum response acceleration in across-wind direction
A6.10.4 Maximum torsional response acceleration
A6.11 Simplified Procedure
A6.11.1 Scope of application
A.6.11.2 Procedure
A6.12 Effects of Neighboring Tall Buildings
A6.13 1-Year-Recurrence Wind Speed
Appendix 6.6 Dispersion of Wind Load
References
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CHAPTER 6 WIND LOADS C6-3
CHAPTER 6 WIND LOADS
Outline
Each wind load is determined by a probabilistic-statistical method based on the concept of
equivalent static wind load, on the assumption that structural frames and components/cladding
behave elastically in strong wind.
Usually, mean wind force based on the mean wind speed and fluctuating wind force based on a
fluctuating flow field act on a building. The effect of fluctuating wind force on a building or part
thereof depends not only on the characteristics of fluctuating wind force but also on the size and
vibration characteristics of the building or part thereof. These recommendations evaluate themaximum loading effect on a building due to fluctuating wind force by a probabilistic-statistical
method, and calculate the static wind load that gives the equivalent effect. The design wind load can
be obtained from the summation of this equivalent static wind load and the mean wind load.
A suitable wind load calculation method corresponding to the scale, shape, and vibration
characteristics of the design object is provided here. Wind load is classified into horizontal wind load
for structural frames, roof wind load for structural frames and wind load for components/cladding. The
wind load for structural frames is calculated from the product of velocity pressure, gust effect factor
and projected area. Furthermore, a calculation method for horizontal wind load for lattice structural
frames that stand upright from the ground is newly added. The wind load for components/cladding is
calculated from the product of velocity pressure, peak wind force coefficient and subject area. For
small-scale buildings, a simplified procedure can be applied.
These recommendations introduce the wind directionality factor for calculating the design wind
speed for each individual wind direction, thus enabling rational design considering the buildings
orientation with respect to wind direction. Moreover, the topography factor for turbulence intensity is
newly added to take into account the increase of fluctuating wind load due to the increase of
fluctuating wind speed.
Introduction of the wind directionality factor requires the combination of wind loads in along-wind,
across-wind and torsional directions. Hence, it is decided to adopt the regulation for the combination
of wind loads in across-wind and along-wind directions, or in torsional and along-wind directions
explicitly. Furthermore, a prediction formula for the response acceleration of the building for
evaluating its habitability to vibration, which is needed in performance design, and information of
1-year-recurrence wind speed are newly added. Besides, information has been provided for the
dispersion of wind load.
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Notation
Notations used in the main text of this chapter are shown here.
Uppercase Letter
A (m2): projected area at height Z
RA (m2): subject area
CA (m2): subject area of components/cladding
0A (m2): whole plane area of one face of lattice structure
FA (m2): projected area of one face of lattice structure
B (m): building breadth
1B (m): building length in span direction
2B (m): building length in ridge direction0B , HB (m): width of lattice structure in ground and width at height H
DB : background excitation factor for lattice structure
1C , 2C , 3C : parameters determining topography factor gE and IE
DC , RC , XC , YC : wind force coefficients
'LC ,
'TC : rms overturning moment coefficient and rms torsional moment coefficient
eC : exposure factor, which is generally 1.0 and shall be 1.4 for open terrain with few
obstructions (Category II). When wind speed is expected to increase due to local
topography, this factor shall be increased accordingly.gC : overturning moment coefficient in along-wind direction
'gC : rms overturning moment coefficient in along-wind direction
fC : wind force coefficient. For horizontal wind loads, wind force coefficient DC defined
in A6.2 with 9.0Z =k shall be used. For roof wind loads, wind force coefficient RC
defined in A6.2 shall be used.
peC : external pressure coefficient
pe1C , pe2C : external pressure coefficients on windward wall and leeward wall
pi
C : internal pressure coefficient
*piC : factor for effect of fluctuating internal pressure
rC : wind force coefficient at resonance
CC : peak wind force coefficient
peC : peak external pressure coefficient
D (m): building depth, building diameter, member diameter
BD (m): building diameter at the base
mD (m): building diameter at height of 3/2H
E: wind speed profile factor
HE : wind speed profile factor at reference height H
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IE : topography factor for turbulence intensity
gE : topography factor for wind speed
gIE : topography factor for turbulence intensity
rE : exposure factor for flat terrain categories
DF : along-wind force spectrum factor
F: wind force spectrum factor
DG : gust effect factor for along-wind load
RG : gust effect factor for roof wind load
H(m): reference height
SH (m): height of topography
TI (kgm2): generalized inertial moment of building for torsional vibration
ZI : turbulence intensity at heightZ
rZI : turbulence intensity at height Z on flat terrain categories
DK : wind directionality factor
L (m): span of roof beam
SL (m): horizontal distance from topography top to point where height is half topography
height
ZL (m): turbulence scale at height Z
M(kg): total building mass
DM (kg): generalized mass of building for along-wind vibration
LM (kg): generalized mass of building for across-wind vibration
R : factor expressing correlation of wind pressure of windward side and leeward side
DR : resonance factor for along-wind vibration
LR : resonance factor for across-wind vibration
TR : resonance factor for torsional vibration
ReR : resonance factor for roof beam
DS : size effect factor
0U (m/s): basic wind speed
1U (m/s): 1-year-recurrence 10-minute mean wind speed at 10m above ground over flat and
open terrain
1HU (m/s): 1-year-recurrence wind speed
500U (m/s): 500-year-recurrence 10-minute mean wind speed at 10m above ground over
flat and open terrain
HU (m/s): design wind speed
*LcrU ,
*TcrU : non-dimensional critical wind speed for aeroelastic instability in across-wind
and torsional directions
rU (m/s): resonance wind speed*TU : non-dimensional wind speed for calculating torsional wind load
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*rU : non-dimensional resonance wind speed
DW (N): along-wind load at height Z
LW (N): across-wind load at height Z
TW (Nm): torsional wind load at height Z
LCW (N): across-wind combination load
RW (N): roof wind load
SCW (N): wind load on components/cladding obtained by simplified method
SfW (N): wind load on structural frames
rW (N): wind load at height Z
SX (m): distance from leading edge of topography to construction site
Z(m): height above ground
bZ , GZ (m): parameters determining exposure factor
Lowercase Letter
Dmaxa , Lmaxa (m/s2), Tmaxa (rad/s
2): maximum response acceleration in along-wind,
across-wind and torsional directions at top of building
b (m): projected width of member
f (m): rise
1f (Hz): The smaller of Lf and Tf
Df , Lf , Tf (Hz): natural frequency for first mode in along-wind, across-wind and torsional
directions
Rf (Hz): natural frequency for first mode of roof beam
aDg , aLg , aTg : peak factors for response accelerations in along-wind, across-wind and
torsional directions
Dg , Lg , Tg : peak factors for wind loads in along-wind, across-wind and torsional
directions
h (m): eaves height
1k : factor for aspect ratio
2k : factor for surface roughness
3k : factor for end effects
4k : factor for three demensionality
Ck : area reduction factor
rWk : return period conversion factor
Zk : factor for vertical profile for wind pressure coefficients or wind force coefficients
l (m): smaller value of H4 and B , minimum value of H4 , 1B and 2B , member
length
a1l (m): smaller value of H and 1B
a2l (m): smaller value of H and 2B
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Hq (N/m2): velocity pressure at reference height H
Zq (N/m2): velocity pressure at height Z
r(year): design return period
Rer : coefficient of variation for generalized external pressure
x (m) : distance from end of component
Greek Alphabet
: exponent of power law for wind speed profile
: exponent of power law for vibration mode
: load combination factor
, L , T : mass damping parameter for vortex induced vibration, across-wind vibration
and torsional vibrationD , L , T : mode correction factor for vortex induced vibration, across-wind vibration and
torsional vibration
D , L , T : critical damping ratio for first translational and torsional modes
R : critical damping ratio for first mode of roof beam
: solidity
: mode correction factor of general wind force
U : 0500 /UU
: first mode shape in each direction
D (Hz): level crossing factor
(): roof angle, angle of attack to member
S (): inclination of topography
(kg/m3): air density
S (kg/m3): building density which is )/( BmDHDM
LT : correlation coefficient between across-wind vibration and torsional vibration
6.1 General
6.1.1 Scope of application
(1) Target strong wind
Most wind damage to buildings occurs during strong winds. The wind loads specified here are
applied to the design of buildings to prevent failure due to strong wind. The strong winds that occur in
this country are mainly those that accompany a tropical or extratropical cyclone, and down-bursts or
tornados. The former are large-scale phenomena that are spread over about 1000km in a horizontal
plane, and their nature is comparatively well known. Down-bursts are gusts due to descending air
flows caused by severe rainfall in developed cumulonimbus. Since the scale of these phenomena are
very small, few are picked up by the meteorological observation network. It is known that tornados are
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small-scale phenomena several hundred meters wide at most having a rotational wind with a rapid
atmospheric pressure descent. The characteristics of the strong wind and pressure fluctuation caused
by tornados are not known. The number of occurrences of down-bursts and tornados is relatively large,
but their probability of attacking a particular site is very small compared with that of the tropical or
extratropical cyclones. However, the winds caused by down-bursts and tornados are very strong, so
they often fatally damage buildings. These recommendations focus on strong winds caused by tropical
or extratropical cyclones. However, the minimum wind speed takes into account the influence of
tornadoes and down-bursts.
(2) Wind loads on structural frames and wind loads on components/cladding
The wind loads provided in these recommendations is composed of those for structural frames and
those for components/cladding. The former are for the design of structural frames such as columns and
beams. The latter are for the design of finishings and bedding members of components/cladding andtheir joints. Wind loads on structural frames and on components/cladding are different, because there
are large differences in their sizes, dynamic characteristics and dominant phenomena and behaviors.
Wind loads on structural frames are calculated on the basis of the elastic response of the whole
building against fluctuating wind forces. Wind loads on components/cladding are calculated on the
basis of fluctuating wind forces acting on a small part.
Wind resistant design for components/cladding has been inadequate until now. They play an
important role in protecting the interior space from destruction by strong wind. Therefore, wind
resistant design for components/cladding should be just as careful as that for structural frames.
6.1.2 Estimation principle
(1) Classification of wind load
A mean wind force acts on a building. This mean wind force is derived from the mean wind speed
and the fluctuating wind force produced by the fluctuating flow field. The effect of the fluctuating
wind force on the building or part thereof depends not only on the characteristics of the fluctuating
wind force but also on the size and vibration characteristics of the building or part thereof. Therefore,
in order to estimate the design wind load, it is necessary to evaluate the characteristics of fluctuating
wind forces and the dynamic characteristics of the building.
The following factors are generally considered in determining the fluctuating wind force.
1) wind turbulence (temporal and spatial fluctuation of wind)
2) vortex generation in wake of building
3) interaction between building vibration and surrounding air flow
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Figure 6.1.1 Fluctuating wind forces based on wind turbulence and vortex generation in wake of
building
Fluctuating wind pressures act on building surfaces due to the above factors. Fluctuating wind
pressures change temporally, and their dynamic characteristics are not uniform at all positions on the
building surface. Therefore, it is better to evaluate wind load on structural frames based on overall
building behavior and that on components/cladding based on the behavior of individual building parts.
For most buildings, the effect of fluctuating wind force generated by wind turbulence is predominant.
In this case, horizontal wind load on structural frames in the along-wind direction is important.
However, for relatively flexible buildings with a large aspect ratio, horizontal wind loads on structural
frames in the across-wind and torsional directions should not be ignored. For roof loads, the
fluctuating wind force caused by separation flow from the leading edge of the roof often predominates.
Therefore, wind load on structural frames is divided into two parts: horizontal wind load on structural
frames and roof wind load on structural frames.
Figure 6.1.2 Classification of wind loads
(2) Combination of wind loads
Wind pressure distributions on the surface of a building with a rectangular section are asymmetric
even when wind blows normal to the building surface. Therefore, wind forces in the across-wind and
torsional directions are not zero when the wind force in the along-wind direction is a maximum.
wind load onstructural frames
wind load on
components/cladding
horizontal wind load
roof wind load
along-wind load
across-wind load
torsional wind loadwind load
simplifiedprocedure
small-scale building
wind load onstructural frames
wind load oncomponents/cladding
vibration direction vorticeswind turbulence
a) fluctuating wind force caused by
wind turbulence
vibration direction
b) fluctuating wind force caused by
vortex generation in wake of building
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Combination of wind loads in the along-wind, across-wind and torsional directions have not been
taken into consideration positively so far, because the design wind speed has been used without
considering the effect of wind direction. However, with the introduction of wind directionality,
combination of wind loads in the along-wind, across-wind and torsional directions has become
necessary. Hence, it has been decided to adopt explicitly a regulation for combination of wind loads in
along-wind, across-wind and torsional directions.
(3) Wind directionality factor
Occurrence and intensity of wind speed at a construction site vary for each wind direction with
geographic location and large-scale topographic effects. Furthermore, the characteristics of wind
forces acting on a building vary for each wind direction. Therefore, rational wind resistant design can
be applied by investigating the characteristics of wind speed at a construction site and wind forces
acting on the building for each wind direction. These recommendations introduce the winddirectionality factor in calculating the design wind speed for each wind direction individually. In
evaluating the wind directionality factor, the influence of typhoons, which is the main factor of strong
winds in Japan, should be taken into account. However, it was difficult to quantify the probability
distribution of wind speed due to a typhoon from meteorological observation records over only about
70 years, because the occurrence of typhoons hitting a particular point is not necessarily high. In these
recommendations, the wind directionality factor was determined by conducting Monte Carlo
simulation of typhoons, and analysis of observation data provided by the Metrological Agency.
(4) Reference height and velocity pressure
The reference height is generally the mean roof height of the building, as shown in Fig.6.1.3. The
wind loads are calculated from the velocity pressure at this reference height. The vertical distribution
of wind load is reflected in the wind force coefficients and wind pressure coefficients. However, the
wind load for a lattice type structure shall be calculated from the velocity pressure at each height, as
shown in Fig.6.1.3.
Figure 6.1.3 Definition of reference height and velocity pressure
(5) Wind load on structural frames
The maximum loading effect on each part of the building can be estimated by the dynamic response
analysis considering the characteristics of temporal and spatial fluctuating wind pressure and the
lattice type structurehigh-rise buildingdomehouse
qZ
qH
qHqH
Z
H
HH
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CHAPTER 6 WIND LOADS C6-11
dynamic characteristics of the building. The equivalent static wind load producing the maximum
loading effect is given as the design wind load. For the response of the building against strong wind,
the first mode is predominant and higher frequency modes are not predominant for most buildings.
The horizontal wind load (along-wind load) distribution for structural frames is assumed to be equal to
the mean wind load distribution, because the first mode shape resembles the mean building
displacement. Specifically, the equivalent wind load is obtained by multiplying the gust effect factor,
which is defined as the ratio of the instantaneous value to the mean value of the building response, to
the mean wind load. The characteristics of the wind force acting on the roof are influenced by the
features of the fluctuating wind force caused by separation flow from the leading edge of the roof and
the inner pressure, which depends on the degree of sealing of the building. Therefore, the
characteristics of roof wind load on structural frames are different from those of the along-wind load
on structural frames. Thus, the roof wind load on structural frames cannot be evaluated by the sameprocedure as for the along-wind load on structural frames. Here, the gust effect factor is given when
the first mode is predominant and assuming elastic dynamic behavior of the roof beam under wind
load.
(6) Wind load on components/cladding
In the calculation of wind load on components/cladding, the peak exterior wind pressure coefficient
and the coefficient of inner wind pressure variation effect are prescribed, and the peak wind force
coefficient is calculated as their difference. Only the size effect is considered. The resonance effect is
ignored, because the natural frequency of components/cladding is generally high. The wind load on
components/cladding is prescribed as the maximum of positive pressure and negative pressure for
each part of the components/cladding for wind from every direction, while the wind load on structural
frames is prescribed for the wind direction normal to the building face. Therefore, for the wind load on
components/cladding, the peak wind force coefficient or the peak exterior wind pressure coefficient
must be obtained from wind tunnel tests or another verification method.
(7) Wind loads in across-wind and torsional directions
It is difficult to predict responses in the across-wind and torsional directions theoretically like
along-wind responses. However, a prediction formula is given in these recommendations based on the
fluctuating overturning moment in the across-wind direction and the fluctuating torsional moment for
the first vibration mode in each direction.
(8) Vortex induced vibration and aeroelastic instability
Vortex-induced vibration and aeroelastic instability can occur with flexible buildings or structural
members with very large aspect ratios. Criteria for across-wind and torsional vibrations are provided
for buildings with rectangular sections. Criteria for vortex-induced vibrations are provided for
buildings and structural members with circular sections. If these criteria indicate that vortex-induced
vibration or aeroelastic instability will occur, structural safety should be confirmed by wind tunnel
tests and so on. A formula for wind load caused by vortex-induced vibrations is also provided for
buildings or structural members with circular sections.
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(9) Small-scale buildings
For small buildings with large stiffness, the size effect is small and the dynamic effect can be
neglected. Thus, a simplified procedure is employed.
(10) Effect of neighboring buildings
When groups of two or more tall buildings are constructed in proximity to each other, the wind flow
through the group may be significantly deformed and cause a much more complex effect than is
usually acknowledged, resulting in higher dynamic pressures and motions, especially on neighboring
downstream buildings.
(11) Assessment of building habitability
Building habitability against wind-induced vibration is usually evaluated on the basis of the
maximum response acceleration for 1-year-recurrence wind speed. Hence, these recommendations
show a map of 1-year-recurrence wind speed based on the daily maximum wind speed observed atmeteorological stations and a calculation method for response acceleration.
(12) Shielding effect by surrounding topography or buildings
When there are topographical features and buildings around the construction site, wind loads or
wind-induced vibrations are sometimes decreased by their shielding effect. Rational wind resistant
design that considers this shielding effect can be performed. However, changes to these features during
the buildings service life need to be confirmed. Furthermore, the shielding effect should be
investigated by careful wind tunnel study or other suitable verification methods, because it is generally
complicate and cannot be easily analyzed.
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CHAPTER 6 WIND LOADS C6-13
Figure 6.1.4 Flow chart for estimation of wind load
Start
Outline of building
6.1.3 Buildings to be designed for particular
wind load or wind induced vibration
(1) across-wind, torsional wind loads
(2) vortex induced vibration,
aeroelastic instability
A6.12 Effects of neighboring tall buildings
A6.1 Wind speed and velocity pressure
A6.11 Simplified procedure
A6.1.1 Velocity pressure
A6.1.2 Design wind speed
A6.1.3 Basic wind speed
A6.1.4 Wind directionality factor
A6.1.5 Wind speed profile factor
A6.1.6 Turbulence intensity and turbulence
scale
A6.1.7 Return period conversion factor
6.2 Horizontal wind load
End
A6.8 Combination of wind loads
Wind tunnel experiment
6.3 Roof wind load 6.4 Wind load on
components/cladding
Wind load on components/claddingWind load on structural frames
A6.2.2 External wind pressure coefficient
A6.2.3 Internal pressure coefficients
A6.2.4 Wind force coefficients
A6.2.1 Procedure for estimating wind force coefficients
A6.2.5 Peak external pressure coefficients
A6.2.6 Factor for effect of fluctuating internal
pressuresA6.2.7 Peak wind force coefficient
A6.4 Across-wind load
A6.5 Torsional wind load
A6.3.2 Gust effect factor
for roof wind loads
A6.3.1 Gust effect factor
for along-wind loads
A6.6 Horizontal wind loads on lattice
structural frames
A6.7 Vortex induced vibration
A6.10 Response acceleration
A6.13 1-year-recurrence wind speed
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6.1.3 Buildings for which particular wind load or wind induced vibration need to be taken into
account
(1) Buildings for which horizontal wind loads on structural frames in across-wind and torsional
directions need to be taken into account
Horizontal wind loads on structural frames imply along-wind load, across-wind load and torsional
wind load. Both across-wind load and torsional wind load must be estimated for wind-sensitive
buildings that satisfy Eq.(6.1). Figure 6.1.5 shows the definition of wind direction, 3 component wind
loads and building shape.
along-wind
across-windwind
torsion
Figure 6.1.5 Definition of load and wind direction
Both across-wind vibration and torsional vibration are caused mainly by vortices generated in the
buildings wake. These vibrations are not so great for low-rise buildings. However, with an increase in
the aspect ratio caused by the presence of high-rise buildings, a vortex with a strong period uniformly
generated in the vertical direction, and across-wind and torsional wind forces increase. However, with
increase in building height, the natural frequency decreases and approaches the vortex shedding
frequency. As a result, resonance components increase and building responses become large. In
general, responses to across-wind vibration and torsional vibration depending on wind speed increase
more rapidly than responses to along-wind vibration. Under normal conditions, along-wind responses
to low wind speed are larger than across-wind responses. However, across-wind responses to high
wind speed are larger than along-wind responses. The wind speed at which the degrees of along-wind
response and across-wind response change places with each other differs depending on the height,
shape and vibration characteristics of the building. The condition with regard to the aspect ratio of
Eq.(6.1) has been established through investigation of the relationship between the magnitude of
along-wind loads and across-wind loads for flat terrain subcategory II and a basic wind speed of 40m/s
assuming 180kg/m3 building density, )024.0/(11 Hf = (Hz) natural frequency of the primary mode
and 1% damping ratio for an ordinary building. Therefore, it is desirable to estimate across-wind and
torsional wind loads even for buildings of light weight and small damping to which Eq.(6.1) is not
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applicable.
Furthermore, for flat-plane buildings with small torsional stiffness or buildings with large
eccentricity whose translational natural frequency and torsional natural frequency approximate each
other, it is also desirable to estimate the torsional wind loads even where Eq.(6.1) is not applicable to
those buildings.
The discriminating conditional formula shown in this chapter was derived for a building with a
rectangular plane. It is possible to apply Eq.(6.1) to a building with a plane that is slightly different
from rectangular by regarding B and D roughly as projected breadth and a depth. For values of B
and D changed in the vertical direction, the wind force acting on the upper part has a major effect on
the response. Therefore, a representative value for the upper part should used for the computation.
Under normal conditions, a value in the vicinity of 2/3 of the building height is chosen in most cases.
The computation of Eq.(6.1) using a smaller value for the upper part yields a conservative value.(2) Vortex resonance and aeroelastic instability
It is feared that aeroelastic instabilities such as vortex-induced vibration, galloping and flutter occur
in buildings with low natural frequency and are high in comparison with their breadth and depth, as
well as in slender members. The conditions for estimation of aeroelastic instability in both across-wind
vibration and torsional vibration for building with rectangular planes as well as the conditions for
estimation of vortex-induced vibrations for a building with a circular plane are given based on wind
tunnel test results and the field measurement results1)-6)
. The method for estimating the wind load for a
building with a circular plan when vortex-induced vibration occurs is shown in A6.7. It may well be
that vortex-induced vibration and aeroelastic instability will occur in a slender building with a
triangular or an elliptical plan. Therefore, attention must be paid to this.
The first condition required for estimating aeroelastic instability and vortex-induced vibration is the
aspect ratio ( BDH/ or m/DH ). Aeroelastic instability as well as vortex-induced vibration does
not occur easily in buildings with a small aspect ratio. Under this recommendation, the aspect ratio for
estimating both aeroelastic instability and vortex-induced vibration was set to 4 or more and 7 or more,
respectively. The second condition for estimating non-dimensional wind speed is ( BDfU/ or
m/ fDU ). The occurrence of aeroelastic instability and vortex-induced vibration is dominated by the
non-dimensional wind speed, which is determined by the representative breadth of the building, its
natural frequency and wind speed. The non-dimensional critical wind speed for aeroelastic instability
depends upon the mass damping parameter, which is determined by the side ratio, the turbulence
characteristics of an approaching flow and the mass and damping ratio of a building. Thus, the
non-dimensional critical wind speed with regard to the estimation of aeroelastic instability of a
building with a rectangular plane was provided as the function for those parameters. The
non-dimensional wind speed for vortex-induced vibration of a building with a circular plan is almost
independent of this parameter. Therefore, the value for non-dimensional critical wind speed is fixed.
The non-dimensional wind speed for estimating aeroelastic instability and vortex-induced vibration is
set at 0.83(=1/1.2) times the non-dimensional critical wind speed. This is because it is known that
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aeroelastic instability or vortex-induced vibration occurs within a period shorter than 10 min, which is
the evaluation time for wind speed prescribed in this recommendation, and that the uncertainty of the
non-dimensional wind speed including errors in experimental values is taken into account.
Furthermore, the damping ratio of a building is required for the computation of the buildings mass
damping parameter. It is thus recommended that the damping ratio of a building be estimated through
reference to Damping in Buildings7)
.
6.2 Horizontal Wind Loads on Structural Frames
6.2.1 Scope of application
This section describes horizontal wind loads on structural frames in the along-wind direction. The
along-wind load is generally composed of a mean component caused by the mean wind speed, aquasi-static component caused by relatively low frequency fluctuation and a resonant component
caused by fluctuation in the vicinity of the natural frequency. For many buildings, only the first mode
is taken into account as the resonant component. The procedure described in this section can estimate
the equivalent static wind load producing the maximum structural responses (load effects of stress and
displacement) using the gust effect factor. The equivalent static wind load is also divided into the mean
component, quasi-static component and resonant component. Although the vertical profiles for these
components are different from each other, it is assumed that all profiles similar to that of the mean
component are provided.
6.2.2 Estimation method
Equation (6.4) for horizontal wind loads is derived from a gust effect factor method, which includes
the effect of along-wind dynamic response due to atmospheric turbulence of approaching wind. The
gust effect factor is a magnifying rate of the maximum instantaneous value to the mean building
responses. Davenport, who first proposed the gust effect factor, calculated this factor based on the
displacement at the highest position of a building8)
. However, in these recommendations the gust effect
factor based on the overturning moment of a base9), which can rationally estimate the design wind load
for a building, was employed. Projected area A is the area projected from the wind direction for the
portion concerned, as shown in Fig.6.2.1, and for wind load at a unit height being taken into account,
projected area A becomes projected breadth B .
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D
wind
Figure 6.2.1 Projected area
6.3 Roof Wind Load on Structural Frames
6.3.1 Scope of application
Roof wind loads on structural frames should be estimated from load effects of wind forces that act
on roof frames. The properties of wind forces acting on roofs are influenced by the external pressures,
which are affected by the behavior of the separated shear layers from leading edges, and the internal
pressures, which are affected by the buildings permeability. This section describes equations to be
applied to roof frames of buildings with rectangular plan without dominant openings, where the
correlation between fluctuating external pressures and fluctuating internal pressures can be ignored.
A light roof like a hanging roof might generate aerodynamically unstable oscillations. These
oscillations may be generated in roof frames that satisfy the conditions of 3/ LfU
and 15.0H
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6.4 Wind Loads for Components/Cladding
6.4.1 Scope of application
Wind loads on components/cladding need to be designed for parts of buildings; finishings of roofs
and external walls; bed members such as purlins, furring strips and studs; roof braces; and tie beams
subject to strong effects of intensive wind pressure. These wind loads are also applied to the design of
eaves and canopies.
6.4.2 Procedure for estimating wind loads
Wind loads on components/cladding are derived from the difference between the wind pressures
acting on the external and internal faces of a building, and are calculated from Eq.(6.6). Peak wind
force coefficients C
C corresponding to the peak values of fluctuating net pressures, defined by thedifference between external and internal pressures, are given by Eq.(A6.15) for convenience. For
buildings such as free-standing canopy roofs, where the top and bottom surfaces are both exposed to
wind, the peak wind force coefficients CC are derived directly from the actual peak values of
pressure differences, as shown in section A6.2.7.
External pressure coefficients provided in the Recommendations correspond to the most critical
positive and negative peak pressures on each part of a building irrespective of wind direction.
Therefore, when the wind loads are calculated by considering the directionality of wind speeds, the
peak pressure or force coefficients for each wind direction are needed, which should be determined
from appropriate wind tunnel experiments or some other method12)
.
The subject areaACdepends on the item to be designed. When designing the finishing of roofs and
external walls, the supported area of the finishing is used, and when designing the supports of the
finishing, the tributary area of the supports is used.
A6.1 Wind speed and velocity pressure
A6.1.1 Velocity pressure
The velocity pressure, which represents the kinetic energy per unit volume of the air flow, is the
basic variable determining the wind loading on a building.. It corresponds to the rise in pressure from
the free stream (atmospheric ambient static pressure) to the stagnation point on the windward face of
the building, and is defined as ( ) 221 U , where U is the wind speed.It is only necessary to consider the velocity pressure as the basic variable of wind loading when
static effects of the wind are examined. However, it is more appropriate to adopt wind speed as the
basic variable when dynamic wind effects are under discussion. Thus, wind speed is adopted in the
recommendations as the basic variable for calculating wind loading. The design velocity pressure, Hq ,
which is based on the design wind speed HU at the reference height H , is defined in Eq.(A6.1).
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Air density varies with temperature, ambient pressure and humidity. However, the influence of
humidity is usually neglected. In these recommendations, the air density is taken as 22.1= (kg/m3),
which corresponds to a temperature of 15C and an ambient pressure of 1013 hPa.
A6.1.2 Design wind speed
The wind speed at a construction site is a function of its geographical location, orography or
large-scale topographic features (e.g. mountain ranges and peninsulas) as well as the ground surface
conditions (e.g. size and density of obstructions such as buildings and trees), and small-scale
topographic features (e.g. escarpments and hills). The height above ground level is also a factor. Of
these factors, the geographical location and large-scale topographical features are reflected in the
values of basic wind speed 0U and wind directionality factor DK . The influences of surface
roughness, small-scale topographical features and elevation are reflected in the wind speed profilefactor HE .
Designers are required to decide the wind load level by considering the buildings social importance,
occupancy, economic situation and so on. This is represented by the return period conversion factor
rWk . The basic wind load defined in 2.2 is that corresponding to the 100-year-recurrence wind speed,
which is calculated from Eq.(A6.2) by substituting 1rW =k . The wind directionality factor DK , a
newly introduced parameter in this version, makes the design more rational by considering the
dependencies of the wind speed, the frequency of occurrence of extreme wind and the aerodynamic
property on wind direction. The wind directionality factor DK is affected by the frequency of
occurrence and the routes of typhoons, climatological factors, large-scale topographic effects and so
on.
If the design ignores wind directionality effects, the design wind speed HU can be calculated by
substituting 1D =K in Eq.(A6.2).
A6.1.3 Basic wind speed
The basic wind speed 0U is the major variable in Eq.(A6.2) for calculating the design wind speed.
The wind speed at a construction site is influenced by the occurrence of typhoon and monsoon, the
longitude and latitude of the location and large-scale topographical effects. The basic wind speed
reflects the effects of these factors. The value of the basic wind speed corresponds to the
100-year-recurrence 10-minute-mean wind speed over a flat and open terrain (category II) at an
elevation of 10m. Figure A6.1.1 shows the procedure for making the basic wind speed map. As the
first step of the procedure, data from different metrological stations were adjusted or corrected to
reduce them to a common base considering the directional terrain roughness. Then extreme value
analyses were conducted for mixed wind climates of typhoon winds and non-typhoon winds. For
typhoon winds, a Monte-Carlo simulation based on a typhoon model was also conducted for each
meteorological station in Japan. Although the analysis was conducted with consideration of wind
directionality effect, the basic wind speed was considered as a non-directional value. Instead, the wind
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directionality effect was reflected by introducing the wind directionality factor, which is defined as the
wind speed ratio for a certain wind direction to the basic wind speed, as defined in A6.1.4.
Records of wind speed and direction
(for all meteorological stations from
1961 to 2000)
Modeling of typhoon pressure fields
(based on data from 1951 to 1999)
Monte Carlo simulation of typhoon
winds (for 5000 years)
Extreme wind probability distribution
due to typhoons
Extraction of independent storm
(including the 2nd higher and less)
Extreme wind probability distribution
due to non-typhoon winds
Synthesis of extreme value
distributions
Evaluation of terrain category
(considering historical variation)
Basic wind speed map
Reduction to the common base
Extreme value probability
analysis for mixed wind climates
Figure A6.1.1 Procedure for making basic wind speed map
1) Data for analysis
Data of wind speed, wind direction and anemometer height from the Japan Meteorological Business
Support Center (Daily observation climate data from 1961-2000, Observation history at metrological
stations) were used for analysis. The daily observation climate data from 1961-1990 and the
Geophysical Review of 1951-1999 by the Japan Meteorological Agency were referred for modeling
the pressure fields and tracks of typhoons, respectively. For homogenization of the wind speed records,
data measured by different types of anemometers were corrected to those of propeller type
anemometers13).
2) Evaluation of directional terrain roughness and homogenization of wind speed
The wind speed records at the meteorological stations were homogenized, that is to say, converted
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into data at a height of 10m over terrain category II by utilizing a method for evaluating the terrain
roughness from the pseudo-gust factor (ratio of daily maximum instantaneous wind speed divided by
daily maximum wind speed) and elevation of the measurement point14)
. The details of the method are
as follows. The pseudo-gust factors were first averaged according to the year and wind direction. Then,
referring to the averaged pseudo-gust factors, a terrain roughness category was identified in which the
same gust-factor was given using the profiles of mean wind speeds (defined in A6.1.5) and turbulence
intensity (defined in A6.1.6). For this calculation, the terrain roughness category was treated as a
continuous variable.
Figure A6.1.2 shows examples of the annual variance of terrain roughness for four dominant wind
directions measured at Fukuoka Meteorological Station, in which the symbols are for the calculated
values and the lines are the results of linear approximation. The value of roughness category was
assumed to be between I and V. This shows that the roughness category changes due to urbanizationand the roughness category varies with wind direction.
Historical changes of the directional terrain roughness were utilized for homogenization of wind
speed records at meteorological stations and calibration of wind speeds near the ground surface in the
extreme value analysis and the typhoon model.
Figure A6.1.2 Examples of evaluation for terrain roughness
3) Extreme value analysis in mixed wind climates
The extreme value analysis in mixed wind climates15)
was applied to extreme wind data generated
by different wind climates, for instance, typhoons and monsoons. In this method, the extreme wind
records were divided into groups and independently fitted by extreme value distributions, and the
combined distribution was obtained assuming the independency of each extreme distribution.
Based on typhoon track data, the measuring records were divided into typhoon and non-typhoon
winds, that is, if it was within 500 km of the typhoon center, the wind climate was considered as
Flatterraincategories V
IV
III
II
I Flatterraincategories V
IV
III
II
I
Flatterraincategories V
IV
III
II
I Flatterraincategories V
IV
III
II
I
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typhoon, and otherwise as non-typhoon. The wind speed data measured in a typhoon area were
analyzed by Monte-Carlo simulation based on a typhoon model to obtain the extreme value
distribution, while those measured in a non-typhoon area were analyzed by the modified Jensen &
Franck method16)
in which wind speed data smaller than the highest value were also included as
independent storms for analysis.
4) Typhoon simulation technique
In Japan, typhoons are the dominant wind climates generating strong winds that need to be taken
into account in wind resistant design, due to their high wind speeds and large influence areas. An
average of 28 typhoons occur annually, of which roughly 10% land. Typhoons sometimes do not pass
near metrological stations, so severe wind damage may occur without large wind speeds being
observed. In order to improve the instability of the statistical data (sampling error), a typhoon
simulation method was adopted for evaluating the strong wind caused by typhoons.Figure A6.1.3 shows a general procedure of this typhoon simulation method. The pressure fields of
typhoons are modeled by several parameters, i.e. central pressure depth, radius to maximum winds,
moving speed, etc. The non-exceedance probability of strong wind in the target area is evaluated by
generating virtual typhoons according to the results of statistical analysis of pressure field parameters.
This Monte-Carlo simulation method is considered in recommendations of other countries. For
example, in the ASCE17)standard, simulation is required as a principle for evaluation of the design
wind speed in hurricane-prone regions. In this standard, the simulation results were adopted as the
value of basic wind speed. In order to improve the accuracy of typhoon simulation18), correlations
between gradient winds and near-ground winds and correlations among parameters of typhoon
pressure fields in each area are considered.
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CHAPTER 6 WIND LOADS C6-23
moving speed and
direction
radius of maximum wind
wind speed fieldgradient wind
surface wind
central pressure
depth
rate of occurrenceinitial position
Probability distributions
statistics of
historical typhoons
return period
pressure field
rate of occurrence
initial position
moving velocity
central pressure depth
radius of maximum wind
correlation of wind speed
and direction based onobserved records
Figure A6.1.3 General procedure for typhoon simulation
The non-exceedance probability of the annual maximum wind speed caused by a typhoon was
obtained from the typhoon simulation. For strong wind not caused by a typhoon, extreme value
analysis was conducted on data observed from 1961-2000. The results obtained from typhoon and
non-typhoon conditions were combined to evaluate the return period of annual maximum wind speed.
Figure A6.1.4 shows an example of the maximum wind speed evaluated at K city.
r
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Figure A6.1.4 Example of maximum wind speed evaluated at K city
5) Map of basic wind speedThe contour line of 100-year-recurrence wind speed was somewhat complicated even though the
data obtained in 4) had been homogenized according to surface roughness, wind direction, etc. This
was assumed to be due to the influences of local topography and structures surrounding the
metrological station and the applicability of the homogenization models. To remove such local effects,
spatial smoothing was conducted.
In addition, the lower limit of wind speed was set to 30m/s. It is difficult to include the effects of
tornado and downburst in the analysis.
6) 100-year-recurrence wind speed in winter
100-year-recurrence wind speed in winter is necessary for combination of wind loads and snow
loads. As for the basic wind speed, 100-year-recurrence wind speed in winter reflects only the effects
of large-scale topography. Figure A6.1.5 is a spatially smoothed wind speed map made for the
100-year-recurrence wind speed at metrological stations during the snow season (from December to
March). The procedure for making this map is the same as that for Fig.A.6.1.1, except that the typhoon
simulation method is not used. Thus, the wind directionality factor should not be used ( 1D =K ) here.
For return period factor rWk mentioned in A6.1.7, there are small differences in U among wind
speeds in winter for different meteorological stations. An average value of 1.1U = can be applied
for calculating rWk in Eq.(A6.12).
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Figure A6.1.5 100-year-recurrence 10-minutes mean wind speed at 10m above ground over a flat
and open terrain in winter (m/s)
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A6.1.4 Wind directionality factor
Meteorological stations in Japan have approximately 70 years of records at most. However, the
annual average number of typhoon landfalls in Japan is only three, so the number of typhoons
included in the records of a particular site is very limited. When the records are divided into 8 sectors
of azimuth, each sector have very few typhoon data, so sampling error is very large. Thus, typhoon
effect should be considered when wind directionality factor is determined. In these recommendations,
Monte-Carlo simulation for typhoon winds and statistical analysis on the non-typhoon observation
data had been conducted to obtain the wind directionality factor.
There are two types of wind directionality factors. One defines a wind directionality factor that
changes with direction, as shown in BS6399.219) and AS/NZS 1170.220),21), except for the
cyclone-prone regions. The other defines a constant reduction coefficient regardless of wind direction,
as in the ASCE
17)
standard. For the latter, it is hard to reflect directional design wind speeds in designpractice. In these recommendations, wind directionality factor was defined for each direction as for the
former type, so as to achieve reasonable wind resistant design.
Wind directionality factor was provided on the assumption that the wind load is calculated
according to the following procedure.
(1) Where the aerodynamic shape factors for each wind direction are known from appropriate wind
tunnel experiments, the wind directionality factor DK , which is used to evaluate wind loads on
structural frames and components/cladding for a particular wind direction, shall take the same value as
that for the cardinal direction whose 45 degree sector includes the wind direction. In this case, the
wind tunnel experiments should be conducted for detailed change of directional characteristics for the
aerodynamic shape factors of the structure.
(2) Where the aerodynamic shape factors in A6.2 are used
1) When assessing the wind loads on structural frames, two conditions are considered: whether or not
the aerodynamic shape factors depend on wind direction.
a) Where the aerodynamic shape factors are dependent on wind directions, four wind directions
should be considered that coincide with the principal coordinate axis of the structure. If the wind
direction is within a 22.5 degree sector centered at one of the 8 cardinal directions, the value of the
wind directionality factor DK for this direction should be adopted (Fig.A6.1.6(a)). If the wind
direction is outside the 22.5 degree sector, the larger of the 2 nearest cardinal directions should be
adopted (Fig.A6.1.6(b)). For lattice structures, the effect of inclined wind on the wind force
coefficient can be considered directly, so the same measures as for above rectangular cylinders are
adopted for the 4-leg square plane (8 directions) and 3-leg triangular plane (6 directions).
b) Where the aerodynamic shape factors are independent of wind directions, e.g. a structure that
has a circular sectional plan, the wind directionality factor DK shall take the same value as for the
cardinal direction whose 45 degree sector includes the wind direction.
2) When assessing wind loads on cladding according to the peak wind pressure coefficient in A6.2,those obtained under the condition of 1D =K should be used for design because the maximum peak
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CHAPTER 6 WIND LOADS C6-27
pressure coefficient of all directions is shown in these recommendations.
The wind directionality factors for the 8 cardinal directions shown in Table A6.1.1 were originally
obtained at 16 directions. When the 16 directional values are converted into 8 cardinal directional ones,
the values are determined to be the maximum of those for the relevant direction and its two
neighboring directions. Therefore, the value for a given direction represents the influence of a 67.5
degree sector centered on that direction. For a building with rectangular horizontal section, the wind
force coefficients for the wind directions normal to the building faces are given by these
recommendations. When the wind direction considered is at an intermediate position between two
cardinal directions shown in the table, the greater value of the two neighboring directions is adopted.
This means that the value considers the influence from a 112.5 degree sector. In addition, considering
the effects of tornado and downburst, which are difficult to take into account, the lower limit of wind
directionality factor is given as 0.85.
KD=0.9
NW
0.85
W
1.0
SW
0.95
NE
0.95
E
0.85
SE
0.85S
0.9
wind directionN
0.9
(a) Where the wind direction falls in a 22.5 degree sector as shown in Table A6.1.1
N
0.9larger value of 0.9 and 0.95
KD= 0.95
NW
0.85
W
1.0
SW
0.95
NE
0.95
E
0.85
SE
0.85S
0.9
wind direction
(b) Where the wind direction does not fall in a 22.5 degree sector as shown in Table A6.1.1
Figure A6.1.6 Selection of the wind directionality factor (when using the wind force coefficient of
buildings with rectangular horizontal sections defined in these recommendations)
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Where wind directionality effects are not considered, this corresponds to the condition where the
wind directionality factors equal unity for all directions. This leads a conservative design compared to
the condition when the wind directionality effects are considered.
Whether or not wind directionality effects are considered corresponds to whether or not wind
directionality factors are adopted. As shown in Table A6.1.1, the wind directionality factors are less
than unity, and are defined as values for evaluating 100-year-recurrence wind loads. It is possible to
achieve a more rational design by considering the orientation of the building plan from the viewpoint
of wind directionality factor. In other words, the wind loads are conservative if wind directionality
factor is not considered. However, the amount of this overestimation depends on the orientation of the
building, and not constant for all buildings. When wind directionality effects are considered, because
the wind directionality factor is less than unity, the wind loads will be smaller than those predicted by
conventional method, in which wind directionality is not taken into account. Designers should beconscious of the fact that safety level decreases when wind directionality factor is utilized.
The wind directionality factors defined in these recommendations are valid only for locations near
major metrological stations. The wind directionality factor defined in Table A6.1.1 can be applied to
construction sites near metrological stations, but they cannot be applied to construction sites far from
metrological stations and influenced by large-scale topography. For these situations, special
consideration should be given, for instance, by not using the wind directionality factors i.e. by setting
1D =K .
A6.1.5 Wind speed profile factor
(1) Effects of terrain roughness and topography on wind speed profile
Wind speed near the ground varies with terrain roughness, i.e. buildings, trees, etc., and topography.
The friction force from terrain roughness and the concentration or blockage effects from topography
influence the atmospheric boundary layer from the ground to the gradient height. In the
recommendations, the influence of surface roughness on the wind speed profile over flat terrain is
expressed by rE , while the influence of small-scale topographical features is represented by gE .
(2) Wind speed profile over flat terrain
Terrain roughness causes a gradual decrease in wind speed toward the ground. The domain than is
influenced by terrain roughness is called the boundary layer, where the wind speed profile changes
with terrain roughness category. The boundary layer depth increases with fetch length, which means
that the wind speed profile extends to a higher elevation downstream. In addition, the boundary layer
tends to develop faster when the terrain is rougher.
For a fully developed boundary layer, the velocity profile can be represented by a power law or a
logarithmic law. The following power law is adopted in the recommendations:
)(
0
0ZZ
Z
ZUU = (A6.1.1)
where ZU (m/s) is the mean wind speed at height Z(m), 0ZU (m/s) is the mean wind speed at height
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0Z , and is the power law exponent.
It has been realized from many observation data that the power law exponent becomes greater as the
terrain becomes rougher.
However, it is rare for the terrain roughness to be uniform over a long fetch. Roughness conditions
usually vary. When the terrain roughness changes suddenly, a new boundary layer develops according
to the new terrain roughness which gradually propagates with elevation and fetch, such that wind
speeds above this new boundary layer remain unchanged after the roughness change. Thus, the wind
speed profile corresponding to the new roughness condition can not be applied to the high elevation.
This tendency is particularly obvious when the wind flows from the sea to city center, where the
roughness changes suddenly from smooth to rough. After a fetch of approximately 3km (or 40H ) the
new boundary layer is considered fully developed. Hence, in the recommendations, the roughness
condition in the region of the smaller of 40H and 3km upstream from the construction site isconsidered when the roughness category, shown in Table A6.2 is to be determined.
The influence of terrain roughness becomes smaller at higher elevations. In the recommendations, it
is assumed that the design wind speed at GZ is not influenced by terrain roughness, and is
considered constant for convenience. However, it does not mean that wind speeds at elevations greater
than GZ are really constant. Since the boundary layer depth becomes greater when the terrain
roughness increases, GZ is assumed to increase with terrain category, as shown in Table A6.3.
However, GZ is defined just for the utilization of the power law for different terrain categories,
because the velocity profile is actually unknown in detail at higher elevations. It is different from the
boundary layer depth.
CFD studies on the wind speed profile in urban area show that the wind speed below a certain
height bZ does not follow the power law when the ratio of building plan area to regional area is over
a few percent, as shown in Fig.A6.1.7. The wind speed profile here is complex due to nearby buildings.
For heights below bZ , the wind speed at bZ is usually the maximum, so the wind speeds in this
region are assumed to equal to that at bZ , which is defined in Table A6.3, in order to arrive at a safer
design. For heights above bZ , the power law can approximate the mean wind speed profile.
Zb
height
Figure A6.1.7 Mean wind speed profile in urban area
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Figure 6.1.8 shows an example of mean wind speed profiles measured in natural wind22)
, in which
the wind speed profiles measured simultaneously at coastal and inland locations are compared. As
mentioned before, the wind speed near the ground decelerates due to the inland terrain roughness. As a
result, there is great difference between the wind speed profiles in the two locations.
The exposure factor rE of the flat terrain, shown in A6.1.5(2) 2), is defined with the above
considerations included. Figure A6.1.9 shows rE for each terrain category. The exposure factor is the
ratio of wind speed at a given height Z for each terrain category to the wind speed at 10m over
terrain roughness category II.
Mean wind speed (m/s)
Figure A6.1.8 Example of mean wind speed profiles measured simultaneously at the coast of Tokyo
bay and a suburban residential area 12km away22)
Exposure factor Er
terrain category
Figure A6.1.9 Exposure factor rE
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Figure A6.1.10 shows an example of terrain roughness categories.
Terrain category I represents open sea or lake, or unobstructed coastal areas on land.
Terrain category II is defined as terrain with scattered obstructions up to 10m high. Rural areas are
representative. Low rise building areas also belongs to this category, if the building area ratio (total
building plan area divided by regional area) is less than 10.0%.
Terrain category III is characterized be closely spaced obstructions up to 10m high, or by sparsely
spaced medium-rise buildings of 4-9 stories. Suburban residential areas, manufacturing districts, and
wooded fields are typical of this category. The area where the building area ratio is between 10% and
20%, or the building area ratio is larger than 10% while the high-rise building ratio (plan area of
buildings higher than 4 stories divided by total area of buildings) is less than 30% belongs to this
category. The example in Fig.A6.1.10(c) is an area with a building area ratio of 30% and a high-rise
building ratio of 5-20%.
(a) Terrain category I (b) Terrain category II
(c) Terrain category III (d) Terrain category IV
(e) Terrain category V
Figure A6.1.10 Example of surface roughness (Photos provided by Kindai Aero Inc.)
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Terrain category IV is mainly where many 4-9 story buildings stand. Local central cities are typical
of this category. Areas with a building area ratio larger than 20%, and a high-rise building ratio larger
than 30% belong to this category.
In terrain category V, tall buildings of 10 or more stories are close together at a high density. Central
regions of large cities such as Tokyo and Osaka belong to this category.
In an area where the building purpose, floor area ratio and building coverage ratio are the same, the
terrain can usually be considered uniform. Typically, in the wide area around the construction site,
the terrain roughness is not usually identical. It is common for several terrain categories to co-exist.
When the terrain roughness changes downstream, a new boundary layer gradually develops, and the
developing process depends on whether the change is from smooth to rough or rough to smooth.
Figure A6.1.11 illustrates approximately the development of a new boundary layer with a terrain
roughness change from smooth to rough. When the terrain roughness changes from smooth to rough,the new boundary layer develops slowly, so the fully developed boundary layer over the new
roughness can not be anticipated if the fetch downstream is not long enough. As a result, a wind speed
profile corresponding to the new roughness category can not be adopted. Thus, if there is a terrain
roughness change from smooth to rough within a distance of the smaller of 40H and 3km upstream
of the construction site, the terrain category at the upstream region before the roughness change will
be adopted as the terrain category for the construction site.
Figure A6.1.11 Developing process of new boundary layer when terrain roughness changes from
smooth to rough
In determining the terrain category for a given wind direction, the upwind area inside a 45 degree
sector within a distance of the smaller of 40H and 3km of the construction site will be counted.
When there is a terrain roughness change upwind of the construction site, a weighting average of
the wind speed profile on roughness and the fetch distance is conducted in AS/NZS 1170.2 20) to
determine the exposure factor.
However, in the recommendations, the overall terrain roughness in the upwind sector is adopted as
the terrain category in this direction if there is no sudden roughness change. Generally, the wind load
will be overestimated when a smoother surface roughness category is utilized.
3 ~ 5kmSmooth Rough
developing internalboundary layer
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For an urban area centered on a railway station, larger buildings are closely spaced near the station.
Figure A6.1.12 shows an example of how to determine the terrain category if a construction site is
near a railway station, in which the roughness changes from smooth to rough downstream. In this case,
where there is a sudden roughness change within a distance of the smaller of 40H and 3km upwind
of the construction site, the smoother terrain category upwind before the terrain roughness change will
be selected.
Wind
Category I
Category III
smaller of
40H and 3kmThe terrain roughness
in this wind direction
should be recognized as
category I.
Figure A6.1.12 Selection of terrain category (with terrain roughness change from smooth to rough)
If the terrain roughness changes from rough to smooth, the terrain category after the terrain
roughness change is selected. However, if there is a smooth area in a rough area, e.g. a park in a
downtown area, it is sometimes necessary to consider the acceleration of wind speed near the ground
downstream.
Generally, careful consideration should be given in the determination of terrain category, because of
the arbitrariness.
(3) Topography factor
When air flow passes escarpments or ridge-shaped topography as shown in Fig.A6.1.13, the flow is
blocked on the front of the escarpment and the mean wind speed decreases. Then the flow starts to
accelerate uphill, resulting in a mean wind speed larger than that of the flat terrain from the middle of
the upwind slope to the top of the topographic feature. If the upwind slope is not large enough, the
mean wind speed is larger than that over the flat terrain over a long region downstream of the hill top.
However, if the upwind slope is sufficiently steep to establish separation downstream of the hill top,
the wind speed downstream of the hill top near the ground is smaller than that of the flat terrain.
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Figure A.6.1.13 Change of mean wind speed over an escarpment (thin solid line and thick solid line
are for the mean wind speed over flat terrain and escarpments respectively)
Equation A6.5 for the topography factor is based on the results of wind tunnel experiments of
two-dimensional escarpments and ridge-shaped topography with different slopes23), 24), 25)
. The
experiments were carried out with an approach flow corresponding to terrain category II. The models
corresponded to escarpments and ridge-shaped topography with heights between several tens of meters
to 100m with smooth surfaces. The ratio of the mean wind speed over the escarpments to the
counterpart over flat terrain was obtained from the experiments. The height Z in Eq.(A6.5) is the
height from the local ground surface over the topographic feature. The slope angle is defined with the
aid of the horizontal distance from the top of the topographic feature to the point where the height is
half the topography height.
Although, the wind speed decreases upwind of the escarpment and in the separation region
downstream of steep topography, the topography factor in these regions is defined as 1 in the
recommendations, as shown in Figs.A6.1.14 and A6.1.15, because only acceleration of wind speed is
considered24)
.
Figure A6.1.14 Wind speed-up ratio over a two-dimensional escarpment with an inclination angle of
60 degrees. The symbols are for the experimental results, and the solid lines are for
Eq.(A6.5)
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CHAPTER 6 WIND LOADS C6-35
Figure A6.1.15 Wind speed-up ratio over a two-dimensional ridge-shaped topography with
inclination angle of 30 degrees. The symbols are for the experimental results, and
the solid lines are for Eq.(A6.5)
Tables A6.4 and A6.5 show the values of the parameters in Eq.(A6.5) for the escarpment and
ridge-shaped topography determined from experiment. For a particular location and a particular slope
angle, not shown in these tables, the topography factor can be obtained by linear interpolation. The
following is an example of the procedure for calculating the topography factor of a 50-degree
escarpment, at a location with a distance ss 6.1 HX = downstream of the top of the escarpment at a
height s5.1 HZ= .
Calculate the topography factor 1gE and 2gE at 1/ ss =HX and 2 for the inclination
angle of 45 degrees from Eq.(A6.5), and then calculate the topography factor 12gE at
6.1/ ss =HX by linear interpolation according to the following equation:
2g1g12g 6.04.0 EEE +=
Calculate the topography factor 34gE for the inclination angle of 60 degrees in the same
way as for the inclination angle of 45 degrees.
Conduct linear interpolation for topography factors 12gE and 34gE , with respect to the
inclination angle to achieve the topography factor at an inclination angle of 50 degrees
and 6.1/ ss =HX from the following equation.
34g12gg3
1
3
2EEE +=
If the inclination angle is less than 7.5 degrees, the topography effect can be neglected.
The topography factor calculated from Eq.(A6.5) is shown in Figs.A6.1.14 and A6.1.15 by a solid
line. It agrees well with the experimental data at all sections with speedup..
Equation (A6.5) is for the condition in which the air flow passes at right angles to the
two-dimensional escarpments and ridge-shaped topography. However, strict two-dimensional hills do
not exist, and flow does not always pass escarpments and ridge-shaped topography at right angles.
However, even in these conditions, Eq.(A6.5) can be applied if the terrain extends a distance of several
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times the height of the topographic feature in the traverse direction. In addition, as has been shown in
experimental and CFD studies, the speed-up ratio of two-dimensional topography is greater than that
of three-dimensional topography, and so application of Eq.(A6.5) to three-dimensional topography is
conservative26)
.
Complex terrain may increase the wind speed in valleys, which is not considered in this equation. In
such cases, it is recommended to investigate the topography factor by wind tunnel or CFD studies
when the construction site is very complex.
Figure A6.1.16 Interpolation procedure for calculating topography factor with inclination angle of
50 degrees and 6.1/ ss =HX
A6.1.6 Turbulence intensity and turbulence scale
Natural wind speed fluctuates with time. The wind speed )(tU at a point, shown in Fig.A6.1.17,
can be separated into a mean wind speed component U and a fluctuating wind speed component
)(tu in the longitudinal direction as well as )(tv and )(tw in the cross wind directions. Usually, the
longitudinal fluctuating wind speed component )(tu is important for design of buildings, so only the
characteristics of )(tu are defined in the recommendations. For long-span structures such as bridges
and for tall slender buildings, the vertical and lateral fluctuating wind-speed components )(tw and)(tv are also sometimes important.
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Figure A6.1.17 Mean wind and component of turbulence
(1) Turbulence intensity
1) On flat terrain
Wind speed fluctuation can be expressed quantitatively by a statistical approach. Turbulenceintensity I indicates the turbulence level and it is defined in the following equation as the ratio of
standard deviation of the fluctuating component u to the mean wind speed U.
UI u
= (A6.1.2)
Turbulence is generated by the friction on the ground and drag on surface obstacles, and is
influenced by the terrain roughness just as is the mean wind speed profile. Figure A6.1.18 shows the
turbulence intensity observed in the natural wind and the recommended values calculated from
Eq.(A6.8).
eq.(A6.8) eq.(A6.8) eq.(A6.8) eq.(A6.8) eq.(A6.8)
Turbulence intensityIrZ Turbulence intensityIrZ Turbulence intensityIrZ Turbulence intensityIrZ Turbulence intensityIrZ
Figure A6.1.18 Observed turbulence intensity27)
and recommended value
The turbulence intensity ZI at height Z above the ground, is defined in Eq.(A6.7), in which the
turbulence intensity rZI on flat terrain expressed in Eq.(A6.8), and the topography factor gIE , shown
in Tables A6.6 and A6.7, is considered separately.
2) Topography factor for turbulence intensity
Not only the mean wind speed, but also the wind speed fluctuation is influenced by topography.
Terrain category I Terrain category II Terrain category III Terrain category IV Terrain category V
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Especially in the separation region, there is an obvious increase in the standard variation of the wind
speed fluctuating component )(tu (fluctuating wind speed hereafter) compared to that on flat terrain,
as Figs.A.6.1.19 and A6.1.20 show. Mean and fluctuating wind speed variation are closely related..
The location of the maximum fluctuating wind speed generally corresponds to the location where the
vertical gradient of mean wind speed is maximum. The region where the fluctuating wind speed is
greater than the flat terrain counterpart is generally inside the separation region when the mean wind
speed is smaller than that on flat terrain.
Figure A6.1.19 Topography factor for fluctuating wind speed on an escarpment with inclination
angle of 60 degrees. The symbols are for the experimental results, and the thick
solid lines are for Eq.(A6.10).
Figure A6.1.20 Topography factor for fluctuating wind speed on ridge-shaped topography with
inclination angle of 30 degrees. The symbols are for the experimental results, and
the thick solid lines are for Eq.(A6.10).
In the recommendations, the topography factor for turbulence intensity is defined as the ratio of the
topography factor for fluctuating wind speed to the topography factor for mean wind speed.
Topography factor for fluctuating wind speed is defined in Eq.(A6.10), in which the values of the
parameters besides 1C , 2C and 3C are identical to those in Eq.(A6.5) for the topography factor for
mean wind speed. Equation (A6.10) is based on the results of wind tunnel experiments on escarpments
and ridge-shaped topography, as for Eq.(A6.5). The experiments were carried out with an approach
flow corresponding to terrain category II. The models corresponded to escarpments and ridge-shaped
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CHAPTER 6 WIND LOADS C6-39
topography with a height of about 50m23), 24), 25)
. Topography factors of mean wind speed and
fluctuating speed are defined to be greater than 1 without considering the decrease in mean wind speed
and fluctuating wind speed due to topography effects24)
. However, when the topography factor for
fluctuating wind speed is smaller than that for mean speed, the topography factor for turbulence
intensity will be smaller than 1.
Fluctuating wind speed near the ground becomes greater on the leeward slope of escarpments or
ridge-shaped topography. In these regions the mean wind speed is smaller, which results in the
maximum instantaneous wind speed being smaller than that for flat terrain in this area, as shown in
Fig.A6.1.21. Because the decrease in mean wind speed is not considered in A6.1.5, the maximum
instantaneous wind speed, and thus the wind load, is possibly overestimated in the separation region if
only the topography factor of fluctuating wind speed is fitted to the experimental data. In order to
reduce this possible overestimation, the actual topography factor for the fluctuating wind speed (
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Eq.(A6.10) for the ridge-shaped topography because of the complexity of the change of fluctuating
wind speed, but the coincidence is good where the topography factor of mean speed is larger than 1.
Although Eq.(A6.10) is obtained from experiments carried out on a two-dimensional escarpment and
ridge-shaped topography with the oncoming airflow passing at right angles, it can be applied to
topography that extends a long distance in the transverse direction several times the height of the
topography26)
. However, if the construction site is in a complex terrain, it is necessary to investigate
the topography factor for fluctuating wind speed by wind tunnel or CFD studies.
(2) Power spectral density
Power spectral density reflects the contribution to turbulence energy at each frequency. In the
recommendations, a von Karman type power spectrum, expressed by Eq.(A6.1.3), is employed to
express the power spectral density of fluctuating component of wind speed )(tu .
6/52
2
uu
})/(8.701{)/(4)(
UfLULfF
+= (A6.1.3)
where
f : frequency
u : standard deviation of fluctuating component of wind speed )(tu
U: mean wind speed
L : turbulence scale
(3) Turbulence scale
Equation (A6.11) is used as the turbulence scale ZL of the wind speed fluctuation )(tu at height
Z.
Turbulence scale is an important parameter in the power spectrum, expressed in Eq.(A6.1.3). It is
the averaging length scale of the turbulence vortices. Figure A6.1.22 shows an example of a profile of
turbulence scale, which can be expressed in Eq.(A6.11) independently of terrain category.
eq.(A6.11)
Figure A6.1.22 Observation of turbulence scale of wind speed fluctuation )(tu
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(4) Co-coherence
Co-coherence of wind speed fluctuation ),,( yzu rrfR is evaluated using Eq.(A6.1.4). It expresses
quantitatively the frequency-dependent spatial correlation of the wind speed fluctuation.
+=
U
rkrkfrrfR
2y
2y
2z
2z
yzu exp),,( (A6.1.4)
where
f : frequency
yz , rr : distance between 2 points in the vertical and horizontal directions
yz ,kk : decaying factors reflecting the degree of spatial correlation of wind speed in the
vertical and horizontal directions
U: mean wind speed averaged at two points
It has been shown by observation that the decay factor is between 5-10.
A6.1.7 Return period conversion factor
Return period conversion factor rWk is defined as the ratio of the r-year-recurrence wind speed
rU to the 100-year-recurrence basic wind speed 0U . In these recommendations, the maximum wind
speed corresponding to an r-year return period should be estimated using Eq.(A6.1.5), assuming a
Gumbel distribution for annual-maximum wind speeds.
b
r
r
a
U +
=
1
lnln1
r (A6.1.5)
where a and b are coefficients. Return period conversion factor krWis calculated approximately in
Eq.(A6.12) by using the parameter U , which is the ratio of the 500-year-re