chapter6 com

8/10/2019 Chapter6 Com

1/81

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


6.2.2 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



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


2/81

C6-2 Recommendations for Loads on Buildings

A6.4.2 Procedure

A6.5 Torsional Vibration and Resulting Wind Load


A6.5.2 Procedure

A6.6 Horizontal Wind Loads on Lattice Structural Frames


A6.6.2 Procedure for estimating wind loads

A6.6.3 Gust effect factor

A6.7 Vortex Induced Vibration


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


A6.8.2 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


A6.9.2 Procedure

A6.10 Response Acceleration


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


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


3/81


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.


4/81


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


5/81


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


6/81


*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


7/81


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


(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


8/81


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


9/81


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


10/81


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


11/81


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.


12/81


(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.


13/81


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






A6.1.6 Turbulence intensity and turbulence

scale


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


for roof wind loads


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


14/81


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


15/81


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


16/81


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


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 .


17/81


D

wind

Figure 6.2.1 Projected area

6.3 Roof Wind Load on Structural Frames


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


18/81


6.4 Wind Loads for Components/Cladding


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.


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


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).


19/81


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.


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).


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


20/81


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


21/81


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


22/81


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.


23/81


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


24/81


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).


25/81


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)


26/81



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


27/81


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)


28/81


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 .


(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


29/81


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


30/81


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


31/81


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.)


32/81


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


33/81


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.


34/81


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)


35/81


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


36/81


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.


37/81


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


38/81


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


39/81


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 (


40/81


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


41/81


(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.


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

chapter6 com

Documents