[SunkenCiv]

Thanks central_va.

http://www.freerepublic.com/focus/chat/2877692/posts?page=19#19

[TigersEye]

The vaulting tube technique is particularly common in Roman North Africa. Here the tubes were used instead of timber, as there was a shortage of wood for temporary supports to place a roof on a building.

Interesting. They are actually a type of concrete form that becomes incorporated into the structure. It sounds like their only structural value is during the construction process.