Load Rating of Steel Girder Structures

Abstract

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 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 application. 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. INTRODUCTORY REMARKS Existing bridges that are rehabilitated are required to be load rated using the Load and Resistance Factor Rating (LRFR) methodology. The Colorado Department of Transportation (CDOT) requires that the load rating be completed using the LRFR methodology, even if the structure was originally designed using the AASHTO Standard Specification. Twin bridges located near Avon, Colorado were constructed in 1972 and carry two lanes of Interstate 70 (I-70) traffic over a railroad line, the Eagle River, and US6. The bridges consist of five-span welded steel plate girders with 56° skewed supports. The bridges are exhibiting signs of fatigue due to poor detailing, particularly at the cross frames, and the high skew angle at the supports. They have been studied for many alternatives of rehabilitation and crack prevention strategies to reduce the distortion induced fatigue with the intent of increasing the service life.