# Influence line question for calculating maximum bending moment

I have the following question:

I have no trouble drawing the influence line but I have problems with finding the magnitude. Can someone please explain how to get $$\frac{8}{3}$$? How should I use the unit relative rotation to find the height of the triangles?

This question is to investigate the moment at an internal point due to the influence of the unit load "1". The solution is illustrated below.

You shall repeat the steps to obtain the coefficient at "E".

• Hi r13 thanks for the help. But why is my diagram (lecturer's solution) showing $\frac{8}{3}$? But your third diagram is showing a unit load? Can you explain the third diagram again and about imposing a unit rotation at the location? Many thanks! May 24 at 14:22
• I've shown two methods to construct the influence line about a point that is subjected to a concentrated load (unit load). The first method consists of 4 steps - 1) place the unit load at the point in question, 2) calculate support reactions due to the unit load, Now disconnect the beam at the joint, then 3) for structure left of the point, use the reaction to calculate the internal moment at the point, and 4) obtain the moment coefficient at the tip of the cantilever by the similar triangle method. My third graph represents steps 3 and 4.
– r13
May 24 at 16:10
• What I didn't show is to finish the diagram by repeating steps 3 and 4 for the structure to the right of the point. Once you complete the task, the coefficients should be identical to your lecture note (-4, 8/3, -8/3).
– r13
May 24 at 16:15
• I've eliminated the second method (previously the last diagram) to avoid confusion.
– r13
May 24 at 16:42
• Hi r13 thanks for the comments, what do you mean by the coefficients of the free end? I think my lecturer is using this method (method 2) May 24 at 16:43