A research on the importance function used in the calculation of the fracture probability through the optimum method

On the basis of the research into original distribution function as the importance function after shifting an appropriate distance, this paper takes the variation of similar ratio of the original function to the importance function as the objective function, the optimum shifting distance obtained by use of an optimization method. The optimum importance function resulting from the optimization method can ensure that the number of Monte Carlo simulations is decreased and at the same time the good estimates of the yearly failure probabilities are obtained.