Concept of Influence line was developed by Winkler in 1867. Influence Line Diagram (ILD) concept is used for analysis of structures that are subjected to variable or moving loads. So in this post you would know What is influence Line Diagram? or what is the difference of influence line diagram with bending moment diagram?.
Structures like bridges, culverts and even roofs of a house is also such structures that have live and moving loads.
What is Influence Line Diagram?
So What is Influence Line Diagram? Influence line is defined as;
“a graph of a response function of a structure as a function of the position of a downward unit load moving across the structure”
A structure is subjected to live load or a dead load i.e. fixed load. Live load includes load of moving objects as well as that of wind load, earthquake load. One important definition of live load is the load that has change in magnitude as well as change in direction in short span of time. Dead load includes the load of structure i.e. self weight and the load of the parts that are fixed with the structure.
The best description of structure response due to dead load is bending moment diagram and shear force diagram. The bending moment diagram and shear force diagram describes variation of bending moment or of shear force across the length of the member.
Live load and influence line diagram
But the best description of structure response for live load is defined by influence line diagram. The influence line represents the variation of either the reaction, shear force, moment or deflection at a specific point in a member as a concentrated force that moves over the member. When the diagram is formulated one can easily determine the most critical live load position for which the structure must be designed for live load.
The ordinates of the influence-line diagram gives you the values of associated reaction, shear or moment, or deflection at the point.
Difference between Bending Moment Diagram and Shear Force Diagram
Influence lines represent the effect of a moving load only at a specified point on a member, whereas shear and bending moment diagrams represent the effect of fixed loads at all points along the axis of the member
INFLUENCE LINE DIAGRAM
BENDING MOMENT DIAGRAM (BMD)
Static and Moving Load
Diagrams show only one point on the beam.
Calculations based on the virtual load.
Straight line only
Calculations do not refer to reactions
Static load only.
Diagram shows the moment at all points on the beam.
Calculations based on real loads.
Straight lines and curves.
Calculations based on the SFD.
Unit : kNm