Name: Arahura Road / Rail Bridge

advertisement
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.
Download