Brittania Bridge cross section

The 1,510 ft long, Brittania Bridge crosses the Menai Straights between the mainland of Wales and the Isle of Anglesy. It was built by Robert Stephenson as an inovative box girder bridge (in riveted wrought iron on masonry towers) to carry trains from the UK mainland to Anglesey and the onto the ferry port at Hollyhead for crossing the Irish Sea , the main route at the time from London to Dublin. Although the, more famous, Stephenson gets most of the credit today the structural research on the box girder was actually carried out by Sir William Fairburn. The bridge was opened in 1850 with the principal constraint on the bridge design being imposed by the British Admiralty to allow the passage of tall sailing vessels along the straights. Trains would pass through the interior of the two box section tubes (one tube up line and one downline) with the longest span measuring 460ft. It used to be the longest box section bridge in the world. When the bridge caught fire in 1970 the tubes were fatally damaged and a new form of bridge was constructed as an arch girder bridge , making use of Stephenson's original towers, and (eventually) carrying both the original rail lines and a road crossing in the form of the A55 North Wales Expressway. By way of a commemoration a thin slice of the original, complex, box section was placed close to the A55 road near the southern end of the bridge. It sits in a small area of parkland next to the Treborth Coastal/Forest Path . It clearly shows the innovative construction of George Stephensons bridge in a setting which allows a real appreciation of the 20th century upgrade. Although a brilliant engineering solution to the problem very few box girder bridges were ever constructed because of the very high cost of that approach, mainly because of the massive amount of wrought iron required. Not shown on the cross section is the wooden roof which was constructed above the tubes to protect the wrought iron from rust. It was this part of the structure which caught fire in 1970 with the intense heat causing the tubes to buckle.