The paper studies two alternative formulations of Reliability-Based Optimization (RBO) problem for truss structures subject to various load cases. The considered RBO problem concerns minimization of structural volume under probabilistic reliability constraints. For the reliability analysis, FORM and Ditlevsen bounds methods are employed. The design variables are taken as cross-sectional areas of truss bars. The constraints imposed on reliability indices of failure elements and failure systems are considered. The failure due to various load cases is modeled as a set of limit states (elemental approach) or as a system of limit states (system approach). A numerical example of spatial truss is presented to illustrate two alternative formulations. Results of optimization are compared and conclusions are drawn.
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