Review of Live Load Impact Factor for Existing Truss Railroad Bridges in the United States

The current American Railway Engineering and Maintenance-of-Way Association (AREMA) Manual provides live load impact formulas for the design of steel railroad bridges. The only variable in those formulas is span length and do not include other parameters that bridge engineers know affects live load impact factor. Years of use in practice and research have shown that these formulas are reliable, safe and simple to apply, though often very conservative. In order to make the nation’s transportation more efficient and energy efficient, a significant effort is underway in the U.S. to enhance its railroad infrastructures. Bridges built before the 1950s, many of which are still in service, were designed to sustain the effects of steam engine hammer blow, and consequently slow speed. Yet, most of these bridges may not be replaced and may be required to carry high speed passenger equipment. This raises the question of what effects higher speed trains will have on old, long span truss steel bridges. This paper presents finding from the detail literature review on the current live load impact factor on truss railroad bridges and its implication to the future.