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

Main features of AS/NZS1170.2:2002 Changes from AS1170.2-1989 Tutorial example 1 – low-rise industrial shed Tutorial example 2 – 50m steel chimney

ABCB approval

New Zealand first use in 2005

New Features : Format of ISO 4354 ‘Simplified’ section in AS1170.2-1989 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

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

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

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/NZS1170.2-2002 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

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= 67- 41 R-0.1 Extreme value Type 3 (not Gumbel) Aust. J. Structural Engineering, I.E.Aust. Vol. 4, pp29-40, 2002

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

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

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

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

Site wind speed Vsit, : Vsit, = VR Md Mz,cat Ms Mt topography Importance Multiplier Mi in AS1170.2-1989- replaced by user-selected ‘design event for safety’ (BCA or AS/NZS1170.0)

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)

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

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 0.90 0.85 0.90 0.90 0.95 0.95 0.95 0.90 Regions B, C and D ‘ … major structural elements …’ N NE E SE S SW W NW 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 Regions B, C and D for cladding N NE E SE S SW W NW 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Terrain - height multipliers Mz,cat Unchanged from AS1170.2-1989 Changes in terrain category - calculation description made simpler (averaging distance based on structure height)

Topographic multiplier Mt : Mt = MhMlee(1 + 0.00015E) 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

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

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

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

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

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

External pressure coefficient Cp.e Section 5.4 and Appendices C to F Section 5.4 - 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

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

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

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

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

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

Appendix C Curved roofs (Table C3) – revised extensively from AS1170.2-1989 Appendix D - some changes for hoardings and walls ( = 0, 45o) - adjustments to monoslope and pitched free roofs - hypar free roofs added (Table D7)

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-1991-1-4.6 - circular discs, hemispheres, spheres from pre-1989 AS1170.2

Dynamic response factor Cdyn AS1170.2-1989 Section 4 - 11 pages AS/NZS1170.2-2002 Section 6 - 8 pages

Dynamic response factor Cdyn AS1170.2-1989 Section 4 - 11 pages Based on mean wind speed AS/NZS1170.2-2002 Section 6 - 7 pages Based on gust wind speed

Dynamic response factor Cdyn AS1170.2-1989 Section 4 - 11 pages Based on mean wind speed Along-wind Gust factor, G - around 2 AS/NZS1170.2-2002 Section 6 - 7 pages Based on gust wind speed Dynamic response factor, Cdyn - around 1

Dynamic response factor Cdyn AS1170.2-1989 Section 4 - 11 pages Based on mean wind speed Along-wind Gust factor, G - around 2 Resonant component not transparent AS/NZS1170.2-2002 Section 6 - 7 pages Based on gust wind speed Dynamic response factor, Cdyn - around 1 Significant resonant component gives Cdyn >1

Dynamic response factor Cdyn AS1170.2-1989 Section 4 - 11 pages Based on mean wind speed Along-wind Gust factor, G - around 2 Resonant component not transparent E factor – Harris form AS/NZS1170.2-2002 Section 6 - 7 pages Based on gust wind speed Dynamic response factor, Cdyn - around 1 Significant resonant component gives Cdyn >1 Et factor - von Karman

Cross-wind Dynamic Response Section 6.3.2 - rectangular cross sections Equations fitted to Cfs (Section 6.3.2.3) Section 6.3.3 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)

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

jdholmes@bigpond.net.au 03-9585-3815 Ph./FAX