Narrows Bridge - 2012 Booklet

History of the Narrows Bridge

As early as September 1849 articles appeared in the Perth local press as to the desirability of constructing a bridge at the Narrows site to improve the communications between Perth and Fremantle in lieu of the “tedious and protracted route via the Causeway bridges” which were built in 1843.

A proposal for a bridge of eleven spans was mooted in 1899. The estimated cost of ₤13,000 pounds was greeted with dismay. The Public Works Department next in 1901 prepared a sketch plan for a timber bridge 900 feet (274m) long with central swing spans but that scheme also did not proceed.

Preliminary reports and sketch plans for a bridge at the site were prepared by the Main Roads Department in the period 1947-1953 but action was postponed because the Departments’s resources were fully committed to the building of the Causeway bridges. Surveys conducted in 1954 revealed that traffic over the Causeway had more than doubled in the preceding five years and as a consequence the MRD proposed to the Government that a bridge should be built across the Narrows.

In August 1954 the Labor Government of the day decided that a bridge should be built as soon as possible. Works Minister John Tonkin announced that foundation test work would commence immediately and the estimated cost of the bridge based on MRD tentative plans would be ₤1,750,000 pounds.

The Government decision was welcomed by Town Planner Gordon Stephenson who forecast a northern approach road to the west of the City of Perth and extensive parking facilities on the reclaimed area of land between the Narrows and the Esplanade.

Due to the difficult foundation conditions revealed and the consensus that the bridge design should provide for slender spans the MRD Chief Engineer E. W. Godfrey was sent overseas in May 1955 to consult with bridge engineering authorities. In September of that year Maunsell, Posford and Pavry of London were appointed design consultants and commissioned to prepare detailed plans for the construction of the bridge.

A design for a prestressed concrete bridge comprising a slender, elegant structure of five spans was subsequently submitted by the Consultants and approved by the Cabinet in April 1956.

Bridge Statistics

The overall length is 1100 feet (335m) ; comprising a centre span of 320 feet (98m) and flanking spans of 230 feet (70m) and 160 feet (49m).

The superstructure consists of 8 rows of I beams each continuous over their full length. The beams vary in depth from 4 feet 11 inches (1.5m) at the abutments to 13 feet 9 inches (4.2m) at the river piers. The webs of the I beams have a thickness of 8 inches (200mm). The beams are fixed at the north abutment and a Demag expansion joint at the south abutment caters for movements of plus or minus 3.5 inches (90 mm) due to temperature, shrinkage and creep.

The overall width of the bridge deck was 90 feet (27.4m) made up of roadway 70 feet (27.4m), footways 8 feet (2.4m), safety fences erected 2 feet (0.6m) from kerbs. The clearance under the central span is 26 feet (8m) above normal water level, road clearance 15 feet (4.5m) for the land spans.

The gradient of the bridge deck comprises a profile of a vertical curve joining tangent lines of a grade of 1:25 on the approach embankment.

The balustrading and safety fences are of anodized aluminium and the lighting comprises aluminium standards at 80 feet (24m) centres with three 80 watt fluorescent lanterns. Services consist of five 30 inch (726mm) water mains, a 30 inch (726mm) sewer main, two 15 inch (381mm) gas mains and telephone and power cables.

Approximate Quantities

Gambia Piles: 180 Nos., 20,960 lineal feet (6390m)
Portland cement concrete : 12,600 cubic yards (9600m³)
Mild steel reinforcing : 1900 tons
Prestressing strand : 325 tons
Roadway surfacing : 18,600 square yards (1728 m²)

Design and Construction

Detailed plans and specifications were prepared by G Maunsell and Partners and their associates, architects Sir William Holford and Partners and prestressing consultant, E W Gifford. The design provided for post tensioned double cantilever beams ( made up of precast units) supported on the shore and river piers with shore and central suspended spans. The beams were made continuous after the suspended spans were lowered into position.

Three tenders were received and the lowest, for ₤1,325,000 pounds, from Christiani & Nielsen A/S of Copenhagen, in association with J O Clough and Son, Perth, was accepted on 12 March, 1957. The successful tenderers proposed to erect the precast units making up the bridge beams in their final position, on trestles supported on a piled staging, prior to post tensioning. This was in lieu of the construction method proposed by the Consultants, which involved assembly in the precast yard and the transfer of the completed beams to their final position using a floating crane which would have had to have a capacity of over 500 tons.

The foundations incorporated Gambia piles, so named because of a previous use in the African location of the same name by the Consultants. They consisted of tubular steel piles 31.75 inch (806 mm) OD and 0.375 inch (9.5 mm) wall thickness. The lower section had a conical point which was filled with reinforced concrete in the preparation yard. The piles were driven with 10 and 12 ton drop hammers striking on a steel anvil embedded in the concrete fill. As pile penetrated the sub strata to depths of 110 to 120 feet (33 m to 36m) extension casings were added by welding. Completed piles were subsequently filled with reinforced concrete. Four piles were load tested to twice their working load of 200 tons.

The staging consisted of a grid of some 1,500 timber piles, up to 80 feet (24m) long, linked with timber bracing. As well as supporting the precast beam units the staging carried the steel rails on which travelled two pile frames, material transporters, an electric powered gantry crane and a monorail concrete support system A 40 foot (12m) navigation gap allowed for the intermittent passage of marine craft when temporary steel bridging beams were lowered onto the river bed.

Early in 1958 the tops of the staging piles near the north bank were observed to be moving laterally out into the river, although they were unloaded at the time. Investigations revealed that the consolidation of the reclaimed Mounts Bay area had caused a wedge shaped layer of mud to displace laterally thus affecting the staging piles. As a result the Consultants modified the north shore pier Gambia piles by increasing the casing diameter from 31.75 inches (806 mm) to 43 inches (1092 mm). The insitu concrete column was replaced by an octagonal shaped precast reinforced concrete column, the lower 5 foot (1.5 m) section of which was grouted into the base of the driven pile, allowing the upper part of the casing to move without affecting the position of the pier support column, and hence, the pier itself.

Precast beam units, weighing up to 20 tons, were manufactured in a yard located on the south shore. They were cast in plywood forms made on site. The precast yard was equipped with a pan concrete mixer capable of producing high strength concrete, an overhead monorail system for concrete distribution and an electric gantry for lifting, stacking and loading units on rail bogies for transport to the bridge site.

The abutments, river and shore piers were of reinforced concrete construction cast in purpose made plywood forms. Most of the concrete was mixed on site but some of the large abutment pours used pre mixed concrete delivered to the site in agitator trucks. All mild steel reinforcement steel was cut and bent on site.

Superstructure beams were post tensioned after the 9 feet 9 inch (2.97m) long I beams were accurately placed on trestles, the 3 inch (75mm) gaps filled with fine aggregate concrete and the longitudinal stressing cables threaded through the anchor blocks and diaphragms. The Gifford Udall stressing system, employing 19 wire 0.69 inch (17.5mm) diameter strands, placed externally to the webs, was used. Due to the need to access anchorages at the ends of the completed beams the suspended beams had to be post tensioned at an elevated position and jacked down to their final location. The prestressing cables were covered by a pumped grout after stressing and the surface of the grout coated with neoprene.

The external beams are clad with exposed aggregate facing panels made from white cement, white sand and white quartz aggregate, matching the grit blasted soffits of the cantilever footways which were also cast in white concrete.

The bridge was completed in September 1959 and officially opened by the Governor, Sir Charles Gairdner on 13 November 1959.

There is no doubt that the designers and the contractors produced a structure of considerable elegance and world wide repute and amply fulfilled the brief that the bridge should harmonise with the aesthetics of such an attractive site.

The Narrows Bridge was a crucial element in and the first manifestation of the implementation of the 1955 Stephenson-Hepburn Report “Plan for the Metropolitan Region, Perth and Fremantle” which changed the development of metropolitan Perth from a east west axis to a north south axis.

At the time of its opening the Narrows Bridge had a number of firsts :

It was the largest precast, prestressed continuous beam bridge in the world.
Its centre span of 320 feet (98m) was the longest of its type in the world.
It was the first bridge in Australia to use a segmental construction method.
It was the first bridge in Australia to use external prestressing cables, allowing for a minimum web thickness.
It was the first major public infrastructure project since the end of World War 2 to be constructed by contract.

In 1999 it received Engineers Australia’s highest accolade, a National Engineering Landmark Award.

Prepared by Don Young 2012

Also available is an ABC Radio Perth item titled Sixty years since the opening of Perth's Narrows Bridge — an engineering feat that transformed the city by Emma Wynne at https://www.abc.net.au/news/2019-11-13/perth-narrows-bridge-60-years-since-construction/11697812. A print of the webpage is available here.