Why we need to consider only 3 cases when we want to find the max positive shear ? IMO , We know that the max shear will only occur when the 67.5kN act on B , am i right ? There is no need to consider 3 different cases .....Or my concept is incorrect ? If so , can anyone show me the example where the max shear will occur at B although the max concentrated load doesn't act on the particular point ?
Sure. Change your wheel loads to (from left to right): 99kN, 99kN, 100kN, 1kN.
When the first wheel has just reached B (Case 1), you have its 99kN at max influence, the second wheel's 99kN at a high positive influence, and the third 100kN wheel at a small positive influence.
Compare to Case 3 when the third (and maximum) wheel reaches B: the 100kN wheel at max influence only provides slightly more positive effect than the 99kN wheel did in Case 1, but instead of having two big wheels with positive influence we instead have two big wheels with negative influence.
It's clear from inspection, without running the numbers, that Case 1 is more onerous. If you change the wheel loads a bit, you can make it less obvious, and need to run the numbers to confirm which is more onerous.