Rakaia Gorge Bridge

The Rakaia Gorge Bridge is located on the Rakaia River in inland Canterbury in New Zealand's South Island. The bridge connects the Selwyn and Ashburton districts and carries State Highway 77 and the Inland Scenic Route. It is one of only two bridges crossing the Rakaia River, and the oldest, with the other being the Rakaia Bridge, carrying State Highway 1.

Rakaia Gorge Bridge
Rakaia Gorge Bridge as viewed from the riverbed to the East, 2025
Coordinates	43°31′3.71″S 171°39′26.27″E
Carries	1 lane of roadway
Crosses	Rakaia River
Locale	Canterbury, Windwhistle
Maintained by	New Zealand Transport Agency
Characteristics
Design	Bollman Truss Bridge

The Rakaia Gorge in flood

The bridge is one of only two Bollman Truss Bridges anywhere in the world, and the only one still carrying vehicles. Construction was completed in 1882.

Description

The Rakaia Gorge Bridge is a unique design, although quite similar to the "Bollman Truss"^[1] bridge design seen in only one other remaining bridge anywhere in the world, the Bollman Truss Railroad Bridge. The design was patented in 1852 by Wendel Bollman, a United States railway engineer. The design involves a series of non-symmetrical trussed beams of iron being jointed to vertical members by hinged pins to facilitate construction, and more than 100 examples were constructed on railways such as the Baltimore and Ohio Railroad among others, all of which are now demolished.^[2][3] Regarding the design, Bollman wrote “This bridge has the advantage of great strength and perfect security, with very little weight of metal". The bridge design enjoyed great success for a number of years before it was superseded by more economic and effective bridge designs. The bridge differs from the works of Bollman in that the diagonal ties are anchored at each end by concrete-filled sockets that are tunnelled into the rock outcrops on which the abutments placed.

The bridge also closely resembles the "Fink truss" bridge design, also used commonly throughout US railroad development.^[1] These similarities may be attributed to the experience of Carruthers, a key engineer involved in the design, in the United State throughout the 1860s, during which time both the Bollman and Fink truss designs were commonplace.

The Rakaia Gorge Bridge spans 57 metres and features a timber deck and handrail on top of it's iron girders and truss. The bridge is also thought to be one of the earliest wrought iron bridges in New Zealand, and one of the few 19th century components of the New Zealand state highway network.^[4]

History

Design and construction

The Rakaia Gorge Bridge was constructed between 1880 and 1882 with the purpose of serving inland farmland and service towns following calls for a second bridge over the river after the construction of one near the river mouth in 1873. In 1877 the Public Works Department (PWD) concluded debate on the bridge and decided to move forward with construction of a single lane road traffic bridge with the opportunity to carry rail if needed.^[4] The opportunity for carrying rail was included due to prospects at the time regarding the South Island Central Trunk Railway, which would run along the foothills of the Southern Alps. This railway never eventuated, and therefore the bridge never ended up carrying rail of any sort.^[1]

At the time of planning and construction, the New Zealand Government had recently been centralised and the PWD restructured, resulting in several high profile engineers being involved in the design and construction. Involved engineers included the Canterbury Provincial Engineer George Thornton, John Carruthers, William Newsham Blair, and Peter Seton Hay. Much initial debate was conducted on the structure of the bridge, with Carruthers suggesting an embankment to block the flood channel, and others considering a suspension bridge. The idea of a suspension bridge was soon discounted due to the large winds experienced in the gorge.^[1]

The first plans for the bridge were registered in July 1876. The final design is referenced on the plans as being designed by a "Wood", although records return no biographical details of Mr Wood, nor his precise role in the project. Carruthers' signature is also on the plans, however it is unknown whether this is purely a "stamp of approval" or evidence of core design involvement.

The Public Works Department had called for tenders twice already before finally finding a willing contractor, and in the end the bridge was constructed by W. H. Barnes of Oamaru.^[4] The iron for the project was produced in England, and delivered to Coalgate by September 1879. Construction took much longer than expected, primarily due to two large slips in 1881 that needed to be cleared by the contractors before work could proceed, and as a result Barnes were penalised by an amount between £300 and £1000. Construction was finally completed in 1882, however road traffic could only begin using the bridge in 1884 when the flood channel was bridged with a timber bridge.^[1] This timber bridge was replaced by a concrete arch bridge in 1945.

As early as 1890, wind resistance of the structures was a concern and further work was conducted by Peter Seton Hay to design additional wind bracing for both bridges. This came as a result of damage from wind gusts in late 1889. No major repair work has been conducted since, with minor upgrades between 2005 and 2011, and regular maintenance from the New Zealand Transport Agency.

Social and economic significance

The bridge initially served the local farming communities, who were experiencing substantial growth in the tail end of the 19th century. The bridge across the Rakaia Gorge further boosted economic activity stimulated by the construction of the Rakaia to Methven branch railway, and assisted in the development of several main roads throughout the 1880s by the Mount Hutt Road Board.

On top of serving farming interests, the bridge is also vital for those coming to Mount Hutt for recreation purposes. As early as 1880, people from across the South Island were coming to Mount Hutt, with the primary intent of scaling Mount Hutt and adjacent mountains. In 1969, plans for a Mount Hutt ski field began, and it became operational within a few years. In 1976 the Rakaia to Methven branch railway closed, hitting the economy hard, however the addition of the new ski field supported Methven and continues to be a strong pillar of it's economy today^[5]. Today, the bridge is primarily used for tourism traffic, either those coming from Christchurch to Mount Hutt, or international tourists following the Inland Scenic Route.

The bridge was registered as a historic place in September 1985 under the Historic Places Act, and is currently categorised as a Category 1 Historic Place by Heritage New Zealand^[2].