Reliability analysis of wood structures and method for determining design strength value of timber

To meet the reliability requirement for structural timber, the performance function for modern wood members was developed and the reliability analysis of the members in bending, compression and tension was conducted. Given a wide range of the coefficient of variation of timber strength, the relationship curves between the resistance partial safety factor and the coefficient of variation which satisfy the reliability requirement were obtained. Following the principle of being safe and reliable, also of being reasonable and economical, the relationship curve under the combination of dead load and residential floor load with a live to dead load ratio of 1.0 was chosen as the reference curve for determining the design strength value. For combinations of dead load and residential floor load or office floor load with a load ratio less than 1.0, a strength modification factor (0.83+0.17ρ, where ρ is the live load to dead load ratio) was introduced; for combinations of dead load and wind or snow load, a strength modification factor of 0.83 was introduced. The relationship between the resistance partial safety factors with respect to the safety class of timber structures was investigated. It is found that timber only with a coefficient of variation of strength smaller than 0.20 is suitable for building structures of safety class one, or the value of the importance factor for design of timber structures can be from 0.8 to 1.2.