Railroad bridges experience dynamic wheel load augment from rolling stock that cross the bridges, due to nominal bridge and suspension dynamics, as well as anomalies such as wheel flats. The level of dynamic augment is particularly high for steam locomotives due to the hammer blow effect associated with the driven wheels. Tests conducted in the early-mid 20th century had quantified some of these effects, and the resulting findings have been part of the impact formulae presented in the AREMA Railway Engineering Manual.

However, there was concern that the impact associated with some of the non-cross counter-balanced, lighter, older locomotives, could be higher than specified by AREMA formulae. This paper describes the methodology and results from a series of tests that evaluated the levels of dynamic augment experienced by railroad track and an exemplar bridge under a set of narrow gauge steam locomotives, and compares the measurements to the design values specified in the AREMA Manual. Vertical and lateral loads on railroad track, and strain levels on multiple critical bridge members were measured under three different classes of light, narrow gauge steam locomotives, over a range of operating speeds and conditions. The tests were conducted on a 120 ft span, through truss bridge, and adjacent track on a tourist railroad.

Dynamic augment values measured during the tests were generally lower than the values expected from AREMA formulae. Similarly, the peak lateral loads measured appear to be nominal and lower than the AREMA prescribed values. However, it should be kept in mind that these results are from tests conducted with three relatively light, narrow gauge locomotives, on specific bridge and track, whereas, the AREMA formulae are intended to cover a wider range of conditions. These tests tend to show that the legacy standards are conservative and are applicable to calculating regulatory required bridge loads where steam locomotives are concerned.

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