A general method for lifetime performance analysis of existing steel girder bridges is presented. Only the superstructure components are considered. The formulation is established by identifying four distinct categories: limit state equations, random variables, deterministic parameters, and constant coefficients. The limit state equations are derived by strictly adhering to the load and capacity formulas and requirements set forth in AASHTO specifications. Generality is pursued by establishing parametric limit state equations such that the formulas are applicable to any type of steel bridge having similar superstructure components. The application of the developed formulation to lifetime performance analysis of four steel girder bridges located in an existing bridge network in Colorado is also presented. Performance analysis results are presented in two main categories: initial reliability indices and lifetime reliability profiles. Once the values associated with random variables, deterministic parameters, and constant coefficients are assigned, component reliability indices for the slab and the girders are calculated for each bridge. Detailed results are presented for an individual bridge and the lifetime reliability profiles are presented for two bridges. For time-variant performance analysis, special emphasis is placed on the corrosion penetration modeling in the girders. An attempt has been made to adopt such models to the atmospheric and environmental conditions of Colorado. Limitations to such an approach are also described. The focus of the paper is on the formulation and the overall methodology rather than the analysis of the results.