ぎ

FOREWORD

The Code of Practice on Wind Effects in Hong Kong 2004, prepared under the direction of theAd hoc Committee on Review of the Code of Practice on Wind Effects, supersedes the Codeof Practice on Wind Effects Hong Kong -1983.

This Code introduces several new concepts that are in keeping with developments inunderstanding of the response of structures to wind action and new wind speed recordsmeasured in Hong Kong.

The main changes in this code devolve from the recognition of the time varying nature of windaction. Accordingly, an assessment of resonant dynamic response is introduced with guidancegiven on the assessment of mean and turbulent wind characteristics.

For the assessment of resonant dynamic response, a signpost is provided to show whetherresonant dynamic effect should be considered or not. In the case that it is, then several newelements of assessment are included in this code to make the estimation of dynamic responsemore precise. These include estimates of turbulence intensity, damping, natural frequency andother descriptors of wind energy parameters. Where the resonant dynamic response is notsignificant, the assessment of wind effects will be broadly similar to the Code of Practice onWind effects Hong Kong -1983.

The two terrain categories used in the former code were replaced with a single general terrainand new guidance on the effect of topogaphy on local wind field is given in this code.

New guidance on wind tunnel testing derived from multi-national research findings and othernational wind codes is also included in this code.

Acknowledgement

The preparation of this code owes a grcat deal to the time and effort given by Dr. K.M. Lam,Professor Alan Jeary, Ir. J. MacArthur, Ir. K.L. Lo, Ir. K.S. Wong, Ir. P.K. Li, Ir. C.C. Wong,h. Y.C. Tsui and the Chairman of the Ad-hoc Committee to review the Code of Practice onWind Effects, h. K.M. Cheung.

A draft of the code has been circulated for general comment to selected practicing engineers,building professionals and Government Departments. All comments and views expressed havebeen taken into consideration in the preparation of the code now published.

Thank is also due to the Hong Kong Observatory for providing the cloud imagery on thecover page which was originally caphred with the Geostationary Meteorological Satellite(GMS-5) of Japan Meteorological Agency.

@ The Government of the Hong Kong Special Administrative Region

First published : December 2OO4

Prepared by: Buildings Department,l2/F-l&E Pioneer Centre.750 Nathan Road,Mongkolq Kowloon,Hong Kong.

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Code of Practice on Wind Effects

ln】臣ong】Kong

2004

愚頓受隆砂8脳韓S

COttWS

1 .

2.

3.

4.

5.

6.

7.

SCOPE

DEFINI口 ONS

CALCLILAT10N OF WIND LOADS

DESIGN WIND PttSSlЛ 述,S

FORCES ON BLIILDINGS

FORCES ON BLIILDING EttNTS

DYNAヽ 伍C EFFECTS

Page

I

2

2

3

4

4

7

9

1 0

1 4

1 7

1 9

Ａ

　

Ｂ

APPENDICES

C.

D.

E.

F.

NECESSARY PROVIS10NS FOR WIND TLINNEL TESTING

WIULTIPLICAT10N FACTOR FOR ttTURN PERIOD GREATER

TIIrAN 50 YEARS

TOPOGRAPHY FACTOR

FORCE COEFFICIENTS

TOTAL PRESSURE COEFFICENTS Cp FOR INDIVIDUAL

ELEMENTS

D Y N A M I C A N A L Y S I S

(111)

SCOPE

This Code of Practice gives general methods for calculating the wind loads to beused in the structural design of buildings or parts of buildings. The Code does not

apply to buildings of an unusual shape or buildings situated at locations where

complicated local topography adversely affects the wind conditions. Experimentalwind tunnel data with reference to local conditions, where available, may be used inplace of the values given in this Code. Necessary provisions for wind tunnel testing

are given in Appendix A.

The design wind pressures given in this Code have been determined from the hourlymean wind velocities and peak gust wind velocities having a retum period of 50years. Design wind pressures on buildings where a longer period of exposure to thewind is required shall be determined from wind velocities having a retum periodgreater than 50 years. Appendix B provides the multiplication factor for design windpressure ofretum period greater than 50 years.

No buildings excep those mentioned in Clause 4.3 and Clause 7.4 shall be designedwith design wind pressures determined from wind velocities having a return period ofless than 50 years.

DEFINITIONS

For the pu{poses of this Code, the following definitions apply:

"Building" means buildings as defined in section 2 ofthe Buildings Ordinance.

"Breadth" means the horizontal dimension of the building normal to the direction ofthe wind.

uDepth" means the horizontal dimension of the building parallel to the direction ofthe wind.

"Frontal projected area" means the area of the shadow projection of the building ona plane normal to the direction of the wind.

"Height" means the height of the building above the site-ground level in theimmediate vicinity of the building up to the top of the building. Masts andother appendages on top of the building should not be included.

■2

1,3

2.

３

　

　

３

CALCULATION OF WII\D LOADS

The design wind force on a building or parts of a building shall be calculated inaccordance with sections 4, 5 and 6 unless the building has significant resonantdynamic response.

A building with significant resonant dynamic response requires a more detailedanalysis than those exhibiting an essentially static type of behaviour.

For the purpose of this Code, a building is considered to be one with significantresonant dynamic response if it has either of the following properties, unless it couldbe justified that the fundamental natural frequency of the building is greater than Illz:-

(a) The height exceeds five times the least horizontal dimension.

(b) Theheightofthebuildingisgreaterthan 100m.

For the purpose of this clause, the least horizontal dimension shall be taken as thesmallest dimension of the rectangular envelope enclosing the main vertical structuralelements of the building.

The along-wind forces on a building with significant resonant dynamic response shallbe assessed in accordance with section 7.

DESIGN WIND PRESSTJRES

Except as provided in Clause 4.3, the design wind pressure q, at height z shall betaken as the value given in Table l.

Where topography is considered significant the design wind pressure shall bemultiplied by a topography factor assessed in accordance with Appendix C.

Temporary buildings or buildings which will remain in position for a period of notmore than one year may be designed with wind pressures of not less than 70 per centof the pressures given in Table l.

No allowance shall be made for the general or specific shielding of other structures ornatural feafures.

3.2

3.3

3.4

4.

4.1

4.2

4.3

4,4

５

　

　

５

Table 1 : Design wind pressure

Height abovesite-ground level

Design wind pressure q,(KPJ

≦5 m

101■

2 0 m

301n

501n

75m

長00 nl

150 rn

200m

2501ttll

30m

40硫

5 0 0 m

1.82

2.01

2.23

2.37

2.57

2.73

2.86

3.05

3.20

3.31

3.41

3.58

3.72

Note : For intermediate values of height, linear interpolation is permitted.

FORCES ON BUILDINGS

The total wind force F on a bullding shall be taken to bc the suIImation of the

prcssurcs acting on the cffcctive pttjected areas of thc bullding ttd shall bcdetemined by the following equation t

^ s lct ZtqA,. (1)

Where C, is the force coefficient for the building, determined in accordance with

Appendix D;

qr. is the design wind pressure at height z, determined in accordancewith section 4: and

Az is the effective projected area of that part of the building

corresponding to qr.

The effective projected area of an enclosed building shall be the frontal projectedatea. The effective projected area of an open framework structure such as signframes and lattice towers shall be the aggregate projected area of all members on aplane normal to the direction of the wind.

5,2

5.3 Every building shall be designed for the effects of wind pressures acting along eachof the critical directions.

6. FORCES ON BTIII,DING ELEMENTS

6.1 The total wind force Fo acting in a direction normal to the individual elements suchas walls, roofs, cladding panels or members of open framework sfuctures shall bedetermined by the following equation:

Fo : cpgr4n Q)

where Co is the total pressut€ coefficient for individual elements, determinedin accordance with Appendix E;

g, is the design wind pressure corresponding to the height z of theelement, determined in accordance with section 4; and

A- is the surface area of the element.

6.2 Except for members of open framework structures, the design wind pressure, q, shallbe a constant value over the lower part of the building. The height up to which thisconstant value occurs is to be taken as the breadth of the building or the actual heightof the building whichever is the lesser. The constant value shall be taken as thedesign wind pressure at this height.

7. DYNAMIC EFFECTS

7.1 The total along-wind force F on an enclosed building with significant resonantdynamic response shall be determined by the following equation :

F : c Cf Xq,A., (3)

where G is the dynamic magnification factor to be determined in accordancewith Appendix F;

Cf is the force coefficient for the structure, determined in accordancewith Appendix D;

q, is the design hourly mean wind pressure at height z given in Table2:and

A, is the effective projected area of that part of the building

corresponding to %

7.2 Pressures and forces on the individual elements such as walls, roofs, cladding panelsof a building with significant resonant dynamic response or members of openframework structures shall be determined in accordance with section 6.

Table 2 : Design hourly mean wind pressure

Note: For intermediate values of height, linear interpolation is permitted.

where the topography is considered significant, the design hourly mean windpressure shall be multiplied by a topography factor assessed in accordance withAppendix C.

Temporary buildings or buildings which will remain in position for a period of notmore than one year may be designed with design hourly mean wind pressures of notless than 70 per cent of the pressures given in Table2.

No allowance shall be made for the general or specific shielding of other structuresor natural features.

In the case of an open framed building with significant resonant dynamic response ora building for which the fundamental natural frequency is less than0.2 FIz, or thecross wind resonant response / torsional resonant response may be significant, theresonant dynamic effects should be investigated in accordance with

7.3

7.4

7.5

Height abovesite-ground level

Design hourly mean wind pressure

7(鮮か

≦5 m

101n

20m

301n

50m

75m

100m

1501n

200m

250m

300m

400m

5 0 0 m

0。77

0.90

1.05

1,15

1.28

1.40

1.49

1.63

1.74

1.83

1.90

2603

2 . 1 3

7.6

recommendations given in published literature and/or through the use of dynamicwind tunnel model studies. The total response of such a building would usually becalculated from the combination of the response in the three fundamental modes ofvibration.

6

Al.

APPEIIDD( A : NECESSARY PROVISIONS FOR WIII{D TUNNELTESTING

Static Structures

(a) The natural wind is to be modelled to account for the variation with height ofhourly mean wind speed and turbulence intensity appropriate to the site.

(b) The instrumentation and its response characteristics are to be appropriate to theloads required.

(c) The measurements should enable peak wind loads consistent with theintentions of this code to be properly determined"

Dynamic Structures

Where resonant dynamic response may be significant, the provisions for staticstructures shall be met and, in addition, the sffucture shall be accurately represented(physically or mathematically) in mass distribution and stiffness in accordance withestablished laws of dimensional scaling and the effect of structural damping shall beappropriate ly refl ected.

Topography Modelling

If the loading on a building may be significantly influenced by the local topography,the effect on the wind properties may be investigated by small-scale wind tunneltesting or established using reliable published data.

Proximity Model

If the loading on a building is significantly influenced by the presence of surroundingbuildings or topographic features, it is necessary to include the models of theseproximity features in the wind tunnel testing. Where the local topography is too largeto be sensibly accounted for in the proximity model, its effects should be accounted foras described in Clause ,43. Where particular adjoining or surrounding buildings couldprovide significant shelter, the effect of their possible removal should also beconsidered.

Model Scale Limitations

The geometric scale and velocity scale employed in the wind tunnel testing shouldmeet the requirement of a minimum Reynolds number. For building models with sharpcomers, the Reynolds number based on the breadth of the building model should not beless than 1x104. A general guide is that the building model should normally be notsmaller than 1:500 in scale and that the test wind speed should be greater than I 0%o ofthe full-scale wind speed.

For rounded shapes sensitive to Reynolds number effects, these conditions may not besufficient and further evidence of the suitability of the test conditions may be required.

7

A2,

A4.

A5,

The blockage in the wind tunnel should normally be less than l0o/o unless ablockage tolerant tunnel is being used. If blockage exceeds 1004, special formsof blockage correction may be required.

Wind Profiles and Design Wind Pressure

The variations of hourly mean wind speed and turbulence intensity with heightin the wind tunnel, with the proximity and test model removed, should besimilar to (after being scaled up with appropriate geometric scale and velocityscale) the full-scale hourly mean wind speed and the turbulence intensity giveninAppendix F.

Calibration between wind tunnel values and full-scale values should be madeso that the peak gust wind pressure at a reference height in the wind tunneltesting should match the design wind pressure given in Table 1. The referenceheight to be used shall normally conespond to 90m (full-scale) or 213 of thebuilding height, whichever is greater.

Where the effect of topography is modelled as described in Clause ,A.3, thewind profiles determined from the small-scale topographic model shall be usedin the building model tests.

For matching purposes, the peak gust wind pressure in the wind tunnel shall becalculated as below:

q : l l2 p v2 ( l+3.7I)2

density of air 0: l.2kg/m3

hourly mean wind velocity

turbulence intensitv

一一　　　　一一　　　　一一

ｐ

　

一Ｖ　

Ｉ

APPENDⅨ B: MULTIPLICAT10N FACTOR FOR RETURN PERIOD GREATER

TIIAN 50 YEARS

Thc design、vind prcssures given in this code havc been detertnined i予oln the hourly mean and

peak gust velocities having a rctum period of 50 years. Thc design wind pressures onbuildings where thc pe五od of exposure to wind is longer than 50 years shatt be muttiplied by

thc following facton―

( 端 ) 2

、vhere R is the period ofexposure to wind in years.

APPENDEX C: TOPOGRAPHY FACTOR

cl. For the purpose of this Code, local topography is considered significant for a sitelocated within the topography significant zone as defined in Figure Cl.

The relevant dimensions of the topography are defined in Figure C2.The effective slope c[" and the eflective slope length L" are defined in terms of thesedimensions by the following :

(a) For shallow upwind slopes 0.05 < cru < 0.3 :

de : 0u and L. : ['o

For steep upwind slopes uu > 0.3 :

0" : 0.3 and L" : FV0.3

When topography is considered significant, the design wind pressure at height z shallbe multiplied by a topography fuctor Sa at that height. The topography factor Su atheight z above site ground level shall be determined by the following equation:-

S u = ( 1 + 1 . 2 u * . s ) 2

where tte is the effective slope of the topographic features as defined in ClauseC2 above.

is a topography location factor, the values of which are given for hillsand ridges in Figure C3 and for cliffs and escarpments in Figure C4.

For cases of complicated topography, specialist advice should be sought and/or windtunnel model testing should be conducted.

Acknowledgement : Figures CI, C2, C3, C4 in Appendix C were modified from figuresextracted from British Standards with the permission of BSI under licence number 2002/SK0004.British Standards can be obtained from BSI Customer Services, 389 Chiswick High Road, LondonW4 4AL, United Kingdom. (Tel +44(0)2089969001).

(b)

10

llVind 0.5 x slope length if C[,u < 0.31.6 x slope height if C[u > 0.3

upwind slopeC[,u > 0.05

downwind slopeO[o > 0.05

H = slopeheight

downwind slopeCfo< o.o5

Lg : slope length

b) Escarpment (0.3 > upwind slope > 0.0b; downwind slope < 0.05) and cliff (upwindslope > 0.3; downwind slope < 0.05)

Figure C1 Definition of significant topography

x < 0

of C[owithlevel

a) Hilland ridge (C[u > 0.05, C[o'0.0s)

Wind ,

b) Escarpment (0.3 > Cfu > 0.05, C[o . O.OS) and cliff (C[r t 0.3, Cf,o < 0.05)

Legends

Lo Length of the downwind slope in the wind directionLu Length of the upwind slope in the wind directionX Horizontal distance of the site from the crestH Effective height of the feature

Ｕ

　

　

　

Ｄ

働　働

Upwind slope H / Lu in the wind direction

Downwind slope H / Lo in the wind direction

Figure C2 Definition of topographic dimensions

x < 0

。ＨヽＮＯ軍何」ＯＥコ０」ＯＯ＞００価”貞め一〇正

2 . 0

1 . 5

1 , 0

0 . 5

0.2

1.5 2.0 2.5

X/Lu ttX/LD

Horizontal position ratios

Figure C3 Topographic location factor s for hills and ridges

。一

ヽＮ

Ｏ軍０」ＯＥＤＯ」め

Ｏ＞００Ｇ

”三め一〇王

2 . 0

4 . 5

1 . 0

0 . 5

0.2

-4.0 -0.5 1.0 1.5 2.0 2.5

X / L eX/Lu

Horizontal position ratios

Figure c4 Topographic location factor s for cliffs and escarpments

APPENDIXD : FORCECOEFFICIENTS

Dl. Enclosed building

Dl.1 The force coeffrcient Clfor an enclosed building shall be-

(a) the product of the height aspect factor C1 and the shape factor C,given in Table Dl and Table D2 respectively; or

O) the appropriate value specified in other lnternational Codesacceptable to the Building Authority.

DLZ The force coefficient shall be applied to the enclosed building as a wholeprovided that :

(a) In the case of a building with isolated blocks projecting above ageneral roof level, individual force coefficients corresponding to theheight and shape ofeach block shall be applied.

(b) In the case of a building composed of similar contiguous structuresseparated by expansion joints, the force coefficients shall be appliedto the entire building.

Dl.3 If the frontal projected area of that part of the building for which Cf operatesis greater than 500 rnl the force coefficient determined by Clause Dl.l maybe multiplied by a reduction factor Ra given in Table D3. This reductionfactor should not be applied to the total wind force calculated in accordancewith Section 7.

D2. Open framework buildings

D2.I The force coefficient Clfor an open framework building shall be-

(a) the value given in Table D4; or

(b) appropriate value specified in other Intemational Codes acceptable tothe Building Authority.

14

Table Dl : Height aspect factors C1 for enclosed building of generally uniformsection

Table D2 : Shape factors C, for enclosed buildings of generally uniform section

Note: When the actual shape of a building renders it to become sensitive to windacting not perpendicular to its face, the diagonal wind effects and torsionalwind effects should be considered.

15

H e i t t h t

BreadthHeight aspect factor C6

1.0 or less2.04.06.0

10.020.0 and over

0。95

1 . 0

1,05

1 . 1

1 . 2

1 . 4

Note : Linear interpolation may be used to obtain intermediate values.

General plan shape Shape factor C,

Rectangular

dｒＣ

Ｓ

　

　

　

Ｖ

ｌｅｓ

　

ｄ。

ｏｒ　　　価

０

０

０

１

２

３

ＦＩＩＩＩＬ

〓

う

一冴

　

　

ｂ

小

―

―

―

―

―

サ

ヽ

ｉ

ｔ

ｒ

ｌ

ジ

1 . 0

1 . 1

1 . 3

「――――

コ

lnterpolatelinearly

Circular

wlnd→

0.75

Other shapesValue of C, for the respectiveenclosing rectangular shape in thedirection of the wind.

Table D3 : Reduction factors

projected area

Ro for enclosed buildings according to frontal

Table D4 : Force coefticients Cl for open framework buildings

Frontal projected area, m2 Reduction factor Ra

500 or less

800

1000

3000

5000

8000

10000

15 000 and over

1.00

0.97

0.96

0.92

0。89

0.86

0.84

0.80

Note : l. Linear interpolation may be used to obtain intermediate values.

Solidity ratio g Force coefficient Cc

０

　

１

　

２

　

３

　

４

　

５

　

８

　

９

　

０

０

　

０

　

０

　

０

　

０

　

０

　

０

　

０

　

１

2 . 0

1 , 9

1 . 8

1 . 7

1 . 7

1 . 6

1 . 6

1 , 8

2 , 0

Note : l. The solidity ratio rp is equal to the effective projected area of the openframework building divided by the area enclosed by the boundary of theframe normal to the direction of the wind.

2. Linear interpolation may be used to obtain intermediate values.

16

El.

APPENDⅨ E: TOTAL PRESSuttE coEコ ■1lCIENTS CP FOR INDIVIDUAL

ELE出 団NTS

The total pressure coefficient Co for individual elements in a particular area of anenclosed building shall be :-

(a) in the case where there is only a negligible probability of a dominant openingoccurring during a severe storm, the value given in Table El; and

(b) in the case where a dominant opening is likely to occur during a severe storm,the value determined with the aid of other published materials acceptable tothe Building Authority or through the use of wind tunnel model studies.

The total pressure coeffrcient Co for individual members of an open frameworkbuilding shall be :-

(a) 2.0; or

(b) appropriate value specified in other Intemational Codes acceptable toBuilding Authority.

Table El : Total pressure coelficients Cp for individual elements of enclosedbuildings with negligible probability of dominant opening

E2。

Walls and claddings(a) edge zones of the building(b) other surfaces

-1,4 or+1.0

-1.O or+1.0

Flat roofs(a) edge zones ofthe roof(b) other surfaces

- 2 . 2

‐1 . 2

Pitched roofs(a) edge zones ofroof(b) ridge zones ofthe roof(c) other surfaces :

(i) wind across ridge, windward surface(iD wind across ridge, leeward surface(iii) wind parallel to ridge

roof angle

10° 30° 60°

由2. 2

- 1 , 4

-■4

- 0 . 8

- 1 . 0

(

i : l- l . 2 o r + 0 . 3 |

- 0 7 |

- 1 . 0 I

rterpolate linearl

‐■0

-1.0

+1.0

- 0 . 8

‐1 . 0

の

17

Canopies(a) edge zones(b) other areas

Notc: 1.

+2.O and-2.0

+1.2 and-1.2

3.

4.

5,

Negative value of Cp indicates that the resultant force is outwards orupwards.

Where altemative coefficients are given the element should bedesigned to accept both loading conditions.

Edge zones of the building are the areas within a distance from theedge of the building equal to 0.25 times the lesser horizontaldimension of the building.

Edge zones of the roof are the areas within a distance from the edgeof the roof equal to 0.15 times the lesser horizontal dimension of theroof.

Ridge zones of the roof are the areas within a distance from the ridgeof the roof equal to 0.15 times the span of the pitched roof.

6. Canopies means any structure which projects more than 500 mmfrom any wall of any building to provide protection from rain orsun and at a height of not more than 7.5m above the level ofground.

7. Edge zones of the canopy are the areas within a distance from the edgeof the canopy equal to 0.2 times the span of the canopy.

18

APPENDD( F : DYIIAMIC AI\ALYSIS

Fl. The along-wind dynamic response of a building shall be assessed using the gust

response factor method. The method involves an assessment of dynamic

magnification factor which represents the amount by which the hourly mean wind

forces shall be multiplied to account for dynamic behaviour. The dynamic

magnification factor G may be taken as the values from Table Fl or Table F2, ot

determined by using the following equation :-

G差 1 + 2 1 h

whcre lh is dに 価rbulence mtellsity at the top of thc building which shall be

taFen as O。1055(h/90)・ n wherc h is equal to tle hcight of the

build士唱in metres.

g v l s t h e p e a k f a c t o r f o r b a c k g r o u n d r e s p o n s e w h i c h i s t a k e n t o b c 3 . 7

gr is the peak factor for resonance response and is equal to

V2bg。(3600 na)Whett naおhe ttndametti natur】髄qucncy ofttebuilding in Hertz which can be taken as 46lh or determined by a moredetailed analysis.

B is a background factor which is a measure of the slowly varyingbackground component of the fluctuating response caused by the lowerfrequency wind speed variation and is equal to

V 3 6 h 2 + 6 4 b 2

where h

Lr'

height of the building in metres

: the breadth of the building in metres

: the effective turbulence length scale

1 +

ｈ

ｂ

　

Ｌ

ヽ１１

１ノ

ｈ

一１０

／１‐―＼

０００〓

is the wind energy spectrum and is equal to , 0'47\,,,

(t * t 'y'u

Where N: effective reduced frequency:gVn

gッ2B+gf bL

19

is the size factor to accountbuilding and is equalto

for the correlation of pressures over a

[1+

3.5 nah ４ｎａｂ一吼

十

一―一―

where na the fundamental natural frequency of the buildingin hertz

- 461h, ot determined by a more detailed analysis

% : the design hourly-mean wind speed at height h whichshall be taken as the values given in Table F3.

E is the wind energy spectrum and is equal to , o'otl,"

(z + N'f '"

Where N : e{l'ective reduced fr ilu|-r

equency =Vh

is the damping ratio of the fundamental mode. This shall normally be taken as1.5o/o for steel structures and 2o/o for reinforced concrete structures. Forparticularly slender buildings, lower values may be appropriate and specialistadvice should be sought. Stocky buildings may have higher damping values.

Table Fl : Dynamic Magnilication Factor G for C= l.Soh

踊ぶ襴ω

20 30 40

200 1.994 1.955 1,922

180 1.983 l,943 1.909

160 1.972 1.930 1.896

140 1.959 1,916 1.882

120 1.945 1.902 1.868

100 1.929 1.886 1.853

Note : For intermediate values, linear interpolation is permitted.

20

Table F2: Dynamic Magnification Factor G for e = 2.0Vo

20 30 40

200 1.907 1.874 1.847

180 1.900 1.867 1=840

160 1.894 1.859 1.832

140 1.886 1.851 1.824

120 1.879 1.843 1.816

100 1.871 1,836 1.808

Note : For intermediate values, linear interpolation is permitted.

Table F3 : Design hourly-mean wind velocity

Hcight above

S推-3TOund level

Design holBriンmean whd velocity

V Km/S)

≦5 m

1 0 m

201n

301■

501n

75m

100 Fn

1501n

200m

250m

300m

400m

>500m

３５

．

８

３８

．

７

４ ‐

．

７

４３

．

６

４６

．

２

４８

。

３

４９

．

８

５２

．

‐

５３

．

８

５５

，

‐

５６

．

２

５８

．

０

５９

．

５

Note : For intermediate values of height" linear interpolation is permitted.

21

Explanatory Materinls to the

Code of Practice on Wind Effectsin Hong Kong 2004

鶴警離監紗言醸鰻§静 置算A t t F t t S N I

@ The Government of the Hong Kong Special Administrative Region

First published : December 2004

Prepared by: Buildings Department,12lF -188 Pioneer Centre.750 Nathan Road,Mongkok, Kowloon,Hong Kong.

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Foreword

The Explanatory Materials give a summary of the background information andconsiderations reviewed by the code drafting committee during the preparing of the Codeof Practice on Wind Effects in Hong Kong 2004, which will be referred to as .the Code'in this document.

As the Code aims to retain the essence of a simple format of its predecessor for ease ofapplication, the Explanatory Materials was set out to accomplish the Code by explainingin depth the major changes in the Code and to address on situations where tfre appticationof the Code may require special attention.

The Explanatory Materials is a technical publication and should not be taken as a part ofthe Code.

Acknowledgment

The compilation of ttre Explanatory Materials to the Code of Practice on Wind Effects

Hong Kong 2004 owes a great deal to Dr. K. M. Lam and Ir. K. L. Lo for their

contribution of manuscripts, and to the Chairmanof the Ad-hoc Committee to review the

Code of Practice on Wind Effects, Ir. K. M. Cheung for his advice and guidance in

formulating the document.

Special acknowledgment is also due to many individuals, in particular Dr. R. Denoon, Ir.

K. S. Wong, Ir. J. MacArthur, Ir. C. C. Wong and Ir. Y. C. Tsui for their valuable

comments offered during the course of compilation of this "Explanatory Materials".

Thank is also due to the Hong Kong Observatory for providing the cloud imagery on the

cover page which was originally captured with the Geostationary Meteorological Satellite(GMS-5) of Japan Meteorological Agency.

CONTENTS

Section I

Section 2

Section 3

Section 4

Section 5

REFERENCES

The Basic Wind Velocity ProfileWind Characteristics in Hong KongReference Wind SpeedHourly Mean Wind Velocity profileGust Wind Velocity ProfileThe Design Velocity and Pressure profiles

Terrain and Topographic EffectTerrain CategorizationTopographic EffectTopography Factor

Dynamic Response of StructuresSignpost to Dynamic SensitivityAlong-wind ResponseCross-wind and Torsional Responses

Force Coefficients and Pressure CoefficientsForce CoeffrcientsPressure CoeffrcientsWind Pressure near Ground Surface

Wind Tunnel TestGeneralStatic StructuresDynamic StructuresTopography and Proximity ModellingModel Scale LimitationsDesign Wind Pressure

ｅ軽１１１２４５

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Section 1 The Basic Wind Velocitv Profile

Wind Characteristics in Hong Kong

1.1 Wind characteristics near the ground are mainly described by the hourly mean

wind velocity profile, peak gust wind velocity profile, turbulence intensity profile, and

directional distribution of wind speed. Two dominant factors shape the extreme wind loading

in Hong Kong. The first is the exposure to severe typhoons. The second is the protection

afforded by one of the most sheltered natural harbours in the world. These two characteristics

tend to interact.

1.2 The wind characteristics for Hong Kong environment have been discussed by

many researchers in past years including Mackey(I3), 6o(14), Chen(I8), Choi(16), Davenport et

al(le), Melbou*"('o), Jeary(zs), and Holmes et al.(3e) However, due to the difficulties involved

in both the understanding of typhoon structure over large hills or mountains and the

measurement of wind characteristics during typhoons, the wind characteristics near theground in Hong Kong associated with building design are still not fully understood.

Reference Wind Speed

1.3 The Hong Kong Observatory was founded in 1883 and has been keeping

almost complete records of wind speed from 1884 onwards. These records have enabledestimates of extreme wind speed at the ground surface to be made. The Hong KongObservatory maintains a large number of measuring stations. Among these stations, the one atWaglan Island is considered to be the principal source of information during the past years.

There are two main reasons for this. Firstly, the Waglan data ue obtained from measurementson an isolated island exposed to the predominant winds. Secondly, data from other sourceswithin the city areas have been subject, over the years, to a changing environment or to theinfl uence of topographical features.

1 .4 Before 1993, the height of the anemometer at waglan Island was 75 metres.As a result of the erection of a new mast in 1993, the anemometer height at Waglan wasadjusted to 82 metres. The steep rocky profile of the island presents a blockage to the wind,and the subsequent speed-up over the island means that the measurements at anemometerheight are actually representative of the wind speeds at a greater height over the open waterapproaching the island. Melbourne suggested that the measurements at anemometer height are

representatives of unobstructed measurements at 90 metres. In practice, this correction makesonly a small difference to the absolute estimates of wind speeds and an effective referenceheight of 90 metres is adopted in the Code for derivation of the design wind speeds.

1.5 The data from measllrements at Wagian lsiand were analysed ushg the

L i e b l c i n B L L I E●est L i n e a r U n b i a s e d E s u m a t。O Tec h n i q u e s ( 2 2 ) t o e S t a b l i s h t h e p r o b a b i l i t y o f

occurrence of certam mean and gust wind speeds.All avallable typhoon data meastlred at

Wagian lsland since 1953 follll the basis Iもr analysis.

Based on these analyses and other sources of published information, the Codehas adopted reference hourly-mean and 3-second gust wind speeds of 46.9 and 65.2 mlsrespectively at a height of 90 m above mean sea level.

1.6

1.7 When comparing the adopted hourly-mean wind speed of 46.9m/s and3-second gust wind speed of 65.2mls with the measurements at Waglan Island for some severetyphoons occurred in Hong Kong in the past years (see Table l.l), it can be seen that theadopted values have demonstrate the expected level of confidence for design purposes.

Table 1.1 Measurement of Severe Typhoon Data at Waglan Island

Typhoon Hourly-mean wind speed Gust wind speed

Wanda(1962)

Rose(1971)

Ellen(1983)

York(1999)

41.4 Hys

39.Om/s

44.2■コ′s

42.5 Hys

60,2m/s

52.4Hコ′s

62.7Hコ′s

65.O Hys

Reference Speed 46.9m/s 65。2 Hys

Hourly Mean Wind Velocity Profile

1.8 The profile of hourly-mean wind velocity against height may be characterised

near the ground surface by a logarithmic relationship and the velocity reaches a value that is

reasonably constant at the gradient height at which the ground friction influence becomes

insignificant. In the Code, a power law profile is used as an arithmetical approximation to

cover the whole range of heights and for use in calculating wind loads on buildings.

1.9

1 . 1 0

The hourly-mean wind velocity v" at height z can be described by thefollowing power law relationship:-

( 1 ・1 )

where v" : the hourly-mean wind speed at the gradie ntheig)tt z "

cr : the power law exponent.

The gradient height is the height at which the ground friction influencebecomes insignificant. Recent field research by the National Hurricane Centre of the UnitedStates has confirmed that the maximum hurricane velocities occur at a height of around 500mabove the ocean(37'3e). These data were obtained by dropping many hundreds of GPS drop-sondes into hurricane eyewalls since 1997 and formed the largest data set yet gathered ontropical cyclone wind profiles. The field data supports the appropriateness of the Codeadopting a gradient height of 500m over open sea condition and modelling a conventionalboundary layer below this level.

t . l 1 The field data refened to in clause I . l0 show that power law exponents of 0. I 0to 0.11 for equation (l.l) are appropriate over deep open water at the design wind speed rangeexpected in Hong Kong. Since Waglan Island, from where the basic reference data werecollected for analysis, is an isolated island exposed to open sea, it is therefore decided to adoptan open sea condition and used a power law exponent of 0.11 for the construction of thevelocity profile in the Code.

r . t 2 In addition to the analysis of waglan Island data, computer simulations usingMonte-Carlo statistical techniques have become a standard tool in the prediction of typhoonstrengths and directionality. Conveniently, most of these simulations assume that gradientbalance occurs at 500 m. These include the works conducted at the University of WestemOntario and by Dr Peter Vickery at Applied Research Associates. The latter have been thesubject of a number of peer-reviewed publications in recent yeals (33X3+X35X301.

1 . 1 3 using equation (1.1) with reference hourly mean wind speed of 46.9rn/s at90m, gradient height of 500m and o value of 0.11, the gradient hourly mean wind speed iscalculated to be 56.6mls. This value is slightly higher than the gradient hourly mean windspeeds predicted using computer simulation techniques, but is considered within the acceptableerror range associated with these techniques.

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The gust wind velocity profile is obtained by applying a gust factor to the

hourly mean wind profile. The gust factor in tum is a function of the turbulence intensity. The

relationship between the gust and the hourly mean wind speed can be expressed through the

following relationship :-

v" =i,G =i,(l+ g"I,)

Gust Wind Velocitv Profile

t . t 4

(1.2)

: the gust factor;: the peak factor which reflects the measured relationship between the peak and

the hourly mean wind speeds measured using a standard anemometer. The

value is normally taken between3.4 and3.7;: the turbulence intensity athei$tz;

= the gust wind velocity atheight z;

: the hourly mean wind velocity at height z.

Based on a review of the analyses by many researchers, the turbulence

intensity at the reference height of 90m is taken to be 0.1055.

r . 1 6 The turbulence intensity defines the degree of gustiness of the wind and is

related to root mean square (RMS) wind velocity. With the RMS wind velocity taken to have a

constant value at different heights, the turbulence intensity also varies with height according to

the power law, but with the power exponent equal to -a. As a result, the turbulence intensity

would vary with height according to the following expression:

(1.3)

l.l7 By combining equations (l.l), (1.2) and (1.3), the gust velocity at any height

can be calculated as :-

where Go,,6 '

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ッz=丁g(こそ戸)α11+gvrgtザまヽ)~αl (1,4)

whcrc Vg : gradient mean wind speed:56.6mls

z, : gradient height : 500m

1, : turbulent intensity at gradient height: 0.087

g" : peak factor:3.7

,r : power exponent:0.11

The gust wind speed at gradient height of 500m is thus calculated to be1 . 1 8

74.9m/sc

The De.sign Velocity and Pressure Profiles

F.F9 There is still considerable uncertainty about wind speeds and profiles intyphoons. The above simplified approach assumes that the wind profile follows a "nomal"

power law until the gradient value of hourly mean wind speed or gust speed is achieved. tndetermining the appropriate design wind speeds for a Code of Practice in typhoon windclimate areas like Hong Kong, it is necessary to obtain an adequate level of reliability. Takinginto consideration the uncertainties inherent in the prediction of typhoon wind speeds and toensure an appropriate level of safety in structural design, the code recommends that the windspeed for design to be increased by 5Yo above the derived 50 year retum wind speed. The useof a higher design wind speed also accounts for the fact that high localized wind pressurecoefficients in excess of code values are often detected durins wind tunnel tests.

1.20 Using equation (1.1), the design hourly mean wind speed u, at height z is thusexpressed as:- \

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where ve

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: hourly mean wind speed at gradient height: 56.6mls: gradient height: 500m

: power exponent for mean wind: 0.1I

軸 e va占筑10n ofdcsign howly mean wind speed witt hdghtおc】culated by cqu抗lon(1.5)and

t h e r e s u l t s a r c t a b u l a t e d i n T a b l e F 3 o f t h e C o d e .

l.2l The design 3 second gust wind speed v, at height z is calculated by combining

equations (1.2), (1.3) and (1.5):-

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whereッ g=hOurly mean wind speed at gradient height=56.6nys

rg=mrbulence intensity at ttadient height=0.087

Zg=留 阻dient height=500m

α =power exponent for mean whd=0.H

1.22 The design hourly-mean wind speed and 3-second gust wind speed at reference

height and gradient height respectively are summarised as follows :-

1.23

Design hourly-mean wind speed at reference height of 90m : 49.2mls

Design hourly-mean wind speed at gradient height of 500m : 59.5m/s

Design 3-sec gust wind speed at reference height of 90m : 68.5m/s

Design 3-sec gust wind speed at gradient height of 500m = 78.7mls

The design wind pressure q" at height z is calculated as :-

7 ,4, =;Pvf

p : density of air: l.2kglm3

The variation of design gust and hourly-mean wind pressure with height are given in Table

and Table 2 in the Code.

Section 3 Dynamic Response of Structures

Signpost to Dynamic Sensitivity

3.1 For the assessment of resonant dynamic response effect of a strucfure, asignpost is to be provided to first determine whether the resonant dynamic response issignificant or not. In the case that it is, then several new elements of assessment are requiredfor the purpose of estimation of dynamic response effect. These include the determination ofturbulence intensity, damping ratio, natural frequency and some other descriptors of windenergy parameters. In the case that the structure is not with significant resonant dynamicresponse, then a quasi-static approach may be adopted.

3.2 Several codes of practice provide signposts to define whether a quasi-staticapproach is suffrcient or a fully dynamic analysis is necessary for determining wind forceacting on a structure. (Table 3.1 refers)

Table 3.1 Various Signposts for Dynamically Sensitive Structures

3.3 The first governing condition relates to an aspect ratio of the structure and theleast horizontal dimension is intended to account for stepped or tapered building profiles. Thiscondition would exclude short squat buildings from the requirement for a dynamic analysis.

Authorised Code Definitions of dynamic sensitive structure

A u s t r a l i a / N e w Z e a l a n d S t a n d a r d

ASNZS l170。 2ぃ1989

Height exceeds 5 times the least plandimension, and the natural frequency in thefrst mode of vibration is less than 1.0 Hz.

ASCE Standそ抑d ASCE 7-02 Height exceeding 5 times the least horizontaldimension or a fundamental natural frequencyless than L0 FIz.

National Building Code of Canada 1995 Height is greater than 4 times the minimumeffective width or greater than 120m.

10

3.4 The second condition relates to the fundamental natural frequency of thestructure, or indirectly to the height of the structure. In general, if the fundamental naturalfrequency is less than I Hz,the building has to be designed for the effect of resonant dynamicresponse.

3.5 Jeary and Yip(ttl carried out a study on three different signposts in 1994. Thesignposts were taken from the proposed ISO Code for wind loading @avenport 1989), theAustralian Wind Loading Code (AS 1170.2 - 1989) and the BRE digest series 346 which wasissued for a basis of the new Eurocodes. The ISO and BRE versions both have formulae toevaluate the signposts, whilst the Australian Code has a simple requirement.

3.6 Seven buildings for which dynamic data were available were used to study theeffect of the different signposts. These included the Jardine House (179m high), Bank ofChina (305m high), Hong Kong Bank (179m high), Peoples College, Nottingham (4.Sm high)and three harmony blocks from Housing Department ranging from 82m to I l8m high. Thesebuildings were chosen to represent a set includes clearly dynamically sensitive (Jardine

Houses, Bank of China and Hong Kong Bank Building) to clearly quasi-static @eople'sCollege) with the three harmony blocks close to the threshold.

3.7 The results from the three sets of signposts are in broad agreement with thethree categories of buildings in the set assumed above. More details of the evaluation andoutcome can be found in the research report by Jeary and Yip (1994)(tt).

3.8 The 1989 Australian approach is chosen as the basis for formulating thesignpost for Hong Kong. The Australian Code required the aspect ratio to be less than 5 and

the natural frequency greater than 1.0 Hz if the structure is not to be classified as with

significant resonant dynamic response. Use of the standard Ellis formula for evaluation of the

fundamental natural frequency i.e. nafural frequency : 46lheight of structure in metres, wouldimply that any building with a height greater than 46 metres would be classified as being

dynamically significant. Although many standard forms of construction in Hong Kong areparticularly stiff and study by Jeary and Yip suggested a l00m restriction is reasonable forHong Kong typical buildings, it is noted that slender buildings with a height of less than 100 m

may also be dynamically significant. [n the Code, a building is therefore considered to be one

with significant resonant dynamical response if it has either one of the following properties

unless it can be justified that the fundamental natural frequency of the building is greater than

I llz-

(a) The height exceeds five times the least horizontal dimension.

●) The height of the building is greater than 100m.

Along-wind Response

3.9 The wind-induced dynamic force on a tall structure may be resolved into two

components: along-wind dynamic force parallel and cross-wind dynamic force normal to the

direction of incident mean wind velocity. The response of the structure to the along-wind

dynamic force is called the along-wind dynamic response, and correspondingly the response of

the building caused by the cross-wind dynamic force is regarded as the cross-wind response.

Torsional d1'namic response of a tall structure may also occur especially when the along-wind

and/or cross-wind dynamic forces and/or the centre of mass do not coincide with the elastic

centre of the structure.

3.10 Most modem wind loading codes in the world provide the gust factor method

for estimating the along-wind dynamic response. The gust factor approach was derived from

the early work of Davenport in the 1960's. It is recognised as a satisfactory assessment method

for the along-wind response where the design peak base overturning moment is determined by

multiplying the mean base overtuming moment by the gust factor. For ease of application, the

Code recommends that the gust factor be defined as a dynamic magnification factor that

represents the amount by which the mean wind forces shall be multiplied to account for the

resonant dynamic behaviour.

3.11 The basic mechanism of along-wind response of a slender structure is

turbulence buffeting. Wind flow is turbulent and the gustiness in the wind produces

fluctuating forces on the structure. The fluctuating along-wind loading acting on a structure is

primarily a function of turbulence intensity and turbulence scale. The turbulence intensity

determines the local magnitude of fluctuating loading while the turbulence scale, in relation to

the size of the structure, determines how well the fluctuations are correlated over the structure.

The dynamic response may be calculated as a sum of quasi-static response for low frequency

component and resonance response at the first nafural frequency. The following gust factor

equation given in Appendix F of the Code is the simplified one.

(3.1)

3.12 The turbulence intensity , I , , at the roof top of the structure, can be assessed by

using the power law expression as discussed in Section l. The two functions undemeath the

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3.16 The parameter, E is a spectrum of turbulence in the approaching wind stream

and it is given by

where N is an effective reduced frequency and is equal to

N: Fro

3 .17 The damping ratio, (, reflects the damping capacrty of the structure and is

defined as a fraction of the critical damping. In general, the damping ratio includes bothstructural damping and aerodynamic damping. The Code recommends 1.5%o of critical

damping for steel sfuctures and 2Yo of critical damping for concrete structures, which aregenerally accepted as reasonable figures for design purposes at design load levels. For very

squat or very slender structures, the value of structural damping may be lower or higher

respectively. As structural damping is amplitude dependent, it is common to use lower values

if assessing dynamic response (e.g. accelerations) at lower return periods.

3 . 1 8 With the assumed fundamental natural frequency of 46h and critical damping

values of l.5Yo and 2o/o as suggested in Clause 3.17, the dynamic response multiplication

factor, G, can be determined from the height, h, and breath, b, of the structure. Variation of G

values with height (h), and breath O) of a structure for critical damping of 1.5%;o and2%o arc

given in Tables Fl and F2 in the Code for designers' easy reference and use. When more

refined estimates of the natural frequency and critical damping value appropriate to the

structure are available, designers should use the basic equation (3.1) to derive the value of G

for the structure.

Cross-wind and Torsional Responses

3 .19 Cross-wind vibration of structures is caused bv the combined effects from

buffeting, vortex shedding and galloping. Due to the complex interaction of these forces, there

is no precise analyical method available to calculate cross-wind response of tall structures.

Saunders and Melbourne (1975) and Kwok (1982) carried out extensive aero-elastic tests of

tall buildings of various sizes in wind tunnels and proposed a spectral method to estimate

cross-wind response of tall buildings. This method is based on the generalized first mode

あ

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14

cross-wind force spectrum measured from wind tunnel tests as well as the random vibrationtheory. This method has been adopted in the Australian Wind Loading Code (1989). In theAustralia/Trlew Zealand Standard (2002), values of the cross-wind force speckum coefficientfor both square and rectangular section buildings are provided in the form of curves which arefunctions of turbulence intensity and the Strouhal Number, and they are obtained from windtunnel tests of isolated buildings under typical wind conditions. Similar data for typical HongKong buildings shapes in the wind regime described in the Code are not available at this time.However, it should be noted that the Aushalia/New Zealand Standard indicates that, forslender exposed buildings, the cross-wind loads can greatly exceed the along-wind loads. Forsuch buildings, it is recommended that specialist advice should be sought.

The torsional dynamic response of a tall building may be especially significantwhen the along-wind and/or cross-wind dynamic forces or the centre of mass do not coincidewith the elastic centre of the building. This may occur, for example, as a result of buildingshape, structural eccentricity and/or uneven load pafferns resulting from the surroundings.Furthermore, when the mass centre of the building does not coincide with the elastic cenhe,coupled translational-torsional vibration may occur. The code-based procedure for evaluatingthe torsional response and the coupled translational-torsional response of tall buildings is stillunder development. Boundary layer wind tunnel tests or specialist advice may have to besought to tackle these problems for buildings of unusual shapes and buildings with complexsurroundinss.

3.20

3.21 The code does not provide any guidance for assessment of the cross-wind andtorsional responses of tall structures, but designers are reminded by Clause 7.3 in the Code thatin the case of a structure for which the cross wind response and/or torsional response may besignificant, the dynamic effects should be investigated in accordance with therecommendations given in published literature and/or through the use of dynamic wind tunnelmodel studies. The total response of a strucfure may normally be taken as a combination of theresponses in the three fundamental modes of vibration.

15

Section 4 Force Coefficients and Pressure Coefficients

Force Coefficients

4.1 The Code adopts the force coefficient method in the determination of totalforce on a bullding due to wind eSttcts. The static wind force acting on a bullding is expressed

as thc product ofthe design wind pressllrc and the force coettcient,which isin ttm a的髄tion

ofthe bullding shape and thc height aspect ratio.A simllar approach was adopted in the Code

of Practicc on Wind Effects Hong Kong-1983.The tttal wind force on a building,F,is thus

expressed as t‐

F=C/Σ?zИz (4.1)

Where C r is the force coefficient for the building, which is a product of the height aspect

faOtor, Co andthe shape factor C" given in Appendix D of the Code;

Q" is the design wind pressure atheightz;

A" is the effective projected area of the building

4.2 In the absence ofaccurate data on irregular shapes, and for the convenience of

application, the Code adopts only a few fundamental shapes i.e. square, rectangular and

circular, and recommends using coefficients for the respective enclosing rectangles for all

other shapes. Nevertheless, allowance is made in Clause D1.1(b) of the Code for designers to

use appropriate values specified in other international codes.

4.3 Contiguous buildings may be regarded as one single building in aerodynamic

terms although they may be categorically structurally independent from each other. It is

therefore recommended in Clause Dl.2(b) of the Code that contiguous buildings may be

considered as a whole building block when considering the wind loading effect. The shape

factor and the height aspect factor for this type of contiguous building structure shall embrace

the overall enclosed building. For dynamically sensitive buildings, care should be taken to

ensure that the differential wind-induced loads and motions between contiguous buildings are

adequately catered for.

The reduction factors for buildings with large frontal area and force coefficients4.4

for open framework buildings remain the same as those given in the Code of Practice on Wind

Effects Hong Kong - 1983.

16

Pressure Coefficients

4.5 The total force on a building element is the sum of the forces acting on the

extemal and intemal faces of the element. Intemal and extemal pressure coefficients should be

chosen to give the most critical positive and negative (suction) forces on the element. The

critical combinations of these coefficients for normal rectangular buildings have been

calculated in the Code and they are given as the resultant pressure coefficients in Table El.

The value of intemal pressure coefficient assumed in deriving the resultant pressure

coefficients is +0.2 or -0.3. The positive pressure coefftcients act towards a building surface

(or downwards in the case of canopies) and negative pressure coeffrcients act away from a

building surface (or upwards in the case of canopies). Therefore, it is necessary to combine the

negative extemal pressure coefficient with the positive internal pressure coefficient to cater for

the worst net negative pressure coefficient, or reverse the above for calculating the worst netpositive pressure coefficient.

4.6 Table El of the Code gives generalised pressure coefficients for elements such

as roofs, cladding and wall panels. Pressure coefficients for design of canopies are also

inhoduced based on the assessment given in the Aushalia/l.,lew Zealand Standard. The wind

loading on the building element, Fo , is equal to the product of the pressure coefficient at the

location, the projected area of the building element and the basic design wind pressure.

Fo : C oQ"A.

Where C p : the pressure coeffrcient for individual elements;

(4.2)

g" : the design wind pressure corresponding to the height z of the element;

A, : the surface area of the element.

Wind Pressure near Ground Surface

4.7 wind tunnel data from building studies indicate that high pressures andsuctions are experienced near the ground surface for tall buildings. These are resulted from theway the wind is channelled down the fagade of a tall building and subsequently accelerated toflow around building corners. This effect has an impact on the design of canopies, claddingsand wall panel elements. In the Code, the design wind pressure, q" for design of roofs,

canopies, wall and cladding panels is adjusted to allow for the effect of larger pressures andsuctions occurred at lower level of a building. Clause 6.2 of the Code specifies a minimum

17

value for qz over the lower part of the building. The design wind pressuta, Q", shall be taken

as constant over a height which is equal to the breadth of the building or the actual height ofthe building whichever is the lesser.

18

5.4

5.6

One important characteristic of wind is how the mean wind speed varies with

height at a particular ground terrain type. The roughness of the ground retards the wind in the

ABL. The wind speed is zero at the ground level. It increases with height above ground until

gradient heighl above which the wind speed is assumed to remain roughly constant with

height. In tropical cycloneso this is a simplification as wind speed can actually decrease at

great heights (>500-600 m), while other extreme wind events such as thunderstorms,

downbursts and tornadoes have quite different structures.

5.5 The variation of hourly-mean wind speed with height, or the hourly mean wind

speed profile, is specified in the Code. It is described by using the power law as:

A similar profile is required in the wind tunnel modelling. The data of hourly mean wind

speed at different heights in the wind tunnel, when plotted in this non-dimensional form,

should fall closely onto the target profile with a power exponent u. AWES-QAM-1-2001

recommends that the simulation is acceptable if the wind tunnel speed data fall within l|Yo ofthe target profile.

Another important feature of wind is its turbulence or gustiness. This plays animportant role in generating peak pressures on a structure and inducing vibrations on a flexiblestructure. On the first level, wind turbulence cari be measured by the turbulence intensity. It isthe ratio of the standard deviation value of wind speed fluctuations to the mean wind speed.The turbulence intensity normally decreases with increasing height. The turbulence intensityprofile is specified in the Code and a power law is used to describe the profile:

5.7 Like the hOurly mean wind specd proilc,the ttrbulcnce intcnsity proilc in thc

wind tunnel to that in the ABL is checked by plotting the measured values in the wind tunnelalongside the target profiles with the height normalised by the reference height. The sameaccuracy within 10% is recommended by AWES-QAM-I-2001 for this profile.

The allowable variations in mean wind speed and turbulence intensity can result in significantdeviation from the intended peak gust pressures (of plus or minus about 30%). There is thus a

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requirement in the Code to calibrate the wind profiles to reproduce the gust pressures as givenin the Code.

5.8 The size of dominant gust eddies in the turbulent wind are important in producingpeak loads on an area of a building. This is measured by the integral 'scales of turbulence andthe turbulence spectra. The usual practice in wind tunnel modelling is to measure the along-wind spectrum Suu(n) at an appropriate height and then match it to the universal windturbulence spectrum. The universal spectrum is on a non-dimensional frequency axis ofnLq,,/V and the value of the longitudinal scale of along-wind turbulence, L,r*, is obtained fromthe best match. Ideally, this value should match with the full-scale ABL value, that is(L".)p(L".). : I. In practice, it is seldom possible to obtain a perfect match and a mismatchof not more than 2 and 3 is considered acceptable respectively for modelling of overall windloads and for cladding pressures (AWES-QAM-I-2001).

5.9 Wind speed, wind pressures and wind loads fluctuate with time due to windgustiness. It is commonly accepted that fluctuations with a period shorter than one secondproduce negligible wind effects on a building structure. The various measuring equipment andtechniques used in the wind tunnel are thus required to be able to measure quantities atfluctuations faster than this one-second criterion. ln the wind tunnel, time and frequency are inthe model scale as determined by the length scale and the velocity scale:

n I tn Lov_ ) "/ t . = - = - = - - = -' ln t, L,Vo 4

where subscript r indicates time, subscript n indicates frequency, and subscript v indicatesvelocitv.

5. r0 For example, if a velocity scale 1:5 is used in a wind tunnel test and the lengthscale is l:250, the model time is 1/50 the full-scale time. Fluctuations faster than l/50 secondneed to be measured. ln order to obtain adequate resolution to detect peaks at this frequency,the response of the instrumentation need to be good up to four times 50 Hz,thatis 200H2.

5.11 During a wind tunnel test, a pitot-static tube is normally placed in the test section

to monitor the wind speed and to provide the reference static pressure in the wind tunnel. Thepitot-static tube should be located outside the zone in which the wind field is modified by thepresence of the wind tunnel model. The pitot-static tube is suitable only for measurements ofmean wind speed in relatively low turbulence conditions. For measurement of gust windspeeds and turbulence intensities, there are a number of other suitable instruments such as hot-wire and hot-film anemometers and miniature pressure probes.

21

5 . 1 2 The pressure at a point on the surface of the test building structure is usually

measured by means of a pressure tap. Pressure at the tap location is commuted through a

length of flexible tubing to a pressure transducer which converts the pressure signal into an

electric signal. Wind pressures can vary greatly over the building surface and pressure taps

need to be placed at a sufficient density in order to accurately measure the pressure

distribution. AWES-QAM-1-2001 recommends that the average pressure tap density should

be higher than one tap per 120 square metres of surface area on the test building.

5 . 1 3 The frequency response of pressure measurement is normally limited by that of

the pressure tubing. The frequency response and the associated distortion to the pressure

signal are greater for longer and larger diameter tubing. There are standard methods to correct

for this tubing response problem including the use of restrictors, leaked tube systems or

mathematical hansform conections.

5.r4 Wind forces and moments on a building model are sometimes measured directly

with force balances. Where dynamic loads are to be measured (either background and/orresonant components), the model-balance assembly must be stiff enough to have a combinednatural frequency lying above and away from the frequency range of the wind loads to bemeasured.

5 . r 5 Model values of wind pressures and wind load obtained in the wind tunnel canbe converted to appropriate full-scale values through the use of loading coefficients. Forexample, wind forces are converted through an equation such as:

( v \ "Fr :C , . l pV ' , q :F . t l + l

\ Y o , )

The choice of the prototype wind speed depends on the full-scale event to be simulated. Toestimate the design wind loads of a normal building, the design wind speed should be usedas ihe prototype wind speed. The determination of wind forces and wind moments by thisconversion is only valid for static buildings that do not exhibit resonant dynamic response towind actions.

5 . 1 6 Wind effects depend on the incident wind directions. It is thus required to makewind tunnel measurements at a number of wind directions in order to obtain the most criticalwind loading cases. ln general, a minimum of 24 wind directions at l5o intervals isnecessary, with smaller intervals around the wind directions causing critical loads. Manywind tunnel laboratories now routinely test at 36 wind directions for pressures and loads.

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Dvnamic Structures

5,17 Many structures, such as tall buildings, towers, cable-suspended bridges andcable-suspended roofs are sensitive to wind-induced vibration that depends to a large extent onthe characteristics of the structures. The most relevant dynamic properties of a structure aremode shapes of vibration, natural frequencies (which depend on the mass distributions andstiffness), modal masses, and damping. They define the mechanical admittance function,which describes how fluctuating deflections are produced from fluctuating forces.

5。18 In modelling the responses of a dynamic structure, the dynamic characteristics ofthc structtre havc to be moddにd in add■lon tt thc whd charact前並ics ttd the aerodynam絶

shape charactcdsticsc ln ttmcmal dynamics,wind―induced auctuating responses are usually

加alysed with the spectrum method in which the response spectrum is obtained ttm ttc

product of the aerOdynarnic force specmm and the mechanical admittce functim of the

s伽限cture.Therefore,simllarity ofresponses requires both similarity of the aerodynarnic force

spccmm and similar,ofthe mechanical admttmce nmction.

5。19 A satisfactory simulation ofthe whd characteristics for static structures discussed

above wiu resutt in the simllarity of the fOrce specttm.The equ】ity of aerOdynamic shape

factors in the protOtype and the modcl is provided by having thc same cxtemal shapes for the

structure ttd the modcl.Thc similariけofthC mechanical admitance function can be achieved

by introttchg the dynamic charac悔n並ics ofthe bulldhg dther physたaltt in the wttd turlncl

model or analytically during the analysis Ofwind ttrIIlel data.

5。20 Aero-elastic tests physically simulate the dynamic characteristics of the prototypebuilding. Full similarity can only be met by a full aero-elastic model that reproduces in theappropriate scale the mass distribution, stiffrress distribution and structural damping at everypart of the prototype building. For a tall building, a rigid model on an elastic base can be analternative to a full aero-elastic model. Most energy of the wind-induced vibrations of a tallbuilding (with the exception of those where torsional response is significant) is conhibuted bythe orthogonal fundamentaltranslational modes whose modal deflection follows very closely alinear shape. It is thus possible to estimate the dynamic behaviour with a "stick model" wherethe dynamic properties of the building are lumped together at the base. A stick model is a rigidmodel of the building mounted elastically on a pivot at the base so that it can vibrate in a linearmode shape. If significant torsional response is likely, then it is necessary to use a threedegree-of-freedom aero-elastic test rig.

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5.21 The high-frequency force-balance (or base balance) technique is the mostcommonly used method of determining wind-induced loads and responses of tall buildings.The technique is based on the measurement of base moments, hence generalized forces, on arigid model of the building and the computation of wind-induced response using the randomvibration theory. Mean and fluctuating wind moments on the whole building are obtainedfrom direct measurements on a lightweight rigid model of the building in the wind tunnelwith a sensitive force balance. The base moments approximate the generalized wind forceson the building of the fundamental vibration mode. This provides the aerodynamic forcespectrum. The dynamic properties of the building are obtained from a dynamic analysis ofthe building structure. Together with an assumed value of damping, the mechanicaladmittance function of the building is obtained. Following the random vibration theory,the measured generalised wind forces and the mechanical admittance function are combinedto obtain the base moment of the prototype building. These base moments are typically thendistributed to provide floor-by-floor loads for structural design.

5.22 The 'high-frequency pressure integration' is the third method sometimes usedto obtain structural loads. Like the high-frequency force balance technique, this employs arigid model and requires the mathematical incorporation of the mechanical admittancefunction. This technique is most commonly employed when testing long-span roofs but hasrecently been used on simple-shaped tall buildings. ln Hong Kong there are practicallimitations to this technique on many buildings where the surface area is high in relation tothe internal volume (e.g. typical housing blocks) as it is not possible to fit sufficient pressuretubes into the model.

5.23 with all the above techniques, appropriate load cases combining responses indifferent modes and from different excitation mechanisms should be provided.

Topography and Proximity Modelling

5.22 The simulation methods described above for static structures produce the correctgeneral wind characteristics over a uniform terrain. If the site is near to or located on a localtopographic feature, the wind characteristics, in particular the mean wind speed profile, may besignificantly modified by that feature. The same applies if the test building is sunounded bysizeable neighbouring buildings.

5.23 In these situations, it is necessary to include a detailed representation of thesuffounding topographic features and/or neighbouring buildings in a region of some distancearound the site. This "proximity model" includes a reproduction of the neighbouring buildings

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in the correct scale and may also include small local topographic features. If too small an areais represented, the simulation may not be able to include all the possible effects on the windcharacteristics from the structures in the proximity. If too large an area is included, the linear

scale of the model has to be reduced, given the dimensions of the wind tunnel. AWES-QAM-

l-2001 recommends that "in general, all major buildings and topographical features within a

radius of 500 mefes of the building site should be modelled to the correct scale, to an accuracy

of l0%o or beffer".

5.24 ln situations where wind characteristics at the site are affected by large-scaletopographic features, it may be necessary to model and study the effects of these features

separately. The topography model is built at a much smaller scale to cover a large area so that

the large-scale topographic features can be included. The wind profiles at the location of the

site are measured in the topography model. Afterwards, these wind profiles are reproduced in

the test section of the wind tunnel at the normal geometric length scale and a model of the

building, with the proximity model of neighbouring buildings if required, is tested under these

wind conditions. It is normal to remove buildings that will be included in the proximity model

from the topography model.

Model Scale Limitations

5.2s Wind flow pattems depend on the Reynolds number (Re) but it is impractical to

achieve the same Reynolds number in the wind tunnel simulation as the full-scale prototype

wind flow. Fortunately, the effect of the Reynolds number on the flow is significant only at

low Reynolds numbers. When the flow is full turbulent, at sufficiently high Reynolds

numberso the flow patterns become almost independent of the Reynolds number. The

Reynolds number effect becomes insignificant and a mismatch of Reynolds number between

the model and the prototype is acceptable. For building shapes with sharp comers, AWES-

QAM-1-2001 recommends the minimum value of Re : 5 x lOa based on the smaller building

width and the mean wind speed at roof height, while the ASCE Manual of Practice on Wind

Tunnel Testing recommends a minimum value of Re : I x l0a. For building shapes of

smooth profiles or with rounded comerq separation of wind flow on the building surfaces

depends on Reynolds' number and turbulence to a much higher degree than on sharp edged

surfaces where the separation is fixed by the sharp edge. ln such cases, the Re mismatch in the

wind tunnel must be addressed and dealt with. One simple and efficient way is to add

roughness to the surfaces of the building model to trigger turbulent flow separation. Care

should be taken to ensure that an appropriate degree ofroughness is used as this can also cause

inappropriate separation behaviour. [n some cases, special larger scale studies will be

necessary to calibrate this.

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5.26 To satisfu the minimum Re requirement, the geometric length scale and velocity

ratio used in thc wind tunnel silnulation ca【 lnot bc too small. A general criterlon for the

VCbCⅢ ratiOおthat i shodd bc grcater航加 1:10.Forthc Lngth sc】 c ofttc b減mttg ttdd,

a scale larger than l:500 is desirable.On thc accllracy ofthe bullding model,AWES由QAM-1-

2001 recoIImends that thc overall dilnenslons ofthe test bullding modei should bc accuratc to

、vithin 20/O alld architecttlral detalls should bc hcluded when their smallest dilnenslon is l

metre or greater.

5.27 0n the other hand,the、 vind ttlIInel lnodels should not be too large othenvisc the flow

will be distorted by blockagc.Wind tullncl blockage is measured by the blockage ratio which

i s t h e r a t i o o f f r o n t a l a r c a o f t h e b u l l d i n g m o d c l s K a n d p r O x i m i t y m o d e l ) t o t t C C r O s s ‐s e c t i o n a l

a r e a o f t h e w i n d t u r l l l c l t c s t s c c j o n . T h e b l o c k a g e r a t i o s h o u l d b c k c p t t o b e l o w 1 0 0 / 0 . A t h i g h c r

blockage,the test rcsu■s may b9打juStCd by appropriate blockage corrcctbns,howcver these

arc not simple to calculate in turbulent boundary layer conditions. The requirement for a

maximum blockage ratio can be relaxed Fblockage toLranttett sectionお employed.

Design Wind Pressure

5.28 [n Clause 5.5 above, it is mentioned that a l0o/o uncertainty is allowed on the

mean wind speed profile and on the turbulence intensity profile. In the conversion of

measured model values of wind pressures and wind loads to full-scale values, the ratio of

prototype velocity to model velocity is important. It is thus desirable to specifi a reference

height at which the velocity value, or the dynamic pressure value, is taken for the model-to-prototype conversion. In the Code, the velocity values are obtained from extreme wind

analysis in Hong Kong and most of the wind data were obtained at a height of 90 m. Hence,

the reference height for the pu{pose of this section is specified to be 90 m, or 213 of the

building height, whichever is greater. The inclusion of the latter height is to account for very

tall buildings where the wind pressures at upper levels play a major part in the overall wind

loads. The use of the gust wind speed, or gust wind pressure, in the calibration is in line with

the Code which tabulates the gust wind pressures at different heights. This should be regarded

as a general guideline, although there are special cases where the use of other matching heightsmay be more appropriate, especially where topography is significant.

26

2.

3.

4.

6.

7.

REFERENCES

l. Building Development Departrnent, Hong Kong (1983) : "Code of Practice on WindEffects". Hong Kong 1983.

Britsh Standards lnsti的1lon(1972):"Code Of Basic Data for the Design of

Buildings'・.CP3 Chapter 5,Part 2,Wind Loads.

Bullding Research Establisttent(1992):Digest 346 Part l to Part 5:W口 le

AsscsslmCnt ofWind Loads".

Bdtish Standard BS6399 Part 2:1995:ⅢCodc ofPractice on Willd LoadsW.

A m e r i c a n S t a n d a r d o 0 0 2 ) A S C E 7 ‐0 2 : いM i n i m u m D e s i g n L o a d s f o r B u l l d i n g s a n d

Othcr Structtres".

Australian Standard (1989): 駈 inimum Design Loads on smcttesW.

ASl170。2-1989.

National Standard of thc PeoplcもRepublic of china(1987):"Load Code for the

Design ofBullding SttcttresⅢ GBJ 9-87.

Davenport A G:(1989):"PrOposed New lnterllational(ISo)Wind Load Standard".

APSO巾 電 2 pp l199-1214.

Deaves D M and Harris R I(1978):"A Mathematictt MOdd Ofthe Smcttre Of

Strong Winds". CIRIA Rcport No.76, Construction lndustly Rcsearch and

lnfollllation Association,London L河(. ISBN.0-86017‐086-1,

Georgiou P N,Suny D,and Davenport A G(1988):"CodiflCation of Wind Loading

in a region with Typh00ns and Hlllsい,Fourth lntemational COnference on Tall

B u l l d i n g s , H O n g K o n g a n d S h a n g h航, A p r i 1 / M a y 1 9 8 8 .

Jeary A P and Yip S(1994)WSignposts in Wind COdesW CPHK Building and

Construction Department July 1994.

Mackey S,ChoiE C C,and Lam R:い Gutt FactorsW.Proceedings Seminar on Wind

Loads on Structllres. IIawali,1970,pp 191-202.

8.

9.

10.

12.

27

14.

15.

13. Mackey, S : "Wind Studies in Hong Kong - Some Preliminary Results" Industrial

Aerodvnamic Abstract 1 970.

Ko K L : "Characteristics of Monsoon and Typhoon winds in Hong Kong from an

engineering view point" Ph.D Thesis,IIK 1972.

Choi E C C : "Gradient Height and Velocity Profile During Typhoons". Proceedings

of the Sixth Intemational Conference on Wind Effects on Buildings and Structures,

Gold Coast Australia. 1982.

Choi E C C : "Commentary on the Code of Practice on Wind Effects Hong Kong -

1983". Hong Kong Institution of Engineers.

Lam R P : "A Review of the full scale Wind Engineering Research at the University

of Hong Kong". Seminar on Wind Engineering, Excelsior Hotel Hong Kong,

28 January 1988.

Chen T Y : "Comparison of Surface Winds in Hong Kong". Royal Observatory

Technical Report No. 41, December 197 5.

Davenport A G, Georgiou P N, Mikitiuk M, Surry D and Lythe G : "The Wind

Climate of Hong Kong". 3rd Intemational Conference on Tall Buildings, Hong

Kong and Guangzhou, pp 454-460. Dec l0-15 1984.

Melboume W H : "Design Wind Data for Hong Kong and Surrounding

Countryside". 3rd lntemational Conference on Tall Buildings, Hong Kong and

Guangzhou, pp 461 -467, Dec 10-l 5 1984.

Associate Committee on the National Building Code, National Research Council of

Canada (1990) : "Supplement to the National Building Code of Canada 1990".

Lieblein Julius (1974), "Efficient Methods of Extreme Value Methodology", USDepartment of Commerce, National Bureau of Standards NBSIR 74-602.

Jeary A P (1995) : "The Description and Measurement of Non-linear Damping inStructures", IWEF Meeting on StrucfuralDamping, Atsugi, Japan, September 1995.

16.

17.

18,

19.

20.

21.

22.

23.

28

27.

28.

29.

30.

31.

24.

25.

26.

32.

33.

34.

Jeary A P (1994) : "Measurement of Turbulence Intensity and their Interpretation",CPHK Building and Construction Departrnent.

Jeary A P (1994) : "The Wind Climate of Hong Kongu, CPHK, Building andConstruction Departnent.

Jeary A P (1994) : "Velocity Profiles During Tlphoon", CPHK, Building andConstruction Deparlrnent.

Lam K M (1995) : "wind characteristics and wind codes". HKU. civil andStuctural Engineering Department.

Lam K M and ram w M (1996) : "state-of-the-Art wind Tunnel Modelling forBuilding Aerodynamics", The Hong Kong tnstitution of Engineers TransactionsVol.3 (1), ppl7-26.

Australian Wind Engineering Society (2001) : AWES-eAM-1-2001, WindEngineering Studies of Buildings.

"ASCE Manual on Engineering Practice No. 67 - Wind Tunnel Model Studies ofBuildings and Structures". ASCE, New York, 1999.

N J Cook:Ⅲ The DesiBIleゴs Guide to Wind Loading of Building StructtresW.

Bullding Research Establishmcnt Report.Butteworths,London,Part l:1986 and

Part 2:1990.

Lam K M and ro A P (1999) : "codification of wind-Induced Dynamic Responsesof Tall Buildings", The Hong Kong Institution of Engineers Transactions Vol.6(l),pp22-28.

Twisdale LA and Vickery PJ (1993), 'Uncertainties in the prediction of HurricaneWindspeeds', Proceedings of Hurricanes of 1992, ASCE,pp.706-715, December.

Vickery PJ and Twisdale LA (1995a),'Prediction of Hurricane Wind Speeds in theUnited States', Joumal of the Structural Division, ASCE, Volume l2l, pp. 169l-1699.

29

36.

37.

38.

39.

40。

35. Vickcry PJ and Twisd】 e LA(1995b),'Windield and Fnhng Modctt for Huricane

Wind Speed Predictions',Jourllal ofthc Struc位田ュl Division,ASCE,Volllme 121,pp.

1700-1709.

Vickery PJ, Skerlj PF, Steckley AC and Twisdale LA (2000),'Hurricane Wind Field

Model for Use in Hurricane Simulations', Journal of Structural Engineering, ASCE,

Y ol. 126, pp.l203-1221.

Powell MD, Reinhold TA and Vickery PJ (2001),'Engineering Implications of New

lnformation on Boundary Layer Profiles in Hurricanes', American Conference on

Wind Engineering, Clemson, Virginia

Powell MD, Reinhold TA & Vickery PJ (2003), 'Reduced drag coefficient for high

wind speeds in tropical cyclones', Nature, Yol. 422 pp.279-283.

Holmes JD, Hitchcock P, Kwok KCS and Chim JKS (2001), Joumal of Wind

Engineering, No. 89, pp. 357-360.

Fok CH, Kwok KCS and Hitchcock PA (2003), 'Studies of topographical effects on

Hong Kong wind climate', I lth Intemational Conference on Wind Engineering,

Lubbock, Texas.

Standards Australia/Standards New Zealand, ASAIZS 1170.2:2002, Structural

design actions, Part2: Wind Actions.

41.

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