Load Rating of Steel Girder Structures

Under bridge rehabilitation of an existing bridge originally designed in accordance to AASHTO Standard Specifications, the Colorado Department of Transportation (CDOT) currently requires a Load and Resistance Factor Rating (LRFR). The dual I-70 bridges being rated are located near Avon, Colorado. They were constructed in 1972 and carry two lanes of traffic over a railroad line, the Eagle River, and US 6. The bridges are five-span welded steel plate girders with 56° skewed supports. These bridges have been studied for the assessment of different structure rehabilitation alternatives, crack prevention strategies to reduce distortion induced fatigue, and crack repair options with the intent of increasing the remaining service life. In this paper we explore live load distribution factor sensitivity when steel girder bridges have a high skew angle and different configurations of cross frames, and their effect on the bridge load rating. A full load rating was performed that included an analysis of the inventory, overload, posting, and permit cases. This task was broken into two subtasks: Distribution Factor (DF) analysis and Load Rating analysis. The DF analysis evaluated live load distribution from AASHTO LRFD Equations and refined FE analysis methods using both SAP2000 and Virtis NSG applications. The results were compared and optimized to more accurately evaluate live load distribution for the rehabilitated structures. The Load Ratings of the rehabilitated bridges using the optimized distribution factors and the bridge load posting requirement for the Colorado State Legal Trucks were evaluated.