It was originally designed and built by the noted railway engineer Robert Stephenson as a tubular bridge of wrought iron rectangular box-section spans for carrying rail traffic.

After many years of deliberation and proposals, on 30 June 1845, a Parliamentary Bill covering the construction of the Britannia Bridge received royal assent.

[3] At the Admiralty's insistence, the bridge elements were required to be relatively high in order to permit the passage of a fully rigged man-of-war.

In order to meet the diverse requirements, Stephenson, the project's chief engineer, performed in-depth studies on the concept of tubular bridges.

[citation needed] Other railway schemes were proposed, including one in 1838 to cross Thomas Telford's existing Menai Bridge.

By 1840, a Treasury committee decided broadly in favour of Stephenson's proposals, however, final consent to the route, including Britannia Bridge, would not be granted until 30 June 1845, the date on which the corresponding Parliamentary Bill received royal assent.

As originally envisaged by Stephenson, the tubular construction would give a structure sufficiently stiff to support the heavy loading associated with trains, but the tubes would not be fully self-supporting, some of their weight having to be taken by suspension chains.

Fairbairn began a series of practical experiments on various tube shapes and enlisted the help of Eaton Hodgkinson "distinguished as the first scientific authority on the strength of iron beams"[7]: 33–37  It became apparent from Fairbairn's experiments that- without special precautions - the failure mode for the tube under load would be buckling of the top plate in compression, the theoretical analysis of which gave Hodgkinson some difficulty.

However, Fairbairn's experiments had moved on from those covered by Hodgkinson's theory to include designs in which the top plate was stiffened by 'corrugation' (the incorporation of cylindrical tubes).

[citation needed] The results of these later experiments he found very encouraging; whilst it was still to be determined what the optimum form of the tubular girder should be "I would venture to state that a Tubular Bridge can be constructed of such powers and dimensions as will meet, with perfect security, the requirements of railway traffic across the Straits" although it might require more materials than originally envisaged and the utmost care would be needed in its construction.

In fact, it should be a huge sheet-iron hollow girder, of sufficient strength and stiffness to sustain those weights; and, provided that the parts are well-proportioned and the plates properly riveted, you may strip off the chains and have it as a useful monument of the enterprise and energy of the age in which it was constructed.

Stephenson, who had not previously attended any of Fairbairn's experiments, was present at one involving this 'model tube', and consequently was persuaded that auxiliary chains were unnecessary.

[3] On 10 April 1846, the foundation stone for the Britannia Bridge was laid, marking the official commencement of construction work at the site.

[3] Working in parallel to the onsite construction process, the two central tube sections, which weighed 1,800 long tons (1,830 tonnes) apiece, were separately built on the nearby Caernarfon shoreline.

The work did not go smoothly; at one point, one of the tubes allegedly came close to being swept out to sea before being recaptured and finally pushed back into place.

The noted engineer Isambard Kingdom Brunel, a professional rival and personal friend of Stephenson's, was claimed to have remarked to him: "If your bridge succeeds, then mine have all been magnificent failures".

[1] These were immortalised in the following Welsh rhyme by the bard John Evans (1826–1888), who was born in nearby Menai Bridge: Pedwar llew tew Heb ddim blew Dau 'ochr yma A dau 'ochr drew Four fat lions Without any hair Two on this side And two over there The lions cannot be seen from the A55, which crosses the modern bridge on the same site, although they can be seen from trains on the North Wales Coast Line below.

[10][11][12] During the evening of 23 May 1970, the bridge was heavily damaged when boys playing inside the structure dropped a burning torch, setting alight the tar-coated wooden roof of the tubes.

However, the structural integrity of the iron tubes had been critically compromised by the intense heat; they had visibly split open at the three towers and had begun to sag.

By the end of July 1970, a total of eight Bailey bridge steel towers had been erected, each being capable of bearing a vertical load of around 200 tonnes.

Then the single-line working was transferred to the new track (on the west side); this allowed the other tube to be removed and replaced with a concrete deck (which is used only for service access) by 1974.

During 2011, national railway infrastructure owner Network Rail, the Welsh Assembly Government and the English Highways Agency undertook a £4 million joint programme to strengthen the 160-year-old structure and improve its reliability.

[16] The work involved the replacement of eroded steelwork, repairs to the drainage system, restoration of the parapets and stonework, and the painting of the steel approach portals of the bridge.