The tubes for the Britannia bridge were built on staging
on the shore, upriver of the bridge. There were four tubes required in
all, each one 470 feet long, and weighing 1300 tons apiece. The tubes were
fabricated of wrought iron plate that was hand-riveted together, some
800,000 rivets being used in each tube. Work began in June, 1847, and
within a year the first tube was finished. It was floated into position on
pontoons in June, 1849.