Reliability Based Design: An Australian Experience

Engineering design involves the application of scientific and mathematical principles to provide functional solutions to technical problems while considering the limitations imposed by regulations, safety, practicality, time and cost. Allowable Stress Design (ASD) traditionally has been used by Australian geotechnical practitioners, and the introduction of Limit State Design (LSD) in the Australian Standards has not resulted in a smooth transition from the conventional method and remains a topic of debate in the industry. Reliability Based Design (RBD) is the next generation of geotechnical design that is not only physically and mathematically self-consistent, but also is compatible with the use of RBD for structures which makes it ideal as an inter-disciplinary common language. Therefore, Reliability Based Design is actively being introduced into design codes around the world as the main concept for the development of the design codes, standards and guidelines. From personal experience and observations, as an Australian geotechnical practitioner, a few examples of the application of ASD and LSD methods in small- and large-size projects are presented. Examples include critical design issues such as misuse of ASD and LSD; poor identification of modes of failure, adoption of design parameters, interpretation of safety factors and interpretation of the contents of key Australian Standards, such as AS 2159, AS 5100.3 and AS 4678. Some thought-provoking psychological biases and fallacies are presented, then some showcase solutions by RBD are offered and finally some suggestions are provided to improve the Australian Standards to benefit from this design method. Some challenges in employing RBD in routine geotechnical practice are discussed; however, detailed descriptions are beyond the scope of this paper.