The classical theory of waterhammer is a well-known and accepted basis for the prediction of pressure surges in piping systems. In this theory the piping system is assumed not to move. In practice however piping systems move when they are loaded by severe pressure surges, which for instance occur after rapid valve closure or pump failure. The motion of the piping system induces pressure surges which are not taken into account in the classical theory. In this article the interaction between pressure surges and pipe motion is investigated. Three interaction mechanisms are distinguished: friction, Poisson and junction coupling. Numerical experiments on a single straight pipe and a liquid loading line show that interaction highly influences the extreme pressures during waterhammer occurrences.