Outside the Limitations

There have been previous Tech Talk articles about compliance with the National Construction Code (NCC). In Queensland we have recently seen an increase in the number of requests by Building Surveyors for certification of designs by Registered Engineers, so it is timely for a reminder on the limitations of projects without requiring additional engineering.

The NCC sets out the minimum performance requirements for all buildings constructed in Australia. Under the NCC compliance can be achieved by using a Performance Solution and/or Deemed-to-Satisfy (DTS) Solution and will typically require Evidence of Suitability of how this is achieved. DTS provisions are the simplest method (it can be likened to following a recipe) and the performance requirement can be satisfied if using prescriptive documents such as AS 1684 or the use of software that follows the ABCB Protocol for Structural Software.

It should be noted that some Australian Standards such as AS 1684 and the ABCB Protocol for Structural Software have limitations of use. Going beyond these limitations will require additional engineering. Table 1 below lists some of the typical limitations.

Table 1 Limitations

Some of the recent requests for engineer certification in Queensland have been when the overall building height has been greater than 8.5m. It should be noted that just looking at a section view may not indicate that the building height exceeds this level. The 2021 edition of AS 4055 clarified this by introducing the averaged ground level to allow for sites with major earthworks as can be seen by the dashed lines in Figure 1

Figure 1 AS 4055 Geometry Limitations

When outside the limitations of AS 4055 the standard wind classifications may no longer apply and AS 1170.2 can be used to determine the Ultimate Limit State Design Wind Speed. This can then be used to adopt an “equivalent” N1-N6 or C1-C4 Wind Classification when referring to AS 1684.

For projects that go beyond the limitations contact your local engineering department at your nail-plate manufacturer as the various States of Australia have different requirements on the forms that can be used for certification and who can sign these forms.

Dean Ashton
‪Structural Engineer
[email protected]   07 3297 3250