Reliability-based calibration of load duration factors for timber structures

The load bearing capacity of timber structures decrease with time depending on the type of load and timber. Based on representative limit states and stochastic models for timber structures, load duration factors are calibrated using probabilistic methods. Load duration effects are estimated on basis of simulation of realizations of wind, snow and imposed loads in accordance with the load models in the Danish structural codes. Three damage accumulation models are considered, namely Gerhards model, Barrett and Foschi's model and Foschi and Yao's model. The parameters in these models are fitted by the Maximum Likelihood Method using data relevant for Danish structural timber and the statistical uncertainty is quantified. The reliability is evaluated using representative short- and long-term limit states, and the load duration factor kmod is estimated using the probabilistic model such that equivalent reliability levels are obtained using short- and long-term design equations. Time variant reliability aspect is considered using a simple, representative limit state with time variant strength and simulation of the whole life time load processes.