Reliability of timber structural systems: a review

The estimation of reliability in timber structures is a procedure complicated by the time-dependent nature of timber failure, coupled with the complex structural behaviour of timber systems. Prior to the implementation of probabilistic concepts, design recommendations given in British Standards were based on permissible stress design, and a similar procedure of allowable stress design employed in the United States. While permissible stress specifications are easy to use, their application results in an inability to achieve a desired uniform level of reliability. Such situations have led to the implementation of probability-based limit states such as load and resistance factor design (LRFD), based on calculating the ‘probability of failure’. The statistical variations and uncertainties in loading and material properties are now established and incorporated into the design process. This change in design philosophy has resulted in the development of new design standards such as Eurocode 5: Design of Timber Structures which is to replace the permissible stress codes of BS 5268. A review of the development of limit states design has been undertaken, and includes the design methods employed prior to the introduction of probability-based design. This review focuses attention on the LRFD format, and although this is not the only probabilistic format available, it is the format that has evolved as the most widely adopted technique