Design Wind regions
Sheds are to be designed for maximum expected wind speeds according to pre-designated regions as per the chart below.
Image from Geoscience Australia www.ga.gov.au
Terrain Categories
These wind speeds are then modified according to the surrounding terrain as below
Terrain Categories Descriptions
TC2 Water surfaces, open terrain, grassland with few, well-scattered obstructions having heights generally from 1.5 m to 10 m.
TC2.5Terrain with few trees, isolated obstructions such as agricultural land, cane fields or long grass, up to 600 mm high.
TC3 Terrain with numerous closely spaced obstructions 3 m to 5 m high such as areas of suburban housing.
Importance Levels for Sheds
A risk factor (importance level) is then applied to the potential ramifications of failure and the building will then be engineered in accordance with the acceptability of potential failure.
The BCA defines four Importance levels:
Level 1 - Buildings with a low degree of hazard to life and other property in case of failure
Level 2 - Default level - buildings not assigned levels 1, 3 or 4
Level 3 - Buildings designed to contain a large number of people
Level 4 - Buildings essential to post-disaster recovery or associated with hazardous facilities
Importance Level Examples
BUILDING DESCRIPTION
Farm Shed
Residential shed or garage
Small school shade structure
Produce sales building
Shearing shed
Large commercial storage warehouse
Large (250+) school assembly shelter
Shed housing hospital emergency generator
Emergency vehicle shed
BCA CLASS
10a
10a
9b
6
8
7b
9b
10a
10a
FAILURE CONSEQUENCES
HUMAN HAZARD
PUBLIC IMPACT
Low Low
Mod Low
Mod Mod
Mod Mod
Sub Mod
Mod Sub
Sub Sub
Sub Ext
Sub Ext
IMPORTANCE LEVEL
1*
2
2
2 or 3
2
3
3
4
4
Ultimate Limit Design State
These factors are all put together to provide the wind load the engineer must design for. Examples below.
Source: steel.org.au (used with permission)