Website Banner. John Monash: Engineering enterprise prior to World War 1.

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Napier Street Swing Bridge
Saltwater (Maribyrnong) River, Footscray
(1895-1958).

Elevation. Based on a copy of the Melbourne Harbor Trust drawing, Contract P, Drawing No.1.

A photograph of the bridge may be seen in the Picture Victoria Collection with ID No. 4854.

Introduction

Monash took pride in his involvement in the design of this bridge while working for the Melbourne Harbor Trust in 1892 (aged 26) under the direction of the Chief Engineer, A M Alexander. The word 'design' as used by structural engineers can mean anything from 'initial conception of form' through to 'computing the necessary sizes for components'. Monash certainly carried out the computations, but it is difficult to know to what extent he shared in the overall conception. The bridge was officially opened in December 1895. It was last swung in the 1930s, presumably because river traffic had become less important. By 1948 it was severely corroded and was replaced by a fixed concrete bridge. Its bluestone abutments still exist and are included in Victoria's Heritage Inventory, with HI Number H7822-0434.

History

In 1889 the Melbourne Harbor Trust sought control of Crown land adjacent to the Maribyrnong (then Saltwater) River to build warehouses. The land was then under the control of the Footscray Council. Influential locals, including the Mayor and Member of Parliament, struck a bargain whereby, in return for the land, the Harbor Trust would provide Footscray with a new bridge across the river, plus a road leading to the City of Melbourne. Both were to be maintained thereafter by the Trust.

Monash had joined the Trust in November 1891 having previously worked for contractors building Melbourne's Princes Bridge and Outer Circle Line. The Trust's Chief Engineer, A M Alexander, probably chose the type and basic form of bridge to used and then handed the detailed design to Monash. The computations, dated February and March 1892, are preserved amongst his personal papers in the National Library of Australia, together with his account of the rationale of the design.

According to the Footscray Independent the bridge was the first of its kind in Victoria. Its physical characteristics were described later in the same newspaper as follows:

[It] … has a length over all of 208 feet, or 196 feet 8 inches [60m] between the abutments, the deck width is 30 feet [9.1m], while on each side there is a pathway 5 feet in width [1.5m], and protected by a lattice handrailing. The deck itself is supported by buckled plates, covered with a layer of concrete six inches [152mm] in thickness and finished with a paving of wooden blocks. The structure is supported on four cast-iron cylinders 8ft. in diameter [2.43m], let down to solid rock and filled with concrete. These cylinders support a frame of plate girders 4ft. in depth [1.22m], which in turn support the roller path 35ft. 9in. in diameter [10.9m] for a live ring of 40 conical rollers. Hand turning gear of a rather novel form has been provided. As the bridge rolls on the top of the live ring, it is geared into the segmental racks on the live ring, it needs only to be equal to an eighth of the circle, because the bridge travels twice as far as the live ring in the same time. Two men can work the centre windlass used in turning. The abutments of the bridge are of Malmsbury bluestone and Harcourt granite; and they are at once handsome and substantial. The foundations which support the masonry are of concrete, built on the solid rock 19ft. 6in. [6m] below low water mark. The total weight of the bridge is about 600 tons [610 tonnes] - the iron work (principally mild steel) weighs 396 tons, and the concrete and wood paving about 200 tons.

The relevant articles may be seen on the National Library of Australia's Trove website, (27 May 1893) and (21 Dec 1895).

The earlier article mentioned that provision had been made for installation of "hydraulic or other" power to turn the bridge in the future.

The contract for the construction was let in November 1892 to H. McKenzie & Sons with completion expected in November 1893. McKenzie at first proposed to have the steelwork fabricated in England, and Monash was hopeful that he would be sent there by the Trust to keep an eye on quality. However, the imported steelwork would have been subject to a 35 per cent import duty, and this persuaded the contractor to have it fabricated in Melbourne.

As often happens, McKenzie's progress was slower than expected. In the meantime the Trust's workforce was downsized in the midst of economic depression and in April 1894, before the substructure was complete, Monash was also retrenched. He contacted his former lecturer at the University, J T N Anderson, and they went into partnership as consultants and contractors. In listing their credentials when seeking appointments they made mention of Monash's experience with the Napier Street Swing Bridge.

The steelwork for the swing platform was completed some time after October 1894 and the superstructure assembled, resting on the roller path. When the heavy deck was added, with its layer of concrete topped by redgum blocks, the steel girders that supported the roller path deflected more than expected under the weight. Rumours to this effect became public towards the end of May 1895. On 29th, Anderson wrote to Professor Kernot at the University that the deflection of a girder "in which we are interested" was "causing anxiety to some people". "We calculate that when loaded to give a stress of about 6.5 tons [100 kPa], the deflection will be about 0.65 inches [16.5 mm] allowing for loose rivetting etc." The observed deflection was 0.75 inches [19.1 mm]. "Is this sufficiently alarming to warrant strengthening?" Kernot checked Monash's calculations and judged the design adequate.

Alexander, who had been publicly credited with the design, happened to have left the Trust and was not present in Victoria. Monash therefore took the responsibility of writing to the Chairman of the Trust:

My attention has been called to certain statements which are in circulation respecting the strength and stability of the Saltwater Swing Bridge now in course of construction, more particularly in regard to the amount of deflection of the girder forming the rollerpath framing. As I understand these rumours, an alarm has been raised that this framing is in a dangerous condition, is unsafe, and requires the expenditure of a considerable sum of money to strengthen it. You will no doubt remember that this structure was one of those designed by me, while Chief Draughtsman of the Trust; and I am therefore in the best position to afford any information and assistance on the point now raised. I have the whole of my original calculations of strength before me, and have again gone carefully into them. I shall be glad to submit them to your Engineer, or to the expert to whom this matter has been referred; and to supply any other information that will be of service to you. I beg now to assure you that if carried out according to design, there is no cause whatever for anxiety in regard to those girders; that they will, under the heaviest load that can possibly be placed upon them have a margin of safety of about four to one, and that the above[?] reported deflections even if correctly observed must in part be due to some other cause in which the strength of the structure is not concerned. I have submitted the question to the highest authority in Victoria (Professor Kernot) and his opinion, which I enclose, fully confirms my view. I also take [?the view that changes to the?] design (viz: the bracing of the central panel of the main girders) were utterly needless, useless, and only serve to add dead weight to the bridge, while atrociously disfiguring the symmetry of the design. John Monash MCE, Assoc. M. Inst. C. E.

The Argus reported that the opening of the bridge, already some 18 months behind schedule, was likely to be delayed for a further period, "as some of the stays and braces are said to be too slight to bear the weight of the superstructure … whether the fault was in the designs or in carrying out the specifications does not yet appear".

The Age was more trenchant in its criticism. "One of the versatile achievements of the Harbor Trust is the construction of a road through Footscray to the Saltwater River at a cost of some thousands for the apparent benefit of the corporation of Melbourne, within the boundary of which the road is situated. Following this up, a swing bridge was designed by Mr. Alexander, who has since left the service of the Trust as chief engineer upon a retiring allowance. A contract for the construction of the bridge at a cost of £18,000 was let to Messrs. McKenzie and Sons, and that it was performed to the satisfaction of the Trust may be judged from the fact that the deposit was returned some weeks ago. There is still, however, a claim for £1300 for extras; but the most perplexing feature of the transaction is that the bridge has turned out to be unworkable. The engineers of the Trust consider an expenditure of £1000 or thereabouts will be required to remedy a deflection of about an inch which occurs when the bridge is moved, rendering its "swing" inoperative. The members of the Trust are evidently unable to blame the contractor, and have decided to call in two engineers from the Railway department to report upon the best means of strengthening the bridge. In the meantime the City Council will gain no advantage from the generosity of the Harbor Trust in expending between £30,000 and £40,000 for its benefit."

Monash wrote to the Editor on 11 June:

Your organ is distinguished as much for the care it takes to secure accurate information, as for its fearlessness in exposing public scandals. We therefore feel sure that you will publish the following facts, which are of value, as a supplement to the brief article on the above subject which appeared in your issue of Friday [7th]. The state of affairs with regard to this Bridge might cast a serious imputation upon its designers. Mr Alexander, the late Engineer in Charge of the Harbor Trust, being absent from the colony, it devolves upon our Mr Monash, who prepared the design under Mr Alexander, to assume any responsibility that has now arisen. The defect which has caused so much consternation is the 'sagging' of the four main foundation girders, which rest upon the central piers, and carry the roller path [bearing?] the whole weight of the moving bridge, which is about 150 tons on each girder. If these girders had been built absolutely straight (as they ought not to have been, but as they no doubt were) they would very naturally sink, as the load was gradually placed upon them. To provide for such sagging the girders should be 'cambered' to a degree which depends upon the workmanship to a large extent. The amount of camber was therefore advisedly left to the contractor, so that he should take full responsibility that when fully loaded the roller path should be perfectly level. That the intention of the designers was not carried out is unquestionably due to the extraordinary action of the Harbor Trust, who dispensed with the services of their engineers while the bridge was in the hands of the contractors. Such a case of 'swapping horses when crossing a stream' is most improper, and shows the small sense of responsibility felt by this body in the expenditure of public money. As for the rumours, that the bridge has been found unsafe and unworkable, we give them the most complete denial. The observed 'sag', though it should not have been permitted to occur, does not constitute the slightest danger, as every part of the structure is amply strong; in this opinion we are confirmed by the highest colonial bridge expert, Professor Kernot. Yet the Harbor Trust commissioners have been on the very point of incurring a further expenditure of nearly £1000 in handling[?] this imaginary danger. If they want to spend their money in strengthening bridges, we can point to several which need it much more than the new Saltwater Bridge.

I have not yet checked whether this letter was published.

However, the Trust went ahead and borrowed one of the Railways Department's bridge experts, J H Fraser. He assisted the current Chief Engineer, J Halliday, to check the design and reported that, while the "stability of the girders was undoubted, there was a weakness in the roller path". The Trust then approved the expenditure of £671 2s. 6d. on remedial measures. "In discussing the report of the committee, Commissioner White, M.L.A., desired to know who was to blame for the weakness of the bridge; and Commissioner Mountain, in reply stated that the bridge was constructed under the supervision of Mr. Alexander, a former engineer of the trust, who had since left the colony" (Trove follow through to page 5). Halliday later lent Monash a copy of the report.

Thus Alexander received public blame for the alleged inadequacy in design, while Monash blamed the contractor for failing to provide an upward camber in the supporting girders, and the Trust for false economy in failing to provide adequate supervision. It is reasonable to speculate that Monash was unconvinced by the report and maintained his pride in the design.

To the frustration of the locals, opening of the bridge was further delayed. Finishing touches included the erection of fences and gates, for which tenders had to be called. McKenzie claimed payment for extra work not included in the original contract, while the Trust was entitled to impose a financial penalty for late completion. These matters had to be thrashed out by negotiation before the bridge could be officially handed over to the Trust. The contractor hired to build the approach roads pulled out, and fresh tenders had to be called. The bridge was finally opened to the public on 18 December 1895 with appropriate ceremony.

Service life

As noted above, the bridge functioned as intended until the 1930s, when the need to swing it ceased. It continued to carry road traffic, but the Trust's responsibility to maintain it ended about 1940 due to the construction of a road connecting Napier Street with Dudley Street. In 1948 a review revealed severe corrosion in the steelwork. The following year a load limit of 10 tons and a speed limit of 10 miles per hour were imposed. It was replaced by a new, fixed, structure under the name "Shepherd's Bridge" in 1958.

Thanks to Dr Max Lay for this information.

Comment. I have not yet tried to find detailed drawings of the turntable, or Fraser's report, and it is difficult to know from the newspaper reports exactly what the problem was. The terms 'strength', 'stability', and 'weakness' are used, but Anderson identified the problem as (slightly) excessive deflection, implying insufficient 'stiffness' - a term that does not appear. At first attention was directed to the supporting girders, but Fraser seems to have suspected the roller track itself. Notes made during one of my hurried visits to the NLA show that Monash's calculations included the main girders, cross girders, longitudinal bearers, and track frame; the stability of the central pier against wind forces; and a graphical analysis of forces in the trusses (Williot-Mohr method). His mechanical calculations included the inertia of the flywheel and the gearing of the turntable.

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