Design and Engineering
The Ha’penny Bridge, erected in 1816, is a single elliptical iron arch spanning 42.8m which was cast at the Coalbrookdale, Shropshire works of Abraham Darby III. It is 3.6m wide and rises 3.35m at the crown above high water, which gives a low rise-to-span ratio.
The bridge is primarily composed of three parallel arch ribs, one on each side and one in the centre. Each arch rib consists of six length pieces which are connected at each joint by six bolts through a single continuous plate. The ribs are cruciform sections. The section shape of members are cruciform because they offer greater resistance than a flat plate. Compression forces are taken by deflection of the arch, while tension forces would be taken by the six bolts at the rib joints. The bridge spans between granite abutments which extend over 4.3m from either quay into the river.
The design is reported to have been overseen by John Windsor, one of the Coalbrookdale foundry foremen. The ribs were cast in eighteen sections primarily for ease of transport from Shropshire to Dublin and for ease of erection. No records exist of the erection procedure. The likely method of construction would have been to construct the whole arch with the central rib only and to join the outside ribs to this by means of the cross and diagonal braces.
Refurbishment and Renewal
A structural investigation carried out by Consulting Engineers Mott MacDonald EPO, on behalf of Dublin Corporation, indicated that while the superstructure was sound the railings and deck were not due to age and corrosion.
The bridge was closed for repair and renovation in early 2001. Extensive work was carried out by Irishenco and sub-contractor Harland & Wolff to repair and renew the cast iron arches and railings. The vertical alignment of the deck was restored to its original profile and wider entrances were created at either end where the toll booths once stood, to allow standing space for pedestrians waiting to cross the road. The new deck was constructed of stiffened steel and covered with a slip-resistant epoxy finish.
Conservation principles required that as much as possible of the original fabric of the bridge be retained, and in fact, 98% of the original cast iron was re-used. On completion of the work the railings of the bridge were repainted in their original off-white colour and the newly strengthened and refurbished bridge was re-opened on 21st December 2001.
The quality of the work was recognised when it received a European Union Cultural Heritage/Europa Nostra Award in 2003. You can view a short documentary on the restoration in our Gallery section.