Bull Island Timber Bridge
This bridge dates from 1907 and connects Bull Island with the mainland. It has replaced two other previous structures, the earliest being built in 1819, its original purpose being to facilitate the construction of the North Bull Wall. The wall itself was constructed to prevent Dublin harbour from silting up. The wall helps maintain an adequate depth clearance for ships to pass through the harbour by creating a barrier for the silt deposits; these deposits built up over time to form the Bull Island as we know it today.
Due to is age (over 100 years) and constant exposure to a harsh maritime climate as well as over half of the structure being submerged in saltwater daily, a company called Carnehill Building Services carried out essential restoration work to the bridge in 2008.
In Ireland it is common practice to use timbers such as Oak, Iroko, Larch, Douglas fir and Cedar for external building purposes; when properly treated these timbers present an adequate resistance to moisture and are used for cladding and general joinery projects. However, when the design for the Bull Wall bridge was being conceived is was clear that timbers with an exceptionally high water resistance would be required due to the fact that the proposed structure would be partly submerged underwater.
From a distance it is possible to assume that the bridge is constructed in one type of timber, when in fact three types are being used, each with its own specific purpose:
A ‘Greenheart’ column and beam assembly forms the superstructure of the bridge. These columns and beams are 230x230mm in section and are set out in rows of four on a grid along the length of the causeway. Greenheart timber (exported from Guyana) has exceptional density and strength. Its heartwood is highly resistant to attack by fungi, marine borers and dry-wood termites, making it a marine and shipbuilders' favorite.
The superstructure supports crossrails made from ‘Ekki’, which are 230 x 75mm in section and are set out at 300mm centres. Ekki, a West African hardwood, is classified as "exceptionally heavy" and is considered to be one of the most durable of all African woods.
It is noted more for its impressive strength and difficulty in working than its appearance. These characteristics have resulted in a long list of practical rather than decorative uses. Ekki is a perfect material for heavy construction or other uses where great strength and durability is needed. Typical applications include wharfs, bridges, sea fences and river pilings because of the wood's strength and resistance to decay.
The bridge supports both pedestrian and vehicular traffic. The carriageway deck is 3100mm wide and is constructed of Ekki due to its hardwearing and durable nature.
Douglas Fir is the only timber used on the bridge which grows in Ireland. Both the pathway (bridge consists of two paths either side of the carriageway) and handrail are constructed in Douglas Fir. Each pathway is 1400mm wide and is built from 200 x 50mm Douglas Fir planks. Douglas-fir, classified as a softwood can be used for external applications as long as its heartwood (as opposed to sapwood) is used. It is used extensively in the construction industry.
The above image shows how decayed sections of the greenheart superstructure had to be replaced with new lengths spliced into the structure as shown.
While only certain sections of the greenheart structure had to be replaced the ekki crossrails were extensively decayed and had to be replaced accordingly.
The above image shows the extensive number of new crossrails with the douglas fir planks for the footpath being laid out.
The above image shows the new ekki carraigeway with the douglas fir timber for the footpath beside.
The bridge begins with a Junction between the new ekki carraigeway and the existing granite kerbing.
Views of the newly restored bridge.