environmental wind conditions and criteria - statement of ......over. generally, the highest wind...

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PLANNING PANELS VICTORIA

MELBOURNE PLANNING SCHEME

AMENDMENT C245

ENVIRONMENTAL WIND CONDITIONS AND CRITERIA

STATEMENT OF EVIDENCE

by

M. Eaddy

and

W. H. Melbourne

Consultants Pty Ltd

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CONTENTS

1. Introduction ........................................................................................................... 3

1.1. Expert Witnesses .................................................................................................. 3

1.2. Scope of Evidence ................................................................................................ 4

2. Proposed Melbourne Planning Scheme Amendment C245 .............................. 5

3. Wind flow Around Buildings and Effects on Public Realm ............................... 7

4. Environmental Wind Criteria ................................................................................ 9

5. Queen Victoria Market Wind Environment ....................................................... 12

6. Environmental Wind Criteria for Queen Victoria Market ................................. 14

7. Summary ............................................................................................................. 18

8. Declaration .......................................................................................................... 19

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1. INTRODUCTION

1.1. Expert Witnesses

The names, addresses, and qualifications of the expert witnesses are as follows:

Dr Michael J. Eaddy B.E.(Hons), M.E.(Dist), PhD

Prof William H Melbourne BE, DIC, PhD, FIEAust, FTSE

We are both directors of the Wind Engineering Consultancy firm MEL Consultants Pty

Ltd that operates a wind tunnel testing facility at 34 Cleeland Road, Oakleigh South.

Michael Eaddy joined MEL Consultants in 2002 as an engineer and become a director of

the company in 2006. He is a member of the Australasian Wind Engineering Society and

Engineers Australia. He has completed numerous wind tunnel and full scale

investigations of environmental wind conditions around buildings and structures within

Australia and overseas. Details of Michael Eaddy’s experience are given in Appendix A.

William Melbourne was the founder of MEL Consultants in the early 1980’s and operated

the company whilst he was a Professor of Fluid Mechanics at Monash University. He has

undertaken and published research in the area of wind engineering and been a member

of national and international wind engineering committees that develop wind engineering

standards and guidelines. He is a Fellow of Engineers Australia and a life member of the

Australasian Wind Engineering Society. Details of William Melbourne’s experience are

given in Appendix A.

MEL Consultants undertakes desktop analyses/assessments and wind tunnel testing of

buildings and structures for wind engineering areas such as, environmental wind

conditions, structural wind loads, and pollutant dispersion. We are consulted by state and

local government, such as the City of Melbourne and the Victorian Department of

Planning to provide expert wind engineering advice.

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1.2. Scope of Evidence

The scope of our statement of evidence is on the requirements of the proposed Planning

Scheme Amendment C245 for the environmental wind criteria. We have been asked to

form a view of the proposed environmental wind criteria in the proposed amendment and

provide an expert witness report on this aspect of the Amendment. We will discuss in this

Statement of Evidence the development of the environmental wind criteria, wind flow

around buildings and the impact on the public realm, and our rationale behind our view

on some applicable environmental wind criteria for the public realm surrounding and

within the Queen Victoria Market as defined by the Development Plan Overlay –

Schedule 11 (DPO11).

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2. PROPOSED MELBOURNE PLANNING SCHEME AMENDMENT C245

The Melbourne City Council Melbourne Planning Scheme Amendment C245 for DPO11

Clause 2.0 proposes that permits issued must achieve the following design requirements

with respect to the environmental wind criteria:

New developments adjoining the proposed public open space shown in Figure 1

and the frontages of Therry Street, Queen Street, the southern side of the New

Franklin Street and Peel Street should be designed to be generally acceptable for

short term stationary wind exposure (where the peak gust speed during the hourly

average with a probability of exceedance of 0.1% in any 22.5o wind direction

sector must not exceed 13ms-1).

New developments adjoining all other public spaces should be designed to be

generally acceptable for walking (where the peak gust speed during the hourly

average with a probability of exceedance of 0.1% in any 22.5o wind directions

sector must not exceed 16ms-1).

DPO11 Clause 2.0 also has the following design requirements that relate to urban

planning/design that would also affect the environmental wind conditions in DPO11

streetscapes;

Discretionary minimum and mandatory maximum podium heights

o Minimum 10m, maximum 20m for podiums fronting Therry Street and

Queen Street north of Franklin Street

o Minimum 20m, maximum 40m for podiums fronting other streets

Orientation of buildings to complement the street system

Construction of buildings to the street edge

Mandatory tower setbacks

o 10m from New Franklin Street

o Towers fronting former alignment of Franklin Street – 6m from podium front

o Towers fronting all other streets – 10m from podium front

o Side and rear boundaries – 10m from side and rear boundaries

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Tower separations

o Setback a discretionary minimum of 24m from an existing or likely future

tower on adjoining sites(s), and must be setback a minimum of 10m

Mid-block publically accessible pedestrian links to enhance pedestrian

permeability of the public realm

Continuous weather protection should be provided to the footpaths of Therry

Street, Queen Street, Peel Street, and to the southern side of the New Franklin

Street to promote pedestrian amenity and provide protection from rain, wind, and

sun

DPO Clause 3.0 requires a development plan to include the following:

A wind effects assessment that demonstrates that wind impacts will not adversely

affect the amenity of the public realm.

A single public submission (submitter 25) has also raised the concern for potential wind

tunnel effects in key streets reducing amenity at street level. Creation of a hostile

pedestrian environment leads to less pedestrians and streets become less attractive and

safe for residents.

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3. WIND FLOW AROUND BUILDINGS AND EFFECTS ON PUBLIC

REALM

In older towns and cities most buildings were of relatively common height and design

uniformity, which allowed the faster moving wind at higher elevations to pass over the top

of cities without affecting the pedestrian level environment. As building technology has

developed and the cost of land in cities increased, developers and architects have

explored taller buildings to make better use of the limited space. This has meant that

taller buildings are now exposed to the increased wind speeds at higher elevations as

buildings rise up from the street line.

Adverse effects on the pedestrian wind environment occur when there is a considerable

height difference between a building and surrounding buildings. A taller building is

exposed to a steadily increasing wind speed up the windward face causing the dynamic

pressure to increase with height up the face. The pressure field created by the wind

induces additional wind flow towards lower levels as shown schematically in Figure 1.

Figure 1: Schematic diagram of wind flow around a building

The downward direction of the additional wind flow starts at approximately two thirds to

three quarters of the height of the windward face of the building, which for a tall building

can be a significant volume of additional wind flow, downwash, being added to that

already in the ground level streetscapes.

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The effects of this additional wind flow on the public realm can create wind conditions

that could disturb the ability of pedestrians to transit through a space to creating

conditions where pedestrians have difficulty maintaining their balance and are blown

over. Generally, the highest wind speeds occur at the corners of buildings as the wind

flow turns the corner, but situations such as two tall buildings on either side of a narrow

laneway would create high wind speeds within the laneway, in arcades through buildings,

or in colonnades.

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4. ENVIRONMENTAL WIND CRITERIA

The basis of the environmental wind criteria used to assess the environmental wind

conditions around Australia has come from the research by W. H. Melbourne and

published in the peer reviewed Journal of Industrial Aerodynamics (3, 1978, pp 241-249)

[now known as the Journal of Wind Engineering and Industrial Aerodynamics] and

included in Appendix A.

The effect of wind on pedestrians is primarily related to the wind speed (peak gust) and

the rate of change of wind speed (gustiness). Other factors such as temperature,

humidity, degree of shade, pedestrians age, and dress are also significant and could be

used to modify the effects of wind speed. Whilst these other factors influence the

perceived wind comfort of pedestrians they are difficult to quantify. The sub-tropical

climate of Melbourne would mean the effects of temperature, particularly winter

temperatures, and humidity would not be as important compared to regions such as

northern Europe and the north of North America and Canada. There would also be an

expectation that pedestrians would also dress appropriately for the climatic conditions.

The wind force felt by a person is related to the dynamic pressure. Whilst it is convenient

to relate the criteria to wind speed, it must be appreciated that the force experienced by a

pedestrian is proportional to the wind speed squared. While it would be better to present

the criteria in terms of velocity pressures, referring the criteria to velocities has become

widely accepted and more easily understood.

W. H. Melbourne’s (1978) criteria were based on two levels of wind speed, an

unacceptable level at which gusts would be strong enough to knock people over and a

level generally acceptable in main public access-ways based on conditions that existed

in Australian Cities during the first half of the 20th century, when building was dense but

heights were restricted to about 30m. It was assumed that pedestrians would be

appropriately dressed for the outside temperature conditions of between 10o and 30o C.

Melbourne’s criteria simply state that in the main public access-ways wind conditions are;

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unacceptable if the peak gust speed during the hourly average with a

probability of exceedence of 0.1% in any 22.5o wind direction sector

exceeds 23ms-1 (the gust wind speed at which people begin to get blown

over);

generally acceptable for walking in urban and suburban areas if the peak

gust speed during the hourly average with a probability of exceedence of

0.1% in any 22.5o wind direction sector does not exceed 16 ms-1 (which

results in half the wind pressure of a 23ms-1 gust)

The environmental wind criteria were extended for more recreational activities;

generally acceptable for stationary short exposure activities (window

shopping, standing or sitting in plazas – less than 15 minutes) if the peak

gust speed during the hourly average with a probability of exceedence of

0.1% in any 22.5o wind direction sector does not exceed 13 ms-1;

generally acceptable for stationary, long exposure activities (outdoor

restaurants/cafes, theatres – longer than 15 minutes) if the peak gust

speed during the hourly average with a probability of exceedence of 0.1%

in any 22.5o wind direction sector does not exceed 10 ms-1.

W. H. Melbourne’s (1978) criteria are based on a gust wind speed within an hourly

average wind speed (average wind speed over 60 minutes). The gust wind speed is a

wind speed averaged over small periods of time to which a pedestrian can respond, that

is of the order of seconds. He refers to an average 2 or 3 second gust wind speed that

has become a useful reference because it is roughly equivalent to the peak gust wind

speed available from climate data recorded by automatic weather stations.

The probability of exceedence of 0.1% relates approximately to the annual (once per

year) maximum mean wind speed occurrence for each wind direction sector.

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In the paper W. H. Melbourne (1978) compares the criteria against other criteria

developed by international researchers and there is good agreement for the level of

acceptable and unacceptable wind conditions in public access-ways.

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5. QUEEN VICTORIA MARKET WIND ENVIRONMENT

The strongest and most frequent winds for the Melbourne wind environment come from

the southwest through west to north-northeast wind directions, with secondary strong

wind from the south sector; east sector winds are relatively light and infrequent. Figure 2

shows an aerial view of Queen Victoria Market.

Figure 2: Aerial view of the Queen Victoria Market

The location of the Queen Victoria Market would mean it has exposure to the strong and

frequent west to north-northeast wind directions over the lower buildings of North and

West Melbourne. The exposure for these wind directions would be similar to the

Docklands Precinct, but without the small fetches of open water within the Docklands

Precinct that exacerbate the wind conditions. The Melbourne Central Business District

and the developing Docklands Precinct provide good protection from the east through

south to west-southwest wind directions for the Queen Victoria Market, which would

disturb and reduce the wind speeds for these wind directions. Previous wind tunnel

model studies have shown, with the above exposure, that the existing wind conditions

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within the DPO11 zone would not achieve the long term stationary criterion for all wind

directions at all locations.

The taller structures, relative to existing buildings, proposed for the site within the DPO11

zone would have exposure to direct wind flow from the strong and prevailing wind

directions for Melbourne.

The DPO11 zone covers the area to the south and east of the Queen Victoria Market so

for the prevailing wind directions any downwash of wind by any proposed built forms

within the zone would impact the wind conditions in Therry Street, Queen Street, the

southern side of the New Franklin Street, Peel Street, and streets outside the DPO11

zone.

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6. ENVIRONMENTAL WIND CRITERIA FOR QUEEN VICTORIA

MARKET

The environmental wind criteria proposed in the Melbourne Planning Scheme

Amendment C245 provides two criteria for the public realm of the DPO11 zone, as noted

in Section 2. The selection of the criteria was based on advice and data from MEL

Consultants and relates to the proposed activation of the public realm areas within the

DPO11. The requirement to achieve the short term stationary wind exposure criterion

along the frontages of Therry Street, Queen Street, the southern side of the New Franklin

Street, and Peel Street and the public open space is expected to enable good pedestrian

activation along these important pedestrian streets and open space areas. If, within these

important areas, activation for cafés or similar long term stationary activities is to be

proposed, then the long term stationary activities criterion would have to be achieved.

Improving the wind conditions locally within the important areas for these types of

activation, from the short term to long term stationary criteria, would be achieved with

minimal local wind break features e.g. vertical screens, green wall planters, landscaping,

rather than the large glass and plastic enclosures that are common in the adverse

Docklands wind environment. The other adjoining public spaces would be expected to be

for pedestrian transit and the criterion for walking comfort is appropriate for these areas.

By requiring these criteria for the development plan, particularly the short term stationary

criterion, in the public realm of the DPO11 zone the potential for ‘wind tunnelling’ and

reduced amenity raised by Submitter 25 would be mitigated.

The criteria for the DPO11 zone differs from the DD01 requirement for active frontages in

the Capital City where the long term stationary activities criterion is required. We know

from experience that many of the active frontages, e.g. northern end of Elizabeth Street

or Spencer Street, defined by DDO1 do not achieve the long term stationary activities

criterion for all wind directions and that lesser criteria have been accepted and reflected

in permits issued for developments in these areas. The intention of requiring the short

term stationary criterion for the DPO11 zone frontages makes the controls realistic,

considers the exposure of the area to the prevailing wind directions, and recognises that

Melbourne is a windy city. Also, achieving the short term stationary criterion along the

frontages by the design of built form that is sympathetic to the wind climate would, as

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discussed above, then require minimal ‘street furniture’ in the form of wind break

screens, to achieve areas for long term stationary activation.

The urban planning requirements of DPO11 with respect to the height and street edge

construction of the podium buildings would be expected to conflict with the required

pedestrian wind comfort criteria. The minimum podium height, 10m or 20m, could still be

too high to allow the required pedestrian comfort criteria, particularly near building

corners, to be achieved and we agree the minimum podium height should be

discretionary. The maximum height of the podiums along the street frontages would be

expected to be academic, since podiums with these maximum heights, with or without

towers above, would be unlikely to achieve the required pedestrian wind comfort criteria.

The podium height and tower form are often traded off against each other to mitigate

pedestrian level wind conditions i.e. a higher podium would require a more aerodynamic

tower and vice versa. The requirement to construct the podium buildings to the street

edges would restrict the potential to incorporate ground level wind mitigation strategies to

improve wind conditions near building corners and outdoor seating areas. Additionally,

the indenting of building entrances into the building faces results in a better outcome for

the transition from the calm internal building environment to the external wind

environment of streets. Therefore, the requirement to construct podium buildings to the

street edge may require some discretion to enable a positive outcome with respect to the

pedestrian level wind comfort. We have no objection to the mandatory requirement for

building podiums to complement the street system.

There are no requirements for the orientation of the towers to complement the street

system and we agree with this approach. Having the flexibility to orient towers away from

the street system is an effective strategy to mitigate the impact of towers on the

pedestrian level wind conditions. The alteration of tower orientation would allow, for

example, the corners of towers to be orientated towards the stronger wind directions that

would induce wind horizontally around the tower rather than towards pedestrian level.

Examples of the application of this strategy in Melbourne are Melbourne Central, Rialto

Towers, and the Commonwealth Bank Building. The Docklands Precinct is an example of

where, for a wind exposed location, the orientation of towers with tall podiums, to

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complement the road system has resulted in a poor outcome with respect to the

pedestrian level environmental wind conditions in the public realm.

The requirement for continuous weather protection, such as canopies, along the

footpaths of Therry Street, Queen Street, Peel Street, and to the southern side of the

New Franklin Street would assist with wind mitigation and would be supported. However,

canopies provide limited wind mitigation, particularly at corners, and should not be relied

upon as a principal wind mitigation strategy.

The mandatory minimum tower setbacks of 10m and 6m from podium edges required by

the amendment would be an acceptable starting point to provide developers with some

guidance, but from an environmental wind conditions perspective are not a mandatory

requirement. The tower setbacks required would depend on the tower built form of the

proposed developments within the DPO11 zone. A rectangular tower form with the faces

orientated towards the strong and prevailing wind directions may require a larger setback

compared to an aerodynamic circular building or a square building with corners pointed

towards the strong wind directions which could have smaller setbacks. The setback

would need to be determined by a wind tunnel model study as this would allow the

assessment of the specific built form of buildings. The urban design may have

requirements for mandatory towers setbacks and we would not object to mandatory

minimum setbacks for urban design reasons.

The proposed tower separations would be supported from a wind effects aspect as the

larger separation would allow the wind to perceive the towers as individual developments

and flow between them. Towers in close proximity are often perceived by the wind as

one single tower and this results in a more adverse impact on the pedestrian level wind

conditions. Additionally, the larger separation would be expected to reduce the wind

speed up between the towers and the impact on resident’s terraces.

The requirement for mid-block publically accessible laneways have been indicated on the

Framework Plan 2015 to be in the north-south direction through the sites. Laneways at

ground level through buildings are subject to the pressure difference between the

windward and leeward sides of the buildings, and this pressure difference would drive

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wind flow the through laneways. The wind conditions in these laneways would be

expected to be above the stationary criteria (and even above the walking criterion) for the

proposed orientations and create wind tunnelling effects. The requirements should allow

the developer/architects to create an effective seal (e.g. revolving doors, effective air-

locks) in these laneways that can be brought into action when wind conditions are

perceived as unpleasant. The effective seal would allow wind conditions in the laneways

to be controlled and achieve the stationary criteria. Without an effective seal the

utilisation of the laneways for stationary activities would be low. The laneways would not

be able to have unobstructed passage at all times.

The developments within the DPO11 will be required to provide a wind effects

assessment that demonstrates that wind impacts will not adversely affect the amenity of

the public realm. A wind assessment should not be required at the development plan

stage since the detailed building designs would not be available. The requirement should

be changed to require the assessment to be done by a wind tunnel model study at the

permit application stage. This should also include a clause that existing and proposed

street trees and landscaping are not to be depended upon for wind mitigation. The

assessment should also include examining the effect of the proposed developments on

the dynamic response to wind of the surrounding existing towers e.g tall towers along

Elizabeth Street.

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7. SUMMARY

In summary, the main points of this evidence are as follows:

The Queen Victoria Market would have similar exposure to strong and prevailing

wind directions as the Docklands Precinct, but without the small fetches of open

water of Docklands that exacerbate the wind conditions.

We support the proposed environmental wind comfort criteria for the DPO11 zone

as it has been stated in the proposed C245 amendment and would propose to

include the long term stationary criterion as a requirement for certain types of

activation, e.g. long exposure activities

A mandatory minimum podium height should not be defined as this will be

determined by the requirement to meet the required pedestrian comfort criteria.

The mandatory maximum podium height at the street frontages would be

academic since it would not be possible to achieve the required pedestrian

comfort criteria with podiums of the required maximum height.

The tower orientations, tower setbacks, and requirement to construct podium

buildings to street edges should be discretionary rather than mandatory, since

flexibility with these parameters is required to develop built form wind mitigation

strategies to achieve the required wind criteria.

We support the proposed minimum separations between the towers.

The mid-block laneways will require effective seals for wind and the requirements

should consider this feature and define required wind criteria for the laneways.

The continuous weather protection requirement should be retained and

unchanged.

The requirement for a wind effects assessment should be changed to a

requirement for a wind tunnel model study that demonstrates that wind impact will

not adversely affect the amenity of the public realm and achieve the required

pedestrian wind comfort criteria at the permit application stage and not at the

development plan stage.

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8. DECLARATION

We have made all the inquiries that we believe are desirable and appropriate and no

matters of significance which we regard as relevant have to our knowledge been withheld

from the Panel.

For MEL Consultants Pty Ltd:

M. Eaddy

W. H. Melbourne

27 April 2016

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Appendix A – Expert Witness Curricula Vitae

Michael J Eaddy

B.E., M.E., PhD, MIEAust

Director

MEL Consultants Pty Ltd

17 Kingston Street

East Malvern

VIC 3145

Senior Research Fellow (2003 - 2009)

Department of Mechanical Engineering

Monash University

Vic 3800

Research and Consulting Fields

Wind Engineering and Industrial Aerodynamics

Environmental Studies

Pressure Measurements and Structural Aeroelastic Modelling

Wind Tunnel Testing

Instrumentation and Acquisition Systems Development

Professional Committees

Australasian Wind Engineering Society (1999 - 2009)

Previous Experience and Qualifications

The University of Auckland, New Zealand (1993 – 1998)

Bachelor of Engineering, Mechanical Engineering, Honours Class 1

Masters of Engineering, Mechanical Engineering, Distinction

UniServices – Wind Tunnel Consulting

Monash University, Australia (1999 – 2004)

Doctor of Philosophy : Lift Forces on Smooth and Rough Circular Cylinders in Low

and High Turbulence Flows.

MEL Consultants Pty Ltd

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Commercial Wind Engineering (2002 - )

Scholarships and Awards

Monash Graduate Scholarship (1999 – 2004)

Publications (including with co-authors)

Published papers:

Over 10 in the Wind Engineering Field

Propriety Reports

Over 150 for Wind Engineering Consulting

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PROFESSOR W H MELBOURNE

BE, DIC, PhD, FIEAust, AFRAeS, FRGS, FTSE

Professor of Fluid Mechanics (1975 - 99)

Chairman, Department of Mechanical Engineering (1976-1994, 1996-98)

Dean, Faculty of Engineering (1994)

Associate Dean, Faculty of Engineering (1995-1996)

Monash University Council (1987-1994)

Founder/Director MEL Consultants Pty Ltd (1981- )

Lawrence Hargrave Medallist (1981)

AGM Michell Award (1993)

Research and Consulting Fields:

Environmental Fluid Mechanics; turbulent flows and their interaction with bluff bodies; the

loading and response of structures to wind action; modelling wind flow over

complex terrain; dispersion of atmospheric pollutants.

International Committees:

International Journal of Wind Engineering and Industrial Aerodynamics, Elsevier Holland

- Regional Editor Australasia (1974 - )

American Council on Tall Buildings and Urban Habitat - Vice Chairman and Editor Wind

Loading Committee (1972 - )

Commonwealth Aeronautical Advisory Council - Coordinator, Low Speed Aerodynamics

(1967-1983)

International Association for Wind Engineering - Chairman (1979-1983)

International Standards Organisation - Chairman 'Wind Action on Structures (2001 - )

Australian Committees:

Standards Association of Australia Committees - BD/5 : General Requirements for

Structural Design, and BD/6 : Loading on Structures (1970 - )

National Committee Thermodynamics & Fluid Mechanics, IEAust (1970-1978, 1991-

1995)

Australian Electrical Services Industry Research Board (1986-1994)

Secretary, Royal Aeronautical Society, Victoria (1964 - 1966)

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Publications:

Texts:

Journal of Wind Engineering & Industrial Aerodynamics, (Proc 4th Asia Pacific

Symposium on Wind Engineering 1997), Guest Editor of Vol 83, 1999.

Tall building design from linear mode force balance model data, Collected Papers of

Habitat and the High Rise, Council of Tall Buildings & Urban Habitat, 557 pp, 1996

Bluff Body Aerodynamics for Wind Engineering, A State of the Art in Wind Engineering,

Wiley Eastern Ltd, pp 47-64, 1994

Designing to Reduce Perceptible Wind-Induced Motions, Structural Systems for Tall

Buildings Monograph, McGraw-Hill, pp 341-352, 1994

A Commentary on the Australian Standard for Wind Loads, (with J D Holmes & G R

Walker), Publisher Australian Wind Engineering Society, 1990

Wind Engineering 1983, (with J D Holmes & P S Jackson) Editors, (Elsevier Proc 6th Int

Conference on Wind Engineering, Gold Coast, Australia, 21-25 March, and

Auckland, New Zealand, 6-7 April, 1983)

Wind loading and wind effects, Editor of chapter in Monograph on Design of Tall

Buildings, Publisher ASCE, pp 145-248, 1980

Architectural Aerodynamics (with R Aynsley & B J Vickery), Applied Science Publishers,

1977

Published Papers:

Over 200 in the aerodynamics field generally and wind engineering in particular.

Unpublished Papers, Lectures, Course Notes:

Over 100.

Major Consulting Reports (Restricted Circulation)

Over 500.

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Appendix A – Melbourne (1978) Environmental Wind Criteria Paper

Journal of Industrial Aerodynamics, 3 (1978) 241--249 241 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Paper 12

CRITERIA FOR ENVIRONMENTAL WIND CONDITIONS

W.H. MELBOURNE

Department of Mechanical Engineering, Monash University, Clayton, Victor& 316~ (Australia)

(Received October 18, 1977)

Summary

Since 1971 a number of authors have published criteria for the acceptability of environmental wind conditions for human comfort for a range of activities.

This paper notes that it is the. forces caused by peak gust wind speeds and associated gradients which people feel most and discusses the relation between peak gust and mean wind speeds. Melbourne's criteria, which have been stated in terms of maximum gust speeds per annum, are shown to define a range of wind-speed probabilities, in particular, the frequency of occurrence of mean wind speeds, which then facilitates comparison between the various published criteria.

It is shown that, in spite of the apparent numerical differences in published wind speed criteria and the various subjective assumptions used in their development, there is remark- ably good agreement when they are compared on a proper probabilistic basis.

1. Introduct ion

In recent literature and at the 4th International Conference on Wind Effects on Buildings and Structures, London, 1975, there has been some debate as to the quantitative values of wind speed to be used in criteria for environmental conditions around new building developments. It was noted by several of the authors at the above-mentioned conference, that in spite of the seeming numerical differences in wind-speed criteria quoted by a number of authors, the differences were, in fact, relatively small [1 ]. The problem is that the phenomenon of wind and frequency of occurrence is very complex and the numerical values developed for these criteria depend on the statistical framework in which they are set.

It is the purpose of this paper to discuss the physical nature and effect Of wind on people in respect of the relationship between mean wind speeds and peak gusts produced in turbulent conditions and the statistical inference of the various ways of expressing the frequency of occurrence of given wind speeds, and hence to permit a comparison of the various published environmental wind criteria.

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2. The reason for needing environmental wind-speed criteria

Whilst involved in the technical argument about criteria, it is important to remember the reason for trying to establish environmental wind-speed criteria.

Briefly, the need has arisen because unacceptable wind speeds can be induced around building developments and one way of avoiding these problems is to conduct wind-tunnel tests from which wind speeds around a proposed development can be estimated. Having obtained the facility for predicting likely wind conditions in a given area, it becomes necessary to develop some criteria as to the f requency of occurrence of wind speeds which are acceptable and unacceptable for a variety of activities.

3. How people feel the effects of wind

There seems little doub t that wind speed and rate of change of wind speed are the primary parameters in any assessment of how wind affects people, Melbourne [2], Hunt et al. [3]. There are, of course, other factors such as temperature, humidity, degree of shade and mode of dress, which are also significant; however, these are factors which can be superimposed on or used to modify the effects of wind speed and as such will no t be dealt with here.

Wind gustiness, or fluctuation of wind speed with time, is a random process and whilst the mean wind speed is a meaningful and simple parameter to ob- tain, the rate of change of wind speed is not. Fortunately, the effect of rate of change of wind speed can be covered generally by the parameter of turbulence intensity of wind speed, that is the standard deviation over the mean of wind speed. Further, in terms of what people feel, it is often convenient to talk in terms of a gust wind speed, that is a wind speed averaged over the smallest periods of time to which a person can respond, of the order of seconds. The mean 2- or 3-second-gust wind speed has become a useful reference in this respect, because it is roughly equivalent to the peak gust speed recorded by the Dines anemometer and the larger cup anemometers.

The wind force felt by a person is related to dynamic pressure. Hence, whilst it may be convenient in one sense to relate criteria directly to wind speed, it must be appreciated that the force felt by a person is proportional to wind speed squared. For this reason a more rational feel for the problem is gained if comparative data are presented in terms of velocity pressures rather than velocities. However, the referring of criteria to wind speed has gained popular acceptance and values of wind speed are more easily remembered than numbers based on the square of wind speed, hence, criteria will be discussed in terms of wind speed.

In concluding this section, it is worth re-casting the opening sentence by now saying that it is the peak gust wind speeds and associated gradients which people feel most.

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4. Relationships be tween peak gust and the mean wind speeds

The peak gust wind speed fi is dependent on turbulence intensity and can be given in terms of the mean u-- and standard deviation ou as

= h-- + 3.50u (1)

For example, for a turbulence intensity ( o u / u ) of 15%, fi = 1.5 u-, and for 30%, ~ = 2.0 u, etc.

As noted, it is the peak gust wind speeds and associated gradients which people feel most and as such it is of interest to know under what conditions they occur. The observations of Melbourne and Jouber t [4] indicated that the areas in full scale which have been classed as having unpleasant or unacceptably high wind speeds were all associated with high mean wind speeds. Later, model- and full-scale measurements by Isyumov and Davenport [ 5] and Melbourne [6] continued to show that the windiest areas were associated with high mean wind speeds, but that the turbulence intensity was important in determining the peak gust wind speeds. In the case of the former, the ratio of peak gust wind speed over mean wind speed f i /u for the three windiest conditions respectively were 1.5, 2.7 and 2.8 and for the latter 1.9, 1.9 and 2.4. For areas and wind directions with lower wind conditions, and obviously for much greater turbulence intensities, this ratio was typically as high as 5.0. This means that to get an accurate prediction of peak gust wind speeds from wind- tunnel model tests, it is essential that mean and rms or peak values for a given probabili ty level be actually measured.

_Although it is possible to have unpleasant areas with low mean wind speeds and high turbulence intensities, the evidence to date does seem to indicate that for areas likely to have unacceptably high wind conditions, such as near corners, in narrow alleys and in arcades, the turbulence intensity is relatively low and that in these areas it would be reasonable to assume that the peak gust wind speeds will be about twice the mean wind speed. This means that wind-tunnel investigations, in terms of exploring and improving likely areas of high wind conditions, can often be reasonably based on very simple and in- expensive model measurements of mean wind speed. However, this does not mean that the need to model the turbulence characteristics of the incident wind stream can be overlooked, as a low turbulence stream would produce quite different f low fields and erroneous information.

5. Melbourne's criteria for environmental wind speeds

Notwithstanding the usefulness of the above very simple tests, to maintain flexibility in the application of environmental wind-speed criteria to all levels of turbulence, the author believes it is necessary to frame the definition in terms of gust wind speeds related to some meaningful return period or frequency of occurrence. Criteria which are defined only by mean wind speeds need to be qualified with respect to turbulence to have any general application.

244

Melbourne's criteria [2,7] were based on two levels of wind speed, an unacceptable level at which wind gusts would be strong enough to knock people over and a level generally acceptable in main public access-ways based on conditions which had existed in the main Australian cities during the first half of the 20th century, when building was dense but heights restricted to about 30 m. Temperatures are typically between 10 ° C and 30 ° C with people appropriately dressed for the outside temperature conditions. These criteria simply state that in main public access-ways wind conditions are

(a) completely unacceptable if the annual maximum gust exceeds 23 m/s (the gust speed at which people begin to get blown over),

(b) generally acceptable if the annual maximum gust does not exceed 16 m/s (which results in half the wind pressure of a 23 m/s gust). Along the lines of Davenport 's [ 8, 9] suggestions for comfor t for activities less than walking in a main public access-way, two additional comfor t criteria have been added to the original criteria as follows:

(c) generally acceptable for stationary short-exposure activities (window shopping, standing or sitting in plazas), if the annual maximum gust does not exceed 13 m/s,

(d) generally acceptable for stationary, long-exposure activities (outdoor restaurants, theatres), if the annual maximum gust does not exceed 10 m/s.

From these basic criteria a probability distribution, or f requency of occurrence, can be developed to suit any turbulence conditions. An example of such a distribution is given in Fig.l, for a turbulence intensity of 30%, where the distributions of the maximum gust speeds per annum, of 23 m/s, 16 m/s, 13 m/s and 10 m/s are shown as normal distributions back to the maximum hourly mean wind speed per annum (i.e. ~ = 2.0 u-for Ou = 0.3 h-, which as discussed in Section 4 is a very typical situation). The upper part of Fig.1 shows the distribution of hourly mean wind speeds for these conditions using a Rayleigh distribution, and the expected maximum wind speeds for periods of a day, week, month and year have been calculated using a method by Davenport [ 10].

Davenport showed that the number of storms, on occasions during which a wind speed u- is exceeded, can be expressed as

Nu = .v/.~_~ vT [F ( 1 2 : +~)-r (i +-i]~ k ] (k-~)/k k/ {-In P(> ~) }J P(>U) (2)

where P(>~-) is the probability of exceeding the mean wind speed W (based on the Weibull distribution), k is one of the Weibull parameters, F is the Gamma function and ~T is the number of independent events per annum. The value of k varies about 1.5 to 2 and vT varies between ,500 and 1000, depending on the local wind climate. From an evaluation of Davenport 's eq. (2) [5] the ranges given in Table 1 can be obtained which express the relation between probability of exceeding a certain hourly mean wind speed and the number of storms per annum during which that mean wind speed is exceeded. Apart f rom

2 4 5

HEM WIND SPEED u rids

5 lO 15 20

i 11_ ~ f I t I I

0

ONCE PER WEEK

0 ONCE PER YEAR

!

O- 1 - -

!

E 0.01 =

~, o .~ l

_1

O. OG01 _ _

25

WIND SPEED u m/s

5 10 1S 20 25 I ~ t I ' - 5 6.5 8 11.5 , " . _ ("-u-)z - \ \ \ \ Ar(a,u ) ' r ~ , 2~" d.

, , \ \ (!, -

- AC~PTJILE \ ~ l t r ~ P T M i . E

\ ~.\ \

_ ~ ~

JinX. SUSt r. . 10 13 16 23 2 s e c e a d

Fig. 1. Probabi l i ty d is tr ibut ions o f Melbourne 's criteria for env ironmenta l wind cond i t ions for dayl ight hours , for a turbulence in tens i ty o f 30%. au = 0 .30~ ' , Q = 2.0h-.

providing a very important link to give information about the maximum wind speeds likely to occur on average for various periods, such as once per year, once per month, etc., this also provides the necessary link to enable the various environmental wind speed criteria to be compared.

One other complication arises in respect of the number of storms per annum which are relevant to the assessment of environmental wind conditions for human comfort. It is obviously conservative to include winds which blow for all hours of the year, day and night, when most areas under consideration will only be occupied for half of the time or less. Although it does not make

246

T A B L E 1

Re la t ionsh ip b e t w e e n p r o b a b i l i t y of exceed ing a m e a n wind speed a n d t he average n u m b e r of s t o r m s per a n n u m dur ing which t h a t m e a n wind speed is exceeded

N u m b e r o f s t o r m s pe r a n n u m dur ing wh ich ~- is exceeded (Nu)

P robab i l i t y of exceed ing an h o u r l y m e a n wind speed ~- ( P ( > ~ ) )

All h o u r s Dayl igh t h o u r s

1, once per a n n u m 0 . 0 0 0 2 5 - - 0 . 0 0 0 5 0 . 0 0 0 5 - - 0 . 0 0 1 o n average

12, once pe r m o n t h 0 . 0 0 3 - - 0 . 0 0 6 0 . 0 0 6 - - 0 . 0 1 2 o n average

52, once per week 0 .015 - - 0 .03 0 .03 - -0 .06 o n average

a great deal of difference, the author prefers to relate criteria and assessment to approximately half the total time, by relating the probability of exceedence to half the yearly cycling rate (i.e. 250--500 independent events per annum) and calling this procedure an assessment of environmental wind conditions relating to "daylight hours"; these ranges are also given in Table 1. Strictly speaking, the cycling rate and evaluation of the wind speed probability distributions should be related to the relevant occupancy times (i.e. daylight hours, afternoon hours, etc.), and in many parts of the world seasonal distributions are also significant. However, for the purposes of this comparison of criteria the simplistic assumptions above described as relating to "daylight hours" will be used in this paper.

6. Comparison of various criteria

Since 1971 several forms of criteria for environmental wind conditions have been published. The criteria developed by Wise [ 11 ], Penwarden [ 12, 13] Davenport [8, 9], Lawson [14] and one by Hunt, Poulton and Mumford [3] are given in terms of mean wind speed at some stated or implied level of turbulence intensity between 15% and 20%. Comparison of these criteria can be made in Fig. 2 with Melbourne's criteria which have been plotted for a turbulence intensity of 15%, i.e. for au/-U = 0.15 and from eqn. (1) u- = ~/1.5.

Wise [ 11 ], in 1971, commented in relation to the Beaufort scale "that wind speeds much above about 5 m/s are likely to give unpleasant disturbance to clothing and hair" and "making reasonable assumptions about metabolic rate, and the thermal resistance of body layers and clothing, speeds of some 5 m/s appeared tolerable at 10 ° C in normal winter clothing". Penwarden [12] in 1973 and again in collaboration with Wise [13] in 1975 prepared a summary of wind effects on people based on a modified version of the Beaufort Scale from which the following three points can be extracted

247

discomfort begins ~ = 5 m/s unpleasant u- = 8--10 m/s dangerous u = 15--20 m/s.

Penwarden and Wise [13] quoted a criterion which they had used at the Building Research Station, that conditions were regarded as acceptable, or no remedial action was required, if u < 5 m/s for 80% or more of the time and vice versa, that remedial action would be taken if u- > 5 m/s for more than 20% of the time. In probability terms this criterion is interpreted as being

acceptable ifP(~ > 5) ~< C.2.

Davenport [8, 9] in ].972 amalgamated work by Wise, Melbourne and Joube~ and suggested criteria for a range of activities; these were related to a Beaufort scale for open-country mean wind speeds at 10 m. These criteria also noted that the relative comfort level might be expected to be reduced by one Beaufort number for every 20 ° C reduction in temperature. In particular Davenport nominated the following hourly mean wind speeds (converted to 2 m) conditions as being tolerable if not exceeded more than once per week, which in probability terms are interpreted as being acceptable for

walking fast if P(~- > 10) ~< 0.05 strolling, skating if P(x > 71~) ~< 0.05 standing, sitting, short exposure if P(~- > 51/2)~< 0.05 standing, sitting, long exposure if P(~- > 31/2) ~< 0.05 Lawson [14] in 1973 used the same Beaufort scale as Penwarden and devel-

oped a figure to take into account the effects of turbulence. A value of fi = 1.7 h-- was used, which from eq. (1) implies a turbulence intensity of about 20%. Lawson quotes Beaufort 4 wind speeds (6--8 m/s) as being tolerable if not exceeded for more than 4% of the time; and Beaufort 6 wind speeds (11-- 14 m/s) as being unacceptable if exceeded for more than 2% of the time. In probability terms these criteria are interpreted as being

acceptable if P(~- > 6-8) ~< 0.04 unacceptable if P(~- > 11--14) ~ 0 .02

Hunt, Poulten and Mumford [3] in 1976 described a range of wind-tunnel tests which were conducted to show how wind affects people's abilities to perform simple tasks, including a simulation of turbulence. Two criteria were developed, firstly that if wind conditions are to be tolerable and for most kinds of performance to be unaffected

< 9/(1 + 3 turbulence intensity)

for turbulence intensity of 15% this becomes u- < 6.2 m/s, and secondly, for safe and sure walking that there must be a low probability (say 1%) of a gust lasting over a few paces (say 5--10 m) exceeding 13 m/s. For a turbulence intensity of 15% the 13 m/s gust becomes a mean wind speed of 13/1.5 = 8.7 m/s. (Hunt used a conversion from Durst to give 9 m/s.) In probability terms

248

O.O01

HOURLY HEM WIND SPEED ~ m/s 5 lO 15 ZO

1 I I

i ~ : = : ~ ~.ONCE PER ~ WEEK

UNACCEPTABLE m ~e ACCEPTABLE ~ A N G E N O U S

0 - 10 13 16 23 m/s YEAR

CRITERIA SYRBOL

HELgOUI~IE AS NOTED ON LINES IN GRAPH

PE~AROEN MO WISE ACCEPTABLE IF P(~ • 5) '~ 0.2 0

DAVENPORT ACCEPTABLE FOR IF Pl,-': • 0.05 X 10) WALKING FAST &u

STROLLING IF P(~ • 7ti) • 0.05 X STANDINO,SITTING IF P(~- • 5~i) • 0,05 X SNORT EXPOSURE STkNDING,SITTING IF P(~ • 3~) '~ O.OS X LONG EXPOSURE

L/~SON ACCEPTABLE IF P(~ > 6 to 8) ~ 0.04 UNACCEPTABLE IF P(~ • 11 tO 14) ) 0.02 '

IAINT.POULTON & MUI@'ORD ACCEPTABLE FOR STROLLING IF P(~ • 6) • 0.1 .~ ACCEPTABLE FOR WALKING IF P(~- • 9) • 0.01 -~-

Fig. 2. Comparison of various criteria for environmental wind condit ions for daylight hours for a turbulence intensi ty o f 15%. au ffi 0.15~-, fi = 1.5h--.

for 15% turbulence intensity, this is interpreted as being

acceptable for strolling if P(~ > 6) ~< 0.1 acceptable for walking if P(h- > 9) ~< 0.01

These criteria in probability terms have been compared in Fig.2 with Melbourne's criteria plotted for a turbulence intensity of 15%.

7. Conclusions

It remains to conclude that the degree of agreement between the criteria when presented in probabilistic terms is quite remarkable for a phenomenon which relies almost completely on subjective assessment. This is particularly so for the earlier attempts by Wise, Melbourne and Penwarden where the criteria were developed entirely independently and in quite different ways. The agreement of the later published criteria, whilst supportive, is not quite so remarkable as there has been a certain amount of influence from the earlier attempts. It seems reasonable to conclude that assessments based on any of these criteria could be said to be made with some consensus of international opinion. However, assessment of the viability of any area in terms of wind environment still relies heavily on the assessment of the use to which the area is to be put and the cost-effectiveness of providing protection from the wind.

249

References

1 Discussion Session 7, Proc. 4th Int. Conf. Wind Effects on Buildings and Structures, Cambridge University Press, London, 1975, pp. 665--666.

2 W.H. Melbourne, Ground level winds caused by large buildings, Monash University, Dept. Mech. Eng., MMER 4, 1971.

3 J.C.R. Hunt, E.C. Poulton and J.C. Mumford, The effects of wind on people; new criteria based on wind tunnel experiments, Building and Environment, II (1976) 15--28.

4 W.H. Melbourne and P.N. Joubert, Problems of wind flow at the base of tall buildings, Proc. 3rd Int. Conf. Wind Effects on Buildings and Structures, Tokyo, 1971, pp. 105--114.

5 N. Isyumov and A.G. Davenport, The ground level wind environment in built up areas, Proc. 4th Int. Conf. Wind Effects on Buildings and Structures, Cambridge University Press, London, 1975, pp. 403--422.

6 W.H. Melbourne, Wind effect measurements on the BHP Building, Melbourne and full scale wind measurements below tall buildings, Syrup. Full Scale Measurements of Wind Effects on Tall Buildings, University of Western Ontario, London, Canada, 1974.

7 W.H. Melbourne, Wind tunnel test expectations, Int. Conf. Planning and Design of Tall Buildings, Lehigh University, ASCE, Vol. DS, 1972, pp. 301--304.

8 A.G. Davenport, An approach to human comfort criteria for environmental wind conditions, Colloquium on Building Climatology, Stockholm, 1972.

9 A.G. Davenport, Approaches to the design of tall buildings against wind, Theme Report at Int. Conf. on Planning and Design of Tall Buildings, Lehigh University, Vol. lb-7, 1972, pp. 1--22.

10 A.G. Davenport, On the statistical prediction of structural performance in the wind environment, Preprint 1420 ASCE National Structural Eng. Meeting, Baltimore, Maryland, 1971.

11 A~F.E. Wise, Wind effects due to groups of buildings, Philos. Trans. R. Soc. (London), A269 (1971) 469--485.

12 A.D. Penwarden, Acceptable wind speeds in towns, Building Sci., 8 (1973) 259--167. 13 A.D. Penwarden and A.F.E. Wise, Wind environment around buildings, Building

Research Establishment Report, H.M.S.O., 1975. 14 T.V. Lawson, The wind environment of buildings: a logical approach to the establish-

ment of criteria, University of Bristol, Dept. of Aeronautical Engineering, Report No. TVL 7321, 1973.

environmental wind conditions and criteria - statement of ......over. generally, the highest wind...

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