Question

Let's just say some random guy approaches you on the street asking what an "influence line" is. He claims to be a structural engineer but has never heard of such a thing. How would you explain it to him? Try to explain in laymen's terms while still appearing competent

Answer 1

Influence line is very important in structural engineering.Simply influence lines are used for representing the variation of reaction forces,shear forces,bending moment and deflection of a member.

Influence lines are lines used for representing the variation of a function at a given point on a structural member due to the application of a unit load at any point on that structural members.When a load is applied or moving through a structural member such as beam or truss,some function may occur due to the application of a load.Usually influence lines are used for representing the variation of functions like reaction,shear force,bending moment,axial force and deflection.Influence lines can be used even when the load applied on the structural member is a unit load or multiple loads.The graphical representation of the variation of the functions such as reactions at supports,shear force,bending moment ,axial force at a given point on a structural member like beam or truss when it is applied by a unit load or multiple loads.Influence lines can be used for representing the variation in functions if the load is static or moving.Lets have look at the concept of influence lines with an example.This example deals only with the variation of reaction forces on a beam when a load is moving through it.

Above given is the basic structure for a beam and here the variation of reactions is represented by a diagram.The diagram is influence line diagram and the lines in the diagram showing the variation of reaction forces is the influence lines.Similarly the variation of shear force,bending moment and deflection can also be represented by using influence lines.

The practical application of this influence line diagram is that if you take a bridge there are so many moving loads that means the vehicles are going through the bridge.Influence lines can be used to represent the variation of different functions such as shear force,bending moment,deflection,reaction forces etc at different points on the bridges .From this diagrams the point of maximum bending moment or deflection can be identified so that the structural engineers can redesign that portions.A designer analyses the beams for various positions of loads so as to get maximum shear force and bending moment values due to moving loads.To identify the positions of loads for maximum shear force and bending moment at specified sections the influence line can be used.