Dr Seifu Bekele ACSEVSeminar2013 Presentation
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Transcript of Dr Seifu Bekele ACSEVSeminar2013 Presentation
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Vipac Engineers & Scientists
Association of Civil Structural Engineers Victoria (ACSEV)
Technical Meeting
Dr. Seifu Bekele
Wind Engineering
Victorian Technology Centre, Melbourne
20th
February 2013
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Overview
Introduction
AS/NZS 1170.2:2011
Example on the use of AS/NZS 1170
AS 4055 - 2012
Example on the use of AS 4055
Wind Tunnel for Structural Study
Vipac Wind Engineering Services
Conclusion
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Introduction
What Can We Do to Minimise Damage to Human Lifeand Properties?
The history of wind and its effect on mankind is as old as the history of mankind.
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Wind Source of Energy
Wind Source of Damage
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Wind Engineering
Study of Wind & Wind Structure Interaction
Full Scale Study
Empirical Formulas
Database and Neural Networks
CWE (Computational Wind Engineering)
Wind Tunnel Testing
Building Codes and Standards Wind loads for housing AS 4055 -2012
Structural design action AS/NZ 1170: 2011
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AS/ NZS 1170.2:2011
Structural design actions
Part 2: Wind actions
Scope
Site wind speed, wind loadLimitation
Not to buildings subjected to wind action of tornadoes
Less than 200m high
Structures other than offshore structures, bridges and transmission
towers
2.1 GENERALThe procedure for determining wind actions (W) on structures and elements of
structures or buildings shall be as follows:
(a) Determine site wind speeds (see Clause 2.2).
(b) Determine design wind speed from the site wind speeds (see Clause 2.3).
(c) Determine design wind pressures and distributed forces (see Clause 2.4).
(d) Calculate wind actions (see Clause 2.5).
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AS/ NZS 1170.2:2011
2.2 SITE WIND SPEED
The site wind speeds (Vsit,) defined for the 8 cardinal directions () at the
reference height (z) above ground (see Figure 2.1) shall be as follows:Vsit, = VRMd (Mz,catMsMt) . . . 2.2
where
VR = regional gust wind speed, in metres per second, for annual
probability of exceedance of 1/R, as given in Section 3Md = wind directional multipliers for the 8 cardinal directions () as
given in Section 3
Mz,cat = terrain/height multiplier, as given in Section 4
Ms = shielding multiplier, as given in Section 4Mt = topographic multiplier, as given in Section 4
Generally, the wind speed is determined at the average roof height (h). In
some cases this varies, as given in the appropriate sections, according to
the structure.
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AS/ NZS 1170.2:2011
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AS/ NZS 1170.2:2011
NOTES:
1) The peak gust has an equivalent moving average time of approximately 0.2 seconds
2) Values for V1 have not been calculated by the formula for VR
3) For ultimate or serviceability limit states, refer to the Building Code of Australia or AS/NZS 1170.0for information on values of annual probability of exceedance appropriate for the design of structures
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AS/ NZS 1170.2:2011
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AS/ NZS 1170.2:2011
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AS/ NZS 1170.2:2011
Terrain Category 2.5 (TC2.5)
Terrain with few trees or isolated obstructions
Large acreage developments with fewer than 10 buildings
per hectare
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AS/ NZS 1170.2:2011
TABLE 4.1
TERRAIN/HEIGHT MULTIPLIERS FOR GUST WIND SPEEDS
IN FULLY DEVELOPED TERRAINSALL REGIONS
NOTE: For intermediate values of height z and terrain category, use linearinterpolation.
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AS/ NZS 1170.2:2011
Terrain Category 1 (TC1)
Very exposed open terrain. Flat, treeless, poorly grassed plains, or rive
canals, lakes enclosed bays less than 10km
Terrain Category 1.5 (TC1.5)
Open water surface extending greater than 10km
Near shore water, sea, leaks and enclosed bay
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AS/ NZS 1170.2:2011
Terrain Category 2 (TC2)
Open terrain, well-scattered obstruction having a height (1.5m to 5m)
Farmland, cleared subdivisions with isolated trees and uncut grass
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AS/ NZS 1170.2:2011
Terrain Category 3 (TC3)
Terrain with numerous closely spaced obstructions having
a height of 3m to 10m
Suburban housing or light industrial area
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AS/ NZS 1170.2:2011 -2012
Effect of Terrain Category
Let the building be 5m height
Assume the building is located in terrain Category 3 Terrain
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AS/ NZS 1170.2:2011
2.4.1 Design wind pressures
The design wind pressures (p), in pascals, shall be determined for structures and parof structures as follows:
p = (0.5air) [Vdes,]2 CfigCdyn . . . 2.4(1)where
p = design wind pressure in pascals
= pe, pi or pn where the sign is given by the Cp values used toevaluate CfigNOTE: Pressures are taken as positive, indicating pressuresabove ambient and negative, indicating pressures below ambient.
air = density of air, which shall be taken as 1.2 kg/m3
Vdes, = building orthogonal design wind speeds (usually, = 0, 90,180 and 270), as given in Clause 2.3
NOTE: For some applications, Vdes, may be a single value or maybe expressed as a function of height (z), eg. windward walls of tallbuildings (>25m).
Cfig = aerodynamic shape factor, as given in Section 5
Cdyn = dynamic response factor, as given in Section 6 [the value is 1.0
except where the structure is dynamically wind sensitive (seeSection 6)]
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AS/ NZS 1170.2:2011
2.4.2 Design frictional drag force per unit area
The design wind frictional drag force per unit area (f), in pascals, shall be taken
for structures and parts of structures as follows:
f= (0.5air) [Vdes,]2 CfigCdyn . . . 2.4(2)
2.5.3.3 Forces derived from force coefficients
Appendices E and F cover structures for which shape factors are given in the
form of force coefficients rather than pressure coefficients. In these cases, todetermine wind actions, the forces (F) in newtons, shall be determined as
follows:
F= (0.5 air) [Vdes,]2 CfigCdynAz . . . 2.5(3)
where
Az= as defined in Paragraph E4, Appendix E, for lattice towers
= lb for members and simple sections in Paragraph E3, Appendix E
= Arefas defined in Appendix F for flags and circular shapes
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AS/ NZS 1170.2:2011
D2 FREESTANDING HOARDINGS AND WALLS
D2.1 Aerodynamic shape factor for normal net pressure on freestanding hoardings and walls
The aerodynamic shape factor (Cfig) for calculating net pressure across freestanding rectangular hoardings or walls (see Figure D1) shabe as follows:
Cfig = Cp,n Kp . . . D2
where
Cp,n = net pressure coefficient acting normal to the surface, obtained from Table D2 using the dimensions defined iFigure D1
Kp = net porosity factor, as given in Paragraph D1.4
NOTES:
1 The factors Ka and Kl do not appear in this equation as they are taken as 1.0.
2 Height for calculation of Vdes, is the top of the hoarding or wall,i.e. height (h) (see Figure D1).
Pressures derived from Equation D2 shall be applied to the total area (gross) of the hoarding or wall (for example, b c).
The resultant of the pressure shall be taken to act at half the height of the hoarding, (h c/2), or wall, (c/2), with a horizontal eccentricit(e).
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AS/ NZS 1170.2:2011
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AS/ NZS 1170.2:2011
TABLE D2(D)
NET PRESSURE COEFFICIENTS (Cp,n)HOARDINGS AND FREESTANDING
WALLSWIND PARALLEL TO HOARDING OR WALL, = 90
D2.2 Aerodynamic shape factor for frictional drag
The aerodynamic shape factor (Cfig) for calculating frictional drag effects on freestanding
hoardings and walls, where the wind is parallel to the hoarding or wall, shall be equal to Cf,
which shall be determined as given in Table D3. The frictional drag on both surfaces shall
be calculated and summed and added to the force on any exposed members calculated in
accordance with Appendix E.
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AS/ NZS 1170.2:2011
Example Free Standing Wall
Let the wall be in Melbourne with the orientation as shown
Height 2m
NE
SWNW
VR = 39 m/s for 50 year wind, imp. Level 1 (25 year life time)
Md = 1.0 (N (1.0), NW (0.95), W (1.0)) Mz,cat = 0.83 (Cat 3, < 3m)
Ms = 1.0 (No shielding)
Mt = 1.0 (No topographic effect, flat land)
Vsit,Nw = 39 x 1.0 x 0.83 x 1.0 x 1.0 = 32.4 m/s
Vsit,=VRMd (Mz,catMsMt)
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AS/ NZS 1170.2:2011
Example Free Standing Wall- Let the wall be in Melbourne with the orientation as shown
- Height 2m
c/h = 1b/c= 10
Cp.n = 1.7 0.5(c/h) = 1.7 0.5 x 1.0 = 1.2 (wind normal)Kp = 1.0 (Solid wall, no porosity)
Cfig = 1.2 x 1.0 = 1.2
p = (0.5 air)[Vdes,]2 CfigCdyn
b = 20m
c= h = 2m
Cp.n = 1.7 0.5(c/h) = 1.7 0.5 x 1.0 = 1.2 (wind normal)
P= 0.5 x 1.2 x 32.42 x 1.2 x 1.0 = 755.83 Pa = 0.8 kPa
Cfig = Cp,nKp . . . D2
where
Cp,n = net pressure coefficient acting normal to the surface, obtained from
Table D2 using the dimensions defined in Figure D1
Kp = net porosity factor, as given in Paragraph D1.4
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AS/ NZS 1170.2:2011
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AS/ NZS 1170.2:2011
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AS 4055-2012
Scope
Site wind speed, wind load
Limitation Total height (ground to roof top) less than 8.5m
Width including verandas less than 16.0 m
Length shall not exceed five time the width (80.0 m)
Roof pitch not exceeding 35o
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AS 4055-2012
Wind Region
Region A, B, C & D
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AS 4055-2012 (Topographic Class)
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AS 4055-2012 (Shielding Class)
In Region A & B, trees and group of trees similar area to the house may be
considered as shielding element
In Region C & D, trees and vegetation shall not be considered as shielding
Three type of shielding:
1) Full shielding (FS)
At least two rows of houses or similar size permanent obstructions
In region A & B permanent heavily wooded areas within 100m of site
FS only for Topographic class T0, T1, and T2
2) Partial shielding (PS)
At least 2.5 houses or sheds per hectare
Wooded parkland and acreage type suburban
PS only for Topographic class T0, T1, T2 and T3
3) No shielding (NS)
No permanent obstructions
Less than 2.5 houses per hectare, row of houses or single houses
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AS 4055-2012 (Wind Classification)
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AS 4055-2012 (Design Wind Speed)
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AS 4055-2012 (Calculation of Pressures)
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AS 4055-2012 (Pressure Coefficients)
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AS 4055-2012 (Pressure Coefficients)
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AS 4055-2012 (Pressure Coefficients)
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AS 4055-2012 Pressures & Forces
Calculation of Pressures
Calculation of Forces
Force = pressure x Area
Uplift force = uplift pressure x Area of the roof
Racking force = area of elevation x Lateral wind pressure
Racking forces are lateral forces transfers to the foundations through
bracing.
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AS 4055-2012 (Ultimate Strength Pressure)
Pressure at roof corner
P = 0.5 x density x Cp x V2/1000
V = 34 m/s for N1 (Table 2.1, page 9)
Cp = -2.61 (Table 3.1, page18)
density = 1.2 kg/m3
P = 0.5 x 1.2* (-2.16)*342 /1000 = 1.81 kPa
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Wind Tunnel Study
Boundary Layer Wind Tunnel
Australian Wind Engineering Society Recommendation
Deaves and Harris (1978)
ESDU (1985 and 1986)
Main Characteristics of
Wind Mean Velocity Profile
Longitudinal TurbulenceIntensity
Integral Length Scale
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Structural Loads
Why we need Wind Tunnel Studies?
Code based estimate
AS/NZ 1170: 2011 (Australia)
Various methods of structural load studies
High Frequency Base Balance
Aeroelastic
Simultaneous Pressure Measurement
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VIPAC Wind Engineering Capability
Pedestrian Level Wind
Cladding Pressures
Structural Loads (Force Balance, Aeroelastic)
Environmental (Dispersion Study)
Wind Noise, Wind Driven Rain
Full Scale Building Components
Topographic Studies
Full Scale Test
Computational Wind Engineering
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Vipac Assessment Tools
Boundary Layer Wind Tunnel
Automotive Wind Tunnel
Air Distribution Lab
Faade Rig
Pressure Rig for Roof Test
Measurement Tools
Flow Measurement (Pitot tube & Hotwire, Cobra probe)
Pressure Measurements (128 simultaneous Pressure transducers)
Force (High Frequency Force Balance, JR3)
Computer Modelling
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Vipac Wind Tunnel Study
Australian Wind EngineersQuality Assurances Manual
ASCE Wind Tunnel Test
Manual
Comparison With Codes
Experienced Wind Engineers
Quality Assurance
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Vipac Wind Tunnel Study
A Wind Tunnel Study is: Reliable
Economical
Time Efficient (Few Weeks)
Vipac has more than 35 years of Wind Tunnel TestExperience
Strong Quality Assurance Program
We welcome new challenges!
Conclusion
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Vipac Engineers & Scientists
Association of Civil Structural Engineers Victoria (ACSEV)
Technical Meeting
Dr. Seifu Bekele
Wind Engineering
Victorian Technology Centre, Melbourne
20th February 2013
Thank You