Queenslanders living in cyclone regions don’t need to be told how important it is to have the right foundations and structural settings for their homes. For those looking to build a new home in this region it is helpful to know where to go for up to date information which outlines what is required by various governmental bodies to meet C2 requirements.
The Building Code of Australia requires buildings to be designed and constructed to resist the design wind forces. This means that all cladding, windows, doors and garage doors must be built to resist the possible combinations of design wind pressures at the design wind speed. It also requires all structural elements to be designed and constructed to resist the wind pressures.
All building work should be undertaken by licensed tradespeople. Building work may also need a building approval and you may check whether this is the case for your house with a building certifier or council or the Building Services Authority.
Site design wind speed or classification
The first step is always to determine the wind classification of your house site:
1. Where the house complies with the scope of AS4055,then the wind classification will be C1, C2, C3 or C4.(An amendment has been proposed to AS4055 that will use the maximum slope of a hill or ridge to determine the topographic class of the site and it is recommended that this be used in practice).
2. In all other cases, AS/NZS1170.2 is to be used and will result in site design wind speeds. These are then used to determine wind pressures assuming a dominant opening on the worst surface for each building element. The site wind classification or wind pressures should then be used to specify the required performance for all structural elements of your house.
When selecting building elements, consideration should be given to wind classification and pressures to determine suitability. This includes external wall and roof cladding, windows, doors, garage doors and soffits.
It is important that each component of your house is matched to the wind requirements for the site. Higher exposure sites will require elements that have a higher wind rating. Loads on building elements are also a function of their location in the building. In cyclone areas it is not possible to predict whether an element will be on a windward, leeward or side face, so all elements must be able to perform regardless of the wind direction. However, their location on a wall can make a difference. For example when ordering doors or windows for houses, it is necessary to indicate the wind classification and whether or not the element will be close to a corner of the building.
All of the building’s structural elements provide a load path to resist the wind forces on your house and transmit them to ground. Each connection in your house structure will be required to resist wind forces when the wind is coming from one or more directions. Assistance from an appropriately qualified person should be obtained for specification details and supervising building work.
Footings provide the link between your house and the ground. Footings transmit lateral forces and should be designed to have sufficient weight or embedment to resist uplift forces.
Sheds are often very light, yet have substantial wind loads. This means that they must be fastened together securely and anchored to a footing system that can resist the substantial uplift loads. If you have a shed, it is vital that it is designed to the correct wind loads and is designed for internal pressures that may result from an opening after failure of windows or doors. All components of sheds – doors, windows or roller doors – must also be specified using the site wind speed.
Many ancillary items including fences, guttering, vegetation and water tanks are also subjected to significant pressures in cyclonic winds. They should all be detailed to reduce the potential for damage to those items or to other buildings if the ancillary items become wind-borne debris that may damage your or your neighbors’ homes. It is very difficult to design cost-effective fencing that can resist the lateral forces of wind loads. However, it is possible to design the base of fence posts so that if they fail, they still remain attached to the ground. In this way if your fencing fails during a cyclone event it does not become wind-borne debris and will still be fastened to its footings.
Guttering is not normally considered a structural element and can fail in tropical cyclones. The cost of its repair can prove substantial where scaffolding is required and the lost guttering can become wind-borne debris. However, increasing (doubling) the number of guttering clips will greatly improve its wind resistance. Full water tanks have sufficient weight to resist the wind forces on most tanks. However, empty or partially full tanks can fail by deformation of the tank itself or by overturning of the complete tank. Tank stands and footings should be appropriately designed and constructed for these severe wind loads. It is best to make sure that your tanks are full before the approach of any cyclone.
Buildings in storm tide zones
If you live near the sea, other features of your house should be planned for to avoid the effects of storm tide. Part 1 in this series provides information on the construction of houses located in storm tide prone areas. The measures in Part 1 should be applied in addition to the recommendations of this guideline. Where construction is in a storm tide prone area, it is recommended that all metal components are well protected against corrosion. This should be considered for both exposed and hidden components.