An analytical method for accumulative plastic deformation under random loadings is described. The analysis is based on a hypothesis of accumulation law and stochastic equations for randomly accumulated plastic deformations. As analytical models, cumulative behaviors of plastic strains for Steel and Steel Reinforced Concrete members under random curvature amplitudes with constant axial force are treated. Analytical expressions and some numerical results are presented with Monte-Carlo simulations. Detailed behaviors on the analytical results are discussed in the following paper [II].