| Historical notes |
The Bridge over the Gwydir River is an iron lattice bridge and was completed in 1881.
Iron lattice bridges are the most significant bridges of the colonial period. They were the major bridges of their time and carried the burgeoning road network across many of the major rivers in New South Wales. Whereas the construction of timber truss bridges extended over a long period, from the 1860s to the 1930s, and was associated with three prominent engineers in the Department of Public Works (Percy Allen, E M de Burgh and Harvey Dare), the construction of the iron lattice bridges was confined to a short period, from 1881 and 1893, and all were designed by bridge engineer John A McDonald.
The lattice truss is an example of British bridge technology and they appeared in large numbers in the British colonies of Australia, particularly in New South Wales and Victoria and a few examples in the other colonies. These bridges were almost the exclusive choice for large rail and road bridges such that in New South Wales, 41 of these bridges (27 for roads and 14 for railways) were built between 1871 and 1893. Most were in the Hunter Valley and New England region.
They were a more economical form of construction than the heavy cellular girder bridges, requiring only about half the amount of iron, but the iron was still an expensive import (rolling marks such as BUTTERLEY, SHELTON and BURBACH appear on iron elements of many of the bridges). In terms of costs, indexed to the year 2000, the iron lattice bridges would have cost around $9,000 per square metre of deck whereas for timber truss bridges this unit cost would only have been around $1,500. Despite the high initial costs of the lattice bridges, their durability and low maintenance costs have meant that in the long term they have been very cost-effective structures. The iron lattice bridges were, to the colonial period, what reinforced concrete bridges were to the period 1930-60 and prestressed concrete bridges have become since the 1970s. |
