Reliability of timber structures under long term loads

The strength of timber is significantly reduced under sustained load. Exist ing experimental data give a reasonable estimate of the strength reduction as a function of time under constant load. In practice, however, the load l evel is variable in time. For this situation the strength reduction must be estimated on the basis of so called damage theories. In engineering design, effects of load duration are accounted for by a stre ngth reduction factor, k(mod), which depends on the type of load acting on the structure. The loads are classified in terms of their duration time. In a load combination the value of k(mod) is based on the load with shortest duration. For several reasons the scientific basis to determine k(mod) is r ather uncertain. In this paper a probabilistic analysis is made with the purpose to calibrat e k(mod) for timber structures under combination of permanent load and snow load. The analyses are based on Swedish data for snow. A theoretical model proposed in [1] describes the damage accumulation. The analyses show that k(mod) is of the order 0.8 for geographic locations with heavy snow loads a s well as for locations with moderate snow loads. The factor k(mod) was als o found independent of the two practically relevant load combinations studi ed. The results can be used as recommendations for appropriate choice of k( mod) in engineering design.