Risk Content in Some Existing Geo-Codes

For many years, civil engineering design was based on the Allowable Stress Design (ASD) approach sometimes referred to as the Working Stress Design (WSD) approach. This traditional approach is deterministic and uses a single factor of safety defined as the ratio of the strength over the applied stress. In this way, ASD provides a certain level of safety by limiting the applied stress to a fraction of the maximum stress that the material can resist. In the 1980s, the emergence of structural reliability led to the development of Load and Resistance Factor Design (LRFD) as the basis of the new structural design codes. Consequently, the single factor of safety was replaced by a set of individual load and resistance factors to separately account for variability and uncertainty of the load and of the resistance. LRFD can be used to design structures with a desired reliability level or a target probability of failure. LRFD has also been used to develop geotechnical engineering codes, especially when it comes to foundations. Since the 1990s, the use of probabilistic methods in geotechnical engineering design has been increasing significantly. Today, the most commonly used tool for reliability-based design is LRFD, which has been adopted by many geo-structures design codes, including AASHTO LRFD Bridge Design Specifications, the Australian Standard for bridge design (AS 5100), the Canadian Highway Bridge Design Code (CHBDC), and Eurocode 7. Nevertheless, it has been recognized that the target reliability in LRFD should be adjusted to consider the consequences of a potential failure. These consequences can be described in terms of loss of life, environmental impact or economic loss or a combination of all three. A review of many existing geo-codes reveals a definite trend towards a risk-informed design where the risk includes not only the probability of failure but also the consequence of the failure. This paper defines risk in civil engineering, summarizes the extent to which the concept of risk is included in some existing civil engineering design codes, applies risk-based design concepts to a simple foundation design problem, and finally presents the authors' opinion on the development of risk-based designs as the next step beyond the reliability-based design era.

Keywords: Risk, reliability, factor of safety, design codes.