When preparing load calculations for a slab, the quick method is to define the tributary areas of a slab for the beams/columns below. These are generally taken as lines at mid-span of the slab panels and, depending on the support conditions, at the edges at angles of 30 & 60 or 45 & 45 degrees.
Where do the 60/30 and 45/45 angle ratios come from? Example distributions are shown in the image below.
I know that in reality the ratios depend on the stiffness and end conditions of the various structural members, I also take it that the tributary areas are somewhat affected by the load they are carrying. Would excessive loading on one panel, compared to the adjacent panel affect the areas?