Standards related to the design of structures generally follow the deterministic approach. The advantage of this approach is that with a limited set of formula to be applied quickly, it allows for a sufficiently secure design with regard to regular loads acting on a structure over its regular useful life.
In limit states, when a structure is subjected to additional loads or quality loss and violates the safety standards according to the deterministic approach, the deterministic approach itself can no longer be applied to determine whether the structure is still sufficiently secure. Limit states in this context are for example ageing material, significant corrosion, and additional loads not accounted for in the initial design.
In this case the probabilistic approach can be applied, calculating the failure frequency for the structure. It has been introduced some 20 years ago into the design and assessment of structures. Compared to the deterministic approach, the calculation of the failure frequency is a complex process that, however, can be carried out safely. A problem has long been the setting of widely accepted limits for the failure frequency.
The ISO has worked on this issue for many years and the standard ISO 16708: „Petroleum and natural gas industries – Pipeline transportation systems – Reliability based limit state methods“ was modified frequently in its draft status over a long period of time. In 2006 it has been adopted as DIN EN ISO 16708 as a national standard in Germany.
The scope of DIN EN ISO 16708 allows for the deterministic and the probabilistic approach to be applied in parallel. The standard has a high level of acceptance among authorities, experts and specialist engineers. In today’s practice the deterministic approach is applied in the design, construction and assessment of entire systems whereas the probabilistic approach is applied for complex issues regarding additional loads, quality loss or economical analyses.
The normative definition of the probabilistic approach along with applicable limits for the failure frequency is highly welcome and widely accepted regarding all complex issues described above. In everyday engineering, however, the deterministic approach is still widely used in the design and construction of structures to allow for a safe and simple process and for legal certainty.