Influence Lines

What am I seeing?

• An influence line shows one response at one fixed place as a unit load moves across the beam.
• The load moves; the section does not. That distinction is the whole idea.
• The influence line for shear at a section jumps by 1 as the load crosses the section.
• The peak ordinate of the moment influence line at section c is c(L−c)/L for a simple span.
• Overhangs create negative regions: load there to relieve the response, not increase it.