Influence Line

Influence Line:

It is the diagram showing the variation in the shear, moment, reaction, stressin a truss member or any other direct function due to a unit load moving across the structure. An influence line is constructed by plotting direct underthe point, where the unit load is placed. An ordinate the height of which represents to some scale the value of the particular functionbeing studied when the load is in that position.

For determinate structures influence lines are always straight lines.

}First>>> Decide how the stress is to be calculated.

}Second>> Move the unit load along the span and observe the effect on the stress in question.

}Third>>> Calculate the controlling ordinates on the influence line and complete the diagram.

Muller-Breslau Principle :

The ordinates of an influence line for any force are proportional to the deflected shape of the structure produce by removing the capacity of the structure to carry the force and then introducing into the modified structure. A displacement the corresponds to the restrained removed.