That rather odd picture was taken inside a brewery that is housed in a nineteenth-century industrial building. The big shiny things are tanks where beer is a-brewing; the piece of wood between them is a post that supports part of the roof. The posts are closely spaced, which is how it can be so slender, and you can see some repairs that were made before the brewery moved in.
Here’s a silly question: how do we know what is structure and what is not? The steel tanks are, quite simply, stronger than the post and could easily carry the roof. They are not, as there is no physical contact between the roof structure and the tops of the tanks. So answer number 1 is: structure is that which is carrying load, regardless of capacity.
Answer 1 is not very satisfactory. We often see pieces of buildings (plaster and lath, for example) carrying load they were not meant to carry and we generally don’t allow that to continue. We want load carried by building elements designed (or at least meant) to do so. So the first draft of answer number 2 is: structure is that which is intended to carry load. But that’s obviously wrong – look at the second sentence of this paragraph – so the second draft of answer number 2 is: structure is that which is intended to carry load and does.
Handrails are intended to carry load and do. Curtain wall mullions are intended to carry load and do. Slabs on grade are intended to carry load and do. Black iron is intended to carry load and does. So answer 2 looks pretty shaky, unless we’re going to define every piece of a building as structure. My four examples in this paragraph all share a trait: the load they carry affects only themselves. Answer number 3: structure consists of those elements which are intended to carry load from other elements and do.
That feels pretty close, although it’s still not exactly right. A wood floor joist carries load from the plywood subfloor, so the joist qualifies but the subfloor does not. Is the subfloor not structure? Same problem with a concrete slab in a steel-frame building.
I’m going to give up here, before I write ten thousand words trying to define something that we all think we know. I’ll end with my favorite answer: structure is that designed by engineers (or our predecessors in the millennia before the profession of structural engineering existed). If I analyze the bracket supporting a bench – which I have done – then that bracket is structure.