Wind Loading Handbook

8/18/2019 Wind Loading Handbook

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AWES HB 001 2012

WIND LO DING

H NDBOOK

FOR

USTR LI

NEW ZE L ND

Background to AS/NZS 1170.2 Wind Actions


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AWES-HB0012012

ID LODIG HDOOK FOR UTRL

D E ELD

Background to AS/NZS 7. Wind Actions

by

J Holmes, K.C.S. wok and J Ginger

wih conibuions from:

AP Jeay, Leich, WH Mebourne, P Muns, L. Nocos,

P Russel, AW Rofal, N Truong and G.S Wood

AUSTRALASIAN WIND ENGINEERNG SOCIETY, 2012


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Publishe 2012©Australasan Wn Enneern Societ

wwwawesorg)

Prnte b:

Unversit Pubshn Servcehe Universt o Sne

Al rhts strctl reserve No part o this book ma be reproucewthout te permission o the pubsher

Dewe ecmal classicaton: 624.175SBN: 978-0-975037614


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Preface

Recent wind events in Australia and overseas have caused catastrophic

resuts in those affected areas, with deats being reported as wel aswidespread destruction. Research and information gathered from these

events have been incorporated into the atest edition of AS/NZS 1170.2 -

201 to now represent a more reaistic determination of wind actions

The Standard applies to structures ranging from 'th lss snsitiv to

wind tion to thos o whih dynmi spons must b tkn into

onsidtion.

This Handbook was prepared by the AWES to provide background

information into wind and its actions, but aso into the derivation of the

Standard and its contents. It covers items sch as:

• Nature of wind loading

• Wind speeds and mutipiers• Shape factors for structures• Dynamic response

In particuar, it equips users with a better understanding of wind and the

Standard to provide them with improved iterpretation and judgment in

determining wind actions on structures

Equaly important, it enabes the user to extend the Standard imitations

whie sti complying with reguations, abeit other information may be

ecessary.

It must be borne in mind: the user is uimatey responsible for their

design, notwithstanding the Standard, and this Handbook xists to ssist

th us s s ptibl to dishg thos sponsibilitis in th

bst intsts o th pojt, th own nd th ommunity

Leo NoicosBEng, FIEAust, CPEg, NPER, RPEQ

Senior Principal Structural Engineer

URS Austraia Pty Ltd


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4

Discaimer: While all due are has been taken in the olletion and preparation of

information in this Handbook no responsibility is assumed by the Australasian Wind

Engineering Soiety or the individual authors or ontributors for any onsequenes arising

from the use of it

Acknowedgements The authors aknowle_dge the indiret ontributions of other members

of subCommittee 806-2 of Standards Australia/Standards New Zealand not listed as

authors or ontributors on the first page and the assistane of Steve Cohard (University ofSydney during the prodution proess for this book


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5

TABLE OF CONTENTS

ntrductin, itry and cpe 9

1.1 ntrouction 912 Nature o win oain 101.3 Histo o Australian an New Zeaan Stanars on win loain 11

1.3.1 Histor o Australian Stanars 11132 Previous New Zeaan Stanas 14

1.4 Scope, an etermination o win actions 1415 Uncertainties in estimation o win oain 161.6 Desin win pressues, oces an oa cases 16

16.1 Desin win pessures 16162 Win irections 17

1.6.3 Frictiona a 17164 Ultimate an seviceabilit imit states 17165 Fatiue 181.6.6 Torsion 19

17 Winbone ebis 20

ind peed and Mutipier

2.1 Desciption o extreme win tpes 2322 Tpes o win spees use in AS/NZS 1170.2 25

23 Importance evels an aveae recurence intevals 2524 Reiona win spees an iection multipiers 27

241 Reional win spees 272.42 Eect o ecorin instument 28243 Win irection mutipies () 28244 cone cateoies an impotance levels 302.45 Win spees o othe juisictions 31

25 Terain cateories an terainheiht mutiplies 31251 Terran cateories 322.5.2 Terainheiht multiplies or snoptic wins ( ct) 33253 Terranheiht mutipiers in cclonic eions (ct) 36

2.6 Shiein multipier 37 27 Topoaphic an hishape mutipies 38

27.1 Site elevation 382.72 Hilshape multiplier () 382.7.3 Leeeect multiplier (ee) 39


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6

erdyamic hape factr fr eced buidig 4

4.

31 Genera 414143444544749

3.11 Loa fuctuatons3.1.2 he qasstea assumpton in AS/NZS 1170.231.3 Duraton of wn loas

32 nternal pressures (Cp3.2.1 Domnant openins3.2.2 Sze of omnant openn an nternal voume32.3 nternal wals an cens

3.3 Externa pressures on recanuar enclose bins 493.3.1 Extenal pressure coeffcents (Cp,e) 50

34 Ajstment factors 51341 Area reucton factor () for roofs an se wals 51

3.42 Acton combnation factor () 513.4.3 Local pressure factor () 52344 Permeabe can reuction factor for roofs an se wals () 53

3.5 Frctiona ra (Cj 533. Attachments to buns 53

3.1 Solar panels attache to roofs 543.2 Balcones an balustraes 5533 Sunshaes 553.4 Parapets 57

hape actr fr ther tructure 9

41 ther enclose structures 59411 Multspan blns 594.12 urve roofs 594.1.3 Bins slos an tanks 0

42 Freestanin wals hoarins an roofs 1421 Was an hoarns 14.21 Free roofs an canopes 2

4.2.3 Attache canopies awnns an carports 243 antevere staum roofs 344 Structural members bun frames clinrca sectons an lattce towers 5

441 penframe structures 54.4.2 Roune cinrcal shapes 544.3 Sharpee cross sectons 7444 attce towers an frameworks 74.45 Ancilares on attce towers 9

4.5 Permeabe (shaecloth) strctures 94 he role of wntunnel tests 70


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f -

.1 Introductio 71

5.2 Dynamic properties of structures 72

2. Natural frequencies 72

.2.2 Structura damping 73

23 Aerodynamic dampig 7

3 Along-wid response of tal buidings and structures 7

31 Introduction 7

3.2 Derivation of a simple gust oading factor formua 76

.3.3 Dyamic response factor in AS/NZS 1702 813 Aternative evauation of the along-wind response of ta structures by

the ESWL method 82

Cross-wind respose mechanisms 8' ,

85

85

87

87

88

Cross-wind excitation due to icident turbulence

..2 Wake excitation

3 Crosswind excitation due to cross-wind motions

. Galoping excitatio

Lock-in effects

Cross-wind response ouidings 88

.6 Cross-wind respose of circuar chimeys masts ad poes 90

6 Basics 90

.6.2 Sinusoida model of css-wid respose 92

63 Radom vibration model 93

7 Iterference effects 9

.71 Interference effects on tal buidings 96

72 Interference effects o vibration of seder towers and masts 97

.8 Combination of aong ad cross-wid response 99

9 Occupat perceptio of motio and acceeration criteria 100

:

A

A2

A3

Itroductio

Dampig measurements and physica mechanisms

Predictors of damping

f

03

03

10

9


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1: INTRODUCTON, HSORY AND SCOPE

1.1 INTRODUCON

This Handbook s intended to support and suppement the Australian/New ZealandStandard for Wnd Actions, AS/NZS 702:20 It provides background for the clauses nthe Standard. In addition, it provides addtonal nformation on shape factors and dynamcfactors, such as structura damping, e. nformaton tat s compatbe wth, but notprovided in, te Standard itsef Although it performs the functons of a commentary, thsHandbook does more than that; however, tere is no direct cause-by causecorrespondence wth the Standard tsef

Ths document is a successor to the 'Commentary to AS 702-989' publshed by th�Australian Wnd Engneerng Society (Holmes, Mebourne and Walker, 990), wchperformed a simlar functon for the 989 Australan Standard.

The Handbook s dvded nto the folowng chapters nd ppendces:

Chapter gves an introducton to wnd loadng, a history of te Standard, and ncudesbackground on Sections 1 and 2 of te Standard tsef

Capter 2 dscusses wnd speeds and multipers incoporatng background to Sections and in AS/NZS 702

Chapter 3 provides background on Section 5 in the Standard e sape factors forrectangular enclosed buidings Additonal nformaton for desgners is gven partcularyon attachments to budngs.

Chapter 4 covers shape factors for structures other tan rectangular enclosed buildngs, andncudes commentary on Appendices C to Fin the Standard

Chapter dscusses the dynamc response of structures to wnd and provdes background toSection 6ynamic response factor n the Standard

Appendix A provdes a more detaied discussion of structura dampng than that gven nChapter , and Appendx B provdes a comprehensve list of references and a bbliograpy.

References to causes, fgures, tabes etc. n AS/NZS 702:20 are gven n italics in thsHandbook. References to sections, fgures and tables in the Handbook are not n tacs


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2 NATUE OF WIND LOADING

Wn oan o stuctres s a complex phenomenon. he wn tsel s ranom, bencompose o a mutte o ees o varn szes an otatonal characterstcs carealon n a eneal steam o a movn elatve to the Earths srace. hese ees vewn ts ust or tubulent chaacter n both Astraa an New Zealan, extreme wnscan be pouce b a varet o mechansms

n Australa, extreme wns whch ae mportant n the esn o stuctures can be classeas 'snoptc wns an smallscale thunestom events. Snoptc wns ae prouce baescale pessure sstems essentall a balance between ools orces assocate wththe earths otaton, an pessure aents. hese storms ma ast o severa as. nasmana an the South slan o New Zealan, locate n the lattues known as the'oan otes, ales proce b arescae snoptc events ae common. hese asoaect the southen coastlne o Austala. Alon the easten coast o New South Waes,stron wns are oten pouce b 'East oast Lows ow pessre sstems n the asmanSea

ropcal ccones are a patcua tpe o severe snoptc stom that occr over the tropcalocean. n the Northen Hemsphere the ae also known b the names o 'hurcanes an'tphoons. n Austraa the aect extensve lenths o the coastnes o Qeenslan, theNorthern erto an Western Australa. n recent ecaes, nomaton ane romsatelte man, an acat lhts n othe countes, has eatl mpove knowlee othese events. n man events, reatve lttle entve nomaton on the wn spees

prouce when topcal cclones make anal n Austaa s avalabe, ue to thespaseness o anemometes

Fo snoptc wns, the ustness o stron wns n the lowe levels o the atmosphee,known as th 'bounar laer, arses om rctonal nteactons wth srace eatues schas veetaton, bns an wate sraces, whch chaacteze the teran n the owerlevels o the bounar ae, n whch most stuctures ae locate, the wn spee aveaeover tme peos o ten to sxt mnutes eneral nceases poessve wth heht, whlethe ustness, o tublece, tens to ecrease wth heht he aveae wn spees areaso aecte sncant b toporaph, such as hls, escarpments an es

hunerstorms are ven b stron convecton o warm most a to hh alttues. Rapcoon s accompane b the release o atent heat hs ener eappears as knetcene o alln an, hal an col ar. he ownrat o col a enerates an outlow stront at roun level he maxmum ust om these events near the oun can excee5 m/s.

Alon the coastal strp o sotheastern Australa, convectve thunerstorms an severeownrats are sual assocate wth co onts However, n nothern an nlanAustrala, severe stoms ae prouce b ocal convecton Althouh relatvel lttle s

known about the vaaton o ust wn spees wth heht n these evets, the ust proeat tme o the peak wns appears to ncease sowl p to about 1 metes heht,eucn n mantue at reate hehts; howeve, the sts are wel corelate (o


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synchronized) over large distances horizontaly, resuting i significant wind loading onhorizontal, ineike structures such as transmission ines. The period of strong winds ismuch shorter than for synoptic events An appropriate averaging time for winds inconvective downdrafts is abot minute The turbulence, or gstiness, superimposed onthe average wind is lower than that for synoptic events, as the effects of terrain and surface

roughness are much lower. Topography is also expected to have a esser efect on windgusts from downdrafts, compared with that on synoptic winds

New Zealand, particuarly in the South Island, experences 'downsope winds on the lee sideof the Alps These are associated with gravity waves above the mountain peaks. They canprodce sustained winds for several hours but affect relatively smal areas

When strong winds interact with a structure, pressres and forces on the surfaces of thestrcture are generated The characteristics of these pressures are influenced by thecharacteristics of the approaching wind and the geometry of the structure Significant

interna pressres may aso be generated if there are openings, or permeabiity, inking theexterio of a building with the interior

Pressures on structures are not steady, bt highy flctuating, partly because of thegstiness in the wind, but aso because of local eddies and vortex generation at the edges ofthe strctures themselves The pressures are also not uniformly distribted spatialy overthe surace of a structure.

Most of the efects described in this section are incorporated into the Standard in someform or other, but mosty in an approximate, or generalized way The complexities of windloading described here should be kept in mind wen applying a design docment ike theAstralian/New Zealand Standard Due to these many uncertainties, the maximum windoads may vary from those assumed in design It shold also be noted that the actualstrength of a structure, or its eements, when constructed, may dier consideraby fromthat assumed at the design stage Ths nonfailure of a strcture due to wind cannotnecessarily be taken as evidence of conservatism of the wind actions Standard

HISTOY OF AUSTALIAN AND NEW ZEALAND STANDADS ON

WIND LOADING

131 History of Australian Standards

AS/NS 1702:201 has a lineage that goes back orty years Australian Standard CA34,Part I published in 197 (Standards Association of Australia, 971) replaced an earlierinterim document, and was the first modern wind loading code or standard pblished ineither Astralia or New ealand. Although in Imperial units, all subsequent Australian andNew Zealand Standards are directy descended from it t contained a contour map of'regiona basic wind speeds in miles per hor with a Oyear return period (applicable to

most sructures), using anemometer data analyzed by Whittingham (1964) As in allsubseqent versions, this wind speed was defined as a gst of 23 seconds duration


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reorded at te tandard meteoroloial heiht o (33t.) 0 m in lat open, terranHowever, reent reear a ound that the averain tme o te pea ut, reorded byte Dine anemometer ued at that time, wa onderably le than 2-3 eond eeSetion 242

CA34 Part alo ave a table o 'reonal ba dein wnd velote or 5, 25, SO and 00-year return period value or 48 tie, town and other entre, or wh te wind peedanalye had been arried out. Value or a number o thee tation were labeed 'hortreord Notaby, oniderin the event that ourred three year later, e value ven orDarwn or 00year return perod wa only 9 mph (53 m/). owever, a ylone atoro .5 wa applied to al loaton noth o 30°S wtn 30 mle o te oatlne. The ourterrain ateore peied were eentialy the ame a toe iven n te urrent Standard.Shape ator (preure and ore oeient) were al iven n an Appendx and werelarey baed on the Brt Code o Prate and Sw Norm o te time, wt vaueobtaned n moot low wnd tunnel. owever, oa preure ator o 5 and 20 were

peied n ede and orner reion but wit no tributary area retriton A eton ondynam repone wa provded, but thi larely oered derptive and reerene matera.

AS 702 973 (Standad Aoation o Autralia, 973) wa eentally a metri veriono CA34, Part -97, althou ome ane to te lited reonal bai wnd peed weremade or ome taton. Dynami repone o tall buldn wa overed ony in annormative Annex (n at extrat rom a onerene paper by B.J Viery).

he ourrene o Cylone 'Tray at Darw, on Critma Day n 974, reulted in a newverion o AS 70.2, wth a ane to te map o reona ba den wind veloite Azonal ytem or the yoneprone oatal trip o northern Autrala wa introdued. Aoin te 975 editon (Standard Aoiaton o Autralia, 975) an nreaed vaue o neativepreure oeent or laddin eement on ide wal o tall buldin wa reommendedin a note An amendment, releaed in 978, ntrodued a new table o external preureoeiient o te roo o buildin with pthe le than 0 deree Thi wa laternorporated into te 98 edton o AS 70.2 (Standard Aoaton o Autrala, 98)

n the 983 edton o AS 702 (Standard Aoaton o Autralia, 983), te wdelyuedtabe in Appendix B o externa preure oeient or pthed roo buldin waextenvely revied to inude value obtaned in turbulent low rom a boundaryayer windtunnel n addtion, an area reduton ator or roo aordin to tributary area, a winddretion reduton ator, a new ytem o movin area or loal preure ator andreved rule or wnd low over earpment, were ntrodued However, the majority oAS 70.2983 wa milar n ormat and ontent to CA34, Part 97

The 989 edton o AS 702 (St�ndard Autralia, 989) wa a major revon o earerveron. t wa ntrodued a part o the onverion to mit tate den n Autralia. talo attempted to provide an alternatve mpler approah or maller owre buldn,and to provde a more aurate determnaton o wnd oad or tal truture wth

niant dynam repone AS 702-989 onted three 'tandaone eton aollow


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3

Setion 2 Simplied Proedure

Setion 3 Detailed Proedure: Stati Analysis

Setion 4 Detailed Proedure: Dynami Analysis

Each section had its own map o egiona wind speeds (pesses in the case o the

Simpiied Section), and mltiplies o teain and topogaphy. A new eate o the 1989Standad was the speciication o highetnpeiod design wind speeds (ie. 1 yeas) otimate imit state design his eiminated the eed o the 'cyclone acto in eaievesions. his concept has since been adopted in the United States

The 1989 Standad also contained nmeos othe changes with evisions to shape actoso mltispan bidings eestanding walls and oos, and biding ames electing theetensive eseach caied ot in the 19s and 198s. he cosswind esponse o talbildings was also incopoated in detai (possibly o the ist time anywhee in the wod)

AS/NS 11.: (Standads Astalia, Standads New ealand, a) was the istcombined Astalian/New ealand wind actions Standad and was also a mao evision inomat compaed to AS 111989

he ollowing majo changes wee aso intodced in :

• A vaiable anna pobabiity o eceedence was adopted o wind speeds. Theseeplaced impotance mltiplies sed in AS 11.1989,

• he sepaate 'simpliied pocede, and detailed pocede: dynamic analysis,sed in AS 11.1989 wee emoved, and a single design method based on a gstwind speed was adopted,

• Diection mtipies o wind speeds o all noncyclonic egions wee intodced,epacing diectional wind speeds o capita cities only in AS 11.1989,

• Methods based on mathematical omlae wee intodced o calcation o hilshape (topogaphic)mtipies and o cosswind dynamic esponse o ta bildings,

• he methods o dynamic esponse sed o along and cosswind dynamicesponse in AS 111989, wee eplaced with appoaches based, as the est othe Standad, on a peak gst wind speed

n addition, nmeos smalle changes, additions and adjstments to the tables o shapeactos wee incopoated

Fo the ist time in its histoy, the edition o the Standad was late spplemented bya seiendly Gide (Holmes and King, 5), containing nine detaied eampes oapplication o the Standad to vaios types o stcte.

he 11 edition o AS/NS 11 had a nmbe o signiicant changes and additionacases have been incopoated he pincipa changes ae as ollows

•

A tosiona loading eqiement in Clause 254 in the om o an eccenticity o %o the beadth, b, appied to the aong-wind oading. his has only been pescibed


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or tall buldings greate than 70 metes n height (see Section 16.6 n thsandbook)

• Wndbone debris mpact oadng criteria have been added n Clause 25 7.• ew wodng n Clause 532 requires designes to teat closed doors and windows,

partcularly oller doors, as potential dominant openings unless t can be

demonstated that they are stucturaly capable o resisting the design wind oads• A new Clause 534 eques consideraton o wnd oads on nternal wals and

celings• A revsed verson o Clause 543 concerned wth the acton combnaton acto• Some changes to Clause 544 and Table 56 on local pessue actors

n an Amendment to AS/ZS 1170.22011, based on recent esearch on the wnd proes ntopca cycones and huricanes, t is proposed to remove Table 41(8 - ie theterain/heght multpliers n Regions and D wl be the same as those speced or RegonsA and B Furthemoe, the terrain category or ovewater wnds wil generaly be treated

as Terrain ategoy 1, rrespective o mt state (i.e on the evel o wind speeds), or o theregion. However, o winds blowng om an ocean etch, the nshoe regon o breakingwaves may be treated as an intermediate Teran ategory Y

13.2 Previous New Zealand Standards

Por to 2002, ew Zealand had sepaate loading Standads dated 1984 and 1992 The wndloadng section (Part 5) o ZS 42031992 (Standads ew Zealand, 1992) was n act anadapton o, and very smar to, AS 1172199. The man dierences wee n the

dieent treatment o topographic eects and multpers, and the lack o a dynamicanaysis method or wind loadng owever, ZS 42031992 reerred the user to AS1170.2-1989 o the atte

n 2002, in c_mmon wth many other standads, a combned Austaliaew Zealand WndActons Standard was publshed. The use o common standads has resulted rom theloser Economc Relatons (ER) ree trade agreement between the two countes datingback to the 1980s The seven wnd egions o ew Zealand n ZS 42031992 weresmpliied to three egions n AS/ZS 1170.22002 Also the 'limt-state multipliers used nZS 4203, to adjust wnd speeds or serviceabty and utmate limt states, were

discontnued Instead average recurence ntevas wee used n AS/ZS 1170.02002Setion 3) as a bass or determining regonal wind speeds or design n ew Zealand.

14 SCOPE J AND DETERMINATION OF WIND ACTIONS

Clause 11 o the Standard limts th coverage to buldngs less than 200m in heght and roospans less than OOm. 'Roo spans should be nterpreted as 'unsuppoted oo spans. Inthe case o tal budngs geater than 200 metres n heght, the dynamic eects ae moresgniicant and complex than can be handled by the Standad In both these cases, wnd

tunnel studes and related processng is nomal pactce. shore structues, bridges andtransmssion lne towers are also excluded. In the case o the ast two, separate Austraan


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and Nw Zaland standards which incorporat wind oad information ar availab. In thcas of bridgs in Austraia, th Bridg Dsign Standard is AS 500 (Standards Austraia,2004) For ovrhad lin dsign, an Australian Nw Zaand Standard, ASNZS 7000:200was issud in 200 (Standards AstraiaStandards Nw Zaand, 200)

Th ffcts of tornados ar also xcldd in Clause 11. n Australia, only about sixtnconfirmd tornados occr on avrag ach yar, ovr th whol country. h risk of adirct strik on an individa structur is minimal; howvr a strctur dsignd to satisfyASNZS 70.2 shold prform satisfactorily in wakr tornados i. Catgoris F and F2on th Fujita scal

Dirct application of ASNZS 702 is on mthod of dtrmination of wind actions, and isth rcommndd mthod by th Buiding Cod of Australia. owvr, hr ar othraltrnativ paths, or spcial studis, that provid an quivant v of confidnc, and arrgardd as accptabl:

• Riab rfrncs usd consistnty with th causs of ASNZS 702• Riab data on wind spd and dirction corrctd for th infunc of trrain,

topography and nighbouring buildings whr ncssary, incuding a dtaildprobabilistic anaysis for th ffcts of wind dirction.

• Wind tunnl tsts carrid ot for a spcific structur, or rfrnc to sch tsts on asimilar structur, togthr with appicabl clauss in ASNZS 702.

• Cacuations by computational fluid dynamics, whih have been aibrated against fullsale or windtunnel measureents

Windtunn tsting in Australia and Nw Zaland shold normally folow th procdurs ofth Quaity Assuranc Manual of th Australasian Wind Enginring Socity (AWES, 200)n particuar, windtunn tsting to dtrmin th ffcts of synoptic winds shal nsurthat th appropriat trrain catgoris ar modld, and th variation of wind spd withhight, and th sca and intnsity of turbunc ar modld with rasonabl accuracyWhr curvd shaps ar involvd, th ffcts of Rynolds numbr shold b takn intoaccount (this usualy xcluds th us of modscal tsting at ow wind spds forstructrs with circular cross scions, such as chimnys) Masurmnt systms for forcand prssur shoud hav appropriat frquncyrspons charactristics Whn a highfrquncy forc balanc approach is sd for tal buildings, statofth art mthods formod shap corrction and assssmnt of torsiona rspons should b adoptd. Whnmodling is adoptd to dirctly dtrmin rsonant dynamic rspons andor aroasticffcts, appropriat scaling of mass, stiffnss and structural damping should b adoptdFinally, whn intrnal prssurs ar incudd in studis for buildings, appropriat scaling ofintrna volms should b adoptd (olms, 2006 Sharma et al 200)

Wind-tunnl tsts ar aso oftn carrid out for cass that are covrd in th Standard.Such tsts can gnrally b xpctd to giv owr dsign wind oads than th StandardHowvr, vn if that is not th cas, th rsults from th wind-tunnl stdis should b

sd in prfrnc to th vals from th Standard.


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6

15 UNCERTAINTIES IN ESTIMATION OF WIND LOADING

n the determination o wind oads, varios paameters are combined regiona windspeeds, mltipiers or terrain, topography and shielding, aerodynamic shape actors, and insome cases, a dynamic response actor able gives estimations o the coeicients ovaiation or these variabes

Table 1.1. Estimated coeffcients of variation for paametes used in AS/NZS 11702

Vs ooParameter Region A ther Regions Region A ther Regions

V R 007 02 02 020Md 005 005 005 005c at 00 00 00 00

020 020 020 020 05 00 05 00C fg 05 05 05 05C dyn 00 00 00 00

he greater ncertainty o the regional wind speeds ( V R) o Regions B C and D relects theact that tropica cyclones are generally too inreqent or analyses o anemomete data tobe able to make accrate predictions here is more ncertainty in the speciied vales otopographic mltipliers in Region A becase o the ncertain efects o topography onwinds at grond evel prodced by thndersorm downdrafts and otlows

The genera poblems o codiication or vaios aspects o wind loads, and the vaiationsbetween national and internationa codes and standards were discssed in a series opapers by Holmes t al., {2005a, Tamra t al. (2005b, Holmes t al (2005b, Letchord tal (2005, an Kaspesky and Gerts (2005

16 DESIGN WIND PRESSURES, FORCES AND LOAD CASES

161 Design wind pressures

Equation 2.4(1 in the Standard is the basic eqation o design wind presses acting on abilding srace (extena or internal his eqation is reprodced in Eqn (

(

n this eqation, the dynamic wind pessre 0.5 airHV d e s 2 represents the additional

presse generated when the wind low is broght to rest at a point in the low, without thedistbance prodced by a arge b body, it reslts om the convesion o momentm

in the ow to a oce per nit area, and essentialy it is a statement o Newtons Second Law


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The value of air density, Pain of 20 kg/m3 , specified in Clause of the Standard, i s anaverage vaue based on a temperature of 2° C, and typical atmospheric pressure at sea

level. Locations at high atitudes such as alpine areas have lower atmospheric pressure

which woud lead to lower air densities, but they also tend to have a lower temperature

than sites at sea level - this is a compensatory factor

Buildings and most other groundbased structures are aerodynamically 'bluff, rather than

streamlined. The effect of the buf body on the pressures and forces induced by the wind

is represented by the aerodynamic shape factor, · This normally takes a positive valueon a windward wal surface, but negative vaues on leeward and sde wals For detaied

expanations of wind fow around bluff bodies, the reader shoud consult textbooks on fluid

mechanics, or genera texts on wind effects of structures (eg Aynsley, Melbourne & Vickery,

1977; Cook, 98 1990 Holmes, 2007; Simiu & Scanlan, 99) Aerodynamic shape

factors, and associated factors ike local pressure factors, used in AS/NZS 11702 have

normaly been derived from windtunne studies, with some input from fulscale

measurements on structures, when they are availabe Aerodynamic shape factors arecovered in detail in Chapters 3 and 4 of this Handbook

The main function of the dynamic response factor, is to allow for possible resonantamplification effects on certain flexible structures with low natural frequencies. However

for the majority of structures that do not fit into this category, may be taken as 1.0.The dynamic response factor is discussed in Chapter of his Handbook.

62

Most of the aerodynamic shape factors provided in the Standard are given for four nominal

orthogonal wind directions Exceptions are freestanding walls and hoardings (Appendix ),individua structural members, and lattice towers (Appendix ), for which oblique winddirections ar� aso required to be considered As outlined in Clause the orthogonawind speeds are taken as the largest site wind speed within a 9 degree sector (ie +/ 45degrees), centred on the nominal wind direction. This process is ilustrated in Figures and in the Standard, and discussed in more detai in Chapter 2 of this Handbook.

63

Wind pressures on a building surface can generaly be assumed to act normal to the surface

(but not necessarily parallel to the wind direction) However, for some situations, the

Standard (Clauses and 5; ection 0 in Appendix requires account to betaken of the frictional drag, ie. the component paralle to the surface Those cases are:

the walls and roo of buildings that are very long in the direction parallel to the wind, and

freestanding roofs of low pitch rictional drag should be considered in conjunction with

normal wind pressures on columns, exposed roof beams, barges, flashings etc

64 Um vby m

Unlike AS 170.2989 in which specific wind speeds for utimate and serviceability imit

states were specified, AS/NZS 170.2:2002 and AS/NZS 170.22011 do not specificaly refer


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8

to these design limit states However AS/NZS 17 (Standards Austaia/Standards NewZealand b) in Scton 2 discusses them in some detail

For ultimate liit states design the designer should efer to one o thee sources todetemine the importance level of the stucture being designed

• the Building Code of Australia (BCA) for buidings in Austalia (Austraian BuildingCodes Board 11)

• Scton 3 o AS/NZS 117 for structures in New Zealand• Appndx F of AS/NZS 117 o nonBCA structures in Australia

nce the impotance level is seected tabes in the above documents give the annuapobabiity of exceedence (l/R). Then Tabl 31 in AS/NZS 117 can be used todetermine the appropriate regional wind speed o design Tabl F2 in AS/NZ117alows a vaiation in 'design working ife to be considered; howeve the BCA does no

recognize 'temporary structures or allow any adjustments o design woking lie

t should be noted that the BCA is only concerned with buildings and life safety and doesnot conside serviceability imit states Howeve guidelines o seviceability limit statesare provided in AS/NZS 117 and in vaious material standards

Suggested seviceabiity limit states citeia (eg deflection imits) associated with an annualprobability o exceedence of 1/5 ae given in Tabl Cl of AS/NZS 117 Howeve theseshould not be regaded as exclusive For example acceeration imits o windinducedvibration of tall buidings are avaiable esewhee including Section 59 of this Handbook

165 Fatigue

Fuctuating ind orces can produce fatigue damage and occasionally ailures in dierentways Highcyce atigue with an efective cycle count o 1 or more can producefailues o stee structues with windinduced stresses below the yied stess This isrelativey common for structures such as lightng poles and is usually accompanied byresonant dynamic esponse which can greatly increase the cycle count at higherfequencies Simpified methods o estimating fatigue life fo stuctues subjected to along

wind dynamic esponse have been descibed by Homes (a) Robertson t al ()and Repetto and Solai (9) However up to now design methods o wind-inducedatigue have been regaded as too compex and equiring uther research beoe they canbe icluded in the Standard

Cyconic events have produced 'lowcycle fatigue faiures on roof and wal cadding with acyce count typically less than 1 Failures typicaly have occured in areas o stressconcentration around fasteners As stated in Scton 255 in the Standad Part 3 o AS (Standards Austraia 199) and the Buiding Code o Austraia both specify testregimes for acceptable peromance o cladding and fastener systems or use in cyclone

prone regions These may change in the future folowing extensive esearch by Henderson(1) using moe reaistic time histoies o fluctuating roo pessures


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9

e rocode (Britis Standards nstitution, 2005, Figur B.3) provides a relationsipbetween te nmber of times a stress is reaced or exceeded in a period o 50 years, andte stress range in a normaized orm i.e as a percentage of te largest value in a 50yearperiod is relationsip is insensitive to te site or location, and cold be applied inAustralia or New ealand for atige anayses.

166 orson

Wind loading can produce eccentric loading wic resuts in a torsion abot a vertica axisaround a centre of stiness o te building. is can be produced in a nmber of ways:

non-uniform distribution o wind pressres, for exampe wen te wind blowsobliqely to te wal of a bilding,

lctating loading de to trbuent gusting in te wind,

dynamic torsion resuting from noncoincidence of te centre of mass of te buildingwit te centre o stiffness

e olowing inormation on te efective eccentricity of wind loading on some actual tallbildings was provided by a windtnne grop:

Reguar 6::2 aspect ratio 80m tal building: 0.8 blong saped pan wit a radised corner 200m tall wit a stepped elevation:

09* rreguar plan sape bt prismatic wer 30m tall (generaly 6::3): 08*

Regular 3::3, 60m ig: 0.4 Reguar 3::3, 80m g: 0.7 wisting irregar plan (truncated aerooil) 64m ig (generally 6::3): 0.23* Aerooilsaped plan, 30m ta (generaly 6::3): 022*

*tese bilding forms strictly do not fal witin te scope o AS/N 70.2

ence, te 20 edition o te Standard, in Claus 254 as introduced a torsionalrequirement in te orm of eccentricity o te resltant orce arising rom alongwindloading. e eccentricity is given as 20% o te crosswind breadt () o te buiding is

reqirement is restricted in AS/NS 702 to rectanglar enclosed bidings of 70 metreseigt or greater owever, tis eigt limit soud not be taen to impy tat torsionawind oading does not exist on oter structres, or bildings of lower eigt (e.g amra tal 2003).

Pea torsion on buidings generally occurs at te same time as te pea along-wind orcedue to te location o te centre o pressure Crosswind orces are generaly not as welcorreated wit torsional moments.


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1.7 WINDBORNE DEBRIS

The 1989 edton of AS 11702 (Cause ntroduced a requrement that n cycloncregons, wndows shall be consdered as potental domnant openngs, unless capable of

resstng mpact by a 4 g pece of tmber of 100 mm x 50 mm cross secton, strng them at

any ange at a speed of 15 m/s Ths statement replaced a smple warnng n the 198

edton (possble debrs effects aso may requre attenton) The 1989 requrement

reflected concern by the standards commttees of the tme about the devastatng effects of

wndborne debrs n several tropca cyclones fro the 1970s onwards (eg Cycone Althea

n 1971 and Tracy n 1974) Creaton of domnant openngs n buldngs by wndborne

debrs had n many observed cases resuted n hgh nternal pressures eadng to roof

falures, and n some cases complete destructon of budngs The 2002 edton (AS/NZS

117022002), n Cause 5 extended the requrement from wndows to the budngenvelope (wndows, doors and claddng)

The 4 g tmber mssle and ts test speed was conceved n the 1970s n Darwn folowng

Cyclone Tracy (Darwn Reconstructon Comsson, 1975) However, at that tme, very

ltte research had been avaabe on the actual speeds reached by tmber roofng members

or other wndborne debrs n cyclones Of couse, athough dstance traveled by such

mssles can be determned n postdsaster surveys, t s usualy not possble to

quanttatvely determne mpact speeds durng suc nspectons

In the early 2000s, extensve research n the Unted States on mssle speeds n hurrcanes

was undertaen One of the ey concusns of ts research s that the horzonta msse

speed s drecty related to the horzonta dstance travelled A ey paper n establshng

horzonta sse speeds as a rato to the wnd gust speeds, n such events, s that by Ln eta (2007) Ths research was based on extensve expermenta tests (wndtunnel and fulscae tests usng a Hercules arcraft) and numercal smuatons

It s noted that the results of the above research on mssle traectores has been adopted n

a new Standard for Storm Sheters, ICC 500, recently publshed n the Unted States

(Internatonal Code Counc, 2008), and the Desgn Gudenes for Queensland Publc

Cyclone Shelters (Department of Publc Wors, Queensand, 2006)

Cause 5ncorporates the same 4 g tmber msse as specfed n the 1989 and 2002

edtons; n addton, a smaller 2 gram stee bal s specfed uthermore, the horontal

mssle speeds for both msses are specfed as 04 tmes the regonal wnd speed; ower

vertcal msse testng speeds are specfed

or Regon C, the horzontal msse speed s therefore 04 x 69 m/s or 276 /s -

consderably hgher than the 15 /s (resultant) speed prevousy specfed However, the

research by Ln et a (2007) clearly ndcates that a mssle speed of 15 m/s n a wndstormproducng 69 m/s gusts wl be attaned n a very short dstance of trave - less than 2 m n

fact 50% of the wnd gust speed s reached n a travel dstance of 78 m, a dstance typcal

of the spacng between buldngs n urban areas


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The horizontal trajectory missiles appy to surfaces which ae subject to positive pessure(i.e wals, steepy pitched roos) The vertical traectory missies apply to suraces on towhich aling objects may and (i.e oofs)

he 4 kg timber missile with a 100 x SO mm cross section is the dominant impact load

The g stee bal missie is 8 mm diameter and has been incuded to ensure the buidingenvelope has a resistance to smal windborne debris. Test specifications and acceptancecriteria are deined in the Queensand Guidelines and the US Standard on Storm SheltersBoth documents requie the test missies to impact at ight angles to the surace heQueensland Guidelines requires a test specimen to be impacted by the timber missileolowed by impact by five steel balls at dieren locations More detaied criteria o theperormance o building facades in windborne debris tests are also avaiable in a standardpublished by the American Society or esting Materials (ASTM 2009).

The impact force applied to the building depends not only on the missile mass and speed,

but aso on the stiness of the buiding at the impact ocation The stier the impactlocatio, the geate the impact force Tests on the debis resistance o building elementshave shown that the critical location is oten near a support

Debris screens can be used to potect windows om windbone debis or a debis screento provide ull potection to a windw, the maximum apeture in the screen would need tobe ess than 8mm lager apertue screens are used which resist the 4 kg missile, then theglazing or insect screen woud need to be capable o resisting the g missie. he gapbetween the debis sceen and the glaing has to be suicient to ensue that whenimpacted the screen delects without breaking the glazing The screen should eithe return

to the wal or overap the wal around the window, to prevent the missile breaking thegazing rom an oblique impact Guidance on debris screen geometry is provided in theQueensland Guidelines

uen builing standards do not require the extenal abric of a building to be resistant towindborne debris, unless the building internal pressure is to be reduced in accordance withClause 532, i.e ignoring the possibility o a dominant opening The vulneability of peopleshelterng within their homes in the cycloneaected egions o Austraia would be greatlyreduced i they had a room within the dweing constructed to esist cyconic winds andwindborne debis

Note tat ause 257 is not itsef a requirement or debris resistance. t merey specifiesthe types and speeds of the missies when debis esistance is speciied elsewhere Theatte may include Clause 532 o the Standad, which is a requirement for intenalpressues in cyconic regions, o a equirement or shelter ooms or buildings in cyconeregions which may be required by builing owners or legislation.

Documents

Wind Loading Handbook