# Need help for Influence line plotting

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Tom on 18 May 2024
Answered: Mathieu NOE on 21 May 2024
I am a civil engineering student specializing in Structural Design. I'm working on a project for a "Prestressed Concrete" course, where I am designing a section for a bridge with prestressed reinforcement. As part of the project, I need to first construct envelopes for the shear force and bending moment for the bridge under a loading pattern where a four-axle truck (in addition to the dead load) traverses the bridge, with different weights for the front and rear axles, thus resulting in different forces. The truck can approach the bridge from either side.
I've attached an image to clarify the loading pattern; 'X' in the image marks the truck's position relative to the bridge's edge.
I have developed MATLAB code that plots the shear and moment envelopes based on the influence line method. The problem is that I am getting non-symmetric graphs, which suggests there is an issue with the code.
In the attached image of the code output for the shear envelope, it can be seen that at a distance of 3 meters relative to the beam center (18 meters), the values obtained are not identical, making the graph asymmetric. I would appreciate any help to correct the code. I am open to different methods that could help solve the issue.
Thank you!!

Mathieu NOE on 21 May 2024
hello
I believe there is no mistake in the code
simply you have a fairly crude spatial resolution (fixed to 0.5 m in the code )
I changed that factor to "dr" to make it simpler to change as it appears in several code lines
I tested dr = 0.5 then 0.05 and finally 0.01; of course it takes a liitle bit more time but it's almost not perceptible
as you improve the spatial resolution, your results becomes closer , but there will always be some minor numerical deltas
here with dr = 0.05 :
improve codes are attached
all the best

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