First, you need to understand the basic theory and procedure of influence line analysis of bridges using MATLAB. The paper by Aldeghaither1 explains this in detail, and you can also refer to other sources such as this and this for more examples and explanations.
Second, you need to define the input parameters for your bridge model, such as the geometry, material properties, boundary conditions, load steps, etc. You can use the data given in the paper or choose your own values. You also need to specify the functions or effects for which you want to obtain the influence lines, such as moments, shears, reactions, etc.
Third, you need to write a MATLAB function that performs the stiffness matrix analysis of your bridge model using the fixed end actions method. This function should take the input parameters as arguments and return the global stiffness matrix and the fixed end actions vector for each load step.
Fourth, you need to write another MATLAB function that calculates the influence lines for the desired functions or effects using the results of the stiffness matrix analysis. This function should take the global stiffness matrix, the fixed end actions vector, and the load and effect positions as arguments and return the influence line values for each load step.
Fifth, you need to write a MATLAB script that calls these two functions and plots the influence lines for different load positions. You can use the built-in MATLAB functions such as plot, xlabel, ylabel, title, legend, etc. to create and customize your plots.
I trust this message finds you well. I would like to extend my gratitude for the valuable information and assistance you provided. Your insights and guidance have been greatly appreciated.