This was written for somewhere else, but why not put it here too?
Perhaps now is a good place (or here is a good time?!?) to say that I am becoming increasingly concerned about "strengthening" of arches. So far as I can tell, all of the testing done has been considering ultimate load. The real concern is loads which will destroy a bridge in time.
I suspect that a hinge is formed locally under the load at rather lower loads than we think. Tests have been, almost universally, carried out using line loads. Real loads are patches and the thrust concentrates under the patch even if it distributes elsewhere.
This makes the result rather different from that obtained when considering an effective strip with parallel sides.
I know of a set of bridges which are breaking up under loads at exactly what Archie-M says is the limit (with a 1.9 load factor). MEXE rates them at 3 times that (so clearly not conservative for a service load).
Just in the last few days I have been thinking again about the breakdown. Try this for a scenario.
Arches have a lot of potential load paths, for example including the stiffness of the spandrels. All of those paths are very stiff and nothing transfers from the primary to the secondary until the primary starts to break down. However, the breakdown is very gradual and not revealed by a 2D analysis.
If the design of a "strengthening" scheme (remember that there is an inevitable stiffening too) is based on 2D, it may well underestimate the local moments under the live load patches. The reinforcing will stiffen the structure and so modify its established distribution system. It might improve things, might make no difference, might actually accelerate deterioration.
Unless a bridge is actually showing signs of failing, I suspect that we are strengthening because we don't understand, rather than because the bridge really needs it. It is more than likely that the result will be a sounder sleep (Phew, I have done something) at the expense of a shorter life for the bridge. Once it has been turned into a reinforced concrete bridge it can never be an arch again. The foundations (for example) were designed for an arch and were never intended to carry a concrete bridge which has more limited flexibility.
This is getting too long. I'll stop there.