Arches are tricky. The undergraduate way to model arch bridges is as three hinged structures - free to rotate at both ends and with a virtual "pin" in the middl

Arches are tricky. The undergraduate way to model arch bridges is as three hinged structures - free to rotate at both ends and with a virtual "pin" in the middle, in order to make the math tractable. That's the kind of approximation that goes into mathematical models.