A Comparison of Load Rating Methods for Steel Truss Bridges

Three methods are available under the American Association of State Highway and Transportation Officials specifications to load rate bridges. These methods are the Allowable Stress Rating, Load Factor Rating, and Load and Resistance Factor Rating methods. General claims have been made that a 10% to 20% increase in live load capacity is gained from using one method over another on steel girder type bridges depending upon the live load to dead load ratio. This paper will present the results of a load rating analysis of two historic steel truss bridges utilizing the aforementioned rating methods. The findings assist the bridge engineer in understanding the effect rating methods have on determining load capacity of tension and compression members of a Warren Pony Truss bridge and a Baltimore Through Truss bridge.