Vipac Engineers & Scientists

Association of Civil Structural Engineers Victoria (ACSEV)

Technical Meeting

Dr. Seifu Bekele

Wind Engineering

Victorian Technology Centre, Melbourne

20th February 2013

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

Introduction

What Can We Do to Minimise Damage to Human Life and Properties?

The history of wind and its effect on mankind is as old as the history of mankind.

– ``

Wind Source of Energy

Wind Source of Damage

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

AS/ NZS 1170.2:2011

Structural design actions Part 2: Wind actions

Scope• Site wind speed, wind load

Limitation• 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).

AS/ NZS 1170.2:2011

2.2 SITE WIND SPEEDThe site wind speeds (Vsit,β) defined for the 8 cardinal directions (β) at the reference height (z) above ground (see Figure 2.1) shall be as follows:

Vsit,β = VR Md (Mz,cat Ms Mt) . . . 2.2

where

VR = regional gust wind speed, in metres per second, for annual probability of exceedance of 1/R, as given in Section 3

Md = wind directional multipliers for the 8 cardinal directions (β) as given in Section 3

Mz,cat = terrain/height multiplier, as given in Section 4

Ms = shielding multiplier, as given in Section 4

Mt = 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.

AS/ NZS 1170.2:2011

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 1170.0 for information on values of annual probability of exceedance appropriate for the design of structures

AS/ NZS 1170.2:2011

AS/ NZS 1170.2:2011

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

AS/ NZS 1170.2:2011

TABLE 4.1TERRAIN/HEIGHT MULTIPLIERS FOR GUST WIND SPEEDS

IN FULLY DEVELOPED TERRAINS—ALL REGIONS

NOTE: For intermediate values of height z and terrain category, use linear interpolation.

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

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

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

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

AS/ NZS 1170.2:2011

2.4.1 Design wind pressuresThe design wind pressures (p), in pascals, shall be determined for structures and partof structures as follows:

p = (0.5ρair) [Vdes,θ]2 Cfig Cdyn . . . 2.4(1)where

p = design wind pressure in pascals= pe, pi or pn where the sign is given by the Cp values used to

evaluate CfigNOTE: Pressures are taken as positive, indicating pressuresabove ambient and negative, indicating pressures below ambient.

ρair = density of air, which shall be taken as 1.2 kg/m3

Vdes,θ = building orthogonal design wind speeds (usually, θ = 0°, 90°, 180° and 270°), as given in Clause 2.3NOTE: For some applications, Vdes,θ may be a single value or may be expressed as a function of height (z), eg. windward walls of tall buildings (>25m).

Cfig = aerodynamic shape factor, as given in Section 5Cdyn = dynamic response factor, as given in Section 6 [the value is 1.0

except where the structure is dynamically wind sensitive (see Section 6)]

AS/ NZS 1170.2:2011

2.4.2 Design frictional drag force per unit areaThe design wind frictional drag force per unit area (f), in pascals, shall be taken for structures and parts of structures as follows:

f = (0.5ρair) [Vdes,θ]2 Cfig Cdyn . . . 2.4(2)

2.5.3.3 Forces derived from force coefficientsAppendices E and F cover structures for which shape factors are given in the form of force coefficients rather than pressure coefficients. In these cases, to determine wind actions, the forces (F) in newtons, shall be determined as follows:

F = (0.5 ρair) [Vdes,θ]2 Cfig Cdyn Az . . . 2.5(3)

where

Az = as defined in Paragraph E4, Appendix E, for lattice towers

= l × b for members and simple sections in Paragraph E3, Appendix E

= Aref as defined in Appendix F for flags and circular shapes

AS/ NZS 1170.2:2011

D2 FREESTANDING HOARDINGS AND WALLSD2.1 Aerodynamic shape factor for normal net pressure on freestanding hoardings and wallsThe aerodynamic shape factor (Cfig) for calculating net pressure across freestanding rectangular hoardings or walls (see Figure D1) shabe as follows:

Cfig = Cp,n Kp . . . D2where

Cp,n = net pressure coefficient acting normal to the surface, obtained from Table D2 using the dimensions defined inFigure D1

Kp = net porosity factor, as given in Paragraph D1.4NOTES: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,θ 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 eccentricity(e).

AS/ NZS 1170.2:2011

AS/ NZS 1170.2:2011

TABLE D2(D)NET PRESSURE COEFFICIENTS (Cp,n)—HOARDINGS AND FREESTANDING

WALLS—WIND PARALLEL TO HOARDING OR WALL, θ = 90°

D2.2 Aerodynamic shape factor for frictional dragThe 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.

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,β=VRMd (Mz,catMsMt)

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 ρair)[Vdes,θ]2 Cfig Cdyn

b = 20mc = 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

AS/ NZS 1170.2:2011

AS/ NZS 1170.2:2011

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

AS 4055-2012

Wind Region

Region A, B, C & D

AS 4055-2012 (Topographic Class)

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

AS 4055-2012 (Wind Classification)

AS 4055-2012 (Design Wind Speed)

AS 4055-2012 (Calculation of Pressures)

AS 4055-2012 (Pressure Coefficients)

AS 4055-2012 (Pressure Coefficients)

AS 4055-2012 (Pressure Coefficients)

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.

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

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 Turbulence

Intensity– Integral Length Scale

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

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

Vipac Assessment Tools

• Boundary Layer Wind Tunnel

• Automotive Wind Tunnel

• Air Distribution Lab

• Façade 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

Vipac Wind Tunnel Study

• Australian Wind Engineers Quality Assurances Manual

• ASCE Wind Tunnel Test Manual

• Comparison With Codes

• Experienced Wind Engineers

Quality Assurance

Vipac Wind Tunnel Study

• A Wind Tunnel Study is: • Reliable • Economical • Time Efficient (Few Weeks)

• Vipac has more than 35 years of Wind Tunnel Test Experience

• Strong Quality Assurance Program

• We welcome new challenges!

Conclusion

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