Load and resistance factor design for wood structures

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Virginia Polytechnic Institute and State University


Uncertainties in engineering design exist due to the random nature of loads and materials, lack of knowledge, and imperfect modelling of design parameters. Conventional design methods based on deterministic procedures do not always yield designs having consistent safety. In recent years considerable research has been conducted in the use of probability theory for modelling uncertainties in engineering designs and several probabilistic design formats have been developed. Probability based design methods provide a unified procedure applicable to all construction materials, all loads, and all types of uncertainties. Code committees are currently working on the development of the new design codes for various construction materials such as steel, concrete, and wood based on probabilistic concepts.

The objective of this study is to study a probability based design format for wood members. Reliability analysis of wood structural elements such as beams, columns, and beam-columns is conducted, and the risk level is measured by the reliability or safety index, β. Wood members subjected to dead plus live load and dead plus snow load combinations are considered. After conducting a reliability analysis of current designs, a target reliability index is selected. The reliability index is then used in conjunction with the predetermined load factors and load combinations to determine resistance factors. Finally, a design format is proposed for Load and Resistance Factor Design for wood structures.