Grafton Bridge

Category
Engineering Work (eg road, bridge, sawmill, dam)

Description
There is a link between engineering heritage in Australia and that in New Zealand in the form of Auckland’s Grafton Bridge, which has a special pride of place in New Zealand, but was designed and built by the Australian company, Ferro-Concrete Company of Australasia Ltd.

It took two and a half years to build, being opened in April 1910, and was claimed to be the biggest span, reinforced concrete arch bridge in the world at that time. The twin, 3 pinned arches, spanned 320 ft (97.6 m) rising 84ft 9 (25.6 m ) above the abutments and to a height of 142 ft (43.3 m) above the valley floor. It was certainly a pioneering structure, being very early in reinforced concrete, showing great engineering enterprise and brilliance of design. In 2003 it is still a vital link in the street system of our largest city, carrying a solid stream of car and van traffic, but heavy transport is excluded.

Two tenders were received by the Auckland City Council in 1907,
one in steel from the Messrs J. McLean & Sons for a steel bridge to be manufactured by the American Bridge Building Company for £28,730-12s 6d, the second for the reinforced concrete bridge for £31,918.
The recommendation from the new City Engineer, Mr W. E. Bush, was that the concrete design be accepted, because the maintenance costs would be much lower.

For the Ferro-Concrete Company, Mr R. F. Moore was Chief Engineer, and Mr Rosegger the Chief Assistant Engineer, who made the calculations.
The timber falsework was massive and consisted of 400,000 super feet (943 cu. m ) of West Australian jarrah and Oregon pine. “The height of the moulds was adjusted most accurately by 160 bottle jacks, which performed their work more effectively than the sand boxes more generally employed” (Cassiers Magazine, August 1910).

Concrete was made of beach shingle, 6:1 mix, using local cement. Preliminary test of concrete were made by Professor Warren of Sydney and a design strength fixed at 500 lbs per square inch (3.4 mPa).

All did not go well for the Ferro-Concrete Company. The site was difficult, being a steep-sided bush clad valley, and there was trouble with the complexity of the formwork and getting it in place. Late in the contract the company could not continue, was declared bankrupt and the job was completed by council labour. The final cost was £35,000, being an over-run of only £3,000 or 9.5%. The arch design was submitted to Professor Moersch of Germany and a clause in the contract stipulated that “...no progress payments should be made on the arch span till it is completed and tested.” This no doubt caused the downfall of the company.

Load tests were carried out on the newly-completed bridge. Firstly one half of the arch span was loaded with 292 tons (297 tonnes) of roading aggregate, giving 113.5 lbs per sq ft (5.4 kPa), and the deflection measured as 1/8 inch. (3.17mm) However temperature movement was also significant. When the road metal load was removed, two steam rollers, of combined weight 32 tons, were run over the bridge and the deflection was measured as 1/12 inch (2.12 mm).
“In all cases the results were extremely satisfactory..........”

The recommendation of Mr Bush that maintenance would be very low was born out by experience. “.. the record of maintenance on the structure over a quarter of a century was excellent, being almost nil,...”
However in 1936 some cracks that had been observed for some time became worse and “ a considerable piece of concrete became detached and fell away from the bottom chord....... of the span adjacent to the main arch at the Grafton Road end.”
The spans away from the main arch were 3 spans of 35 to 42 ft (11 to 12.8 m) and 6 spans of 75 to 80 ft (23 to 24.4 m) made up of concrete Vierendeel girders with a top and bottom chord and vertical posts, being 9ft deep. The appearance was of a number of square openings with big chamfers at the corners, and the end openings, adjacent to the supports, were filled in.

The detailed study that followed demonstrated that the shear stresses in these Vierendeel frames was excessive and there was surprise that the structural performance had been so good. The remedy was to erect two steel trusses on the deck to support the damaged area and to fill in the openings with concrete, with special emphasis on the shear reinforcement, thus turning the frames into large I section beams. Since that time maintenance has again been of a minor nature associated with the ravages of time, such as cover to steel on beam edges. Repair details are given in a paper (c. 1938 ) by A. J. Dickson, City Engineer and at 36, youngest President of NZIE. (now IPENZ.)

Now 93 years since it was opened, the bridge spans motorway lanes in Grafton Gully, standing tall over the modern system. It is still giving good service and will for many years to come, as a live tribute to the pioneer engineers who designed and built it.

Location
Between Symonds Street and Park Road, Grafton, Auckland

Region/s
Auckland

Access Info
The Bridge is a public road and can be walked over. Access to view underneath can be made through the Symonds St Cemetery. No underneath access is available on the opposite side.

Nature of Engineering
Building and Construction, Infrastructure (incl. Road, water, ports)