When you're talking about something structural you're talking about the way it's built. The simple look of a building can mask its complex structural design.
A skeleton is the structural framework of a body. While we all look different on the outside, for the most part, we all share the same structural components — we're built the same way. Some buildings, such as the Eiffel Tower, reveal their structural design completely. They are nothing but structure. Other buildings work hard to hide their structural qualities. You can discuss the structural nature of ideas too, as in "structural economics" or even human relationships.
