A rational procedure for determining design load combination and safety format is proposed, in which reliability-based structural design is developed on the basis of limit state probabilities. Load factors or safety factors associated with each design load combination in a safety format are optimized numerically through the objective function so that uniform level of structural reliabilities evaluated by limit state probability can be realized to the target, regardless of structural types and

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... mensions. Conceptual diagram of limit state probabilities is introduced to interpret the structural reliability, and the methodology to estimate the factors is showed with numerical examples using structural models and load combinations.