Pretty, right? It’s the rear of the sanctuary of the Hallgrímskirkja in Reykjavik. That clean gothic vaulting is reinforced concrete. And that sentence contains everything that’s wrong with the concept of “structural honesty” in architecture.
The vault form was developed thousands of years ago for use with unreinforced masonry, and is a fantastic way to span a roof using only compression in the structure. Reinforced concrete, on the other hand, is a modern material capable of withstanding any loads we design it for. There’s no structural reason to use that geometry with that material. There’s no mystery as to why reinforced concrete was used: it’s the common building material in Iceland. I’d guess that there’s no mystery as to why the gothic vaulted form was used: it’s associated with churches in most people’s minds and so the architect felt it was appropriate. Maybe he also thought it was good-looking.
Because the vaulting isn’t acting solely in compression, it doesn’t need buttresses, and there aren’t any. Because it’s cast-in-place concrete, there are no visible joints.
A strict interpretation of structural honesty leads to the conclusion that this is a terribly dishonest building. The concrete is formed in imitation of stone and the geometry is based on non-existent structural action. A more liberal interpretation might say that there is honesty in the fact that the unneeded buttresses are missing and in the exposed face of the structural concrete. And my interpretation is that a theory that gives two contradictory results on a single building isn’t worth using.