Arahura Road / Rail Bridge Report by Paul Mahoney to the IPENZ National Engineering Heritage Committee, 2000. History The bridge was built under the direction of the Public Works Department 1. It was opened on 3 December 1893 for road traffic, and in 1894 as part of the Greymouth to Hokitika railway 2. The design was a typical standard PWD bridge of its time involving no notable engineering challenges. The reason for adopting a single lane combined road and rail design was economic; to minimise costs since so many bridges had to be built on the West Coast. The only design highlight is the use of cast iron cylinders specified in preference to the more usual driven hardwood piles. This reflected the foundation difficulties in the Arahura River, as with some others on the West Coast. For road transport, the bridge provided a significant benefit, replacing a ford and ferry service3. Has been in continuous use by road and rail since that time, 106 years. It is a key component of the main transport link between Greymouth and Hokitika. Fabric (a) As-built This is a single-lane ‘combined road and rail’ timber truss bridge 4. There are seven 24.35 meter trusses giving a length of 170.45 meters, and on the ends 11 timber stringer spans totalling 29 meters. The truss design is a ‘PWD standard 80 foot Howe truss’ built of Australian Hardwood with some iron tension members, principally the bottom chord and droppers. It is founded on pairs of concrete filled cast iron cylinders sunk as caissons into bedrock in the river bed. There are good engineering records of this bridge. (b) Modifications & maintenance No major modifications have been made since the bridge was built 5. It has been continuously maintained which has involved replacing numerous timber and iron members as they became too badly decayed or rusted 6. Community While there was some celebration when the railway opened in 1894, like most public utilities it has been largely taken for granted for most of its life. Its on-going community value is far more clearly understood in the context of the result if the bridge been suddenly lost – the severance of road and rail communication with Hokitika and beyond. In recent times, as other such bridges have been replaced, the community has had mixed feelings about this bridge. The bridge may be considered as antiquated in being (1) single lane, (2) combined road & rail, and (3) being built of timber. However, for these same three reasons, tourists find it a fascinating cultural curiosity Historical significance Generally there is nothing nationally notable about the history of this bridge. However its history has strong representative values that become exceptional when linked to its rarity (considered below): It is representative of significant national organisations: Public Works Department, NZ Railways system, NZ road system. It is representative of a very important and common national bridge design: The standard PWD timber Howe trusses were built in large numbers throughout NZ for sixty from 1870s to 1930s. 7 Significance of the fabric It is representative of economic and political development: The key Government role in upgrading transportation as part of building the economy, the very strict policy requirement to economise on bridge design because of the large number required relative to the traffic expected. Timber truss bridges were for a long time the most common type of long-span bridge in NZ and hundreds were built. Very few now remain in either road or rail use and these are all under threat. 8 Because this bridge has not been modified and has been maintained in the tradition in which it was built, it represents a pristine example of NZ th 19 century bridge technology. Today this bridge is a unique survivor, the only wooden truss road-rail bridge remaining in NZ. Community significance The bridge if kept in use has economic value as a key bridge. Because of its uniqueness and ‘antique’ appearance, it has significant potential community value as both a West Coast regional ‘antiquity’ and a national tourism landmark. Throughout the world, bridges have a powerful potential to become community landmarks. Community esteem could be enhanced by better publicising the heritage values of this bridge with a plaque. References: 1 A copy of the 1893 tender documents is held at National Archives, Wellington. 2 Appendices to the House of Representatives, 1894-95, Section D-2, page 253. 3 West Coast Times, 23 November 1894, P3C6. 4 This description is based on PWD plan 23661 held at National Archives, Wellington 5 NZR File BR-WC 2336 held in National Archives, Wellington. Interview by Paul Mahoney with Dave Pawson of Greymouth, retired Foreman of Works, 12 March 1992. Inspection of bridge by Paul Mahoney, 13 March 1992. 6 NZR Ways & Works Manual 1958, section B3 describes the strict maintenance regime for wooden bridges. 7 Personal Correspondence with Mr G G Thornton of Wellington, NZ bridge heritage expert. 8 List of surviving bridges compiled by Mr G G Thornton of Wellington, NZ bridge heritage expert.