Further steps towards a quantitative approach to durability design

Abstract
This paper presents further steps in the development of reliability-based approaches for the durability design and service life prediction of building components which integrate the requirements of safety, serviceability and durability. In general, the load and resistance should be modelled as stochastic processes and the resulting durability problem is formulated in a time-dependent probabilistic format. Using the classical reliability approach, the resulting time-dependent reliability problem is transformed into a time-independent reliability problem through the adoption of an extreme-value probability distribution for the maximum lifetime load. The resistance degradation and its variability are included in the model, and the probabilistic design problem is transformed into a deterministic (or semi-probabilistic) problem using the first-order second moment theory. This semi-probabilistic integrated approach to durability design and prediction overcomes the shortcomings of the empirical factorial approach and the complexities of a fully time-dependent probabilistic method. An alternative approach using stochastic process theory is proposed to formulate the durability design problem as a crossing problem for which the probability of failure within the component lifetime is obtained from the first-passage probability for the stochastic process. In addition, a service life-based formulation of the durability design and prediction problems is presented in order to illustrate its equivalence with the performance-based formulation. It is shown that in principle the same probabilistic approaches used for the development of structural design approaches for safety and serviceability are also applicable for durability design. The durability design objective is to keep the probability of failure within a specified time interval (or service life) below a certain threshold value that depends on the consequences of failure of the component or system. It is expected that in the near future, further simplifications of the proposed approaches will be made leading to practical and reliability-based methods to durability design or service life prediction. These simplified methods will be implemented in the design of durable new structures and optimal life-cycle maintenance management of existing structures