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 environ- mental 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 be- tween 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 nu- merical differences in wind-speed criteria quoted by a number of authors, the differences were, in fact, relatively small [1 ]. The problem is that the phenom- enon 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 in- duced 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 unac- ceptably 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 condi- tions 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 fre- quency 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 un- acceptable 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 con- ditions 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 appropri- ately 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 occur- rence, 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 vari- ous 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 dis- tributions should be related to the relevant occupancy times (i.e. daylight hours, afternoon hours, etc.), and in many parts of the world seasonal distri- butions 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 turbu- lence 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 in- tensity 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 cri- teria were developed entirely independently and in quite different ways. The agreement of the later published criteria, whilst supportive, is not quite so re- markable as there has been a certain amount of influence from the earlier at- tempts. 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.