When a joist hangs on a header, it exerts a downward force (action) on the header and the header exerts an upward force (reaction) against it. Newton's third law states that "for every action, there is an equal and opposite reaction."
Reaction is the force generated at bearing supports that counteracts weight of member and loads applied to it.
- End reaction is reaction from support at end of member.
- Intermediate reaction is reaction from support somewhere in middle of member.