Back Issues: Suspended heritage – Ōpiki bridge a monument to industrious ambition

For 105 years, the Ōpiki Suspension Bridge has featured in Manawatū’s landscape.

Variously named Tane Suspension Bridge, Rangitāne Swing Bridge, Akers Bridge and Ōpiki Toll Bridge, it has a category 1 listing with Heritage New Zealand, and appears on the heritage lists of both the Horowhenua and Manawatū district councils.

It is also the subject of Molly Akers’ book Suspended Access: Ōpiki Toll Bridge 1918-1969.

In concluding her book in 2003, Molly wrote that seen from the present bridge, the old bridge is: “A ‘giant necklace’ suspended by the pillars of history, with its dangling strands of rusting steel, still held in place by the 16 cables of ‘ancient chain’, hopefully to dangle for many more generations to view.”

Sadly, these cables soon began snapping.

The Tane Hemp Company Ltd, whose owners included the prominent Seifert and Akers families, erected the bridge. Established in 1915, the mill had struggled to access the local railway station, and the significant investment in this bridge resolved that.

The mill’s concrete chimney, built in 1918, survives nearby. However, ‘‘yellow leaf’’ flax disease, combined with a post-WWI slump in demand for flax, saw the mill close in 1921. Thereafter, until 1969, the bridge served road-users instead.

The bridge’s backstory adds to its uniqueness. Its owners explored British and US designs before concluding that a suspension bridge was the most practical. They chose Pahiatua-based Joseph Dawson (1843-1923) to design and build it, and this became his 16th bridge.

The son of a Hobart hotelier, Dawson was a farmer before arriving in New Zealand in 1869. In 1917, the Christchurch Star described him as “a noted bridge-builder” and a “self-taught engineer”.

The article, quoting former minister of roads and bridges, A.W. Hogg, stated that Dawson was an ordinary pick and shovel worker, until someone asked him if he could build a brewery. This, in 1878, became the malt house for Christchurch’s Crown Brewery.

The Star also stated Dawson had a talent for engineering, despite having no training, and that he developed the idea for steel suspension bridges.

As he was not a certificated engineer, he struggled against officialdom, “but he had a friend in the late [NZ Prime Minister] Mr Seddon. He resembles a famous English engineer who was once asked by a judge during an arbitration case where his diploma was. ‘Here, my lord,’ was the reply of the self-taught man as he pointed to his head.”

Dawson built on a day-labour principle (i.e. no clerk of works and contractor’s profits), and claimed his bridges cost only £8 per foot, whereas wooden bridges cost £14 per foot.

The Star added: “Mr Dawson is a type of man who belongs to the past, yet keeps looking forward, and is probably better equipped for the practical work of bridge-building than many highly-educated engineers. He has constructive ability of a high order, and Pahīatua is largely occupied by premises erected by him.”

The highly destructive, 7.6 magnitude, 1934 Pahīatua Earthquake doubtlessly tested those – but Ōpiki Bridge survived.

By 1897, Dawson had constructed a 2.1-metre model of his suspension bridge design. This he exhibited widely, including at an exhibition described in the King Country Chronicle in May 1911.

It also described the recent opening of Dawson’s Taoroa Suspension Bridge over the Rangitīkei River (at Mōkai Gravity Canyon), which he warranted to carry “anything but a railway train”. However, a sheep truck destroyed it in 1960.

Dawson patented his side truss bridge design in 1907 and his suspension bridge in 1911. These were spreading around the country, with the Ashburton Guardian reporting on Dawson’s designs in 1912, that “a reinforced concrete bridge, to last over 200 years, and costing no more than £8 per foot of its length” should impress those seeking a bridge for the Ashburton River.

The article then described features common to Ōpiki Bridge.

Finally came Ōpiki Bridge, with the Manawatu Times reporting in 1917 that Dawson was erecting it “for Messrs Seifert’s Flaxmilling Co”. It was “475 feet in length, or the longest heavy traffic bridge in New Zealand”.

It was warranted to carry a tram load of eight tons; and one concrete tower was nearly finished. “Considering the weather, the state of the river, and the difficulty in securing material, surprising progress is being made with the work.”

The of April 12, 1918, described its official opening three days earlier, adding that as well as being New Zealand’s longest suspension bridge, it was probably also the longest of its kind in Australasia.

“The ropes, of the best plough steel, are hung on four towers of reinforced concrete, each pair of towers being connected with three cement arches. The towers measure 70ft from the foundations, and stand 62ft above the flooring of the bridge. They are crowned with iron saddles, each saddle having 24 wheels, on which the cables rest. Each cable has a breaking strain of 60 tons, making the breaking strain of the 16 cables employed 960 tons…”

Further details from other sources included that the towers were made of Wilson’s Portland Cement; while those troublesome suspension cables were recycled from the gold-mining operations at Waihi. Also: “The cables are anchored to massive concrete foundations. The cost of the structure was £3000.”

The Dominion reported on the same day that the war had made some items unpurchaseable, and labour was limited.

“It was a huge work, the building of the four white concrete monuments, and it needed no ordinary strength to haul 16 heavy steel ropes over the saddles on their summits. But it was done. The towers on which the ropes are suspended, strongly built of reinforced concrete, are double, two on each side of the river, each pair connected by three arches, and their total weight is 288 tons.

“Besides this, 80 tons of buried concrete are used for the anchors. The length of the bridge is 473ft, and the width 11ft. At the top of the towers the cables rest on saddles armed with 24 wheels. Each cable measures 750ft, so that altogether 12,000ft of steel rope has been used.

“The timber used for flooring, joists, and chords is black birch, hard as flint, and more durable than ironbark. The weight of the bridge, as hung from the ropes, is 80 tons.”

Dawson’s first bridge collapse, in February 1922, occurred while 150 sheep were crossing it. This was the newly-built Ngaipu Bridge in Wairarapa, and seven sheep died.

Steel clips on the cables near the anchor had slipped, pulling the cables out of their sockets. Small wonder the carrying capacity of these bridges required close control.

Engineering historian Geoffrey Thornton states that Ōpiki Suspension Bridge had the second-longest main span of any pre-1940 bridge – and the other is now gone.

The bridge, nowadays owned by Horizons Regional Council, remains as a monument to one of the longest main-span road bridges ever built in New Zealand.

It is also a monument to Dawson, who learned his trade ‘’on the job”, and who optimistically predicted that his concrete bridges could last 200 years.

Val Burr is a historian and a heritage researcher in Manawatū.