What are the considerations for applying the impact factor (for AASHTO LRFD) when placing axle loads on a User Defined truck?  

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Answer:

You should apply the impact factor when you put the axle loads on User Defined truck.
The impact factor is not applied to the lane load. You can check the vehicle value by converting the moving load to static loads.
(menu-results-moving tracer-"Write Min/Max Load to File")
You can also check lane factor on '.MOUT' file which you can get after performing analysis.