AS 1170.2 - 1989 - Wind Loads

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Australian Standard



SAA Loading Code

Part 2: Wind loads

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was approved on behalf of the Council of Standards Australia on 19 December 1988 and published on 20 March 1989.

The following interests are represented on Committee BD/6: Association of Consulting Engineers, Australia

Association of Consulting Structural Engineers, Australia Australian Clay Brick Association

Australian Construction Services (Department of Administrative Services) Australian Council of Local Government Associations

Australian Federation of Construction Contractors Australian Institute of Steel Construction

Australian Mining Industry Council Building Management Authority, W.A. Bureau of Meteorology

Bureau of Steel Manufacturers of Australia

CSIRO, Division of Building, Construction and Engineering Department of Local Government, Qld

Electricity Supply Association of Australia Engineering and Water Supply Department, S.A. James Cook University of North Queensland

Master Builders’ Construction & Housing Association, Australia Monash University

National Association of Australian State Road Authorities Public Works Department, N.S.W.

University of Melbourne University of Newcastle

Additional interests participating in preparation of Standard: Road Construction Authority

University of Sydney

University of Western Australia

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Australian Standard



Minimum design loads on

structures (known as the SAA

Loading Code)

Part 2: Wind loads

First published as part of SAA Int. 350—1952. Revised and redesignated AS CA 34.2—1971. Revised and redesignated AS 1170.2 — 1973. Second edition 1975. Third edition 1981. Fourth edition 1983. Fifth edition 1989. Incorporating: Amdt 1 — 1991 Amdt 2 — 1993 Amdt 3 — 1993

PUBLISHED BY STANDARDS AUSTRALIA (STANDARDS ASSOCIATION OF AUSTRALIA) 1 THE CRESCENT, HOMEBUSH, NSW 2140

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PREFACE

This Standard was prepared by the Standards Australia Committee for Loading on Structures to supersede AS 1170 — 1983, Minimum design loads on structures, Part 2 — Wind forces. This Standard is intended to be used for the determination of the minimum wind loads in structural design, and is in a limit states format.

It provides a simplified procedure (Section 2) for the determination of wind loads on a limited range of small buildings and structures, and a detailed procedure (Sections 3 and 4) for determination of wind loads on a wide range of structures, varying from those less sensitive to wind action, to those for which dynamic response must be taken into consideration. It permits wind tunnel tests or similar determinations of wind loads on structures.

Explanatory material for this Standard are given in Appendices C to F, which correspond to Sections 1 to 4.

The Standards Australia Committee has considered exhaustive research and testing information from Australian and overseas sources in the preparation of this Standard with a view to reducing the design wind loads by the maximum extent consistent with safety. The design wind loads prescribed in this Standard are the minimum for the general cases. These will be circumstances arising in particular cases, which will result in additional loads requiring to be taken note of in the design of structures in those cases. Designers must be alert to the conditions to which their particular structure is exposed and must take note of all the provisions in clauses and notes under the clauses.

This Standard differs from the previous Standard as follows:

(a) Windspeeds are specified for the serviceability and ultimate strength/stability limit states, and for permissible stress design.

(b) Return periods and windspeed contours (isopleths) have been deleted.

(c) Regional boundaries have been included (boundaries of the tropical cyclone regions are slightly modified).

(d) Direct shielding allowance is separately identified and extended.

(e) A more rational system of multipliers for wind speed and external pressures is provided. (f) Methods of calculating wind loads on cantilevered roofs, attached canopies, awnings, carports, circular cross-sections, such as bins, silos and tanks, and lattice towers have been added. Existing data on pressure and force coefficients have been revised in the light of recent research.

(g) Dynamic analysis has been expanded (replaces Annex: ‘Notes on Wind Forces on Tall Buildings’ in the previous edition).

(h) References to other publications are listed numerically at the end of this document. (i) Statements expressed in mandatory terms in Notes to tables and figures are deemed to

be requirements of this Standard.

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For the purposes of determining forces and pressures, wind tunnel tests or similar tests employing a fluid other than air, shall be considered properly conducted only if the natural wind has been modelled for the appropriate terrain categories to take account of — (a) the variation of wind speed with height; and (b) the scale and intensity of the longitudinal

component of turbulence. Notice shall be taken of —

(i) the effects of Reynolds number where curved shapes are involved;

(ii) the appropriate frequency response of force and pressure measuring systems; and

(iii) scaling of mass, length, stiffness, and damping, where measurements of dynamic response are involved.

1.5.3 Wind tunnel tests on a specific structure.

Where properly conducted wind tunnel tests on a specific structure have been carried out, or when reference to such tests on a similar structure is used, the loads thus determined shall be used instead of those determined through the provisions of this Standard.

1.6 DEFINITIONS. For the purpose of this

Standard, the definitions below apply.

Awnings — a roof-like structure, usually of limited

extent, projecting from a wall of a building.

Canopy — a roof adjacent to or attached to a

building, generally not enclosed by walls.

Cladding — the material which forms the external

surface over the framing of an element of a building or structure.

Dominant opening — an opening in the external

surface of an enclosed building which directly influences the average internal pressure in response to external pressures at that particular opening. Dominant openings need not be large.

Drag — a force acting in the direction of the

windstream.

Enclosed buildings — buildings which have full

perimeter walls (nominally sealed) from floor to roof level.

Escarpment — a long (steeply sloping) face between

nominally level lower and upper plains with average slopes of not greater than 5%.

Force coefficient — a coefficient which when

multiplied by the incident wind pressure and an appropriate area (defined in the text), gives the force in a specific direction.

Free roof — a roof (of any type) with no enclosing

walls underneath, e.g. freestanding carport.

Freestanding walls — walls which are exposed to

the wind on both sides, with no roof attached, e.g. fences.

Freestream dynamic pressure — the theoretically

computed incident pressure of a uniform air stream of known density q = 0.0006 x V2

(at ambient temperature and barometric pressure).

Gable roof — a ridged roof with end walls triangular

from lowest points up to the ridge.

Hip roof — a traditional roof with sloping ridges

rising up from external corners (valleys rise up from any return corners).

Hoardings — free standing (rectangular) signboards,

etc, supported clear of the ground.

Immediate supports (cladding) — those supporting

members to which cladding is directly fixed (e.g. battens, purlins, girts, studs).

Lattice towers — three-dimensional frameworks

comprising three or more linear boundary members interconnected by linear bracing members joined at common points (nodes), enclosing an open area through which the wind may pass.

Lift — a force acting at 90° to the windstream.

Major offshore structures — major navigation

structures, drilling platforms and major structures on small islands, reefs or shoals.

Monoslope roof — a planar roof with no ridge,

which has a constant slope.

Obstructions — natural or man-made objects which

generate turbulent windflow, ranging from single trees to forests and from isolated small structures to closely spaced multi-storey buildings.

Permeability — an aggregation of small openings

and cracks etc, which allows air to pass through walls or roofs etc, under the action of a pressure differential.

Pitched roof — a bi-fold, bi-planar roof with a ridge

at its highest point.

Porosity (of cladding) — the ratio of the area of

openings divided by the total surface area.

Pressure — air pressure in excess of ambient.

Negative values are less than ambient, positive values exceed ambient. Net pressures act normal to a surface in the direction specified within the text.

Pressure coefficient — the ratio of the average

pressure acting at the point on a surface, to the freestream pressure of the incident wind.

Reliable data (wind speeds and directions) — it is

the professional responsibility of the user to assess data other than that presented within this Standard.

Reliable references (wind pressures and loads) —

reference material or other material judged to be reliable by the professional user.

Ridge (topographic feature) — a long crest or chain

of hills which have a nearly linear apex, with sloping faces on either side of the crest.

Roughness length — a theoretical quantification of

the (wind) turbulence inducing nature of a particular type of terrain.

Sufficient (meteorological information) — the

assessment of an appropriately qualified and experienced professional.

Terrain — the surface roughness condition when

considering the size and arrangement of obstructions to the wind.

Topography — major land surface features

comprising hills, valleys and plains which strongly influence wind flow patterns.

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Tornado — a violently rotating column of air,

pendant from the base of a convective cloud, and often observable as a funnel cloud attached to the cloud base.

Tributary area — the area of building surface

contributing to the force being considered.

Tropical cyclone — an intense low-pressure centre

accompanied by heavy rain and gale-force winds or greater. It forms over warm tropical oceans and decays rapidly over land. In the southern hemisphere, winds spiral clockwise into the centre.

Troughed roof — a bi-fold, bi-planar roof with a

valley at its lowest point.

1.7 NOTATION. Unless a contrary intention is

stated, the notation used in this Standard shall have the following meanings with respect to a structure, or member, or condition to which a Clause is applied. The SI system of units is used throughout.

A = the surface area of the element or the tributary area which transmits wind forces to the element

Ar = the gross plan area of the roof including

eaves, canopies, awnings etc

Az = the area of a structure or a part of a

structure, at height z, upon which the design wind pressure (pz) operates, being —

(a) when used in conjunction with the pressure coefficient (Cp), the area upon

which the pressure acts, which may not always be normal to the windstream; (b) when used in conjunction with a drag

force coefficient (Cd), the projected area

normal to the windstream; and

(c) when used in conjunction with a force coefficient, (CF,x) or (CF,y), the areas as

defined in applicable clauses.

a = the dimension used in defining the extent of application of local pressure factors

B = a background factor, which is a measure of the slowly varying background component of the fluctuating response caused by the lower frequency wind speed variations

B1 = a regional multiplying factor

B2 = a terrain and height multiplying factor

B3 = a topographic multiplying factor

B4 = an area reduction factor for roofs

b = the horizontal breadth of a vertical structure normal to the windstream; or the average breadth of a vertically tapered structure over the top half of the structure; or the nominal average breadth of a horizontal structure; or the average diameter of a circular section

bs = the average breadth of shielding buildings,

normal to the windstream.

Cd = the drag force coefficient for a structure or

member in the direction of the windstream

= Fd

A_zq_z

CF,x = the force coefficient for a structure or

member, in the direction of the member’s

x-axis

= Fx

A_zq_z

CF,y = the force coefficient for a structure or

member, in the direction of the member’s

y-axis

= Fy

A_zq_z

Cf = a frictional drag force coefficient

Cfs = the cross-wind force spectrum coefficient

generalized for a linear mode

Cp = a pressure coefficient

Cp,c = a pressure coefficient for the windward edge

of a roof supported by a cantilevered beam

Cp,e = an external pressure coefficient

Cp,i = an internal pressure coefficient

Cp,l = a net pressure coefficient for the leeward

half of a free roof

Cp,n = a net pressure coefficient for canopies, free

standing roofs, walls, etc

Cp,w= a net pressure coefficient for the windward

half of a free roof

Cp1 = an external pressure coefficient for a bin,

silo or tank of unit aspect ratio

c = the net height of a hoarding, bin, silo or tank

D = a building spacing parameter

d = the minimum roof plan dimension or, the depth or distance to which the plan or cross-section of a structure or shape extends parallel to the wind stream

da = the along-wind depth of a porous wall or

roof surface

E = a spectrum of turbulence in the approach windstream; or the modulus of elasticity

e = the base of Napierian logarithms (≈ 2.71828)

F = the wind force acting normal to the surface of a building element

Fd = the drag force acting parallel to the wind

stream

= the hourly mean drag force acting parallel to

F_d

the windstream

Ff = the resultant frictional force acting parallel

to the windstream

Fx = the wind force component resolved along

the x-axis of a body

Fy = the wind force component resolved along

the y-axis of a body

= the hourly mean net horizontal force acting

Fz

on a building or structure at height z

f = a friction stress

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G = a gust factor

gf = a peak factor (fluctuating response)

gv = a peak factor for the upwind velocity

fluctuation

H = the height of a hill, ridge or escarpment

h = the height of a structure above ground

hc = the height from ground to the attached

canopy, etc

he = the eaves height of building

hi = the developed height of the inner layer,

which is equal to z for the calculation of xi

hs = the average height of shielding buildings

ht = the height to the top of a structure above

ground

I = the second moment of area

Ka = an area reduction factor

Kar = an aspect ratio correction factor for

individual member forces

Ki = a factor to account for the angle of

inclination of the axis of members to the wind direction

Kl = a local pressure factor

Kp = a reduction factor for porous cladding

Ksh = a shielding factor for multiple open frames

Kt = a type factor for topography

k = a mode shape power exponent

kc = a multiplier for Cp,e(on circular tanks, bins

and silos)

Lh = a measure of the effective turbulence length

scale

Lu = the horizontal distance upwind from the

crest of a hill, ridge or escarpment to a level half the height below the crest

L*

= the effective horizontal length of the upwind slope of a hill, ridge or escarpment

l = the length of a frame member; or the length of a cantilevered roof beam

ls = the average spacing of shielding buildings

Ma = the along-wind base overturning moment

Ma = the mean base overturning moment for a

structure in the along-wind direction

M^a = the design peak base overturning moment

for a structure in the along-wind direction

Mc = the cross-wind base overturning moment

Mc = the mean base overturning moment for a

structure in the cross-wind direction

M^c = the design peak base overturning moment

for a structure in the cross-wind direction

Mi = a structure importance multiplier

Mo = the upstream terrain category gust wind

speed multiplier at the beginning of each new terrain inner layer for height z

Mo = the upstream terrain category hourly mean

wind speed multiplier at the beginning of each new terrain inner layer for height z

MR = the resultant vector base overturning

moment

M^R = the peak resultant vector base

overturning moment

Ms = a shielding multiplier for both gust and

hourly mean wind speeds

Mt = a topographic multiplier for gust wind

speeds

Mt = a topographic multiplier for hourly mean

wind speeds

Mx = the gust wind speed multiplier at a

distance x from the start of new terrain category for height z

Mx = the hourly mean wind speed multiplier

at a distance x from the start of the new terrain category for height z

M(z,cat) = a gust wind speed multiplier for a

terrain category at height z (for upwind distance of at least (2500 + xi) m)

M(z,cat) = an hourly mean wind speed multiplier

for a terrain category at height z (for upwind distance of at least (1500 +

xi) m)

Mα = a component of the base overturning

moment acting in the αdirection

m = the mass per unit length

mo = the average mass per unit height of a

structure

N = an effective reduced frequency

n = the number of spans of a multi-span roof; or the first mode frequency of a structure

na = the first mode frequency of a structure

in along-wind direction

nc = the first mode frequency of a structure

in cross-wind direction

ns = the number of upwind shielding

buildings within a 45° sector of radius 20ht

pc = the maximum design wind pressure at

the leading edge of a roof supported by a cantilevered beam

pd = the ultimate limit state design wind

pressure

pe = the external wind pressure

pi = the internal wind pressure

pn = the net wind pressure

pz = the design wind pressure at height z

p′ = the net basic wind pressure

qh = the free stream gust dynamic wind

pressure at the top of a structure

qh = the free stream hourly mean dynamic

wind pressure at height h

qz = the free stream gust dynamic wind

pressure resulting from Vz

qz = the free stream hourly mean dynamic

wind pressure resulting from Vz

R = the return period

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r = the corner radius of a structural shape, or a roughness factor, or the rise of a curved roof

S = a size factor

Sr = the Strouhal number

s = a position factor for topographic effects

u*

= the friction velocity

V = the basic wind speed

V = the hourly mean wind speed

Vcrit = the critical wind speed, for ‘Lock-in’

Vh = the design gust wind speed at height h

Vh = the design hourly mean wind speed at

height h

Vp = the basic wind speed for permissible

stress methods

Vs = the basic wind speed for serviceability

limit state

Vu = the basic wind speed for ultimate

strength limit state

Vz = the design gust wind speed at height z

Vz = the design hourly mean wind speed at

height z

w = a factor to account for the second order effects of turbulence intensity

wc = the width of canopy, etc, from the face

of the building

x = the distance downwind from a change in terrain category to the structure under consideration; or the horizontal distance from a structure to the crest of a hill or a ridge

x = the mean value of random variables

x^ = the peak value of random variables

xi = the distance downstream from the start

of the new terrain roughness to the developed height of the inner layer (hi)

ÿ^ = the peak acceleration at the top of a structure in cross-wind direction

z = the distance or height above the ground; or the effective height of an escarpment

zg = the gradient height, at which terrain

influence ceases

zo = a characteristic terrain roughness length

z1, z2 = the effective heights of a hill, on each

side of it

zo,r = the larger of the two roughness lengths,

at the change in terrain

α = the angle of slope of a roof; or the direction of a resultant vector base overturning moment with respect to the along-wind direction

αmax = the angle to the along-wind direction of

the plane of the maximum resultant vector base overturning moment β = the fractional porosity of a wall; or the

angle from the wind direction to a point on the wall of a circular bin, silo or tank δ = the actual solidity ratio for an open

frame

δe = the effective solidity ratio for an open

frame

εa = a structural load effect derived from the

mean along-wind dynamic response = a structural load effect derived from the εc

mean cross-wind dynamic response ε

^

c = a structural load effect derived from the

peak cross-wind response, and proportional to the peak cross-wind base overturning moment M^c

ε

^

t = the total combined peak load scalar

effect

ζ = a fraction of the critical damping capacity of a structure

η = a multiplier, used to calculate σv

=

θ = the angle of deviation of the wind stream from the axis of a structural section

λ = a spacing ratio for open frames υ = the kinematic viscosity

ρ = the atmospheric density

σv = the standard deviation of a wind gust

component

σv/Vz = the turbulence intensity in the approach

flow at height z

σx = the standard deviation of a set of values

of variable ‘x’

τo = the surface friction shear stress

φ = the upwind slope of a hill, ridge or escarpment or the angle between the windstream and the plane of a structural member

φd = the average downwind slope measured

from the crest of a hill, ridge or escarpment to the ground level at a distance of 5H

φ′ = the lesser value of φor 0.3 ψ(z) = a mode shape

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SECTION 2. SIMPLIFIED PROCEDURE

2.1 INTRODUCTION. This Section sets out a

simplified procedure for the determination of wind loads on a limited range of small buildings and structures, which satisfy the limitations set out in Clause 2.2.

This Section must not be used in conjunction with other sections of this Standard except where specific reference to other sections is made (see Clause 1.3.1). The loads specified in this Section are ultimate strength limit state design wind loads.

If loads are required for permissible stress design, the ultimate strength loads given in this Section shall be divided by 1.5.

If loads are required for serviceability limit state design, the ultimate strength loads specified in this Section shall be multiplied by the serviceability multiplying factors given in Table 2.7.

(See Paragraph D2.1 of Appendix D.)

2.2 LIMITATION. The simplified procedure shall

only be applied to the determination of wind loads on buildings, their attachments, and miscellaneous structures which satisfy all of the relevant following conditions:

(a) The overall height does not exceed 15.0 m.

(b) The buildings are rectangular in plan, or a combination of rectangular units.

(c) The roof pitch of buildings does not exceed 30°. (d) The ratio of the height (ht) to the minimum roof

plan dimension (d) is less than five for enclosed buildings, and less than one for free standing roofs.

(e) The gross roof plan area of buildings does not exceed 1000 m2

.

(f) The consequences of failure of the building in social and economic terms is not high relative to that normally associated with small buildings. Structures that have special post-disaster functions, e.g. hospitals and communications buildings, shall not be designed using this Section.

The minimum roof plan dimension of a building is the minimum width of the roof in plan (see Figure 2.2(A)).

The height (ht) is the height from the lowest point at

ground level to the highest point of the structure (see Figure 2.2(B)).

The gross roof plan area of buildings is the total area of the roof in plan, including any attached canopies, awnings, carports, etc.

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2.3 PROCEDURE.

2.3.1 General. The ultimate limit state design wind

pressures (pd) shall be obtained by multiplying the net

basic wind pressures (p′) given in Clause 2.4 by the appropriate multiplying factors given in Clause 2.5, using Equation 2.3.1.

pd = p′B1B2B3B4 . . . (2.3.1)

where

pressure, in kilopascals

p′ = the net basic wind pressure, in kilopascals

B1 = a regional multiplying factor (see

Table 2.5.1)

B2 = a terrain and height multiplying factor

(see Table 2.5.2)

B3 = a topographic multiplying factor (see

Table 2.5.3)

B4 = an area reduction factor for external

pressures on roofs (see Table 2.5.4). The net basic wind pressure (p′ ) shall be the worst case of combined internal and external basic pressures, or windward and leeward basic wall pressures, as appropriate. In combining external and internal basic pressures, the worst combination of the basic pressures prescribed within the limits given in this Section shall be used. Where only one limit of basic pressure is given, it shall be assumed that the other limit is zero.

2.3.2 Forces (F) on elements of buildings. The

wind force (F) due to wind pressure acting on an element of a structure shall be calculated from Equation 2.3.2.

F = pdA . . . (2.3.2)

where

F = the wind force acting normal to the surface of a building element

pressure, in kilopascals

A = the surface area of the element or the tributary area which transmits wind forces to the element, in square metres. Resultant forces on complete buildings shall be computed from the summation of forces acting normal to all the individual surfaces of the building.

2.4 BASIC PRESSURES.

2.4.1 External pressures on rectangular buildings. 2.4.1.1 General. The basic external pressures on buildings are a function of the location of the building, the direction of the wind relative to the orientation of the building, and the geometry of the building.

The geometry of the building is defined in terms of the windward roof slope in the direction of the wind and the following factors:

(a) ht = the height of the building, as described in

Clause 2.2.

(b) d = the minimum roof plan dimension in the direction of the wind (see Figure 2.4.1.1).

2.4.1.2 Windward sections of roofs. For the

windward sections of roofs the basic pressures given in Table 2.4.1.2 shall be used.

TABLE 2.4.1.2

EXTERNAL BASIC PRESSURES FOR WINDWARD SECTIONS OF ROOFS Windward

roof slope (α) degrees

Basic pressure, kPa

max.neg. max.pos. max.neg. max. pos.

≤10 15 20 25 30 -0.95 -0.75 -0.45 -0.35 -0.25 0 0 0.25 0.35 0.35 -1.4 -1.1 -0.75 -0.55 -0.35 0 0 0 0 0.25

NOTE: For intermediate values of α and ht/d, linear

interpolation is permitted.

The windward section of the roof is the windward half of the roof, or the section windward of the highest horizontal ridge at right angles to the wind direction where such ridges are present.

Where the windward roof slope varies, the basic pressures used in the design vary according to the roof slope in the direction of the wind, otherwise the minimum roof slope in the direction of the wind on the windward section of the roof shall be used for negative pressures, and the maximum roof pitch shall be used for positive pressures.

The basic pressures given in Table 2.4.1.2 shall be assumed to act over the whole of enclosed monoslope roofs, and over all of the area of roofs having slopes in the direction of the wind which are nominally zero.

2.4.1.3 Leeward sections of roofs. For the leeward

sections of roofs the basic pressures given in Table 2.4.1.3 shall be used.

TABLE 2.4.1.3

EXTERNAL BASIC PRESSURES FOR LEEWARD SECTIONS OF ROOFS Leeward roof slope (α)

degrees

≤15 ≥20 -0.55 -0.65 -0.75 -0.65

NOTE: For intermediate values of α and ht/d, linear

interpolation is permitted.

2.4.1.4 Walls and undersides of eaves. For walls and

undersides of eaves, the basic pressures given in Table 2.4.1.4 shall be used.

TABLE 2.4.1.4

EXTERNAL BASIC PRESSURES FOR WALLS AND UNDERSIDES OF EAVES

Location Basic pressure, kPa

Windward: (a) normal building (b) highset building Leeward Side 0.75 0.85 -0.55 -0.7

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NOTE: α ≤30°; ht≤15 m.

NOTE: For wind blowing end-on to a gable roof, the windward roof slope shall be taken as zero.

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A highset building is an elevated building with a clear, unwalled space underneath the first floor level, with a height from ground to underside of floor of at least one third of the total height of the building. For the design of unenclosed eaves, the use of net basic pressures obtained from Clauses 2.4.3.2 and 2.4.3.3 is permitted.

2.4.1.5 Local negative external pressures. Cladding

and its immediate supports within 0.2√Ar of edges,

corners, ridges, etc, shall also be designed for the external local basic pressures given in Table 2.4.1.5, where Aris the gross plan area of the roof including

attached canopies, awnings, etc.

TABLE 2.4.1.5

BASIC LOCAL NEGATIVE EXTERNAL PRESSURES FOR CLADDING AND ITS

IMMEDIATE SUPPORTS Location Tributary area (A)

m2

Roof <0.01Ar ≥0.01Ar≤0.04Ar >0.04Ar -1.9 -1.45 -0.95 -2.1 -2.1 -1.4 Walls <0.01Ar ≥0.01Ar≤0.04Ar >0.04A -1.4 -1.05 -0.7 -1.4 -1.05 -0.7

The tributary area is the area contributing to the force being considered. For example, the tributary area for a cladding fastener will be the area of cladding supported by a single fastener; for a purlin it will be the span between supporting rafters times the distance between purlins.

2.4.1.6 Local positive external pressures. Wall elements with tributary areas less than 0.01Ar, as

defined in Clause 2.4.1.5, shall be designed for the following basic local positive external pressures: (a) For normal buildings: 0.9 kPa.

(b) For highset buildings: 1.05 kPa.

For definition of highset building, see Clause 2.4.1.4.

2.4.1.7 Under floor pressures of highset buildings.

Highset buildings shall be designed for under floor basic pressures of 0.85 kPa and - 0.65 kPa.

2.4.2 Internal pressures. Both cladding and

structure shall be designed for the internal basic pressures given in Table 2.4.2.

TABLE 2.4.2

INTERNAL BASIC PRESSURES

Openings

Basic pressure, kPa max. neg. max. pos.

No dominant openings Dominant openings: (a) Normal building (b) Highset building -0.35 -0.7 -0.7 0.25 0.75 0.85

Internal pressures based on dominant openings shall be used when the area of a permanent opening in one

wall exceeds 4 times the sum of the permanent openings in other walls and the roof.

In tropical cyclone-prone regions C and D, as defined in Clause 2.5.1, internal pressures based on dominant openings shall be used for calculating both ultimate strength and permissible stress design loads unless windows are protected against impact of debris by screens or shutters capable of resisting a 4 kg piece of timber of 100 mm× 50 mm cross-section striking them at any angle at a speed of 15.0 m/s. This requirement does not apply to the calculation of serviceability design loads.

For definition of high set buildings, see Clause 2.4.1.4.

2.4.3 Unenclosed attached canopies, awnings,

carports and eaves.

2.4.3.1 General. Unenclosed attached canopies,

awnings and carports with a roof slope of less than 5° and attached to buildings satisfying the limitations in Clause 2.2, shall be designed using the basic pressures given in Clauses 2.4.3.2 and 2.4.3.3. For the design of unenclosed eaves, the use of net basic pressures given in these Clauses is also permitted.

2.4.3.2 Main structural components. The main

structural components shall be designed for the net basic pressures given in Table 2.4.3.2 acting on the roof.

TABLE 2.4.3.2

NET BASIC PRESSURES (p′) FOR

UNENCLOSED CANOPIES, AWNINGS, CARPORTS AND EAVES

Net basic pressure (p′), kPa Upwards Downwards ≤1 2 ≥5 <0.1 0.5 0.75 1.0 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.60 1.05 1.05 1.40 1.60 1.25 0.60 0.60 0.60 LEGEND:

hc = the height from ground to the attached canopy, etc

he = the eaves height of the building, to which the canopy, etc, is attached

wc = the width of the canopy, etc, from the face of the

building.

NOTE: For intermediate values of hc/he and hc/wc, linear interpolation is permitted.

2.4.3.3 Roof cladding and its immediate supports.

Roof cladding and its immediate supports shall be designed for the same net basic pressures as given for the main structural components in Table 2.4.3.2. Within a distance from an edge of 0.2 times the length of the canopy, awning, carport or eave (measured parallel to the wall to which it is attached), the roof cladding and its immediate supports shall also be designed for the local net basic pressures given in Table 2.4.3.3.

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TABLE 2.4.3.3

LOCAL NET BASIC PRESSURES (p′) ON

ROOF CLADDING OF CANOPIES, ETC Tributary area

(A) m2

Net basic pressure (p′), kPa Upwards Downwards <1 2 >5 <0.01Ar ≤0.1 0.5 0.75 1.0 2.1 2.1 2.1 2.1 2.1 2.1 2.1 3.2 2.1 2.1 2.8 3.2 2.5 1.2 1.2 1.2 ≥0.01Ar≤0.04Ar ≤0.1 0.5 0.75 1.0 1.6 1.6 1.6 1.6 1.6 1.6 1.6 2.4 1.6 1.6 2.1 2.4 1.9 0.9 0.9 0.9 NOTES:

1. For intermediate values of hc/heand hc/wc, linear interpolation is permitted.

2. For definition of tributary area, see Clause 2.4.1.5. 3. For definition of hc, heand wc, see Clause 2.4.3.2. 2.4.4 Free standing roofs.

2.4.4.1 General. Monoslope, pitched and troughed

free roofs, whose height (ht) is less than their

minimum plan dimension (d), shall be designed for the net basic pressures (p′) and frictional forces (Ff)

given in Clauses 2.4.4.2 and 2.4.4.3.

2.4.4.2 Main structural components. The main

structural components shall be designed for the following net basic pressures (p′ ) and frictional forces (Ff):

(a) Net basic pressures (p′ ) acting upwards and downwards at right-angles to the roof surface as given in Table 2.4.4.2.

TABLE 2.4.4.2

NET BASIC PRESSURES (p′) ON FREE

STANDING ROOFS Roof type Roof pitch,

degrees

Net basic pressure (p′), kPa Downward Upward Pitched and troughed Monoslope Monoslope all ≤15 30 0.95 0.85 1.70 1.60 1.60 2.80 NOTE: For monoslope roof pitches between 15°and 30°, linear interpolation is permitted.

(b) Frictional forces (Ff) acting parallel to the roof

surface and given by Equation 2.4.4.2.

Ff= fArB1B2B3B4 . . . (2.4.4.2)

where

Ff = the resultant frictional force

f = a friction stress, in kilopascals = 0.011 for wind parallel to

corrugations and ribs

= 0.021 for wind at right angles to corrugations

= 0.042 for wind at right angles to ribs

Ar = the gross plan area of free standing

roof

B1,B2, = the multiplying factors given in

B3and B4 Clause 2.5

2.4.4.3 Free roof cladding and its immediate

supports. In general, free roof cladding and its immediate supports shall be designed for the same net basic pressures given for the main structural components in Table 2.4.4.2. Within 0.2√Arof edges,

corners, ridges, etc, where Aris the gross plan area of

the roof, they shall also be designed for local net basic pressures acting upwards as given in Table 2.4.4.3.

TABLE 2.4.4.3

LOCAL UPWARD NET BASIC PRESSURES

(p′) ON FREE STANDING ROOFS

NOTE: For roof pitches between 10° and 30°, linear

Roof type Roof pitch, degrees Tributary area (A) m2

Net basic pressure (p′) kPa All ≤10 Corner < 0.01Ar Other < 0.01Ar ≥0.01Ar≤0.04Ar 4.2 2.8 2.1 Pitched and troughed 30 < 0.01Ar ≥0.01Ar≤0.04Ar 3.2 2.4 Monoslope < 0.01Ar ≥0.01Ar≤0.04Ar 5.6 4.2

interpolation is permitted, except that it shall be assumed that the higher local pressures in corner regions do not occur for roof pitches greater than 10°.

For definition of tributary area, see Clause 2.4.1.5. 2.4.5 Freestanding walls on ground. Freestanding

walls on ground shall be designed for net basic pressures (p′) of 1.8 kPa.

2.4.6 Rectangular hoardings and signs. Hoardings

and signs shall be designed for net basic pressures (p′) of 1.8 kPa.

A hoarding or sign is defined as a rectangular plate mounted vertically with a clear space underneath of at least 0.4 times the depth (i.e. vertical dimension) of the plate. If the clear space under the sign is less than this, the hoarding or sign shall be treated as a freestanding wall (see Clause 2.4.5).

2.5 MULTIPLYING FACTORS.

2.5.1 Regional multiplying factor (B1). The

regional multiplying factors (B1) given in Table 2.5.1

shall be used in the different wind risk regions as shown in Figure 2.5.1.

TABLE 2.5.1

REGIONAL MULTIPLYING FACTORS (B1)

Region Factors (B1)

A—Normal B—Intermediate C—Tropical cyclone D—Severe tropical cyclone

1.0 1.5 2.3 3.3

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FIGURE 2.5.1 MAP OF AUSTRALIA SHOWING BOUNDARIES OF REGIONS FOR THE CHOICE OF REGIONAL MULTIPLYING FACTOR (B1)

(see Paragraph D2.5.1 of Appendix D)

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2.5.2 Terrain and height multiplying factor (B2).

To account for the surrounding terrain and the height of the building, the terrain and height multiplying factors (B2) given in Table 2.5.2 shall be used.

TABLE 2.5.2

TERRAIN AND HEIGHT MULTIPLYING FACTORS (B2)

NOTE: For intermediate values of ht, linear interpolation is Building

Height (ht)

m

Factors (B2)

Suburban Open rural terrain

Exposed Sheltered Regions A and B Regions C and D

≤4 7 10 15 0.85 0.90 1.00 1.15 0.55 0.60 0.65 0.75 1.15 1.30 1.45 1.60 1.10 1.20 1.30 1.45 permitted.

The height (ht) of the structure or building shall be

used when selecting (B2) factors for components

situated lower on the structures.

Open rural terrain refers to isolated structures in designated rural areas, and buildings on the edge of designated rural areas or adjacent to the sea or other large expanses of water.

Suburban terrain refers to buildings within designated urban areas and at least 500 m from the edge of designated rural areas in the case of buildings of overall height less than 10 m, and at least 2500 m from the edge of designated rural areas in the case of buildings of overall height equal to or greater than 10 m.

Within suburban terrain, buildings shall be classed as sheltered if surrounded to a depth of at least two rows by buildings of similar or greater height and size at an average density of not less than 10 buildings per hectare; and classed as exposed if adjacent to areas in which the average density of buildings of similar or greater height and size is less than 2.5 buildings per hectare.

For intermediate densities of buildings and within the transition region between suburban terrain and designated rural areas, linear interpolation of values of B2is permitted.

2.5.3 Topographic multiplying factor (B3). Where

structures are located on the upper levels of hills and ridges or near the edges of escarpments, the topographic multiplying factors (B3) given in

Table 2.5.3 shall be applied.

TABLE 2.5.3

TOPOGRAPHIC MULTIPLYING FACTORS (B3) Topographic features Maximum incline Location Factors (B3) Escarpment Escarpment Hill or ridge Hill or ridge < 1 in 7.5 < 1 in 15 < 1 in 7.5 < 1 in 10 J T J T 1.0 Escarpment Escarpment Hill or ridge Hill or ridge ≥1 in 4 1 in 7.5 1 in 4 1 in 7.5 J T J T 1.3 Escarpment Hill or ridge Hill or ridge ≥1 in 4 1 in 3 1 in 5 T J T 1.6 Hill or ridge Hill or ridge > 1 in 1.5 1 in 2 J T 2.0 Hill or ridge ≥1 in 1.5 T 2.4 NOTES:

1. The maximum incline is the maximum slope of the hillside in terms of the average slope over a change in elevation of 20% of the effective height of the hill.

2. The effective height is the height of the hill above the general level of the adjacent terrain. In Figure 2.5.3(a), z1 and z2are the effective heights of the hill (in metres), on each side of it. In Figure 2.5.3(b), z is the effective height of the escarpment (in metres).

3. Locations T and J are defined in Figure 2.5.3.

4. For topographic features with effective heights of less than 10 m in open rural terrain and 25 m in suburban terrain, it is permitted to take the topographic multiplying factor (B3) equal to 1 for buildings and their attachments.

5. The topographic multiplying factor (B3) for locations other than T and J is equal to 1.

6. For intermediate values of average upwind slopes, linear interpolation is permitted.

2.5.4 Roof reduction factor (B4). The external wind

forces on major roof supporting structures, excluding cladding and its immediate supporting members, shall be multiplied by the roof reduction factor (B4) given

in Table 2.5.4.

TABLE 2.5.4

ROOF REDUCTION FACTORS (B4)

NOTES:

Tributary area (A) m2 Factor (B4) ≤ 10 25 ≥100 1.0 0.9 0.8

1. For intermediate values of tributary area (A), linear interpolation is permitted.

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2.6 FATIGUE LOADING. In tropical cyclone regions C and D, as shown in Figure 2.5.1, cladding and its connections shall be designed to resist the fatigue loading sequence given in Table 2.6.

(See Paragraph D2.6 of Appendix D.)

TABLE 2.6

FATIGUE LOADING SEQUENCE

LEGEND:

Range Number of cycles

0 to 0.4pd 0 to 0.5pd 0 to 0.6pd 0 to 1.0pd 8 000 2 000 200 1

pd = the ultimate limit state design wind pressure, as

specified in Clause 2.3.

NOTE: These requirements will normally be met by testing three samples each of which should pass. If only one sample is tested, the final cycle should be increased to 1.3pd, and if two samples are tested, it should be increased to 1.2pd. 2.7 SERVICEABILITY DESIGN LOADS. Where

serviceability design loads are required, they shall be obtained by multiplying the ultimate limit state design loads by the multiplying factors given in Table 2.7.

TABLE 2.7

SERVICEABILITY MULTIPLYING FACTORS

Region Serviceability multiplying factor A—Normal B—Intermediate C—Tropical cyclone D—Severe tropical cyclone

0.6 0.4 0.4 0.35

2.8 FARM BUILDINGS AND TEMPORARY

STRUCTURES.

2.8.1 Farm buildings. In the design of structures

such as farm buildings, which present a low degree of hazard to life and other property in the case of failure, the calculated forces on the structure obtained from Clauses 2.4 to 2.7 may be multiplied by 0.8.

2.8.2 Temporary structures. Where the structure is

of a temporary nature and is to be erected for a period of less than 6 months, the calculated forces on the structure obtained from Clauses 2.4 to 2.7 may be multiplied by 0.65.

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SECTION 3. DETAILED PROCEDURE: STATIC ANALYSIS

3.1 LIMITATION. Wind pressures, forces, and

moments on structures and components may be calculated by the static analysis procedure set down in this Section unless the structure is wind sensitive. A wind sensitive structure or component is defined as one in which additional loads occur as a result of the dynamic interaction of the wind and structure. The dynamic analysis procedure set down in Section 4 shall be used for wind sensitive structures.

The static analysis procedure shall not be used for the design of main structural components of any structure having both the following properties:

(a) Height or length-to-breadth ratio greater than 5. (b) A first-mode frequency of vibration of less than

1 Hz.

Pressures and forces on parts of walls, roof cladding, canopies, awnings, windows, doors and their supporting framework shall be determined using the static analysis procedure set out in this Section. (See Paragraph E3.1 of Appendix E.)

3.2 GUST WIND SPEED.

3.2.1 General. The design gust wind speed (Vz) at

height z shall be used to determine wind loads on a structure or part of a structure with the static analysis procedure set out in this Section.

3.2.2 Derivation of design gust wind speed (Vz).

The design gust wind speeds (Vz) shall be determined

from the appropriate basic wind speed shown in Figure 3.2.2 for the appropriate limit state given by Equation 3.2.2.

Vz = VM(z,cat)MsMtMi . . . (3.2.2)

where

Vz = the design gust wind speed at height z,

in metres per second

V = the basic wind speed, (Vu), (Vp) and (Vs)

(see Figure 3.2.2), in metres per second

M(z,cat) = a gust wind speed multiplier for a

terrain category at height z for upwind distance of at least (2500 + xi) m (see also Clause 3.2.6,

Tables 3.2.5.1 and 3.2.5.2)

Ms = a sh ield in g mul ti pl ier ( see

Table 3.2.7)

Mt = a topographic multiplier for gust

wind speeds (see Table 3.2.8)

Mi = a structure importance multiplier

(see Table 3.2.9).

NOTE: M(z,cat)may change from the tabulated values if the structure site is within the transition zone near the edge of a terrain boundary (see Clause 3.2.6).

Irrespective of the calculation in this Clause, the design gust wind speed (Vz), determined by

Equation 3.2.2, shall be not less than the following: (a) Ultimate limit state . . . 30 m/s. (b) Permissible stress method . . . 25 m/s. (See Paragraph E3.2.2 of Appendix E.)

3.2.3 Wind direction. At least four wind directions,

equally spaced, shall be considered when calculating wind loads on structures using the detailed procedure. Where sufficient meteorological information is available, the basic wind speed (V) at a site may be adjusted for specific wind directions, in region A for

Vs, Vpand Vu, and in region B for Vs. For some of the

major population centres this is given in Table 3.2.3. Directional wind speeds shall be corrected for terrain, height, shielding and local topography, as indicated in Clause 3.2.2.

Where pressure coefficients or force coefficients and associated multiplying factors are given for only four orthogonal directions relative to the major axes of the structure, the wind speed for any given orthogonal direction shall be taken to be the largest corrected directional wind speed from a 90° sector, symmetrically positioned about the orthogonal direction being considered.

TABLE 3.2.3

BASIC WIND SPEEDS (V) IN (m/s) WITH WIND DIRECTION FOR SOME OF THE MAJOR POPULATION CENTRES Wind

direction

Adelaide Brisbane Canberra Melbourne Perth Sydney

Vs Vp Vu Vs Vp Vu Vs Vp Vu Vs Vp Vu Vs Vp Vu Vs Vp Vu NE E SE S SW W NW N 31 30 30 30 38 38 36 33 34 34 33 33 41 41 39 37 42 42 40 40 50 50 48 45 30 30 32 32 38 38 30 30 49 49 49 49 49 49 49 49 60 60 60 60 60 60 60 60 30 30 30 30 30 35 38 30 33 33 33 33 33 38 41 34 40 40 40 40 41 46 50 42 30 30 30 32 35 38 34 37 33 33 33 34 38 41 36 38 40 40 40 42 46 50 44 46 30 31 30 30 34 38 34 30 33 33 33 33 35 41 38 33 40 40 40 40 43 50 46 41 31 30 36 36 35 38 35 30 33 33 39 38 38 41 38 33 40 40 48 47 47 50 47 40 NOTES:

1 Wind direction in this Table indicates the direction from which the wind blows. 2 For intermediate wind directions, linear interpolation is permitted.

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FIGURE 3.2.2 BOUNDARIES OF REGIONS A, B, C AND D (see Paragraph E3.2.1 of Appendix E)

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Where pressure coefficients or force coefficients and associated multiplying factors are given for 8 or 16 separate wind directions, they shall be used with the corresponding corrected sector wind speeds derived from Table 3.2.3.

Where directional wind speed data are not available, or their use is not allowed (as in tropical cyclone regions C and D), or for Vpand Vu in intermediate region B

(which includes Brisbane), the basic wind speed may be multiplied by 0.95 for the determination of resultant forces and overturning moments on complete buildings and major framing elements.

NOTE: A reduction factor of 0.95 should not be used simultaneously with values from Table 3.2.3, except for Vpand

Vuin Brisbane.

As an alternative to the methods outlined in this Clause, a detailed probability analysis to allow for the directional effects of wind is permitted.

(See Paragraph E3.2.3 of Appendix E.)

3.2.4 Terrain category. Terrain, over which the

approach wind flows towards a structure, shall be assessed on the basis of the following category descriptions (see also Figures E3.2.4(A) to (D) of Appendix E):

(a) Category 1 — exposed open terrain with few or no obstructions and water surfaces at serviceability wind speeds (Vs) only.

(b) Category 2 — open terrain, grassland with few well scattered obstructions having heights generally from 1.5 m to 10.0 m and water surfaces at wind speeds (Vu) and (Vp).

(c) Category 3 — terrain with numerous closely spaced obstructions having the size of domestic houses (3.0 m to 5.0 m high).

(d) Category 4 — terrain with numerous large, high (10.0 m to 30.0 m high) and closely spaced obstructions such as large city centres and well-developed industrial complexes.

Selection of terrain category shall be made with due regard to the permanence of the obstructions which constitute the surface roughness, in particular vegetation in tropical cyclonic regions shall not be relied upon to maintain a wooded terrain roughness.

A roughness length (zo) is defined for each terrain

category in Table 3.2.4. TABLE 3.2.4 ROUGHNESS LENGTH (zo) Terrain category Roughness length (zo) metres 1 2 3 4 0.002 0.02 0.2 2.0

3.2.5 Terrain and structure height multiplier

(M(z, cat)). The variation of terrain multipliers with

height (z) shall be taken from Tables 3.2.5.1 and 3.2.5.2.

Designers shall take account of probable future changes to terrain roughness in assessment of terrain and structure height multipliers M(z,cat).

TABLE 3.2.5.1

TERRAIN AND STRUCTURE HEIGHT MULTIPLIERS FOR GUST WIND SPEEDS IN

FULLY DEVELOPED TERRAINS

ULTIMATE LIMIT STATE AND PERMISSIBLE STRESS DESIGN — REGIONS A AND B ONLY SERVICEABILITY LIMIT STATE DESIGN —

ALL REGIONS Height (z) m Multiplier (M(z, cat)) Terrain Category 1 Terrain Category 2 Terrain Category 3 Terrain Category 4 ≤3 5 10 15 20 30 40 50 75 100 150 200 250 300 400 500 0.99 1.05 1.12 1.16 1.19 1.22 1.24 1.25 1.27 1.29 1.31 1.32 1.34 1.35 1.37 1.38 0.85 0.91 1.00 1.05 1.08 1.12 1.16 1.18 1.22 1.24 1.27 1.29 1.31 1.32 1.35 1.37 0.75 0.75 0.83 0.89 0.94 1.00 1.04 1.07 1.12 1.16 1.21 1.24 1.27 1.29 1.32 1.35 0.75 0.75 0.75 0.75 0.75 0.80 0.85 0.90 0.98 1.03 1.11 1.16 1.20 1.23 1.28 1.31 TABLE 3.2.5.2

TERRAIN AND STRUCTURE HEIGHT MULTIPLIERS FOR GUST WIND SPEEDS IN

FULLY DEVELOPED TERRAINS

ULTIMATE LIMIT STATE AND PERMISSIBLE STRESS DESIGN — REGIONS C AND D ONLY

Height (z) Multiplier (M(z, cat))

m Terrain Categories 1 and 2 Terrain Categories 3 and 4 ≤3 5 10 15 20 30 40 50 75 ≥100 0.90 0.95 1.00 1.07 1.13 1.20 1.25 1.29 1.35 1.40 0.80 0.80 0.89 0.95 1.05 1.15 1.25 1.29 1.35 1.40

NOTE TO TABLES 3.2.5.1 AND 3.2.5.2: For intermediate values of height z and terrain category, interpolation is permitted.

3.2.6 Changes in terrain category. The wind speed at

a site shall be adjusted for changes in terrain roughness through a correction to the wind speed multiplier

(M(z,cat)). There is an upper limit to the developed height

of the inner layer (hi) which is a function of xi, and

which is independent of whether the flow is from the rougher terrain or to the rougher terrain. With reference to Figure 3.2.6 the corrected wind speed multiplier is given by Equation 3.2.6(3).

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xi = . . . (3.2.6(1))

Conversely:

hi = . . . (3.2.6(2))

For gust wind speeds:

Mx = Mo for x < xi Mx = . . . (3.2.6(3)) for 0 < (x - xi) < 2500 m Mx = M(z,cat) for (x - xi) > 2500 m where

xi = the distance downstream, in metres, from

the start of the new terrain to the developed height of the inner layer (hi),

given by Equation 3.2.6(1)

zo,r = the larger of the two roughness lengths, in

metres, given in Table 3.2.4, at the change in terrain

hi = the developed height of the inner layer, in

metres, which is equal to z for the c a l c u l a t i o n o f xi, g i v e n b y

Equation 3.2.6(2)

Mx = the wind speed multiplier at a distance x

from the start of new terrain category for height z

Mo = the upstream terrain category gust wind

speed multiplier at the beginning of each new terrain for height z

M(z,cat) = the downstream terrain category gust

wind speed multiplier for each new ter-rain for height z and (x - xi) > 2500 m,

given in Tables 3.2.5.1 and 3.2.5.2

x = the distance downwind, in metres, from a change in terrain category to the structure under consideration

Fully developed gust windspeed multipliers M(z,cat) only

apply at a structure site when the terrain category at the site is uniform upstream for a distance greater than (2500 + xi) metres.

When there is terrain of more than one roughness length upwind of the structure site, corrected wind speed mul tipl iers (Mx) sh all be co mput ed u sing

Equation 3.2.6(3).

The extent of upwind terrain to be considered need not exceed the larger of either 2500 m or 50 times the structure height (ht), provided that the terrain at that limit

is Terrain Category 3 or less rough, (assume the windspeed multiplier (Mo) to be the value for fully

developed terrain at that limit).

If the terrain at that point is rougher than Terrain Category 3, the upwind limit shall be extended until Terrain Category 3 or terrain of less roughness is encountered, or alternatively fully developed Terrain Category 3 may be arbitrarily assumed upwind of that point.

AS 1170.2 - 1989 - Wind Loads

Australian Standard



SAA Loading Code

Part 2: Wind loads

Australian Standard



Minimum design loads on

structures (known as the SAA

Loading Code)

Part 2: Wind loads

PREFACE

CONTENTS

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