Reliability of timber structural systems: a review

The estimation of reliability in timber structures is a procedure complicat ed by the time-dependent nature of timber failure, coupled with the complex structural behaviour of timber systems. Prior to the implementation of pro babilistic concepts, design recommendations given in British Standards were based on permissible stress design, and a similar procedure of allowable s tress design employed in the United States,While permissible stress specifi cations are easy to use, their application results in an inability to achie ve a desired uniform level of reliability. Such situations have led to the implementation of probability-based limit states such as load and resistanc e factor design (LRFD), based on calculating the 'probability of failure'. The statistical variations and uncertainties in loading and material proper ties are now established and incorporated into the design process,This chan ge in design philosophy has resulted in the development of new design stand ards such as Eurocode 5: Design of Timber Structures which is to replace th e permissible stress codes of BS 5268. A review of the development of limit states design has been undertaken, and includes the design methods employe d prior to the introduction of probability-based design. This review focuse s attention on the LRFD format, and although this is not the only probabili stic format available. it is the format that has evolved as the most widely adopted technique.