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

[Introduction.]   [Main JM Index.]  [Bridges Index.]  [People Index.]
[Abbreviations.] [Units & Currency.] [Glossary.]

Tooronga Road Bridge.

Introduction.

View along creek to what appears to be an arched bridge. The "arch" is very flat in the middle and tightly curved at the ends. However, this is a facade concealing haunched girders. The bridge is being tested. A steam roller stands at mid-span.

Load Test (1914?)
University of Melbourne Archives NN/980
Reinforced Concrete & Monier Pipe Construction Co Collection

University of Melbourne Archives UMA holds 17 images relating to this bridge, with Location Numbers NN/967 to NN/983. These show stages of construction and views of the load test involving two steam rollers.

This bridge, carrying Tooronga Road over Gardiner's Creek, between the Melbourne suburbs of Hawthorn and Malvern, was completed late in 1914 or early in 1915, and demolished in the 1960s to make way for the South Eastern (now 'Monash') Freeway. If its form is interpreted as a portal frame, it is an early example of this type. John Monash carried out the initial conceptual design. Detailed dimensioning was completed by J A Laing.

Simple diagram showing a "portal frame" comprised of a deep haunched girder and colums.

The deck was supported by eleven parallel girders, 3'-2½" (980mm) deep and 12.5" (320mm) wide, supported on 'legs' of the same width (see figure above). The legs were founded on slabs resting on 'solid' Schist rock at about creek level. The clear span between the inside faces was 60 feet (18.3m).

The deck slab works with the girders to form T-girders. The abutment face walls do the same for the columns.

The structural simplicity of the 11 portal frames was of necessity complicated by the presence of the deck slab joining the tops of the horizontal girders; retaining walls joining the inner edges of the legs; and horizontal base slabs on the rock foundations. (These are seen edge-on in the figure above as thick black horizontal and vertical lines.) The walls retained the earth filled in behind them to support the approach roads. The base slabs were provided with a key (short vertical projection) to prevent sliding.

Perhaps for 'architectural' reasons, the outer girders on each side were shaped to suggest a flat elliptical arch (photo above).

Design and winning of contract.

The earliest document in the RCMPC project file is a list of "Notes taken in the field", dated 14 March 1911, followed by Monash's rough computations. He was already thinking of a portal frame at this stage, though he used a simpler concept for his structural analysis. He estimated the cost of construction at £2420 for a span of 50 feet (15.2m). A notation "for loan" against this figure suggests that he planned to borrow to finance the project.

Monash sent a rough estimate of price to E F Gilchrist, City Engineer of Malvern. He had had contact with Gilchrist since at least 1900, in regard to a bridge project at Charlton, and later at Warrnambool. The City Engineer of Hawthorn at the time was James Meldrum, who had been Engineer for Nathalia in 1901 where M&A had proposed a Monier arch bridge, and Engineer for the Numurkah Water Trust in 1905 when RCMPC built the Wunghnu Service Reservoir. Appointed to Hawthorn in 1907, Meldrum had continued to show interest in reinforced concrete for tanks and drainage.

In January 1913, RCMPC's sales representative C H Goodwill reported that, after long consideration, the two Councils were close to action. He thought Gilchrist favoured reinforced concrete while Meldrum wanted a brick arch. Monash found this hard to believe and in private communication was assured that it was not the case. The Hawthorn Council had instructed Meldrum to call tenders for a brick arch, but he was confident they would accept an alternative bid "for a cheaper structure if such is forthcoming".

About the middle of the year, Gilchrist left Malvern and J Nelson Muntz became acting City Engineer.

Meldrum completed his own design for the bridge in December 1913 and showed Monash the drawing and specification. Although correspondence about this time refers to a brick arch, as required by Council, the drawing shows a somewhat idiosyncratic reinforced concrete arch along the lines of M&A's Monier arches. Both the structure and the reinforcement are massive. Meldrum estimated its cost at £4000.

Nelson Muntz lent Monash a tracing of the drawing, from which he made a blueprint. This provided handy information concerning the site and the proposed straightening and widening of the creek. JM calculated that the cost of constructing Meldrum's version would be £4349, and decided to set RCMPC's tender at £3514.

In mid-January, Monash checked the stability of the 'legs' as abutments. He imagined the ends of the girders to be brackets, attached to open boxes formed by the walls, legs and foundation slabs. The weight of the central portion of the beam would cause these 'boxes' to tilt inwards, while the weight of the earth resting on the base slabs would more than balance this effect. Satisfied with this, he handed over to J A Laing, who repeated the calculations, introduced curves at the ends of the girders (JM's initial scheme had been rectangular) and pared down the cost. Laing then prepared a tender drawing showing an architectural elevation of the bridge and simple details. To preserve RCMPC's intellectual property, only a sketchy indication was given of the layout of reinforcement and no sizes were shown.

In his letter of tender, Monash offered to guarantee his alternative design fully for the loadings and tests specified and to place this in legal form. He pointed out that it provided a considerably increased waterway compared with Meldrum's arch, because of the increased height of the soffit. Another advantage was that a complete concrete deck would be provided over the whole area of the bridge, while in Meldrum's scheme (as in the Monier arch bridges) the road pavement would rest on earth fill, liable to settlement.

When tenders were opened, RCMPC proved to have won by a very small margin. The figures, as published in Cazaly's Contract Reporter of 12 March 1914 were:

RCMPCSpacer.£3,514
W Williams £3,516
Stone & Siddeley £5,268
H Cooper £5,468
D Spence £6,500

RCMPC were then told that Council wanted the bridge built in halves longitudinally, so that traffic could continue to cross during construction. Brick carters would be particularly affected by complete closure, which would mean a long hilly detour requiring the use of extra horses per wagon to deliver bricks to the northern suburbs. Monash replied that, with the new bridge directly on the centreline of the old, this would be difficult and costly. He suggested that the old bridge be cut down to 11 feet width, and traffic be allowed during daylight only. He estimated the extra cost, including lighting and watchmen, to be over £50, but offered to do it for £25. The Council decided on total closure.

Construction.

The first requisition for materials was issued on 19 May 1914. The initial two weeks were devoted to preliminary earthworks. Laing completed the final design calculations on 5 June and the working drawing, showing full details of reinforcement, was ready on 8 July. The full specification followed the next day.

Please refer to caption. The reinforcement in the thin, deep columns is grid-like. The girder has frequent stirrups along its full length.

Extract from the working drawing of 8 July 1914.
Half longitudinal-section showing outline and reinforcement
J Thomas Collection

By 24 July, excavation for the south side abutment was down to rock. On 24 August, Laing reported that 4000 cubic yards of excavation had been completed. The wing walls, the base plate, and the counterforts (legs) for the north abutment had been cast. The value of work done totalled £820. RCMPC were entitled to claim 90% of this as a progress payment. However, their letter missed the deadline for the Council meeting, so a new claim for £950 was put in on 9 September.

On 7th, Meldrum pointed out that he had not yet received a copy of the working drawings, as required by the specification. Laing replied, in line with RCMPC policy:

As you are aware, the existence of this Company depends entirely upon our ability to propound safe, and, at the same time, more economical designs of engineering structures than our competitors in this line of business, and consequently it would be most disastrous to us if, by any mischance, our drawings should come into the hands of any of our competitors. It is for this reason, which we venture you will consider reasonable, that we are writing to ask you to treat this drawing as strictly confidential and to have it filed in such a way that no unauthorised person can have access to it. I would esteem it a favor if you would let me have a line stating that you have given effect to this.

Claims for progress payments show that by 29 September all wingwalls and both abutment retaining walls were complete (£700 claimed). By 5 November, half the deck was in place (£670).

The two Councils decided that medallions, cast in mortar and featuring their coats of arms, should be placed on the inside of the parapets. P T Fairway questioned this decision, fearing they would "almost immediately be broken by boys or vandals". Caution prevailed and the medallions were placed on the outer sides (photo at top of page).

By 19 November all concrete work was complete (£560 claimed) and on 12 December a further £380 was claimed, with the total work done so far valued at £3200. The last requisition for materials was issued on 14 December 1914. The RCMPC project file does not contain a record of the official completion date or of the load test.

Technical.

RCMPC's analysis

The analysis of even simple portal frames was not in RCMPC's repertoire. As mentioned above, Monash roughly sized the girders on the assumption that they were partially fixed against rotation at their ends. He assumed a bending moment at the ends of WL/40 (tension on upper face) and of WL/10 at mid-span (tension on lower face) with W and L based on the 60-foot length (18.3 m). His instructions to his assistant engineers for detailed design assume points of contraflexure near the ends of the girders. Laing assumed these to be 10 feet (3.05m) from the face of the abutments, leaving a 40-foot length of girder in between that could be considered simply-supported. The maximum bending moment in this section under uniformly distributed load was therefore WL/8 (with W and L based on 40 feet).

Warning for today. Analysis using assumed points of contraflexure must not be attempted without a thorough theoretical and practical understanding of structures. If the locations are badly chosen, portions of the structure may be over-stressed. Most forms of construction, including properly designed reinforced concrete, possess a certain amount of ductility. The over-stressed portions yield slightly, stretching without failing, and the load is transferred to less highly stressed portions. However, there are limits to this ductility beyond which catastrophic failure will occur. Materials such as cast-iron, stone, and unreinforced concrete are not ductile. Even steels may exhibit brittle behaviour, especially at low temperature and under three-directional tension.

The use of this approximation by RCMPC in 1913 is understandable. Convenient hand methods of analysis such as 'slope-deflection' and 'moment distribution' (Hardy-Cross) had not yet been developed. Factors of safety were more than double those now adopted. Assumed points of contraflexure were still used in the 1930s for very large assemblages such as the frames of multistorey buildings, e.g. the Empire State Building. The introduction of computers has since made this procedure unnecessary except, perhaps, for back-of-the-envelope conceptual work.)

In this scheme the columns, assumed unable to rotate at their bases, support horizontal cantilevers reaching to about quarter-span. A shallow girder of about half the length of the bridge spans between the cantilevers.JM's early calculations suggest that he briefly contemplated using beam-and-cantilever construction (see figure), involving real hinges. He made a sketch from the German serial Beton und Eisen, Vol. 1, p.2, 1912 showing a form of seating suitable for supporting the end of the beam on the cantilever. However, he chose not to do this, presumably realising that it would be an unnecessary expense in a bridge of this size, would raise challenging questions regarding the shear strength of the overlapping portions (of reduced depth) at the hinge location, and probably necessitate a lateral joint in the road surface.

The Council Design

Meldrum's design was for a reinforced concrete arch of 60 feet span, 18" (457 mm) thick at the crown and 3 feet (914 mm) thick at the haunches. The abutments were to be 18 feet (5.5 m) thick at foundation level. The form seems to have been inspired by M&A's previous Monier arches, but is very much heavier, the arch being about three times as thick as M&A's. The drawing shows the main reinforcement as 1¼" bars at 6" centres (32mm at 150mm crs), top and bottom. These were to be held in place by rectangular frames, made from angle iron, lying across the width of the arch, within the concrete. The top and bottom angles were to be drilled to allow the main bars to pass through. Instead of splicing, main bars were to be linked end-to-end by standard hooks that would simply interlock. The centre half of the span was to be solid, but the outer portions and abutments were to be of diaphragm construction, implying lost formwork. The longitudinal diaphragms are shown equipped with diagonal shear reinforcement in both directions forming a criss-cross pattern extending into the solid part of the arch. The spandrel walls were to be reinforced and provided with internal counterforts to resist the outward pressure of retained fill.