AS 4055-2012 Wind loads for housing.pdf

AS 4055—2012

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AS 4055—2012

Australian Standard®

Wind loads for housing

This Australian Standard® was prepared by Committee BD-099, Wind Loads for Housing. It was approved on behalf of the Council of Standards Australia on 23 November 2012. This Standard was published on 24 December 2012.

The following are represented on Committee BD-099:

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• • • • • • • • • •

Australian Building Codes Board Australian Window Association Concrete Masonry Association of Australia Cyclone Testing Station Engineers Australia Forest and Wood Products Australia Housing Industry Association Master Builders Association Roofing Tile Association of Australia Think Brick Australia

This Standard was issued in draft form for comment as DR AS 4055. Standards Australia wishes to acknowledge the participation of the expert individuals that contributed to the development of this Standard through their representation on the Committee and through the public comment period.

Keeping Standards up-to-date Australian Standards® are living documents that reflect progress in science, technology and systems. To maintain their currency, all Standards are periodically reviewed, and new editions are published. Between editions, amendments may be issued. Standards may also be withdrawn. It is important that readers assure themselves they are using a current Standard, which should include any amendments that may have been published since the Standard was published. Detailed information about Australian Standards, drafts, amendments and new projects can be found by visiting www.standards.org.au Standards Australia welcomes suggestions for improvements, and encourages readers to notify us immediately of any apparent inaccuracies or ambiguities. Contact us via email at [email protected], or write to Standards Australia, GPO Box 476, Sydney, NSW 2001.

AS 4055—2012

Australian Standard®

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Wind loads for housing

Originated as AS 4055—1992. Previous edition 2006. Third edition 2012.

COPYRIGHT © Standards Australia Limited All rights are reserved. No part of this work may be reproduced or copied in any form or by any means, electronic or mechanical, including photocopying, without the written permission of the publisher, unless otherwise permitted under the Copyright Act 1968. Published by SAI Global Limited under licence from Standards Australia Limited, GPO Box 476, Sydney, NSW 2001, Australia ISBN 978 1 74342 323 3

AS 4055—2012

2

PREFACE This Standard was prepared by the Standards Australia Committee BD-099, Wind loads for housing to supersede AS 4055—2006. This Standard will be referenced in the National Construction Code (NCC) 2013 edition, thereby superseding in part the previous edition, AS 4055—2006, which will be withdrawn 12 months from the date of publication of this edition. The objective of this Standard is to provide designers, builders and manufacturers of building products that are affected by wind loading with a range of wind speed classes that can be used to design and specify such products for use in housing that are within the limitations in this Standard.

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This revision aims to improve modelling of topographic effects and also to harmonize with recent changes to AS/NZS 1170.2:2011, including Amendments No.1 and No.2. This edition differs from the previous edition as follows: (a)

The Scope of the Standard has been revised to include the limitation to Class 1 and Class 10 buildings as defined by the NCC. This has always been the intention of this Standard as reflected in the definition of ‘house’, but the limitation is more obvious when presented in the Scope.

(b)

Table 2.1 presenting wind speeds for each wind classification has been split into a Non-Cyclonic regions table and a Cyclonic regions table for clarification. The wind speeds for each wind classification remain unchanged.

(c)

Table 2.2 presenting the Wind Classification for sites has been changed to include a new Topographic Class T0 and to harmonize with changes adopted by AS/NZS 1170.2, terrain category multipliers.

(d)

Definitions for terrain categories have been revised to be compatible with those in AS/NZS 1170.2:2011 (as amended). The revised definitions are intended to clarify the differences between the categories. International research has shown that the wind speeds over water are appropriate for Terrain Category 1 multipliers, so AS/NZS 1170.2 has included water bodies in Terrain Category 1 for all wind regions. In the case of wind blowing over large bodies of water, including seas and oceans, shoaling waves can introduce a near-shore roughness that means this water can be considered as Terrain Category 1.5. This change has followed through to this Standard. Terrain Category 4 is not applicable to this Standard as in Terrain Category 4, a house is embedded within the Terrain Category 4 roughness and its wind force evaluation may require special techniques.

(e)

The calculation of topographic class had previously used the average of the maximum and minimum slope on a topographic feature to determine an average slope. While the average slope characterized a conical hill well, it significantly underestimated the slope of a ridge or escarpment. The maximum slope is now used to characterize the topographic feature. This will better represent the slope of a ridge or escarpment without significantly changing the characterization of a conical hill. This change was recommended as a result of observation of significantly higher levels of wind damage on ridges and escarpments in cyclonic and non-cyclonic wind storms.

(f)

In AS/NZS 1170.2:2011, the topographic multiplier for flat land applies to hill slopes of less than 1:20 and this revision introduces a new Topographic Class T0 to represent slopes from 0 to 1:20. This Class has a topographic multiplier of 1.0. The topographic multiplier for T1 has been changed to 1.1 and includes wind for slopes from 1:20 to 1:10. Hill slopes have also been expressed in degrees.

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AS 4055—2012

(g)

Shielding classifications have been harmonized with those in AS/NZS 1170.2 as appropriate for houses. For Regions A and B, large trees and heavily wooded areas can offer shielding and have been explicitly included, whereas in Regions C and D, the long duration of the wind event means that trees will be denuded before the arrival of the peak gust.

(h)

Shielding classifications are linked to the topographic classes. AS/NZS 1170.2:2011 also links shielding with topography by allowing shielding only on slopes of less than 1:5. This has also been incorporated into this Standard by allowing full shielding only for those topographic classes with slopes of less than 1:5. This change in both Standards are based on wind-field models of hills and damage surveys following cyclonic and non-cyclonic wind events.

(j)

Houses in the first row adjacent to wide, open areas are classed as ‘Not Shielded’, the second row from wide open areas is classed as ‘Partial Shielding’ and subsequent rows as ‘Full Shielding’ where there are sufficient houses.

(k)

Pressure zones on roofs and walls have been defined, named and illustrated on diagrams. Edge and corner zones are subject to higher pressures due to the local pressure factors defined in AS/NZS 1170.2. An additional zone on the windward corners of low slope roofs allows for the RC1 zone introduced to AS/NZS 1170.2:2011 based on recent international research.

(l)

The combination factor (Kc) from AS/NZS 1170.2:2011 has been applied to all pressures for walls and roofs. This has reduced some of the design pressures in the Standard.

(m)

A more detailed commentary has been added (Appendix A) to clarify the relationship of this Standard to AS/NZS 1170.2 and to give background to some of the clauses.

(n)

The example of topographic classes (Appendix B) has been changed to reflect the changes to definition of topographic classes.

(o)

The example of terrain categories and shielding (Appendix C) has been changed to reflect the changes to definition of terrain categories and Shielding.

(p)

In checking Tables 5.2 to 5.13, some minor discrepancies were found between values in the previous edition and those calculated from the formulae in Appendix A. These discrepancies were corrected and while there may be small differences in racking forces between this document and AS 1684, this will not affect the use of AS 1684.

(q)

References to the differential AS/NZS 1170.2 were included.

pressures

on

photovoltaic

solar

panels

in

The term ‘informative’ has been used in this Standard to define the application of the Appendix to which it applies. An ‘informative’ appendix is only for information and guidance. Notes to the text contain information and guidance. They are not an integral part of the Standard.

AS 4055—2012

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CONTENTS Page SECTION 1 SCOPE AND GENERAL 1.1 SCOPE ......................................................................................................................... 5 1.2 LIMITATIONS ............................................................................................................ 5 1.3 NORMATIVE REFERENCES .................................................................................... 5 1.4 DEFINITIONS............................................................................................................. 6 1.5 NOTATION ................................................................................................................. 7

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SECTION 2 WIND LOADS 2.1 CLASSIFICATION ..................................................................................................... 9 2.2 RELATIONSHIP TO WIND REGION AND SITE CONDITIONS ............................ 9 2.3 SELECTION OF TERRAIN CATEGORY ................................................................ 12 2.4 SELECTION OF TOPOGRAPHIC CLASS .............................................................. 13 2.5 SELECTION OF SHIELDING CLASS ..................................................................... 14 SECTION 3 CALCULATION OF PRESSURES AND FORCES 3.1 PRESSURE ZONES .................................................................................................. 16 3.2 PRESSURE COEFFICIENTS.................................................................................... 17 3.3 CALCULATION OF PRESSURES ........................................................................... 20 3.4 CALCULATION OF FORCES.................................................................................. 21 3.5 PRESSURES FOR TYPICAL APPLICATIONS ....................................................... 21 SECTION 4 UPLIFT FORCES .............................................................................................. 24 SECTION 5 RACKING FORCES 5.1 RACKING FORCES ................................................................................................. 25 5.2 AREA OF ELEVATION ........................................................................................... 25

APPENDICES A COMMENTARY ....................................................................................................... 42 B WORKED EXAMPLE FOR THE DETERMINATION OF TOPOGRAPHIC CLASS ....................................................................................................................... 53 C WORKED EXAMPLES FOR THE SELECTION OF TERRAIN CATEGORY AND SHIELDING CLASS........................................................................................ 57 D WORKED EXAMPLE FOR RACKING FORCES .................................................... 61

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AS 4055—2012

STANDARDS AUSTRALIA Australian Standard Wind loads for housing S E C T I O N

1

S C O P E

A N D

G E N E R A L

1.1 SCOPE This Standard specifies site wind speed classes for determining design wind speeds and wind loads for NCC buildings Classes 1 and 10 within the geometric limits given in Clause 1.2. The classes are for use in the design of housing and for design, manufacturing and specifying of building products and systems used for housing.

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Wind loads for houses not complying with the geometric limits given in Clause 1.2 are outside the scope of this Standard. NOTES: 1 Commentary on the clauses of this Standard is given in Appendix A. 2 A worked example for the determination of topography is given in Appendix B. 3 Worked examples for the determination of terrain category and shielding class are given in Appendix C. 4 A worked example for racking forces is given in Appendix D. 5 Where houses do not comply with the geometric and other limitations of this Standard, use AS/NZS 1170.2.

1.2 LIMITATIONS For the purpose of this Standard, the following conditions (geometric limits) shall apply (see Figure 1.1): (a)

The distance from ground level to the underside of eaves shall not exceed 6.0 m. The distance from ground level to the highest point of the roof, not including chimneys, shall not exceed 8.5 m.

(b)

The width (W) including roofed verandas, excluding eaves, shall not exceed 16.0 m, and the length (L) shall not exceed five times the width.

(c)

The roof pitch shall not exceed 35°.

The tables in Section 5 are based on floor to ceiling height of 2.4 m and a floor depth of 0.3 m (floor level down to ceiling below). If wind loads on houses are determined using this Standard, design parameters shall be derived from this Standard only. Where wind loads on buildings are determined using AS/NZS 1170.2, design parameters in that Standard only must be used. NOTE: Clause 3.2.3 in this Standard refers to pressures on solar panels given in AS/NZS 1170.2. These parameters are referenced in this Standard.

1.3 NORMATIVE REFERENCES The following are the normative documents referenced in this Standard: AS/NZS 1170 1170.2

Structural design actions Part 2: Wind actions

ABCB NCC

National Construction Code

www.standards.org.au

© Standards Australia

AS 4055—2012

6 E ave s 9 0 0 m m m a x.

R o of p i tc h 3 5° m a x.

R o of p i tc h 3 5° m a x. H e i g ht to to p of r o of, ridge or gable a n d 8. 5 m m a x.

H e i g ht f r o m g r o u n d l eve l to u n d e r s i d e of e ave s exc e pt fo r g a b l e e n d s 6.0 m m a x.

One or t wo s to r ey 16.0 m m a x.

16.0 m m a x.

H e i g ht to e ave s exc e pt g a b l e e n d s 6.0 m m a x.

H e i g ht a t a ny s e c ti o n th r o u g h th e h o u s e 8. 5 m m a x.

(a) S e c ti o n s L

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W 16.0 m m a x.

L

5W

W 16.0 m m a x. L Ed g e of e ave s L

E x te r n a l wa ll

L W 16.0 m m a x.

( b) Pl a n vi ew

FIGURE 1.1 GEOMETRY

1.4 DEFINITIONS For the purpose of this Standard, the definitions below apply. 1.4.1 Bottom of hill, ridge or escarpment Area at the base of the hill, ridge or escarpment, where the average slope is less than 1 in 20. 1.4.2 Height Distance from ground level to the underside of eaves or to the highest point of the roof neglecting chimneys; or the height of each storey at external walls (see Figure 1.1). 1.4.3 House Class 1 or 10 building as defined by the National Construction Code (NCC) with the geometric limitations specified in Clause 1.2.

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7

AS 4055—2012

1.4.4 Length Maximum overall distance between outside edges of the external walls of a house or shape (see Figure 1.1). 1.4.5 Obstruction Natural or man-made objects that generate turbulent wind flow, ranging from single trees to forests and from isolated small structures to closely spaced multi-storey buildings. 1.4.6 Plan Basic rectangular, square or L-shaped layout, or simple combinations of these (see Figure 1.1). 1.4.7 Racking forces Forces that occur in walls parallel to the wind direction. 1.4.8 Width Maximum distance from wall to wall in the direction perpendicular to the length, including roofed verandas but excluding eaves (see Figure 1.1).

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1.5 NOTATION Unless otherwise stated, the notation used in this Standard shall have the following meaning: C1 to C4

=

cyclonic wind classes

C1serv to C4serv

=

cyclonic wind classes for serviceability

Cp

=

pressure coefficient (external, internal or net, as appropriate)

Cp,e

=

external pressure coefficient

Cp,i

=

internal pressure coefficient

Cp,n

=

net pressure coefficient

d

=

average horizontal distance measured from the crest of the escarpment or hill to the near top-third zone, in metres

FS, PS, NS

=

shielding classes, full shielding, partial shielding and no shielding

G

=

dead load; or permanent action (self-weight), in kPa

G

=

wind pressure zone more than 1200 mm from edges of roofs or external corners of walls

H

=

height of a hill, ridge or escarpment, in metres

H0

=

maximum distance from the ground to the underside of the bearer in the lower floor, in metres

h

=

average roof height, in metres

h0

=

half the height of the wall (half of the floor to ceiling height) in metres

Kc

=

combination factor

Kl

=

local pressure factor

L, M, T, O

=

lower, middle and top third of hill, ridge or escarpment and over-top zone for escarpments

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AS 4055—2012

8

L

=

length of a house; or lower part of a hill, ridge or escarpment, in metres

Ms

=

shielding multiplier

Mt

=

topographic multiplier

M6.5,cat

=

terrain category multiplier at height (h)

N1 to N6

=

non-cyclonic wind classes

N1serv to N6serv

=

non-cyclonic wind classes for serviceability

p

=

design wind pressure acting normal to a surface, in kilopascals

qu

=

free stream dynamic gust pressure, in kilopascals

NA

=

Not applicable

RC

=

pressure zone on roofs within 1200 mm of external corners

RE

=

pressure zone on roofs within 1200 mm of roof panel edges

SC

=

pressure zone on walls within 1200 mm of external corners of the house

TC1 to TC3

=

terrain categories

T0 to T5

=

topographic classes

Vh

=

design gust wind speed at height (h), in metres per second

Vh,s

=

design gust wind speed at height (h) for serviceability limit state, in metres per second

Vh,u

=

design gust wind speed at height (h) for ultimate strength limit state, in metres per second

W

=

width of a house, in metres

Ws

=

serviceability wind action, in kPa

Wu

=

ultimate wind action in kPa

α

=

angle of roof pitch

φa

=

maximum slope through the top half of the hill, ridge or escarpment

γ

=

load factor

ρair

=

density of air, which shall be taken as 1.2 kg/m 3

© Standards Australia

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9

S E C T I O N

2

W I N D

AS 4055—2012

L O A D S

2.1 CLASSIFICATION The system of 10 classes is set out in Tables 2.1A and B together with the associated design gust wind speeds (Vh) for the serviceability and ultimate limit states. This incorporates both non-cyclonic (N) and cyclonic (C) winds. TABLE 2.1A DESIGN GUST WIND SPEED (Vh) FOR NON-CYCLONIC REGIONS A AND B Design gust wind speed (V h ) at height (h) m/s

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Wind class Serviceability limit state (V h,s )

Ultimate limit state (V h,u )

N1 N2 N3

26 26 32

34 40 50

N4 N5 N6

39 47 55

61 74 86

TABLE 2.1B DESIGN GUST WIND SPEED (Vh) FOR CYCLONIC REGIONS C AND D Design gust wind speed (V h ) at height (h) m/s Wind class

C1 C2 C3 C4

Serviceability limit state (V h,s )

Ultimate limit state (V h,u )

32 39 47 55

50 61 74 86

NOTE: Section 3 may present different pressures for the same wind speed depending on classification.

2.2 RELATIONSHIP TO WIND REGION AND SITE CONDITIONS The selection of wind speed class for a house depends on the conditions at the site of the house. The class shall be determined from Table 2.2 using the following site conditions determined as stated: (a)

Geographic wind speed region of the site as defined in Figure 2.1 (Region A, B, C or D, as given in AS/NZS 1170.2).

(b)

The terrain category that surrounds or is likely to surround the site within the next 5 years, as defined in Clause 2.3 (TC1, TC2, TC2.5 or TC3).

(c)

The topographic class of the site, as defined in Clause 2.4 (T0, T1, T2, T3, T4 or T5).

(d)

The shielding class of the house, as defined in Clause 2.5 (FS, PS or NS).

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© Standards Australia

AS 4055—2012

10

TABLE 2.2 WIND CLASSIFICATION FROM WIND REGION AND SITE CONDITIONS Wind region

TC T0

T2

T3

T4

T5

PS

NS

FS

PS

NS

FS

PS

NS

PS

NS

NS

NS

3

N1

N1

N1

N1

N2

N2

N2

N2

N2

N3

N3

N3

N4

2.5

N1

N1

N2

N1

N2

N2

N2

N3

N3

N3

N3

N4

N4

2

N1

N2

N2

N2

N2

N3

N2

N3

N3

N3

N3

N4

N4

1.5

N2

N2

N2

N2

N3

N3

N3

N3

N3

N3

N4

N4

N5

1

N2

N3

N3

N2

N3

N3

N3

N3

N4

N4

N4

N4

N5

3

N2

N2

N3

N2

N3

N3

N3

N3

N4

N4

N4

N4

N5

2.5

N2

N3

N3

N3

N3

N3

N3

N4

N4

N4

N4

N5

N5

2

N2

N3

N3

N3

N3

N4

N3

N4

N4

N4

N5

N5

N6

1.5

N3

N3

N4

N3

N4

N4

N4

N4

N4

N5

N5

N5

N6

1

N3

N4

N4

N4

N4

N4

N4

N5

N5

N5

N5

N6

N6

3

C1

C1

C2

C1

C2

C2

C2

C2

C3

C3

C3

C3

C4

2.5

C1

C2

C2

C2

C2

C2

C2

C3

C3

C3

C3

C4

NA

2

C1

C2

C2

C2

C2

C3

C2

C3

C3

C3

C4

C4

NA

1.5

C2

C2

C3

C2

C3

C3

C3

C3

C4

C4

C4

NA

NA

1

C2

C3

C3

C3

C3

C3

C3

C4

C4

C4

NA

NA

NA

3

C2

C3

C3

C2

C3

C3

C3

C4

C4

C4

C4

NA

NA

2.5

C2

C3

C3

C3

C3

C4

C3

C4

C4

C4

NA

NA

NA

2

C3

C3

C4

C3

C4

C4

C4

C4

NA

NA

NA

NA

NA

1.5

C3

C4

C4

C4

C4

NA

C4

NA

NA

NA

NA

NA

NA

1

C3

C4

C4

C4

NA

NA

NA

NA

NA

NA

NA

NA

NA

B

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T1

FS A

D

LEGEND: FS PS NS N C N/A TC

Topographic class

= = = = = = =

© Standards Australia

Full shielding Partial shielding No shielding Non-cyclonic Cyclonic Not applicable, that is, beyond the scope of this Standard (use AS/NZS 1170.2) Terrain category

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20° Marble Bar W i t te n o o m Millstream

Region C

Kalgoorlie

Esperance

A l b a ny

PER T H

M o r awa

M u l l ewa

Iva n h o e D e r by

Region A

Cairns

4 5°

Sale

Wa g g a Wa g g a

CA N B ER R A

B o u r ke

40°

© Standards Australia

FIGURE 2.1 BOUNDARIES OF REGIONS A, B, C AND D

N o r fo l k I s l a n d Region B 30° L o r d H owe I s l a n d Region A

Wo l l o n g o n g

3 5°

Corindi

N ewc a s t l e

HOBART

Region B BRISBANE

25 °

Bundaberg

SY D N E Y

L a u n c e s to n

Kyo g l e

To owo o m b a

10 0 k m

Biloela

R o c k h a m pto n

20°

Region C

M a c k ay

Mareeba

11° McDonnell Creek M o r e to n

C o l l i n sv i l l e

Tow n sv i l l e

At h e r to n

A D EL A I D E

Geelong

Mount Gambier

To r r e s S t r a i t I s l a n d s Region B

Region B

142 °

NOTE: This map is from AS/NZS 1170.2. The wind direction sub-regions of Region A have been removed for clarity.

3 5°

Ceduna

Wo o m e r a

N o r m a n to n

Alice Springs

Region A

Region C

Karumba

B u r ke tow n

Borroloola

Adelaide Rive r

Nhulunbuy

Ku n u n u r r a

W y n d h a m (C)

Broome

G a s c oy n e J u n c t i o n Cally tharra Springs

Green Head

k m 10 0

30°

C

Po r t H e d l a n d

B

D

Region C

DA RW I N

Region B

R e g i o n s a r e m a r ke d wi th th e l e t te r s A , B, C a n d D. C oa s t a l r e g i o n b o u n d a r i e s a r e s m o oth li n e s s e t i n f r o m a s m o oth e d c oa s tli n e by 50, 10 0, 150 a n d 20 0 k m. I s l a n d s wi thi n 50 k m of th e c oa s t a r e d e s i g n ate d i n th e s a m e r e g i o n a s th e a d j a c e nt c oa s t.

G e r a l d to n

Region B

27°

25°

C a r n a r vo n

Exmouth

Region D

O n s l ow

Karratha

S m o ot h e d coastline

50 km 10 0 k m 15 0 k m

Cocos Islands Region C

Christmas Island Region B

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11 AS 4055—2012

AS 4055—2012

12

2.3 SELECTION OF TERRAIN CATEGORY The terrain category for a housing site is a measure of the lowest effective surface roughness from any radial direction within a distance of 500 m of the proposed housing site. It shall be based on the likely terrain five years after design. Substantial wellestablished trees may be considered as obstructions for evaluation of terrain category in all wind regions.

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The terrain category for a housing site shall be identified by the notation TC1, TC1.5, TC2, TC2.5 or TC3 and shall be determined as follows: (a)

Terrain Category 1 (TC1) Very exposed open terrain with few or no obstructions and enclosed limited sized water surfaces at serviceability and ultimate wind speeds in all wind regions, e.g. flat, treeless, poorly grassed plains, or river, canals, lakes and enclosed bays, extending less than 10 km in the wind direction.

(b)

Terrain Category 1.5 (TC1.5) Open water surfaces subjected to shoaling waves at serviceability and ultimate wind speeds in all wind regions, e.g. near-shore water, large unenclosed bays on seas and oceans, lakes and enclosed bays extending greater than 10 km in the wind direction.

(c)

Terrain Category 2 (TC2) Open terrain including grassland with well-scattered obstructions having heights generally from 1.5 m to 5 m with no more than two obstructions per hectare, e.g. farmland and cleared subdivisions with isolated trees and uncut grass.

(d)

Terrain Category 2.5 (TC2.5) Terrain with a few trees or isolated obstructions. This category is intermediate between TC2 and TC3 and represents the terrain in developing outer urban areas with scattered houses, or large acreage developments with fewer than 10 buildings per hectare.

(e)

Terrain Category 3 (TC3) Terrain with numerous closely spaced obstructions having heights generally from 3 m to 10 m. The minimum density of obstructions shall be at least the equivalent of 10 house-size obstructions per hectare, e.g. suburban housing or light industrial estates.

In urban situations, roads, rivers, small lakes or canals less than 200 m wide shall be considered to form part of normal ‘Terrain Category 3’ terrain. Parks and other open spaces less than 250 000 m2 in area shall also be considered to form part of normal ‘Terrain Category 3’ terrain provided they are not within 500 m of each other, or not within 500 m of open country. Housing sites less than 200 m from the boundaries of open areas larger than 250 000 m2, e.g. golf courses, that are completely surrounded by urban terrain, shall be considered to have the terrain category applicable to the open area itself. Shielding provisions may still apply to these sites. Housing sites less than 500 m from the edge of a development shall be classified as the applicable terrain that adjoins the development, i.e. TC1, TC1.5, TC2, TC2.5 or TC3, as applicable. NOTES: 1 For commentary on terrain categories, see Appendix A. 2 For worked examples, see Appendix C. 3 Terrain Category 4, as defined in AS/NZS 1170.2, is not applicable to this Standard. 4 The terrain categories are the same as those in AS/NZS 1170.2. 5 Vegetation is considered differently for terrain categories in this Clause and for shielding classes in Clause 2.5.

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AS 4055—2012

2.4 SELECTION OF TOPOGRAPHIC CLASS The topographic class determines the effect of wind on a house because of its location on a hill, ridge or escarpment and the height and maximum slope of the hill, ridge or escarpment. The topographic class for a housing site shall be identified by the notation T0, T1, T2, T3, T4 or T5 and shall be determined from Table 2.3 and Figure 2.2 for all sites in the top two thirds of a ridge, hill or escarpment. NOTES: 1 The method defined in Table 2.3 and Figure 2.2 is suitable for the purpose of either mapping the wind classes of an area or assessing the wind class of an individual site. 2 For a worked example to determine topographic class, see Appendix B.

The bottom of a hill, ridge or escarpment shall be that area at the base of the hill, ridge or escarpment where the average slope is less than 1 in 20, e.g. creek, river valley or flat area. The maximum slope of a hill, ridge or escarpment ( φa) shall be the slope measured as the steepest slope through the top half of the hill, ridge or escarpment.

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NOTES: 1 Often the maximum slope will not occur at the actual proposed housing site and should be appraised by considering the adjacent topography 2 For an example of the classification of topography, see Appendix B.

The top-third zone (T) extends for an equal distance (d) either side of the crest of an escarpment as shown in Figure 2.2. The value of d is the average horizontal distance measured from the crest of the escarpment to the near top-third zone. A rise in terrain shall be considered an escarpment where the maximum slope on one side of the ridge is greater than 1 in 10 and on the other side is less than 1 in 20 (see Figure 2.2(b)). The over-top zone (O) of an escarpment shall be taken to extend to a distance of 4H past the crest of an escarpment. TABLE 2.3 TOPOGRAPHIC CLASSIFICATION FOR HILLS, RIDGES OR ESCARPMENTS Site location (see Figure 2.2)

Maximum slope

( φa )