Nowadays, several optimal parameter design tools have been developed for the nonlinear structures. These traditional approaches manage only the expectation values of the design variables, the loading conditions, and the assembly processes, although it is sure that there is variation due to the manufacturing or the assembling errors. The neglect sometimes causes the failure of the structures. There is no doubt that it is a better way to carry out optimal design of structure with consideration of the variation of design parameters and assembly processes. In this study, a technique of reliability design with considering the variation of the design variables was introduced into the optimization method of the SDSS (Statistical Design Support System) to achieve the robust and optimal parameter design for the nonlinear structures, where the second moment method was applied to evaluate the structure reliability. The SDSS was proposed by Yokohama National University, and it is an integrated design system based upon the response surface methodology and the design of experiments. The parameter optimization can be carried out with this system basing upon a small number of case studies. As an application of the proposed approach, the design optimization of a hydraulic servo valve was carried out, and the variation of assembly factors was considered.
