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It is well known that environmental loads, e.g., wind and ice, acting on power transmission lines are highly uncertain, as are the structural strengths of the towers supporting the lines. The design of such systems must take uncertainty into account in order to achieve acceptable reliability at a reasonable cost. The paper presents a simulation-based methodology for the optimal design of a transmission line which considers uncertainties in both environmental loads and structural resistance. The methodology is developed and illustrated for the simple problem of determining the optimal span length required for designing against tower failure. Wind, ice, and tower resistances are simulated over the extent of the transmission line and over the design life of the transmission system. Total expected system cost, along with the estimated probability of lifetime failure, are produced for a range of possible span lengths, allowing an informed decision regarding the optimum span length for the tower strength limit state.