Civil Engineering Reference
In-Depth Information
15.2.5
Australian/New Zealand Standard AS/NZS 1170.2
The current edition of the joint Australian and New Zealand Standard for Wind Loads
was issued in 2002 as the first edition of a combined Standard, replacing separate
documents from the two countries. It is a comprehensive document of 88 A4 pages and is
supported by a separate Commentary, and a Guide for designers is also available (Holmes
and King, 2005). It uses a notation and format based on the 1997 International Standard,
ISO 4354.
AS/NZS 1170.2:2002 has an indirect legal status by being called up in the Building
Codes of Australia and New Zealand. These documents and Part 0 of the joint Standards
on Structural design actions must be consulted to obtain the appropriate annual
probability of exceedence for the importance and use of the building, before use of Part 2,
Wind actions.
The basic wind speed in AS/NZS 1170.2 is a 3-s gust measured at 10 m height in open
country terrain, and values are specified for a range of annual probability of exceedence
from 1/5 to 1/2000, for four Regions. For most buildings, excluding those with large
numbers of occupants and important post-disaster facilities, the annual risk of exceedence
for ultimate limit states wind speeds is specified, in the Building Code of Australia, as
1/500.
15.2.6
British Standard BS6399: Part 2:1997
Part 2 of the British Standard BS6399—Loading for buildings is the 'Code of practice for
wind loads' which replaced CP3: Chapter V: Part 2 in 1995. The significant difference
between BS6399: Part 2 and the earlier code of practice is that the basic wind speed is an
hourly mean, instead of the 3-s gust speed used in earlier editions. However, the mean
wind speed is subsequently converted into a gust speed for calculation of design loads, to
take advantage of the quasi-steady model of wind loads. The stated reasons for using the
hourly mean are that it allows more accurate treatment of topography and that it provides
a starting point for calculations involving fatigue and dynamic response.
BS6399: Part 2 provides two alternative methods of calculating wind loads:
1. a 'standard method', which does not use directional wind speed and coefficient data;
2. a 'directional method', which is more complex but generally less conservative.
In this comparison, only the standard method will be discussed, as the other standards do
not have equivalent methods to the directional method.
15.3
Basic wind speeds or pressures
Table 15.1 summarizes the basic wind speed characteristics used, or recommended, in the
six documents. In all cases, the standard meteorological reference position of 10m height
in flat, open country is used.
The ISO Standard, as previously discussed, does not give basic wind speeds or
dynamic pressures. However, it provides a useful conversion method between wind
