Influence line

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Influence lines play an important part in the design of bridges, industrial crane rails, conveyor belts, and other structures where loads move across their span.

An influence line represents the variation of either the reaction, shear, moment, or deflection at a specific point in a member as a concentrated force moves over the member. Once this line is constructed, one can tell at glance where the moving load should be placed on a structure so that it creates the greatest influence at the specified point. Furthermore, the magnitude of the associated reaction, shear, moment, or deflection at the point can than be calculated from the ordinates of the influence line diagram.