john holmes (jdh consulting) as/nzs1170.2 wind actions standard
TRANSCRIPT
John Holmes(JDH Consulting)
AS/NZS1170.2 Wind actions Standard
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)
• 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
ISO 4354 w = qref CexpCfigCdyn
AS/NZS1170.2-2002p = (0.5air)[Vdes,]2CfigCdyn
ISO 4354 w = qref CexpCfigCdyn
qref = reference dynamic pressure (non-directional)
Cexp = exposure factor
Cfig = shape factor
Cdyn = dynamic response factor
AS/NZS1170.2-2002p = (0.5air)[Vdes,]2CfigCdyn
Vdes, = design wind speed (directional)
- incorporates exposure effects
Cfig = shape factor
Cdyn = dynamic response factor
Cexp ~ [Mz,cat Ms Mt ]2
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
Sydney Area Extreme Wind speedsMascot 1969-92; Bankstown 1970-91; Richmond 1970-92
0102030405060
0 1 2 3 4Log10(return period)
Win
d s
pe
ed
(m
/s)
Synoptic
Downbursts
Combined
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, :
Vsit, = VR Md Mz,cat Ms Mt
wind direction
(Eq. 2.2)
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)
h
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 claddingN 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,e Ka Kc K Kp
area reduction
Cfig = Cp,i Kc
Cfig = Cp,e Ka Kc K Kp
Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
action combination
Cfig = Cp,e Ka Kc K Kp
local pressure
Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
local pressure
Cfig = Cp,e Ka Kc K Kp
Aerodynamic shape factor Cfig
Cfig = Cp,i Kc
porosity
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 buildingsFlat, gable and hipped roofs
• Appendix C - other enclosed buildingCurved 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
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)
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 :
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
Example : portal frame
Kc=0.8
Kc=0.8 Kc=0.8
Kc - 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:Kc = 0.95
Example : portal frame
Kc=0.8
Kc=0.8 Kc=0.8Kc=0.95
Kc=0.95Kc=1.0
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
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
