Influence Lines for statically determinate beams
This video (given below) by Dr. Structure explains the influence line of moments considering the case of statically determinate beams.
Influence lines are great tool to determine the internal force in a structure subjected to moving load. In this example a bridge is shown having mid-span denoted by point G. In order to get maximum positive and negative moment at this point both quantitative and qualitative methods are presented.
Qualitative method is quick and can be of great use in order to understand how influence line will work. Quantitative method is more rigorous and is used to give exact results.