The tubes forming the viaducts, rest upon two abutments and three piers, called respectively the Anglesea abutment and pier, the Carnarvon abutment and pier, and the Britannia or central pier, built upon the Britannia rock in the middle of the straits, which gives name to the bridge. The Anglesea abutment is 143 feet 6 inches high, 55 feet wide, and 175 feet long to the end of the wings, which terminate in pedestals, supporting colossal lions on either side, 25 feet 6 inches in length, 12 feet 6 inches high, and 8 feet broad, carved out of a single block of Anglesea marble. The space between the Anglesea abutment and pier is 230 feet. This pier is 196 feet high, 55 feet wide, and 32 feet long. The Carnarvon abutment and pier are of the same dimensions as those above described, on the opposite shore. The Britannia pier is 240 feet high, 55 feet wide, and 45 feet long. This pier is 460 feet clear of each of the two side piers. The bottom of the tubes are 124 feet above low water mark, so that large ships can pass under them, under full sail.

There are two tubes, to accommodate a double track (one would have done in this country, but in England they do nothing by halves), and each is 1513 feet long. The total length of the bridge is 1841 feet. These tubes are not round or oval, but nearly square at the termini; the bridge being constructed on the principle of the arch. A section of one of the tubes at the Britannia pier is in the form of a parallelogram, where it is 30 feet high, gradually diminishing towards each end to 20 feet. The tubes are riveted together into continuous hollow beams; they are of the uniform width of 14 feet 8 inches throughout; they are constructed entirely of iron, and weigh about 12,000 tons, each tube containing 5000 tons of wrought iron, and about 1000 tons of cast iron. The tubes were constructed each in four sections; the sections extending from the abutments to their corresponding piers, each 250 feet long, were built in situ, on immense scaffolding, made of heavy timbers for the purpose, even with the railway; but the middle sections, each 470 feet long, were built on piers on the Carnarvonshire shore, then floated into the stream, and elevated to their position; each of these sections weighed 1800 tons.