1. AS/NZS1170.2 Wind actions Standard
John Holmes
(JDH Consulting)

2. Main features of AS/NZS1170.2:2002
Changes from AS
Tutorial example 1 – low-rise industrial shed
Tutorial example 2 – 50m steel chimney

3. ABCB approval

4. New Zealand
first use in 2005

5. New Features : Format of ISO 4354
‘Simplified’ section in AS eliminated
Dynamic analysis replaced with ‘dynamic response factor’
Contains design wind speed data for both Australia and New Zealand
Re-analysis of wind speeds for Region A

6. New Features :
Return period determined elsewhere (BCA or AS/NZS1170.0)
Structural importance multiplier removed
Wind direction multipliers introduced for whole of Region A
New shape factors : high-pitch gable roofs, curved roofs, pitched-free roofs, hypar free roofs, tower ancillaries, flags
Cross-wind response of chimneys

7. ISO 4354 AS/NZS1170.2-2002 w = qref CexpCfigCdyn
p = (0.5air)[Vdes,]2CfigCdyn

8. ISO 4354 AS/NZS1170.2-2002 w = qref CexpCfigCdyn
qref = reference dynamic pressure (non-directional)
Cexp = exposure factor
Cfig = shape factor
Cdyn = dynamic response factor
AS/NZS
p = (0.5air)[Vdes,]2CfigCdyn
Vdes, = design wind speed (directional)
- incorporates exposure effects
Cfig = shape factor
Cdyn = dynamic response factor
Cexp ~ [Mz,cat Ms Mt ]2

9. Extreme value Type 3 (not Gumbel)
Regional Wind speed VR
3-second gust at 10 metres in open country
functions of return period given in Section 3.2
e.g. Region A (most of Australia, N.Z.):
VR= R-0.1
Extreme value Type 3 (not Gumbel)
Aust. J. Structural Engineering, I.E.Aust.
Vol. 4, pp29-40, 2002

11. Regions C, D, (B) Needs comprehensive re-analysis
Monte-Carlo analyses using historical cyclone tracks, probabilistic models of central pressure, radius to maximum winds etc..
U.S. relies on this method for hurricane regions of Gulf of Mexico, Atlantic coast in ASCE-7
Regional Factors : FC = 1.05, FD =1.10

12. Site wind speed Vsit, : (Eq. 2.2) Vsit, = VR Md Mz,cat Ms Mt
wind direction

13. Site wind speed Vsit, :
Vsit, = VR Md Mz,cat Ms Mt
terrain-height

14. Site wind speed Vsit, :
Vsit, = VR Md Mz,cat Ms Mt
shielding

15. Site wind speed Vsit, : Vsit, = VR Md Mz,cat Ms Mt topography
Importance Multiplier Mi in AS
replaced by user-selected ‘design event for safety’
(BCA or AS/NZS1170.0)

16. Site wind speed Vsit, : Vsit, = VR Md Mz,cat Ms Mt
Md is in Section 3
Mz,cat Ms and Mt in Section 4 (Site Exposure Multipliers)
Design wind speed Vdes, :
Maximum Vsit, within 45o of the normal to building wall
(Figure 2.3)

17. Average roof height is used to calculate the wind speed Vdes, and hence p (for all wind directions)

18. Wind direction Multiplier Md (Table 3.2)
seven sub-Regions 5 Australia, 2 New Zealand
Region A4 (north of 30th parallel) :
N NE E SE S SW W NW
Regions B, C and D ‘ … major structural elements …’ N NE E SE S SW W NW
Regions B, C and D for cladding
N NE E SE S SW W NW

19. Terrain - height multipliers Mz,cat
Unchanged from AS
Changes in terrain category - calculation description made simpler (averaging distance based on structure height)

20. Topographic multiplier Mt :
Mt = MhMlee( E)
Elevation and mountain lee effects are included
(mainly NZ)
Hill-shape multiplier Mh
Non linear variation with height, z, - falls off more rapidly near the ground
Simple formula given - easier for spreadsheets or computer programs

21. Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
area reduction
Cfig = Cp,e Ka Kc Kl Kp

22. Aerodynamic shape factor Cfig
action combination
Cfig = Cp,i Kc
Cfig = Cp,e Ka Kc Kl Kp

23. Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
local pressure
local pressure
Cfig = Cp,e Ka Kc Kl Kp

24. Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
porosity
Cfig = Cp,e Ka Kc Kl Kp

25. Internal pressure coefficient Cp.i
Section 5.3, Tables 5.1(A) and 5.1(B)
Diagrams showing wind direction in relation to permeability and openings
Some values changed for dominant openings cases

26. External pressure coefficient Cp.e
Section 5.4 and Appendices C to F
Section rectangular enclosed buildings
Flat, gable and hipped roofs
Appendix C - other enclosed building
Curved roofs, multi-span , bins, silos and tanks
Appendix D – walls, hoardings and canopies
Appendix E – exposed structural members, frames, lattice towers,
Appendix F – flags and circular shapes

27. Rectangular enclosed buildings
Section 5.4
Walls : Tables 5.2(A), 5.2(B) and 5.2(C)
Roofs : Tables 5.3(A), 5.3(B) and 5.3(C)
Table 5.3(A) flat roofs:
Positive pressures on downwind roofs reduced

28. Rectangular enclosed buildings
Significant changes to Table 5.3(C) for downwind roof slope for  > 25o (depends on b/d ratio)

29. Kc - combination factor
Allows for reduction in peak load when one or more building surfaces contributes to peak load effect
4 cases : Kc = 0.8 to 1.0
note that Kc.Ka 0.8
when more than one case applies – use lowest value of Kc

30. Kc - combination factor :
Example : portal frame
Kc=0.8
Kc=0.8
Kc=0.8
Because of portal frame action, roof and wall pressures act in combination.
Case (b) in Table 5.5 applies.
Kc = 0.8 for external wall and roof pressures

31. Kc - combination factor :
Example : portal frame
Kc=0.8
Kc=0.95
Kc=0.8
Kc=0.8
Kc=1.0
Kc=0.95
With dominant opening, internal pressure can contribute > 25% of net load across surface.
Case (d) in Table 5.5 applies for positive internal in combination with negative external pressures: Kc = 0.95

32. Appendix C
Curved roofs (Table C3) – revised extensively from AS
Appendix D
- some changes for hoardings and walls ( = 0, 45o)
- adjustments to monoslope and pitched free roofs
- hypar free roofs added (Table D7)

33. Appendix E Appendix F - flags from Eurocode prEN-1991-1-4.6
Cd for rough circular cylinders at high Re revised
- many ‘rounded’ shapes removed (unreliable)
lattice tower data (including antennas) from AS3995
interference effects of ancillaries
Appendix F
- flags from Eurocode prEN
- circular discs, hemispheres, spheres from pre AS1170.2

34. Dynamic response factor CdynAS
Section pages
AS/NZS
Section pages

35. Dynamic response factor CdynAS
Section pages
Based on mean wind speed
AS/NZS
Section pages
Based on gust wind speed

36. Dynamic response factor CdynAS
Section pages
Based on mean wind speed
Along-wind Gust factor, G - around 2
AS/NZS
Section pages
Based on gust wind speed
Dynamic response factor, Cdyn - around 1

37. Dynamic response factor CdynAS
Section pages
Based on mean wind speed
Along-wind Gust factor, G - around 2
Resonant component not transparent
AS/NZS
Section pages
Based on gust wind speed
Dynamic response factor, Cdyn - around 1
Significant resonant component gives Cdyn >1

38. Dynamic response factor CdynAS
Section pages
Based on mean wind speed
Along-wind Gust factor, G - around 2
Resonant component not transparent
E factor – Harris form
AS/NZS
Section pages
Based on gust wind speed
Dynamic response factor, Cdyn - around 1
Significant resonant component gives Cdyn >1
Et factor - von Karman

39. Cross-wind Dynamic Response
Section rectangular cross sections
Equations fitted to Cfs (Section )
Section for circular cross-sections (new)
Very approximate - if cross-wind response dominates should either:
i) design out (e.g. add mass or damping)
ii) seek expert advice
iii) commission wind-tunnel tests
iv) use specialist code (CICIND or EN)

40. Design Guide published in 2005
9 example calculations
(low-rise, high-rise, chimney, free-roof etc…)
Frequently-asked questions

41.
Ph./FAX