Define the Influence line-beam shear?
1. Replace point of interest with a link element of nominal length. If the point of interest is a reaction, place a hinge at that point and lift the hinge upward a unit distance.
2. Push the two ends of the beam together at the link until it is vertical. Thus the total beam length has not changed. The slope of sections to the left/right is the same.
3. Determine the ratio of δleft and δright.