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John Holmes (JDH Consulting )

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 :.

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John Holmes (JDH Consulting )

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

  2. 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

  3. ABCB approval

  4. New Zealand • first use in 2005

  5. 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

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

  8. ISO 4354 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,catMs Mt ]2

  9. 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

  10. 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

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

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

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

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

  15. 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)

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

  17. 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

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

  19. 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

  20. areareduction Aerodynamic shape factor Cfig Cfig = Cp,i Kc Cfig = Cp,e Ka Kc KlKp

  21. action combination Aerodynamic shape factor Cfig Cfig = Cp,i Kc Cfig = Cp,e Ka Kc KlKp

  22. local pressure local pressure Aerodynamic shape factor Cfig Cfig = Cp,i Kc Cfig = Cp,e Ka Kc KlKp

  23. porosity Aerodynamic shape factor Cfig Cfig = Cp,i Kc Cfig = Cp,e Ka Kc KlKp

  24. 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

  25. 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

  26. 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

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

  28. 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

  29. 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

  30. 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

  31. 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)

  32. Appendix E • 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

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

  34. 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

  35. 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

  36. 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

  37. 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

  38. 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)

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

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

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