Optimization method of step-by-step equivalent plane method

The invention discloses an optimization method of a step-by-step equivalent plane method. The optimization method comprises the following steps: step 1, determining a normal random variable and establishing a performance function of each failure mode in a portal frame structure; step 2, obtaining a reliability index of a single failure mode; step 3, obtaining a linear structure performance function with a unit coefficient vector; step 4, sorting the failure modes according to a descending order of the correlation coefficients; and step 5, calculating the failure probability of the series portal frame structure system. According to the method, one failure mode is used for equivalently replacing two failure modes one by one, the problem of solving the reliability index of the multi-failure-mode series portal frame structure system at a time is converted into the problem of solving a series of reliability indexes of the two failure modes, and the complexity of the solving process is reduced; and the method has r