Hi<br><br>i think that you need to change your point of view when studying influence lines.<br><br>When you plot a BMD, you plot bending moment among the x axis. function of the bending moment is only dependant on x (for a given load of course), so M = function(x)<br><br><br>Now, lets make it more complex. suppose that you have a point loading on a beam. but you don't know exactly where it is. To be able to formulate some sort of solution, say it is place somwhere on the beam, at an unknown position "a". <br><br>You can now write M = function (x,a)<br><br>Now you can plot several BMD, depending on the position of the loading (the "a" parameter).<br><br>Ok, now that we made it complex, lets make it easyier:<br><br>Say that you dont want to know the moments on all the beam. but only at a particular point of the beam, say at position "x_fixed".<br><br>The previous moments function becomes M = function(x_fixed,a).<br><br>The only parameter you can vary is "a". and the results that you will get are only valid for the particular x position you chose earlyier (x = x_fixed).<br><br>This is called an influence line, at a particular x ordinate...<br><br>For i don't know what reason, influence line have always been confusing... don't worry.<br><br>