Story: Bridges and tunnels
Page 3 – Types of bridges
The earliest bridges in New Zealand were simply the trunks of trees laid across a stream. Many Māori paths crossed such bridges.
British settlers brought a long tradition of bridge-building, especially arched stone bridges and, later, iron bridges. The first constructed in New Zealand was a humpback bridge over the Waitangi River in 1830.
The simplest type of bridge was the girder or beam bridge, which stretched from one side of a river to the other. Where the river was too wide, the beams rested on supporting piers in midstream. This style of bridge was common in New Zealand because it was normally the cheapest. It was used for the long bridges crossing the wide braided rivers of Canterbury.
Where builders found it difficult to extend a beam bridge by erecting supporting piers, beams could be made stronger and longer by using a truss. This was a triangle where the two side struts in effect held up the bottom beam. Sometimes the deck was at the base of the triangles; sometimes it ran along the top. Truss bridges developed in North America, where there was abundant wood, and they were relatively common in 19th-century New Zealand because of the availability of wood. The first was built over the Hutt River at Petone in 1847. Wooden truss bridges had a narrow carriageway, and as native timber often rotted, few remained in the 2000s.
Suspension bridges are most common as ‘swing bridges’ on tramping (hiking) tracks. The bridge is suspended from two metal cables, and can span a considerable distance. Cheap to build, they were quite common in the 19th century. The first was probably the Waiwakaiho River bridge, built in New Plymouth in 1842. It was suspended from the anchor chains from the Fifeshire, which had been wrecked earlier that year in Nelson.
The Kawarau suspension bridge was the site of New Zealand’s first bungy operation – with a 43-metre leap – in 1988. The Kawarau bridge was built in 1880 from a design and specifications drawn up by the engineer Harry Higginson in a remarkable 16 days. Another bungy bridge is the 1901 Skippers bridge near Queenstown, the highest suspension bridge in the country at 91.4 metres.
Stone arch bridges were less common in New Zealand than in Britain because of the relative lack of building stone. As well as some stone bridges, a few timber and iron arch bridges were built. Hamilton’s Victoria Road bridge over the Waikato River (1908) was a steel arch. There were more concrete arch bridges. Most arched bridges had the deck on top of the arch, but sometimes the arch was above the deck – a ‘bowstring arch’. The Ōpawa River bridge at Blenheim and the Balclutha bridge over the Clutha River/Mata-Au are good examples of bowstring-arch bridges.
Wood, being cheap and plentiful, was at first the main material used to build bridges. Tōtara was common in the 19th century, but it tended to rot, so later Australian hardwoods were imported and used. Wooden bridges were still built in the 1930s in rural areas and often painted in red oxide. By the 1950s many had rotted, and temporary bailey bridges were used until concrete replacements were built.
Even when bridges were not built wholly of wood, the decking was often wooden. The planks on the bridge over the Hokitika River became loose and the structure became known as ‘the longest xylophone in the world’.1
Because of a lack of both masons and suitable stone, few stone bridges were built in New Zealand. The exceptions were in South Canterbury where there were 28 masonry bridges, most built in the 1870s and 1880s, and in North Otago where limestone was used for the Waianakārua River bridge, the oldest on State Highway 1. There were brick bridges in Christchurch and Akaroa.
Wrought-iron balustrades were first used in the 1860s. The first significant use of iron girders was on a road–rail bridge over the Waitaki River in 1876. Iron girders became common in the first half of the 20th century. Iron trusses were also used, notably on the Rangitātā River bridge near Arundel in 1872.
Concrete was excellent for beam bridges and arches. The first reinforced concrete bridge was built over the Leith in Dunedin in 1903. In 1910 the Grafton bridge in Auckland became the world’s largest reinforced concrete arch bridge; 21 years later the Kelburn viaduct was built in Wellington. Taranaki was especially forward-looking in using concrete arched bridges. In 1954 another major development occurred when the Hutt estuary bridge used post-tensioned pre-stressed concrete. Pre-stressed concrete made possible slim and elegant constructions like the 1987 Hāpuawhenua viaduct on the North Island main trunk railway line.