options for a wind loading code

8/15/2019 Options for a WInd Loading Code

http://slidepdf.com/reader/full/options-for-a-wind-loading-code 1/10



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:



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



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.



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 .



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:



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.



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.



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



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

options for a wind loading code

Documents