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 Frames 6.3.1 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
A6.4.2 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 circular
sections
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
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 the maximum 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 building’s 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.
Notation
Notations used in the main text of this chapter are shown here. Uppercase Letter
A(m2): projected area at height Z R
A (m2): subject area C
A (m2): subject area of components/cladding
0
A (m2): whole plane area of one face of lattice structure
F
A (m2): projected area of one face of lattice structure B(m): building breadth
1
B (m): building length in span direction
2
B (m): building length in ridge direction
0
B ,BH(m): width of lattice structure in ground and width at height H
D
B : background excitation factor for lattice structure 1
C ,C2,C3: parameters determining topography factor Eg and EI
D
C ,CR,CX,CY: wind force coefficients '
L
C ,CT' : rms overturning moment coefficient and rms torsional moment coefficient e
C : 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.
g
C : overturning moment coefficient in along-wind direction '
g
C : rms overturning moment coefficient in along-wind direction f
C : wind force coefficient. For horizontal wind loads, wind force coefficient CD defined in A6.2 with kZ =0.9 shall be used. For roof wind loads, wind force coefficient CR defined in A6.2 shall be used.
pe
C : external pressure coefficient pe1
C ,Cpe2: external pressure coefficients on windward wall and leeward wall pi
C : internal pressure coefficient *
pi
C : factor for effect of fluctuating internal pressure r
C : wind force coefficient at resonance C
ˆ
C : peak wind force coefficient
pe
ˆ
C : peak external pressure coefficient
D(m): building depth, building diameter, member diameter B
D (m): building diameter at the base m
D (m): building diameter at height of 2H/3 E: wind speed profile factor
H
I
E : topography factor for turbulence intensity
g
E : topography factor for wind speed
gI
E : topography factor for turbulence intensity
r
E : exposure factor for flat terrain categories
D
F : along-wind force spectrum factor F: wind force spectrum factor
D
G : gust effect factor for along-wind load
R
G : gust effect factor for roof wind load H (m): reference height
S
H (m): height of topography
T
I (kgm2): generalized inertial moment of building for torsional vibration
Z
I : turbulence intensity at height Z
rZ
I : turbulence intensity at height Z on flat terrain categories
D
K : wind directionality factor L(m): span of roof beam
S
L (m): horizontal distance from topography top to point where height is half topography height
Z
L (m): turbulence scale at height Z M(kg): total building mass
D
M (kg): generalized mass of building for along-wind vibration
L
M (kg): generalized mass of building for across-wind vibration
R: factor expressing correlation of wind pressure of windward side and leeward side
D
R : resonance factor for along-wind vibration
L
R : resonance factor for across-wind vibration
T
R : resonance factor for torsional vibration
Re
R : resonance factor for roof beam
D
S : size effect factor
0
U (m/s): basic wind speed
1
U (m/s): 1-year-recurrence 10-minute mean wind speed at 10m above ground over flat and open terrain
1H
U (m/s): 1-year-recurrence wind speed
500
U (m/s): 500-year-recurrence 10-minute mean wind speed at 10m above ground over flat and open terrain
H
U (m/s): design wind speed *
Lcr
U ,UTcr* : non-dimensional critical wind speed for aeroelastic instability in across-wind and torsional directions
r
U (m/s): resonance wind speed *
T
* r
U : non-dimensional resonance wind speed D
W (N): along-wind load at height Z L
W (N): across-wind load at height Z T
W (Nm): torsional wind load at height Z LC
W (N): across-wind combination load R
W (N): roof wind load SC
W (N): wind load on components/cladding obtained by simplified method Sf
W
(N): wind load on structural frames rW (N): wind load at height Z S
X (m): distance from leading edge of topography to construction site Z(m): height above ground
b
Z ,ZG(m): parameters determining exposure factor
Lowercase Letter
Dmax
a , aLmax (m/s2), aTmax (rad/s2): maximum response acceleration in along-wind, across-wind and torsional directions at top of building
b(m): projected width of member f (m): rise
1
f (Hz): The smaller of fL and fT D
f , fL, fT(Hz): natural frequency for first mode in along-wind, across-wind and torsional directions
R
f (Hz): natural frequency for first mode of roof beam aD
g ,gaL,gaT: peak factors for response accelerations in along-wind, across-wind and torsional directions
D
g ,gL,gT: peak factors for wind loads in along-wind, across-wind and torsional directions
h(m): eaves height 1
k : factor for aspect ratio 2
k : factor for surface roughness 3
k : factor for end effects 4
k : factor for three demensionality C
k : area reduction factor rW
k : return period conversion factor Z
k : factor for vertical profile for wind pressure coefficients or wind force coefficients l(m): smaller value of 4H and B, minimum value of 4H ,B1 and B2, member
length a1
l (m): smaller value of H and B1 a2
H
q (N/m2): velocity pressure at reference height H Z
q (N/m2): velocity pressure at height Z r(year): design return period
Re
r : coefficient of variation for generalized external pressure x(m) : distance from end of component
Greek Alphabet
α : exponent of power law for wind speed profile β : exponent of power law for vibration mode γ : load combination factor
δ
,δL,δT: mass damping parameter for vortex induced vibration, across-wind vibration and torsional vibrationD
φ ,φL,φT: mode correction factor for vortex induced vibration, across-wind vibration and torsional vibration
D
ζ ,ζL,ζT: critical damping ratio for first translational and torsional modes R
ζ : critical damping ratio for first mode of roof beam ϕ: solidity
λ: mode correction factor of general wind force U
λ : U500/U0
μ: first mode shape in each direction D
ν (Hz): level crossing factor
θ(°): roof angle, angle of attack to member S
θ (°): inclination of topography ρ(kg/m3): air density
S
ρ (kg/m3): building density which is /( ) B mD HD M LT
ρ : 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
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 and their 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
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 on structural frames
wind load on components/cladding
horizontal wind load roof wind load
along-wind load across-wind load torsional wind load wind load simplified procedure small-scale building wind load on structural frames wind load on components/cladding
vibration direction vortices wind turbulence
a) fluctuating wind force caused by wind turbulence
vibration direction
b) fluctuating wind force caused by vortex generation in wake of building
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 wind directionality 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 structure high-rise building dome house qZ qH qH qH Z H H H
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 same procedure 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.
(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 at meteorological 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 building’s 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.
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/cladding Wind 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
pressures
A6.2.7 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
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-wind wind torsion Ｄ Ｈ Ｂ
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 building’s 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, 1/(0.024 )
1 H
f = (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
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 results 1)-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 (H/ BD or H/Dm). 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 (U/f BD or
m
/fD
U ). 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
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 building’s mass damping parameter. It is thus recommended that the damping ratio of a building be estimated through reference to “Damping in Buildings” 7).
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, a quasi-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.
Ａ ＷD Ｄ Ｈ Ｂ 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 building’s 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 m/ρL<3, 1UH/ fR1L> and 15IH <0. , where m is mass per unit area, ρ is air density, L is span length, UH is design wind speed, fR1 is frequency of first unsymmetrical vibration mode and IH is turbulence intensity at reference height10),11). In addition, note that large amplitude vibration may occur on large-span roofs with light weight because the deflection or oscillation-induced wind force due to mean wind pressure seems to make the stiffness weak. In these cases, wind tunnel tests must be carried out to ensure that aerodynamic instability such as self-excited oscillation does not occur within the design wind speed. 6.3.2 Procedure for estimating wind loads
The equivalent static wind loads on roofs can be estimated by the gust effect factor method, which includes the effects of fluctuating external pressures and fluctuating internal pressures for roof responses. The gust effect factor is only formulated under the condition where beam oscillation is dominated by the fundamental mode. The equivalent static wind load distribution that produces the maximum load effect on a roof is not strictly similar to the mean wind pressure distribution. However, to simplify the procedure, the wind load can be estimated by multiplying the gust effect factor by the mean wind force distribution.
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 CˆC corresponding to the peak values of fluctuating net pressures, defined by the difference 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 CˆC 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 area AC depends 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
( )
1 2 ρU2, 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, qH, which is based on the design wind speed UH at the reference height H , is defined in Eq.(A6.1).
Air density ρ varies with temperature, ambient pressure and humidity. However, the influence of humidity is usually neglected. In these recommendations, the air density is taken as ρ =1.22(kg/m3), which corresponds to a temperature of 15°C 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
U
0 and wind directionality factor KD. The influences of surface roughness, small-scale topographical features and elevation are reflected in the wind speed profile factorEH.Designers are required to decide the wind load level by considering the building’s social importance, occupancy, economic situation and so on. This is represented by the return period conversion factor
rW
k . 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 krW =1. The wind directionality factor KD, 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 KD 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 UH can be calculated by substituting 1KD = in Eq.(A6.2).
A6.1.3 Basic wind speed
The basic wind speed U0 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
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
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 urbanization and 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
Fl at t erra in cat eg ories V IV III II I Flat t erra in cat eg ories V IV III II I Fl at t erra in cat eg ories V IV III II I Fl at t erra in cat eg ories V IV III II I
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.
moving speed and direction
radius of maximum wind
wind speed field gradient wind surface wind central pressure depth rate of occurrence initial 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 on observed 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.
Figure A6.1.4 Example of maximum wind speed evaluated at K city
5) Map of basic wind speed
The 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 (KD =1) here. For return period factor krW mentioned in A6.1.7, there are small differences in λU among wind speeds in winter for different meteorological stations. An average value of λU =1.1 can be applied for calculating krW in Eq.(A6.12).
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)
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 ASCE17) standard. For the latter, it is hard to reflect directional design wind speeds in design practice. 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 KD, 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 KD 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 KD 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 KD =1 should be used for design because the maximum peak
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 0.85 S 0.9 wind direction N 0.9
(a) Where the wind direction falls in a 22.5 degree sector as shown in Table A6.1.1
N
0.9 larger 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 0.85 S 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
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 be conscious 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
1
D =
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 Er, while the influence of small-scale topographical features is represented by Eg. (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:
α ) ( 0 0 Z Z Z Z U U = (A6.1.1)
0
Z
, andα
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 is considered 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 ZG is not influenced by terrain roughness, and is considered constant for convenience. However, it does not mean that wind speeds at elevations greater than ZG are really constant. Since the boundary layer depth becomes greater when the terrain roughness increases, ZG is assumed to increase with terrain category, as shown in Table A6.3. However, ZG 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 Zb 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 Zb, the wind speed at Zb is usually the maximum, so the wind speeds in this region are assumed to equal to that at Zb, which is defined in Table A6.3, in order to arrive at a safer design. For heights aboveZb, the power law can approximate the mean wind speed profile.
Zb
height
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 Er of the flat terrain, shown in A6.1.5(2) 2), is defined with the above considerations included. Figure A6.1.9 shows Er 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 A6.1.10 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
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 1170.220) 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 ~ 5km
Smooth Rough
developing internal boundary layer
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 3km
The 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.
