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

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Presentation on theme: "John Holmes (JDH Consulting) AS/NZS1170.2 Wind actions Standard."— Presentation transcript:

1 John Holmes (JDH Consulting) AS/NZS Wind actions Standard

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) Wind direction multipliers introduced for whole of Region A Structural importance multiplier removed 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 = q ref C exp C fig C dyn AS/NZS p = (0.5  air) [V des,  ] 2 C fig C dyn

8 ISO 4354 w = q ref C exp C fig C dyn q ref = reference dynamic pressure (non- directional) C exp = exposure factor C fig = shape factor C dyn = dynamic response factor AS/NZS p = (0.5  air) [V des,  ] 2 C fig C dyn V des,  = design wind speed (directional) - incorporates exposure effects C fig = shape factor C dyn = dynamic response factor C exp ~ [M z,cat M s M t ] 2

9 Regional Wind speed V R 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.): V R = R -0.1 Extreme value Type 3 (not Gumbel) Aust. J. Structural Engineering, I.E.Aust. Vol. 4, pp29-40, 2002

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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 : F C = 1.05, F D =1.10

12 Site wind speed V sit,  : V sit,  = V R M d M z,cat M s M t wind direction (Eq. 2.2)

13 Site wind speed V sit,  : V sit,  = V R M d M z,cat M s M t terrain-height

14 Site wind speed V sit,  : V sit,  = V R M d M z,cat M s M t shielding

15 Site wind speed V sit,  : V sit,  = V R M d M z,cat M s M t topography Importance Multiplier M i in AS replaced by user-selected ‘design event for safety’ (BCA or AS/NZS1170.0)

16 Site wind speed V sit,  : V sit,  = V R M d M z,cat M s M t M d is in Section 3 M z,cat M s and M t in Section 4 (Site Exposure Multipliers) Design wind speed V des,  : Maximum V sit,  within  45 o of the normal to building wall (Figure 2.3)

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

18 Wind direction Multiplier M d (Table 3.2) seven sub-Regions 5 Australia, 2 New Zealand Region A4 (north of 30 th 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 M z,cat Unchanged from AS Changes in terrain category - calculation description made simpler (averaging distance based on structure height)

20 Topographic multiplier M t : M t = M h M lee ( E) Elevation and mountain lee effects are included (mainly NZ) Hill-shape multiplier M h 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 C fig C fig = C p,e K a K c K K p area reduction C fig = C p,i K c

22 C fig = C p,e K a K c K K p Aerodynamic shape factor C fig C fig = C p,i K c action combination

23 C fig = C p,e K a K c K K p local pressure Aerodynamic shape factor C fig C fig = C p,i K c local pressure

24 C fig = C p,e K a K c K K p Aerodynamic shape factor C fig C fig = C p,i K c porosity

25 Internal pressure coefficient C p.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 C p.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 Table 5.3(A) flat roofs: Positive pressures on downwind roofs reduced Section 5.4 Roofs : Tables 5.3(A), 5.3(B) and 5.3(C) Walls : Tables 5.2(A), 5.2(B) and 5.2(C)

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

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

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

31 K c - combination factor : 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:K c = 0.95 Example : portal frame K c =0.8 K c =0.95 K c =1.0

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

33 Appendix E C d 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-1989 AS1170.2

34 Dynamic response factor C dyn AS Section pages AS/NZS Section pages

35 Dynamic response factor C dyn AS Section pages Based on mean wind speed AS/NZS Section pages Based on gust wind speed

36 Dynamic response factor C dyn AS 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, C dyn - around 1

37 Dynamic response factor C dyn AS 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, C dyn - around 1 Significant resonant component gives C dyn >1

38 Dynamic response factor C dyn AS 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, C dyn - around 1 Significant resonant component gives C dyn >1 E t factor - von Karman

39 Cross-wind Dynamic Response Section rectangular cross sections Equations fitted to C fs (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 example calculations (low-rise, high-rise, chimney, free-roof etc…) Frequently-asked questions

41 Ph./FAX


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