This major article was first published in 1938 in Wonders of World Engineering, a multi-part series. The story is one of the most detailed – yet readable - discussions on the engineering of the Sydney Harbour Bridge ever produced.
And the bridge itself? It remains an engineering marvel.
The wonderful sheet of water lying inside Sydney Heads, Australia, has every requisite that a first-class harbour ought to have. It is easy of access and free from sunken dangers. It is large—big enough to accommodate the combined navies of the world—yet its shape—a number of inlets all branching away from a central channel—gives perfect shelter from all the winds that blow. In addition its waters are uniformly deep, its shores steep and the holding ground excellent.
Its natural and overwhelming advantages induced Governor Phillip, sent out from England to form a settlement at Botany Bay, to remove on January 26, 1788, to a new site in one of the harbour's many inlets. He christened this Sydney Cove, after Thomas Townshend, first Viscount Sydney, who was then Secretary of State for the Colonies. The century and a half which have elapsed since Phillip first hoisted the Union Jack there has seen his tiny settlement grow into a city of well over a million inhabitants. This rapid growth, in recent years, has been directly responsible for spanning the main channel of Sydney's magnificent harbour, from north to south, with a miracle of modern engineering—Sydney Harbour Bridge.
From the Heads at its entrance, facing eastward into the Pacific Ocean, the main channel of Sydney Harbour runs inland, first south-westward and then westward, for some thirteen miles. The original settlement, forming the nucleus of modern Sydney, was on the South shore, about five miles from the entrance—and here the distance to the north shore is only about 600 yards. The harbour is so extensively indented, however, that although its area is only about 22 square miles, its shore line is more than 180 miles long. Thus rapid communication with the north shore – which offers many advantages as a residential neighbourhood—is impracticable by land.
Until recent years, therefore, communication was effected by ferry-boats, plying to and fro busily and carrying as many as 40,000,000 passengers a year. As in the parallel instance of New York and its neighbour Brooklyn, the necessity for a connecting bridge (or, alternatively, a subway) between Sydney proper and North Sydney, across the harbour, became greater every year. At the present rate of growth, for instance, the population of Sydney, already 1,255,000 will have exceeded 2,000,000 by 1950.
As long ago as 1815 some residents of Sydney put forward a proposal for bridging the harbour; but, as the type of bridge then proposed (a low wooden one on piled foundations) would have completely sealed the inner half of the harbour to shipping, the plan was at once turned down by Governor Macquarie. Other schemes of the kind were submitted, at intervals, during the last century and met with the same fate ; but in 1900 the New South Wales Government, observing the enormously accelerated rate at which the capital was spreading, instituted a competition for the best design of a harbour bridge complying with various rather stringent conditions. This met with a ready response, and many designs were submitted; but, in the event, no action was taken.
In 1911 and later years the question of connecting and extending the main and suburban railways on either side of Sydney Harbour formed the subject of a long inquiry conducted by the Public Works Committee. An interim report recommended the building of a subway, under the harbour, connecting Sydney with North Sydney. But further investigation showed that the erection of a bridge, although fraught with serious difficulties, was a preferable solution.
In 1915 a final report to this effect, by Dr. J. J. C. Bradfield, the Government's engineer, led to the passing of an Act authorizing the proposed extension and coordination of the railway systems and as a corollary, the building of a high-level rail and road bridge across Sydney Harbour from Milsons Point to Dawes Point—immediately to the westward of Sydney Cove. In January 1923 the New South Wales Government invited tenders for the building of such a bridge, stipulating that these must conform with the general principles governing its position, span, layout and so forth laid down in Dr. Bradfield's report, but leaving the competing firms a reasonably wide scope for the design.
The width between the two shores at the points named is slightly more than 1,600 feet. As several important docks and wharves are situated higher up the harbour than the site of the proposed bridge, it was necessary for the lowest portions of this to have sufficient height above water to clear the masts of the largest vessels. For the same reason, it was stipulated that neither the finished design of the bridge nor the operations performed while it was being built should occasion any obstruction to the passage of shipping. This ruled out any reduction of the main span by the building of a mid-channel pier or piers and it also forbade the designer to support his main span from below while it was under construction.
It was laid down that provision should be made for a central, roadway wide enough to take six lines of traffic, and flanked on either side by two lines of railway and a footway. These had to be designed to bear the following loads. The footway must be able to support a loading of 100 lb, per sq. ft. The roadway must be able to bear, in any given area measuring 30 feet by 12 feet, the weight of a motor lorry (12 feet by 6 feet wheelbase) whose axle-loads were 18,000 lb. (front axle) and 36,000 lb. (rear axle)—a total of rather more than 24 tons. Each of the four railway tracks must be able to bear the weight of a train composed of two 160-tons locomotives, each 65 feet long, followed. by carriages weighing 2,200 lb. per foot of length.
Such were the conditions governing the design of Sydney Harbour Bridge. In combination they presented a problem of considerable difficulty and complexity. The wide main span, great height above water, and heavy designed load, recalled two famous engineering triumphs of former days—the Britannia and Forth Bridges—but in the present instance the obstacles to be overcome were decidedly greater. In the Britannia Bridge the width of the shore-to-shore span was much less (900 feet as against 1,650 feet). It was halved by the presence, in mid-channel, of the Britannia Rock, on which a central pier could be erected; and the consequent division of the main span into two portions had enabled Robert Stephenson to block one half of the navigable channel for as long as he pleased, while his enormous tubular girders were being hoisted into position, keeping meanwhile the other half open for navigation.
Again, although in the Forth Bridge the length of the two main spans (1,710 feet each) slightly exceeded the span of the proposed Australian bridge (1,650 feet), this was offset by the fact that the Forth Bridge was required to carry only two lines of railway (as against four) and no roadway. In addition, the isolated situation of the Forth Bridge made it a comparatively simple matter to dispense with central supports for the main spans (while these were under construction) by forming the spans of double, symmetrical cantilevers which could be built out from both sides of the piers simultaneously. The weight of one incomplete truss was thus balanced by that of the other. The cantilever principle was, however, seriously considered for the Sydney bridge and more than one contractor submitted designs incorporating cantilevers.
The contract for the new bridge was finally allotted (March 1924) to the British firm of Dorman Long & Co., of Middlesbrough. Their consulting engineer, Mr. Ralph Freeman, had prepared seven individual The New South Wales Government ultimately selected the last-named type, the total estimated cost being £4,217,721. The structure was to be chiefly of steel, but the design also embodied granite-faced concrete piers and pylons, the architectural details of these features conforming to designs provided by Sir John Burnet and Partners. Expert advice as to the reinforced concrete portions was furnished by Dr. Oscar Faber, O.B.E., M.Inst.C.E.
"Light" and "Heavy" Shops
IT was a condition of the tender that the steelwork should be fabricated on the spot at Sydney, thus providing employment for Australian labour. Dorman Long & Co. therefore erected extensive workshops on a site at Milsons Point, near the north end of the new bridge. The two main workshops, known respectively as the "light" and "heavy" shops from the nature of the work undertaken in them, were each some 500 feet long by 150 feet wide. Their equipment comprised electric travelling cranes capable of hoisting 120 tons each (some of the heavier portions of the bridge required two such cranes to handle them effectively), planing machines able to edge-plane plates more than 60 feet long, a wall-planing machine able to finish surfaces 12 feet by 10 feet, a plate-straightening machine able to tackle pieces 12 feet wide and 2 1/4 inches thick, high-speed drills, and all the thousand and one special items of machinery required for a modern engineering work of first-class importance.
In addition, a quarry for the supply of the necessary granite and crushed stone was opened at Moruya, a coastal settlement some 150 miles south of Sydney, and a small township was built to accommodate the workmen (most of whom, recruited specially for the work, came from the Granite City—Aberdeen) and their families. Here again the plant required was shipped out from England, but the transport of the stone from Moruya to Sydney was performed by, a little fleet of three steamers of 400 tons carrying capacity, built in New South Wales. On their return trip to Moruya these vessels brought cargoes of food supplies and drinking water, it being impossible to procure sufficient food and water locally for the 240 men employed at the quarry and for their dependents.
After a journey of some 12,000 miles, the components of the great bridge, produced at Middlesbrough, arrived at Milsons Point in the form of steel plates and bars. The total weight of steel to be worked into the huge structure was 51,000 tons, of which 38,000 tons were required for the main arch.
Apart from a small amount bought from an Australian firm, and a few special pieces such as the main bearings of the arch, the whole was manufactured by Dorman Long & Co. from their own raw materials. Once on the spot the material was straightened, planed, finished, drilled and rapidly built up into an endless series of intricate components, all destined to form part of one great whole, remarkable alike for its huge size and for the accurate finish and fitting of even its smallest parts.
The design of the bridge provided for a single great central arch of 1,650 feet span, crossing Sydney Harbour from north to south in a single stride, and rising at its centre to a height of 440 feet. From this, the trackway was suspended at a mean height of 170 feet above sea-level, passing off the arch at either end on to a series of five approach spans, formed of horizontal girder structures on granite piers.
These approaches, with a span of 238 feet, presented their own problems—the northern approach had to be set out in a marked curve—but by comparison with those involved in the erection of the main arch, such difficulties as they introduced appeared almost trifling. For the engineers at the time, as for the amazed visitors to-day, the enormous main arch was the central feature of the whole bridge—the one by which it must stand or fall.
The main arch is really two arches side by side, exactly parallel, 98 feet apart, and connected by cross-girders. Each of these arches consists of an upper and a lower member (termed the "upper and lower chords", although each is really a series of short chords along an enormous imaginary arc) formed of tubular box-girders, and connected by vertical struts (open lattice-work girders).
These vertical struts, which gradually diminish in height from 188 feet at the abutments of the arches to 60 feet at the crowns, divide each arch into twenty-eight panels, and each panel is traversed by a diagonal strut running from the head of each vertical strut to the foot of the next innermost. Seen in profile, therefore, each arch appears to be composed of a series of huge capital Ns, gradually diminishing in size.
A leading feature of the design is that the whole weight and thrust of the gigantic arch are transmitted to its piers through four enormous hinges, one at either end of each of the two lower chords. Mounted in this manner, the arch is free to rise in the centre when expanded by heat—a necessary precaution in a metal structure of such enormous size. The amount of expansion is much greater than might be imagined. The total length of the whole bridge, from one end to the other (approximately 3,770 feet) is roughly a foot greater by day than at night, and special expansion joints are fitted at intervals along this length to allow for expansion. It has, however, little effect upon the height of the track-way above the water, as the upward lift of the main arch is largely neutralized by the downward expansion of the vertical hangers by which the trackway is suspended.
The work of clearing the sites for the approaches had begun in July 1923, long before the contract for building the bridge was awarded; but the constructional work dates from January 1925, when a start was made upon the spans of the southern approach. The site for the northern spans became available some months later. This part of the work, being carried out on land, and with the spans supported on timber falsework during construction, presented little difficulty.
With the great main arch (whose weight was to be taken by four hinge-bearings, two on either side of the harbour) the initial step was to ensure that the foundation sites for those bearings were secure, and the bearings themselves set down in perfect alignment. In the position selected for the bridge, the shores of the harbour are of sand-stone rock. To keep the dimensions of the arch as small as possible, the sites selected for the bearings were not more than 50 feet from the waters of the harbour.
Special cofferdams having been placed to ensure that no water should find its way into the workings, the sites for the bearings were excavated to a depth of 40 feet over an area 90 feet long, and 40 wide. On the floor of these pits exploratory holes were then drilled to a further depth of 75 feet.
Bearing Surface 504 Square Feet
The result of these tests was satisfactory, and indicated that the sandstone on which the thrust of the bearings (approximately 13 tons per square foot) would ultimately come was entirely solid, without cracks or fissures. The excavations were then carefully filled up with strong concrete—four parts crushed granite, two parts sand, and one part cement.
On the foundations thus formed the hinge-bearings were installed. Except in principle, these structures were as different from the ordinary conception of a hinge as can well be imagined. Manufactured throughout by the Darlington Forge Co., Ltd., each weighed 300 tons, and had a bearing surface of 501 square feet. Their main components were machined true to 1/1000th inch and the bolts were made with a limiting error of 1/10,000th inch. The precautions taken to get the bearings into true alignment were extreme. On this depended the success or failure of the whole structure.
The pairs of bearings were situated at the corners of an enormous rectangle, the distance between the members of a pair being nearly 100 feet, while a third of a mile of water divided one pair from the other. Yet it had to be ensured that the axis of one bearing, if produced, passed exactly through the axis of its neighbour; that the common axis of the northern pair was exactly parallel with that of the southern pair; and that both axes were exactly horizontal, as they would form the only fixed 46 points of reference in the whole structure.
From these bearings were to start, sloping upwards and outwards over the water at an initial angle of 30°, the two widely separated halves of the giant arch. These halves were destined ultimately to meet in mid-air at a point some 900 feet away and 400 feet above the water. The maximum permissible horizontal deviation when they should meet was not much more than four inches, but no appreciable deviation occurred.
A fixed mark was therefore set up on either shore midway between the bearing sites; the two marks were connected by direct triangulation, a base line 1,200 feet long being measured off eastward along the northern shore. Angles to the marks were taken from either end of the base line by theodolite. As a check, a second base line of much the same length as the first, but designedly not run quite parallel with it, was also laid off, and a second triangulation was made. The relative positions and bearings of the two fixed marks having been accurately established, the exact sites of the four bearings were next laid off from these by direct measurement.
The bearings themselves, which had meanwhile been installed on a temporary steel framing above the concrete foundations, were then adjusted bodily into exact position by hydraulic jacks, and there secured. So well was the work performed, that when the two halves of the arch ultimately met the error of alignment was so small as to be negligible.
The main problem, however, had yet to be tackled. That was to support the enormous weight (about 14,000 tons each) of the two half-arches while building them out into mid-channel. The natural plan, to support them from below, was out of the question, as it involved obstructing the free navigation of the harbour.
Strain Taken by Anchored Cables
Telford, long before, had proposed to build arched bridges, of wide span, without using any support from below —support being provided by wire cables, led from points on shore over a tall pylon at either end of the span. This plan was afterwards used with success by Brunel, and it may perhaps have provided a hint for the method adopted at Sydney.' This was to take the strain caused by the immense overhanging weight of the half-arches, as they were built out, by wire cables anchored in the ground.
The design of the bridge provided for a granite-faced concrete pylon at either end, located immediately in rear of the giant would reach a height of 285 feet, but, while the great arch was in progress, their erection was stopped at 155 feet from the ground to afford clearance for the anchorage cables and to permit the erection of the giant creeper cranes.
A long way in rear of the pylons were dug the anchorages for the supporting cables. It takes much anchoring, even in sandstone rock, to hold a pull of 30,000 tons or so, and each anchorage took the form of a U-shaped tunnel, the sides of the U being about 100 feet apart, and driven into the rock at an angle of 45° from the vertical. The inner curve of this tunnel was lined with grooved sheets of steel, against which, and the sandstone backing it, the cables exerted their enormous pull.
The point at which the cables were attached to the Half-arches was that at which the outermost of the vertical struts between the two chords met the upper chord. This point gave the maximum leverage around the hinge-bearing that could be obtained without building up a special structure for the purpose above the end of the arch. In all, 128 wire cables were used, each 1,200 feet along, of 2¾ inch diameter and each able to sustain a strain of 360 tons. The maximum strain imposed on them, however, was about 125 tons. Each cable ran from the top of one end-post of the arch, over the pylon, into and through the tunnel, and so to the top of the other end-post. Special steel saddles, to prevent any chafing of the wires, were installed at the pylons and the tunnel entrances.
Work on each half-arch was begun by getting the outermost panel into place and bolted. During this time the fabric was prevented from turning round the hinge-bearing by a set of eight temporary wire cables, anchored to the end-post a little below its head. It was impossible, at the moment, to fit the full complement of 128 permanent cables, as their points of attachment to the end-posts were occupied by a sloping steel ramp, extending from the head of the pylon to the upper chord. Up this ramp, as soon as the first panel was completed, there slowly hauled themselves two enormous electric creeper cranes, weighing 602 tons each and capable of handling loads of 122 tons. As soon as these were safely in position straddling the upper chords, the ramps were demolished and the work of attaching the permanent cables began.
It was absolutely necessary to ensure, first, that every cable was made fast to the arch without any possibility of the attachment giving way, and, secondly, that each wire was taking its fair share, and no more or less, of the total strain. An unequal distribution of the load would mean that, as the strain on the cables gradually increased with the outward growth of the half-arches, the cable most overloaded would ultimately snap. This would at once increase the load on some other cable, already dangerously near breaking point. That, too, would give way and transmit its load to break another—and the whole giant fabric, crowded with workmen, would come crashing down hundreds of feet into the water below.
Closing the Gap
To safeguard these vital points, the end of each cable was spread out and secured, by a plug of fused white metal, into a metal socket, this plan providing an attachment equal in strength to the cable itself. These sockets were secured, by equally strong nuts and bolts, in a gigantic steel container, being arranged in eight rows of sixteen. The sides of this container formed the end bearings of a steel hinge-pin 12 feet long and 27 in. in diameter. The central bearing of the hinge was in the end-post of the arch. Thus the - container was free to rotate slightly about a pivot and to maintain an equal distribution of the load on all the cables. Extra cables were added as the half-arches progressed outwards, care being taken to adjust the tension in each new cable so that all cables were equally stressed.
Exact adjustment of the cables was effected by the nuts and bolts. After each cable had been given a provisional loading of about 10 tons, the final tensioning was effected by a hydraulic jack. A pressure gauge on this indicated when the desired tension had been reached. The nuts were then set up and the jack removed.
The first panels being thus firmly anchored, the sides of the great arch began to extend outwards over the harbour panel by panel, the outermost panel always topped by the creeper cranes. These, built and tested in England, were really a battery of cranes on one platform; for, in addition to the main jib-crane (105 feet long, and also carrying a supplementary 20-tons hoist), they embodied two 212-tons derrick cranes and a 5-tons wall crane. With these cranes, gradually advancing on four-wheel bogies along the upper chords as the work advanced, every piece of material was picked up, lowered into place and built in. No use was made of chain or rope slings throughout the work, every piece incorporating its own special lugs for hoisting.
The provision of hinge-bearings at the feet of the half-arches enabled these to be moved slightly up or down, similarly to the halves of a bascule bridge. Since it was better and simpler to lower than to raise them, provision had been made, when adjusting the tension of the anchorage cables, to ensure that when the half-arches were completed (at the fourteenth panels) there should be a slight gap between the two inner faces, which could be closed by slightly easing the cables. The upper ends of the outer end-posts, therefore, had been raked back slightly from the vertical—a distance of 2 ft. 6 in. at a radius of 188 feet.
It was calculated that, after having made allowance for the inevitable slight stretch of the cables under their enormous load and for the equally inevitable slight deformation of the girders as the weight came farther and farther off the line of the hinge-bearings, there would be a gap of about 3 feet between the inner ends of the half-arches, with a possible variation of about a foot more or less, depending upon the temperature.
The meeting of the north and south portions of the arch, in August 1930, showed how accurately the work had been planned and executed. So perfectly had the alignment of the hinge-bearings been secured that the provision made for forcibly aligning the half-arches laterally by hydraulic jacks (which could eliminate a divergence not exceeding 41 in.) proved superfluous. The gap between the two faces was well within the limits allowed for, and the sole observed deviation was a slight inequality in the height of the two ends above water. This was due to a difference of expansion, the sun's heat having a greater effect upon the northern side of the arch.
The gap at the summit of the arch was next closed by slacking off the anchorage cables, a delicate process which occupied a fortnight. On August 19, 1930, the hoisting of flags on the creeper-crane jibs, and an answering roar of sirens from the shipping in the harbour, indicated that the halves of the great arch were in permanent contact, and its lower chord was completed. This was effected by uniting the halves of the chord with steel pins of 8 in. diameter, forming a hinge-bearing.
The arch, however, was not yet in its final condition as a two-hinged structure. It was necessary to bring the upper chord into a state of compression. This was effected by forcing the butting faces of its two halves slightly apart by hydraulic jacks, and inserting steel filling-pieces, carefully machined to the exact thickness required. The completed joints of upper and lower chords were then covered with plating. The weight of the bridge, the main load of 28,000 tons, remained, as before, distributed over the four hinge-bearings at the extremities of the lower chord.
Safeguards Against Contraction
The great arch once finished, the creeper cranes, which had met at the summit, reversed their progress, moving slowly down the slopes of the upper chord as they hoisted into place the long vertical hangers, weighing anything up to 35 tons each, by which the horizontal trackway was suspended from the arch.
Meanwhile, the wire cables, already slacked well off so that no contraction through cold could cause any tendency to open up the central arch joints, were finally removed and shipped back to England. The creeper cranes were dismantled after they had finished their homeward journey and come to rest over the first panel of the arch. Finally, the pylons were completed to their full height of 285 feet, in the form of twin towers, each pierced for a footway and one railway track. The two inner railway tracks and the roadway run in the space between them.
The completion of a continuous thoroughfare, at rail level, between Dawes Point on the south side of the harbour, and Milsons Point on the north, took place on February 17, 1931. But much still remained to be done, and Sydney Harbour Bridge was not ready for its formal opening until more than a year later, on March 19, 1932.
The city of Sydney could already claim to have the finest harbour in the world. With the completion of the bridge her inhabitants obtained the world's largest arched bridge. The magnificence of Australia's finest bridge is such as to make an indelible impression on any one seeing it for the first time.