The AREMA Manual for Railway Engineering recommended values for impact only consider ballast and open deck steel structures and concrete ballast deck structure. AREMA does not suggest using a 200% impact factor for all structures with the rail fixed directly to the deck. An impact factor of 200% is recommended for the design of reinforced concrete slab track systems on ground. This factor recognizes the challenges created by impulse loadings with minimal dampening in this application. Bridge spans provide greater dampening and distribution of loading. Therefore lower impact factors are appropriate for concrete decks with direct fixation on most bridges. The dampening provided is directly related to length. The dampening provided by short spans may warrant a higher impact. An appropriate analysis for the impact for direct fixation of rail on a bridge would depend on the system used to attenuate impulse loading and would only apply to the members directly supporting the rail.
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