Bridge brought down its constructor

Part of Grafton Bridge spans a graveyard – and a graveyard is what the bridge proved to be for its constructor, the Ferro-Concrete Co of Australasia.

Grafton_1.jpgBeset by difficulties on the two-and-half-year project, the company went out of business before the project was completed in early 1910, according to the Institute of Professional Engineers’ website.

Auckland City Council received two tenders in 1907 for the building of the bridge, which was to replace a wooden pedestrian structure. John McLean & Sons (whose rip-roaring 70-year history was described in Contractor in 2001) proposed to erect a £28,730 steel bridge to be manufactured by the American Bridge Building Co, while Ferro-Concrete said it would erect a reinforced concrete structure for £31,918.

The city engineer recommended that Ferro-Concrete’s bid be accepted as the maintenance costs would be much lower – a shrewd assessment, as the bridge required almost no maintenance until 1936.

A magazine article in 1910 reported that the bridge’s timber formwork was massive and consisted of 400,000 super feet (943 cubic metres) of West Australian jarrah and Oregon pine. The height of the moulds was adjusted accurately by 160 bottle jacks, which proved more effective than the more commonly used sand boxes.

Concrete was made of beach shingle, using local cement, in a 6:1 mix. The design strength was fixed at 500 lb per sq inch (3.4Mpa).

Grafton_2.jpgFerro-Concrete had difficulty coping with the steep, bush-clad gully and getting and keeping the complex formwork in place. It was eventually unable to continue, was declared bankrupt, and the project was completed by council labour at a final cost of $35,000 (a $3000 overrun).

The arch design was submitted to an expert in Germany for review and IPENZ’s website article says this may have sealed Ferro-Concrete’s downfall since a clause in the contract stipulated that “no progress payments should be made on the arch span till it is completed and tested”.

Load tests on the newly completed bridge proved extremely satisfactory. One half of the arch span was loaded with 292 tons (297 tonnes) of roading aggregate, giving 113.5 lb per sq foot (5.4Kpa), and the deflection was measured as one-eighth of an inch (3.17mm). Then when the road metal road was removed, two steam rollers with a combined weight of 32 tons were run over the bridge and the deflection was measured as one-twelfth of an inch (2.12mm).

The first signifcant maintenance of the bridge occurred in 1936 after cracks which had been observed for some time became worse and a sizeable piece of concrete fell away from the bottom chord of the span next to the main arch at the Grafton Rd end.

When Grafton Bridge opened in April 1910 it was claimed to be the biggest span, reinforced-concrete arch bridge in the world. The twin, three pinned arches spanned 97.6 metres, rising 25.6 metres above the abutments to a height of 42.3 metres above the gully floor.

“It was certainly a pioneering structure, being very early in reinforced concrete, showing great engineering enterprise and brilliance of design,” IPENZ says.

Contractor Vol.33  No.3  April 2009
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