A girder is supported by two horizontal cylinders. The green cylinder represents a load on the deck between the two supports. The bending moment acting on the given cross-section of the girder is represented by the height of the triangle, the tip of which runs along a parabola while the load is shifted. A dashed red line represents the allowable moment, which is proportional to the thickness of the girder, which in turn is proportional to the strength. You can change the mass of the loading cylinder or shift it on the deck.