Parker Gun Collectors Association Forums - Unique Culvert on Logging Rd

I was never an expert in physics. But I've always appreciated someone with the education and experience who could explain complicated things so that someone like me could understand it. So my questions are:
1. Did someone shape that mass of metal into that shape because it would be better able to support the forces thrust upon it?
2. Or did the forces of nature (frost, heat waves, flood impacts/damage) reshape the culvert and the road crews simply supported it with rock and fill to keep the road relatively level?
3. Did a cagey experienced old road construction guy, without enough budget just know how to keep using a frequently blown out and damaged massive steel culvert?
4. Did they use their practical engineering background to know that a triangular or arch-shaped piece of metal would support 80,000 lb tractor trailer?
5. Why has it not collapsed?

It's questions like these, not politics or other socially popular discussions, that cause me to pause an otherwise awesome bird hunt, take a picture and try and figure out what in heck heavy equipment operators are doing driving over something like this.

One of you metalurgist, physics experts on this forum (whom I greatly respect by the way) must have the answers...please. I estimate the height of the culvert (in it's present shape) to be 12 feet.

p.s. We're going to rename this covert based on some of the responses here such as, "The Pipeular Covert" or "The Art Where You Find It Covert", or "The Spec'ed Culvert Covert" or "The Edvard Munch Anxiety Illness and Death Covert" - all good possibilities!