My interest in Elgin Cathedral led me to have studied the construction of various arches for different purposes.
Actually that is not entirely true, because long before that I took great interest in arched bridges, and how they stand the test of time, with a just a little maintenance. Also how a bridge constructed without the arch principle is almost guaranteed to fall, or require redone.
There was one in America recently if you recall, which was built alongside an existing bridge. They wondered why the new one fell and the older one still stands? The older one is arched, too far in my opinion, but arched never the less. The bridge, that which fell, was reliant totally on the strength of the steel. To me, that is a definite no, no. Always arch.
We have such a bridge over the river Spey near Fochabers, that is poorly built, where an arch would have been much easier anyway. One must consider sideways movement caused by wind contributing to weakening the structure as well. An old steel arch bridge still stands beside it, but is too narrow for modern traffic, and so it is not kept up to a traffic carrying standard.
Take a look at the viaducts and bridges on the link, and see the wisdom of an arch. Building one is easy, (Though it may not look that way) and keeping them in good repair is just a matter of understanding the structure.
They can come unstuck though, as the one at Cullen, Banffshire did in the late 1800’s. It was really the earth embankment that gave way, and allowed the inadequate pillar to cave. It was repaired properly, and the entire viaduct still stands today, though without trains to run over it.
Arches can be subtle though. There is a steel rail bridge still used every day on the Aberdeen – Inverness line, near the old Rothiemay station, (Closed) and I suspect that young engineers take a glance, and think it’s just a steel support reliant on the steel. It takes closer study to see the arch. It is a beautifully constructed arch bridge, over the river Deveron.