In following applet you can stretch out a circular arc to a barrel vault by moving the sleder.

The Roman could build immens barral vaults. By using cassette ceilings they could economise on material and weight. Walls in brick and concrete with an thickness of several meters supported the vault.[br]On the Forum Romanum in Rome you can still admire the side aisles of the [b]basilica of Maxentius[/b]. The basilica dates from the early 4th century, was 80 m long and this lenght was crossed just by 3 adjacent barrel vaults, separated by walls with a thickness of 6 m.