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OPTIONS FOR A
WIND
LOADING CODE
-Discussion paper for IPENZ Conference, February 1985.
R C Cooney BE(Hons) MIPENZ
Head of Structures Division
Building Research Association o f NZ
bstract
The New Zealand loadings code, NZS 4203 - Code of practice for general structural
design and design loadings for buildings, i s currently being completely revised,
including part
4
wind loads.
-
Apart from the basic wind speed data, there i s nothing unique about wind loads on
buildings in New Zealand, and hence there may not be sufficient reason for
continuing to have a different code from other countries, particularly those we have
strong associations with, such as Australia.
This paper briefly reviews the present situation and details options for the wind
loading part of the revised loadings code. It i s intended to generate discussion in
order that the code revision committee can obtain an indication of the format
preferred by designers for the future New Zealand wind loading code.
1.
Introduction
The Standards Association of New Zealand (SANZ) i s currently preparing a
completely revised edi tion of the New Zealand loading code, NZS 4203 (I ). Naturally
this
revision includes Part 4 wind loads, this part hereinafter being referred to as
the present code .
In 1983 the outgoing wind loading subcommittee of the SANZ NZS 4203 committee
prepared an internal repo rt on the present code. That report t o SANZ identi fied
technical deficiencies with the code, indicated developments wi th other overseas
wind codes, br ie fl y gave some options for a future wind code in New Zealand, and
recommended that one of a number of overseas codes be adopted for future use as
the NZ wind loadings code.
It
i s the purpose of this paper to publicly give some of the background to the
revision of the wind code, to discuss details of the options available and to indicate
a di rection in order that the subject can be discussed and the preferences of code
users ascertained.
2. The present code
Before presenting the options, it
i s
relevant t o mention some of the deficiencies o f
the present code highlighted by the previously mentioned report.
The present code
i s
largely an amalgamation of the 1972 British code (2) and the
1975
Australian code (3), both o f which have their origins in research conducted
during the
1960's and early 1970%. Dur ing the last decade a great deal of more
refined research into wind loads on buildings has occurred in North America,
England and Australia, particularly in the area of low-r ise buildings. This research
has resulted in significant amendments to the wind codes in those countries during
this period. Of di rect relevance to our present code, the Australian code was
substantially revised in 1979, 1981 and 1983, and signif icant amendments wi l l soon
be published for the Bri tish code. Consequential amendment to our present code has
not taken place and thus many parts of i t are now out of date with respect to
current research, some differences being of a conservative nature, but others not.
The following i s an indication of some areas where the source data in overseas codes
has been substantially amended:
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Provisions to allow for the variation of basic wind speeds with direction.
The variation of wind speed with height above ground.
Force reduction factors relating to tributary supported areas of roofs.
Average external wall and roof pressure coefficients.
Force coefficients for some special building shapes.
Local external pressure coefficients for the edges of wall and roof surfaces.
Internal pressure coefficients.
The effect of topography on wind speed.
In addition, we in New Zealand have a further 10 years of wind speed data wi th
which to refine and extend the present basic wind speeds. Better reference
documents are also now more freely available t o support a wind code.
The sourcing of a code from different overseas codes demands a reasonable
.
commitment to consequential change when the source codes are changed. This i s not
without i t s problems, but i f t
i s
not done, such a code can quickly become obsolete
and find disfavour with users.
3. he options
or
new
code
In broad terms the options are;
-
To ~ r e ~ a r euniaue code from basic data.
-
To take the besi from overseas codes and amalgamate, again to produce a
unique code.
To adopt another country s code to be used with the appropriate New Zealand
basic wind speed data.
In choosing from these options we must consider:
-
Whether a real need
i s
established to change from our present code.
Validity of the source data to be used in preparing
a
new code or adopting an
overseas code.
The acceptance of a new format and terminology by present code users.
The availabil ity of resources to achieve the preparation, validation, and
ongoing updating of a unique code on an appropriate t ime scale.
With regard to preparing a unique code, t
i s
doubtful whether we have the real need
to do so, le t alone the resources. We in New Zealand have, a t considerable effort,
often produced unique design codes, but to do so in this instance does not seem at a ll
justified.
As to producing a unique code by amalgamating parts from other codes, this s t i l l
would require a substantial resource, and again
t
i s
doubtful whether the resource i s
available. But perhaps more relevant i s our less than satisfactory experience with
our present code which has demonstrated a need for a commitment and o resource
to be available for consequential updating.
This leaves us with the option of using another country s code. In deciding on a group
of countries from which the selection i s made we must first consider;
The language
The format and appropriateness
The terminology used
The supporting research base
- The supporting base for amendment/revision and the likely frequency of
amendment
Any social or economic factors
The method of adoption for use in New Zealand
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Those wind codes in the wor ld which are wri tten in English are largely based on, or
are, those from the Uni ted States of America, Canada, England and Australia. This
ref lec ts the f ac t that much of the fundamental wind engineering research related to
buildings i s carried out in those countries.
Leaving aside the problem of adequately translating codes written in other than
English and communicating w ith countries w ith such codes, it i s unlikely that a wind
code from a non-Engl
ish
speaking country would be sufficiently more advanced to
justify i t s consideration in
this
exercise.
Those codes written in English will now be discussed in turn.
3.1
The Nor th American Codes:
Compared w it h our present code, the Nor th American codes are of a somewhat
dif ferent format, they use slightly different terminology the
USA
one i s in
Imperia l units) and, more significantly , use a dif fe rent basis for the basic wind
speed fastest m ile and
10
minute mean), all of which i s quite unfamiliar t o us.
The New Zealand wind speed data presently based on
3
second gusts) could be
produced in the same format i f necessary.
The No rth Americans do have a
large and effective research base and their codes are revised at regular, well
spaced intervals. They also have thei r research basis we ll documented 4).
3.2 The British code:
In terms of format, we in New Zealand have traditiona lly largely followed the
British
code although our present code does owe some parts t o the Aust ralian
one.
The B ri tish code has had one dis tinct advantage over recent years, and that i s
it has a very good source guide 5).
This
handbook has been an invaluable
reference for designers, not only in Britain, but also New Zealand and other
countries.
The
British
code has remained without amendment since 1972, and
t h i s
year
the f i r s t amendments wi l l appear. t might have been amended earlier had the
supporting research e ff or t not been signif icantly reduced. This reduced
research effor t, i f
i t
i s going to be relati vel y permanent, must be taken int o
consideration when we consider whether we use the
British
code in future.
Another advantage of the Br it ish code is i t s terminology and style of writing.
As codes go it
i s
comparatively easy to read and understand, an important
factor in any code.
The handbook to the
British
code
i s
currently being updated in line w ith the
most recent research and to reflect the amendments currently being
processed. In addition, supporting documents have been produced by the
Building Research Establishment 6, 7, 8).
t i s proposed in the next few years to produce a completely revised
British
wind loading code in limit state format. t will also be in a number of parts
relating t o di ffere nt types of structures.
The current New Zealand wind speed data can be used directly with the British
code.
One of the amendments to the British code replaces the previous topography
factor SI) and escarpment rule which are the same as in our present code)
with a completely new method of assessing the change of wind speed with
topography. This i s a significant advance and would be welcomed by designers
in New Zealand.
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However, even wi th the amendments, the Br iti sh Code does appear t o be
behind current research in some areas such as the average and local external
pressure coefficients for walls and roofs for low-rise buildings.
3.3
The Aus tra lian code:
The Australian code has been in
i t s
present format since 1975, although
amendments have been incorporated in revised edit ions i n 1979, 1981 and 1983.
The forma t and terminology is similar in a number of respects to that of th e
Br iti sh code, but there are a number o f signif icant differences. These
differences have increased wit h successive editions of the code, largely
reflecting the significant amount of individual research that has occurred in
Australia.
The Australian code contains much more def init ive detail in parts than the
North American and Brit ish codes, and as such i s more complicated t o use. In
part icular , the wind profi les (wind speed wit h height above ground) are much
more detailed wi th regard t o ground roughness, and the local pressures a t
edges of walls and roofs are also more detailed.
It
i s also quite well supported
by reference documents (9).
The Austral ian code, lik e the Br it is h and New Zealand ones, is the last tha t
w il l be produced in the present format. Completely new codes are cu rrently
being prepared by the Standards Association o f Aust ralia (SAA) t o replace it
but these are some tim e away yet. These w il l be in lim it state f ormat and
incorporate the latest international research informaton. One code w il l be for
low-rise buildings of simple geometry, and the other f or more complicated
buildings and
i f
more detailed analysis
i s
necessary or desired (10). This type
o f subdivision is oft en seen as being desirable for a number o f str uc tu ra l design
codes, no t just for a wind code, in order to provide for the apparently
con tradic tory wishes o f designers for sim pli cit y and more detailed data.
I t is to be hoped that the new codes wi ll be wri tt en in a more easily understood
manner than the present Austral ian code, perhaps more in the manner o f the
Br iti sh code.
In deciding to go to a l im it st ate f orm at the Australians have chosen particula r
loading levels associated with selected risk levels and building performance.
This aspect will need consideration by us i f we are t o use tha t code i n order
that it i s consistent wi th other N.Z. codes.
The Australian code i s supported by an ac tive research base, and wh ils t there
might be some criticism that revised editions have been published quite
frequent ly i n recent years, the code i s expected t o b e revised less frequently
in future.
The New Zealand basic wind speed data does require factoring slightly (by
dividing by 1.09) i f i t is t o be used wi th the present Austra lian code. This is
because the basic wind speeds in the Australian code are generally refe renced
to ground roughness
2
conditions whereas the New Zealand basic wind speeds
are referenced t o ground roughness I.
There is one other consideration w it h regard t o the Austra lian code, namely
CER. As a result o f the New Zealand and Australian Governments agreements
wi th respect to CER SANZ has decided (11) that;
New Zealand Standards should harmonise with Australian Standards in
preference to International or other overseas Standards where there i s
disharmony between the two, and tha t SANZ urge SAA t o align w it h I S 0
Standards where applicable .
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In addition,
SANZ
and SAA have agreed (I 2) that;
Every ef fo rt w il l be made to achieve a harmonised approach in the tw o
Member States on standardization matters. Further, that where one
Member State indicates an interest in, or concern about standardization
act iviti es of the other Member State, then every f ac ili ty w il l be offered
the concerned Member State to participate in the particular
standardization ac ti vi ty on an equal footing wi th a ll other interests.
The effect of these decisions might well be that there i s l i t t le practical option
for a New Zealand wind code, that we w il l simply have to use the Austra lian
code. I f this i s the decision, then the form at for use in New Zealand w il l also
require consideration.
One practical possibility
i s
for the New Zealand basic wind speed data (and
possibly that also from Pacif ic Island countries) t o be included in the
Australian code and some other minor aspects of that code to be adjusted to
suit both countries. That code could then simply be ci ted as a means of
compliance with the Model Building Bylaw (or proposed National Building
Code). However
t
i s possible that there may be difficulties in this, in that i n
response to the question Can Trans Tasman Standards be produced? SANZ s
reply ( I2) is;
No. The SAA Charter provides for
t
to publish Australian Standards
and the Standards Ac t provides for SANZ t o publish New Zealand
Standards. However, the content o f Australian Standards and New
Zealand Standards can be identical and the Memorandum provides for
such harmonized standards to be clea rly identi fied fo r the benefit of
exporters.
This
statement will require further investigation since
t
would require both
countries t o simultaneously publish similar documents.
4.
ummary
W i t h
the complete revision of the New Zealand loadings code (NZS
4203)
currentl y being undertaken, the oppor tunity has been taken t o consider the
practical options available for what the wind loading part of that code might
be.
The adoption of an appropriate wind code from another country, with
appropr iate basic wind speed data fr om New Zealand being used, would appear
to be the best option with such a code being selected from those of North
America, Br itain or Australia. The format and terminology o f the Nort h
American codes probably counts against them. The style of the
British
code
i s
very familiar to New Zealand designers, t
i s
easy t o use and
i s
supported by
good reference documents. The format of the Australan code i s only slightly
dif fe rent from ou; present code although
t i s
more complicated in parts. It is
quite well supported by reference documents and usually i s revised to
represent the state-of -the -art, more so than the British code. Of necessity,
the impact of CER and the SANZISAA agreements must be taken into account
when considering the Australian code.
The fac t that both Brita in and Australia are currently preparing completely
new wind codes in li m it sta te format should also be taken in to account:
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5
ecommendat ons
i)
ii)
i i i)
iv)
Local Authorities could permit the use, for an inter im period unti l a
revised NZS 42 3 s published, of either the amended version of the
Br it ish code soon to be available), or the Australian code, w ith the New
Zealand basic wind speed data factored i f the Australian code
s
used).
The revised edition of NZS 42 3 should, i f practicable, use the
Australian wind code as the basis for defining wind loads on buildings,
the Australian code being that which s current when the revised edition
of NZS
42 3 s
published.
If i i ) s accepted then a New Zealand representative should at an early
stage be appointed to serve on the appropriate SAA project committee
to ensure that harmony between the countries s achieved and that a ll
background information s available for use in subsequent interpretation
of the code in NZ.
Again i f ii) s accepted then serious consideration should be given to
only SAA publishing a wind loading code which would include the NZ
basic wind speed data and which could be used in the building control
system in both countries.
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7
eferences
New Zealand Standard. NZS 4203: 1976), Code of Practice for General
Structural Design and Design Loadings for Buildings: Standards Association of
New Zealand.
Brit ish Standard code of Practice CP3: Chapter
V:
Part 2: 1972. Code of Basic
data for the design of buildings Chapter
V
Loading, Part 2. Wind Loads:
Brit ish Standards Institution.
Australian Standard AS 1170, Par t 2-1983, SAA Loading Code Part 2 - wind
forces: Standard Association of Australia.
Mehta,
K.
C. 1984, Wind load Provisions ANSI A58.1-1982. Journal of
Structural Engineering, Vol.
110 4) Apri l 1984. American Society of C ivi l
Engineers.
Newberry, C. W and K. J. Eaton, 1974), Wind Loading Handbook. Building
Research Establishment Report, England.
Building Research Establishment, 1984). The assessment of wind loads, Digest
19, England.
Bui lding Research Establishment, 1984). The assessment of wind speed over
topography, Digest 283, England.
Building Research Establishment, 1984). Wind loads on canopy
roofs
Digest
284, England.
Holmes, J. D. 1983). Wind loads on low rise buildings - A review, CSIRO,
Highett.
B Holmes
J. D. 1984). Format and data requirements for the Australian
Standard on wind forces, Proceedings of Workship on wind engineering and
industrial aerodynamics, 3 t o
5
July 1984, CSIRO, Highett.
Pers. Comm. SANZ.
Standards Association of NZ, 1984). Trans-Tasman Memorandum
of
Understanding to ease barriers to trade. Standards: Vol. 30, 9) September
1984, 16-17.
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S e r i e s OPY
R
Optfans
f o r
a wind lo adi ng code
BR NZ
conference
p per
S R I S
QD
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BUILDING RESEARCH ASSOCIATION OF NEW ZEALAND INC.
HEAD OFFICE AND LIBRARY MOONSHINE ROAD JUDGEFORD.
The Building Research Association of New Zealand is
an industry-backed independent research and testing
organisation
set
up to acquire apply and distribute
knowledge about bui lding which wi ll benefit the
industry and through it the community a t large.
Postal Address:
B R A N Z
Private
Bag
Por irua
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