Is your Importance Level 1 building really that unimportant?
The Insight
The NCC defines four Importance Levels (ILs) for buildings in Australia, and these in turn set a number of the parameters for structural design including minimum return periods for wind, earthquake and snow loading. The requirements become more onerous as the classification moves from IL 1 to IL 4.
IL 1 is defined as a building or structure that presents ‘a low degree of hazard to life and other property in the case of failure’.
If the structure is not habitable, the automatic assumption may be that it does not present a hazard to life in the case of failure. This is often incorrect.
The Impact
The consideration of the hazard needs to go further than potential occupants (or lack thereof) as structural failures can have flow on impacts on both life and property.
This is particularly relevant in cyclonic areas where a failing structure, even an unimportant one, can create additional windborne debris which can have a devastating impact on surrounding property and property occupants.
In such circumstances, it is appropriate to consider the likely failure mechanism of structures. For instance, fence structures which undergo a ductile failure of the fence posts and essentially lay down without detachment of the posts from the footings or panels from the posts, may be considered as presenting a low risk. Conversely a fence structure in which panels detach from the posts and create debris can present a significant risk.
When considering the use of IL 1 design parameters, engineers must have a clear understanding of the potential failure mechanism of the structure, the circumstances in which such a failure may occur, and the associated broader impact on life and property.
Only when the engineer is satisfied that the hazards presented are low when considered in this context should design parameters be based on IL 1 requirements.