The arch itself would be made by a pyramid type roll wuth dies sized for the tubing angle iron "C" channel I beam etc that was to be used to power the roll again a cylinder could be connected to a series of bell-cranks and fulcrums and possibly sprockets &chains to turn linear motion into rotary motion this would entail losses of mechanical advantages or require a reversing ratcheting sprag & pawl to maintain the mechanical advantage while making the roll rotate in more than a short arc.
Or in the Hosfeld bender the arch could be made by making a series of very slight bends for example 30 6° bends would result in a 180° arch the distance or width could be anything from 1/2" per bend = 15" nominal arch to something like 10" per bend spaceing to = a 300" arch with small bends the arch would look almost circular