Abstract
The probability density function of load effect S, strength R as random variables is needed in order to solve the optimal value of importance factor. In the first case we assume that the probability density functions of S and R are normal distributions because it is probable that variables S and R determined many factors have a normal distribution asymptotically by the central limit theorem. In this case a solution of equation (19) in earlier paper is given by equation (N-10). This solution is shown in Fig.1. In second case, we assume variables S and R have log-normal distributions, that is, the distribution functions of log S and log R are normal distributions. The solution in this case is given by equation (LN-21) and this result is shown in Fig.4.
