A railing looks like the simplest component on a building: a line of posts, a handrail, some infill. Yet almost every visible feature of a finished railing system — the thickness of the posts, the span between them, the size of the glass panels, the depth of the base shoe, the way the whole assembly is anchored into a slab — can be traced back to two numbers that appear long before the first shop drawing is issued: the required height and the required load. Designers who treat height and load as a final code check usually end up redrawing details late in the project. Designers who treat them as…