Dr Seifu Bekele ACSEVSeminar2013 Presentation

5/18/2018 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.

``

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


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 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 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

Dr Seifu Bekele ACSEVSeminar2013 Presentation

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