AS-NZS-1170.2-SUPP-1-2002.pdf

as/nzs 1170.2 supp 1:2002 as/nzs 1170.2 supplement 1:2002 structural design actionswind actionscommentary (supplement to as/nzs 1170.2:2002) as/nzs 1170.2 supp 1 accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 this joint australian/new zealand standard was prepared by joint technical committee bd-006, general design requirements and loading on structures. it was approved on behalf of the council of standards australia on 29 march 2002 and on behalf of the council of standards new zealand on 28 march 2002. it was published on 4 june 2002. the following are represented on committee bd-006: association of consulting engineers australia australian building codes board australian institute of steel construction building research association of new zealand cement and concrete association of australia csiro building, construction and engineering cyclone testing stationjames cook university electricity supply association of australia housing industry association institution of engineers australia institution of professional engineers new zealand master builders australia new zealand heavy engineering research association steel reinforcement institute of australia university of canterbury new zealand university of melbourne university of newcastle additional interests participating in the preparation of this standard: bureau of meteorology curtin university of technology monash university national institute of water and atmospheric research, nz university of queensland keeping standards up-to-date standards are living documents which reflect progress in science, technology and systems. to maintain their currency, all standards are periodically reviewed, and new editions are published. between editions, amendments may be issued. standards may also be withdrawn. it is important that readers assure themselves they are using a current standard, which should include any amendments which may have been published since the standard was purchased. detailed information about joint australian/new zealand standards can be found by visiting the standards australia web site at .au or standards new zealand web site at www.standards.co.nz and looking up the relevant standard in the on-line catalogue. alternatively, both organizations publish an annual printed catalogue with full details of all current standards. for more frequent listings or notification of revisions, amendments and withdrawals, standards australia and standards new zealand offer a number of update options. for information about these services, users should contact their respective national standards organization. we also welcome suggestions for improvement in our standards, and especially encourage readers to notify us immediately of any apparent inaccuracies or ambiguities. please address your comments to the chief executive of either standards australia international or standards new zealand at the address shown on the back cover. accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 as/nzs 1170.2 supplement 1:2002 structural design actionswind actionscommentary (supplement to as/nzs 1170.2:2002) originated in australia as part of as 1170.21989. originated in new zealand as part of nzs 4203:1976. previous new zealand edition nzs 4203:1992. as 1170.21989 and nzs 4203:1992 jointly revised, amalgamated and redesignated in part as as/nzs 1170.2 supplement 1:2002. copyright standards australia/standards new zealand all rights are reserved. no part of this work may be reproduced or copied in any form or by any means, electronic or mechanical, including photocopying, without the written permission of the publisher. jointly published by standards australia international ltd, gpo box 5420, sydney, nsw 2001 and standards new zealand, private bag 2439, wellington 6020 isbn 0 7337 4474 5 accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:20022 preface this commentary was prepared by the joint standards australia/standards new zealand committee bd-006, general design requirements and loading on structures, as a supplement to as/nzs 1170.2, structural design actions, part 2: wind actions. this commentary supersedes, in part, as 1170.21989, minimum design loads on structures, part 2: wind loads and, in part, nzs 4203:1992, code of practice for general structural design and design loadings for buildings (vol. 2). the commentary provides background material and guidance to the requirements of the standard. the clause numbers of this commentary are prefixed by the letter c to distinguish them from references to the standard clauses to which they directly relate. where a commentary to a certain clause is non-existent, it is because no explanation of the clause is necessary. appendix ca includes a list of additional definitions used in this commentary, and appendix cb includes a list of additional symbols used in this commentary. notwithstanding the general copyright provisions applicable to all australian standards as detailed in the standard, the standard contains intellectual material provided by other parties and permission to reproduce that material may be conditional on an appropriate royalty payment to standards australia, or the other party, or both. acknowledgement standards australia wishes to acknowledge and thank the following members who have contributed significantly to this commentary: john holmes bill melbourne chris letchford andrew king steve reid accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:20023 contents page section c1 scope and application c1.1 scope.5 c1.2 application5 c1.3 referenced documents.5 c1.4 determination of wind actions5 c1.5 units7 c1.6 definitions.7 c1.7 notation.7 section c2 calculation of wind actions c2.1 general.8 c2.2 site wind speed 8 c2.3 design wind speed 8 c2.4 design wind pressure and distributed forces11 c2.5 wind actions.12 section c3 regional wind speeds c3.1 general.14 c3.2 regional wind speeds (vr).14 c3.3 wind direction multiplier (md) 14 c3.4 factors for regions c and d (fc, fd).15 section c4 site exposure multipliers c4.1 general.16 c4.2 terrain/height multiplier (mz,cat).16 c4.3 shielding multiplier (ms) 24 c4.4 topographic multiplier (mt) 25 section c5 aerodynamic shape factor c5.1 general.28 c5.2 evaluation of aerodynamic shape factor 28 c5.3 internal pressure for enclosed rectangular buildings28 c5.4 external pressures for enclosed rectangular buildings.29 c5.5 frictional drag forces for enclosed buildings.30 section c6 dynamic response factor c6.1 evaluation of dynamic response factor32 c6.2 along-wind response for tall buildings and towers34 c6.3 crosswind response35 c6.4 combination of along-wind and crosswind response38 accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:20024 appendices cadefinitions.41 cbnotation.42 ccadditional pressure coefficients for enclosed buildings .44 cdfreestanding walls, hoardings and canopies 46 ceaerodynamic shape factors for exposed structural members, frames and lattice towers49 cfflags and circular shapes .53 cgaccelerations for wind sensitive structures.54 accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 copyright 5 standards australia/standards new zealand australian/new zealand standard structural design actionswind actionscommentary (supplement to as/nzs 1170.2:2002) s e c t i o n c 1 s c o p e a n d a p p l i c a t i o n c1.1 scope this commentary is intended to be read in conjunction with as/nzs 1170.2:2002. the commentary includes explanations of the provisions and in some cases suggests approaches that may satisfy the intent of the standard. commentary clauses are not mandatory. lists of references are also given at the end of each section for further reading. the principles for determining wind actions given in the standard are generally applicable to wind loads on any structure. the specific data given for the cases outlined (e.g., aerodynamic shape factors) are based on research involving wind tunnel testing, measurements in the field and calculation using established principles of fluid mechanics. they are generally relevant for simplified situations; however, the values derived are applied to real design cases using methods that account for such uncertainties (see also clause c1.4). the standard does not attempt to predict the effects of possible future climatic changes, as the evidence for changes in wind speeds is inconclusive. for the effects of tornadoes, wind speeds can be significantly higher than those given in the standard. the effect of debris is also significant. such wind events are not covered due to their small nature and low occurrence in populated areas of australia and new zealand. structures that extend over considerable distances, such as powerlines or pipelines, may be affected by tornadoes if the usual path of such events is across the structure. the methods given in the standard are not sufficient for some structures such as electricity transmission structures, bridges, lattice towers, cranes and similar, as specific details necessary to perform calculations are not given. in these cases, the standard may be used to determine site wind speeds or design wind speeds, but other standards should be referenced for detailed factors. c1.2 application c1.3 referenced documents documents referred to in this commentary are listed at the end of each section. c1.4 determination of wind actions this clause includes the performance statement for wind forces and states that the remainder of the standard is a solution that is deemed to comply with this statement within the context of the standard. a general background to the wind loading of structures is given by holmes (ref. 1). accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 copyright 6 the following methods can be regarded as satisfactory means of establishing alternative solutions to those given in the standard, and where used, these methods should be thoroughly documented via a special study in order to achieve an equivalent level of confidence in the resulting loads: (a) reliable references used consistently with the clauses of the standard. (b) reliable data on wind speed and direction corrected for the influence of terrain, topography and neighbouring buildings where necessary. (c) the use of a detailed probability analysis for the effects of wind direction as an alternative to the methods for determining the effect of direction on wind outlined in section 2 and clause 3.3. (d) wind tunnel or similar tests carried out for a specific structure or reference to such tests on a similar structure, together with applicable clauses of the standard. (e) calculations by use of computational fluid dynamics, which have been verified and calibrated against full-scale or wind-tunnel measurements. where properly conducted wind tunnel tests on a specific structure have been carried out, or where reference to such tests on a similar structure is used, the forces so determined should be used instead of those determined through the provisions of the standard. testing by wind tunnel or other method for the purposes of determining forces and pressures, wind tunnel tests, or similar tests employing a fluid other than air, shall be considered properly conducted only if the natural wind has been modelled for the appropriate terrain categories to take account of (a) the variation of wind speed with height; and (b) the scale and intensity of the longitudinal component of the turbulence. notice shall be taken of (a) the effects of reynolds number where curved shapes are involved; (b) the appropriate frequency response of force and pressure-measuring systems; and (c) scaling of mass, length, stiffness and structural damping where measurement of dynamic response is involved. wind tunnel testing for buildings in australia and new zealand should normally follow the procedures of the quality assurance manual of the australasian wind engineering society (ref. 2). further information is given in asce and in reinhold (refs. 3 and 4). uncertainties for engineering quantities in the estimation of wind loading, the design wind pressure is a value calculated from a number of input variables. table c1.4 gives estimates of the coefficient of variation for some of the variables given in the standard. accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 copyright 7 table c1.4 coefficients of variation for engineering quantities given in the standard coefficient of variation v50v500quantity region aother regionsregion aother regions vr0.05 to 0.100.1 to 0.150.1 to 0.150.2 to 0.25 mz,cat0.100.15 ms0.20 mt0.100.15 cfig0.15 cdyn0.10 md0.05not applicable c1.5 units the units used in this standard are those generally used in engineering practice. a consistent use of units reduces the chance of errors occurring. si units are metres, seconds, newtons and pascals. c1.6 definitions see appendix ca. c1.7 notation see appendix cb. references 1 holmes, wind loading of structures, spon press, london, 2001. 2 awes, quality assurance manual, wind engineering studies of buildings, australian wind engineering society, 2001 (awes-qam-1-2001). 3 asce, wind tunnel model studies of buildings and structures manual and reports on engineering practice, no 67, american society of civil engineers, new york, n.y., 1999. 4 reinhold, t., (ed.), wind tunnel modelling for civil engineering applications, proceedings, international workshop on wind tunnel modelling criteria and techniques in civil engineering applications, gaithersburg, maryland, u.s.a., cambridge university press, april 1982. accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 copyright 8 s e c t i o n c 2 c a l c u l a t i o n o f w i n d a c t i o n s c2.1 general the general layout of the standard is based on the format set out in iso 4354 (ref. 1). the only departure is that the national wind information is given in terms of wind speed rather than as a wind reference pressure. it was recognized that the site exposure factors are universal in their derivation and for that reason they are kept separate. this arrangement provides for direct comparison between national codes and, at the same time, allows for the site exposure factors to be used in other calculations. iso 4354 (ref. 1) gives the values for the effects of the site as cexp, which effectively equals the square of the factors covered in section 4 (mz,cat ms mt)2. it also assumes an all- direction wind effective reference pressure (qref) which effectively equals (0.5kair) vr2. therefore, this standard relates to iso 4354 (ref. 1) as follows (see also equation 2.4(2): qref cexp = (0.5kair) vdes,2. . . c2.1(1) the notation used for pressure in this standard is p instead of the w used in iso 4354. in iso 4354, the definitions of reference pressure and exposure factor do not provide for different types of wind, such as synoptic, thunderstorm, downburst or cyclone (e.g., profile shape is not allowed for). this is incorporated into the exposure factor (part of mz,cat in as/nzs 1170.2). strictly speaking this belongs in the wind speed section (i.e., in the determination of qref) to enable true international standardization of the exposure factor. c2.2 site wind speed to determine the site wind speed (vsit,), the regional wind speed (vr), based on real gust wind speeds measured at a large number of stations across the region, is multiplied by factors that take the following effects into account: (a) mdthe variation of wind speed at a location with compass direction depending on the type of commonly occurring wind events and the weather patterns in which they arise. (b) mz,catvariation of wind speed with height (related to the speed for terrain category 2 at 10 m height). this includes consideration of the type of wind event, e.g., cyclones or synoptic winds such as thunderstorms. (c) variation of mz,cat at a site due to changes in roughness of the surrounding land or water. this effect also varies with height. the factor varies with direction as the terrain roughness varies with direction. (d) mtmodification of wind caused by topographic features such as mountains and hills. again the factor may vary with direction at a site. (e) msshielding caused by adjacent structures. c2.3 design wind speed the site wind speed (vsit,) is independent of the type or shape of the proposed structure. the design wind speeds (vdes,) are the wind speeds for directions normal to the faces of the structure in its chosen position on the site. the design wind speed for a structure includes both the directional considerations and variations with height. therefore, the value depends on the height, shape and configuration of the structure. accessed by university of south australia on 29 jan 2008 as/nzs 1170.2 supp 1:2002 copyright 9 either the design may use detailed direction effects, in which case the most severe site wind speed profile for each of the four orthogonal axes of the building are required to be used during the design process; or the design may use a simplified, more conservative, non- directional approach. the conservatism is introduced by using a non-directional wind speed, which is the maximum specific directional wind speed, and by assuming this value applies for all directions. the effects of height, terrain, topography and shielding should still be considered to determine the pressures on the faces of the structure. where the form of the structure is complex, more directions (e.g., 16 directions) could be considered where data is available. one example of this is the design of hoardings and walls, where values are given for the 45 degree wind direction. determine design wind speed using one of the following procedures: (a) where a structure orientation is