This study develops practical models and methods for the assessment of safety and capacity rating of existing steel plate girder bridges based on the reliability methods. One of the main objectives of the study is to propose a practical but realistic limit state model for safety assessment and LRFR rating criteria, which explicitly incorporates the degree of deterioration and damage as well as actual condition of steel plate girder bridges in terms of the damage factor and the response ratio. The damage factor proposed in the paper is defined as the ratio of the current estimated stiffness to the intact base-line stiffness of a member. Also, an attempt is made in the paper to improve the reliability analysis of the plate girder bridges by simultaneously considering flexure and shear failure modes as a simple system reliability model. Based on the observation and the results of applications to existing bridges, it may be concluded that the proposed methods for the assessment and capacity rating models, which explicitly account for the uncertainties and the effects of degree of deterioration or damage, provide more realistic and consistent safety-assessment and capacity rating.