05 November, 2014

Branching out - a grand old bridge

The Leigh Grand Junction Bridge which today crosses the Leigh River east of Mount Mercer, is the second bridge to stand on this site. The 40 year history of the first bridge is described in my two previous posts here and here. The second bridge will be the focus of this post.
By 1908 it had become clear that the first bridge - built in 1873 - was in desperate need of replacement. In November that year, Buninyong Shire got the ball rolling when they approached the Leigh Shire to discuss the issue. The following month, the Leigh Shire engineer (presumably CAC Wilson) reported to council that the bridge was in bad repair and should be replaced by an iron girder bridge.
Over the next year and more, decisions were made, costs determined and most importantly, plans were drawn up. The end result however was not the iron girder structure recommended, but rather a revolutionary, steel-reinforced prestressed-concrete bridge. The technology was relatively new to Victoria as the first concrete bridge had been completed just over a decade earlier in 1899.

Charles Corbett Powell Wilson, civil engineer. Image held
by the State Library of Victoria
This revolutionary design was the brainchild of Charles Corbett Powell Wilson, shire engineer from 1908 for both Meredith and Buninyong, and then, from 1910 upon the retirement of his father CAC Wilson, for Leigh Shire also. CAC was a pioneer in the use of concrete to build bridges and no doubt passed this interest on to his eldest son.
 Between them, these two men were responsible for the design and construction of scores of bridges and a vast array of community facilities across the three shires. For over 90 years from 1864 to 1938 they served as engineers in one shire or another and amongst their contributions were a number of bridges which spanned the Barwon, Moorabool, and Leigh Rivers and their tributaries (post to follow). Few however, were as impressive as the Leigh Grand Junction Bridge.

Leigh Grand Junction Bridge looking downstream, photograph taken
by Colin O'Connor, Copyright Department of the Environment
By the time the bridge was completed in the first months of 1911, the total cost had been reckoned at £1,118 of which the government agreed to pay £150. The remainder of the cost was to be divided between the three shires, with Buninyong paying half and the other two shires one quarter each. As per my previous post, after significant debate, this was the eventual outcome.
As to the bridge itself, the end result was a concrete structure with a carrying capacity of 30 tons. Coming in at 165 feet in length, it was however only 8 feet wide - a single lane. Unusually for bridges in the area, it stood some 35 feet above the river below (or by today's measurements: about 50.3m long, marginally less than 2.5m wide and over 10.5m high). The significant elevation was a necessary requirement due to the Leigh River having quite a steep, narrow channel at this point.
Today, the Leigh Grand Junction Bridge is one of the oldest surviving examples of a true concrete-reinforced girder bridge in Australia and not surprisingly, is heritage listed.

Looking upstream from the picnic area
Structurally, the bridge has four spans, each 38 feet 6 inches in length, which are supported by three slender piers and at its northern end, by a bluestone abutment, with concrete serving at the southern end. The columns of these piers are reinforced with "mild" or low-carbon steel (suitable for many purposes, including bridge-building) and are further strengthened by interconnecting diaphragms (concrete panels) at two levels.
In a clever move of both economic efficiency and structural enhancement, the steel used to reinforce the four T-section, concrete girders which span the bridge was recycled cable from the Melbourne tramways. The girders are connected to the piers below by distinctive triangular fillets and the whole is topped by a concrete slab sealed with bitumen, flanked by metal guard rails.
Protection during times of flooding is provided by cut-waters - triangular projections on the upriver face of the outermost column of each pier, which reduce pressure on the bridge during times of high water flow.
Bridge detail. Click to enlarge
As a further measure of economy, the mortar used in the construction process was made onsite by local council workers using sand taken from the river, with outside contract labour used only to construct the temporary timberwork required during construction. In total, completion of the bridge took about four months.
Aesthetically, the Grand Junction Bridge was described by the media of the day as "light and graceful". It was noted that the site had been a popular picnic spot in previous years and it was felt that the new bridge would increase the natural beauty of the area - already known for its impressive rock formations and lush growth -  and would encourage picnickers to return once again.
Rock face and picnic area downstream of the bridge today
Of course, an event such as the building of a new bridge - especially one which was so crucial to movement between three shires - could not go unremarked. Naturally, there was an official opening. This took place on 6th May, 1911 in the presence of such dignitaries as the Mayor of Buninyong, the presidents of each of the three shires and a large contingent of locals, all of whom were in imminent danger of losing their hats  to the roaring gale which was, in the words of The Ballarat Star, "howling furiously about the high wall of rock, and stirring the sluggish waters below into unwonted activity".
So, under less than ideal conditions, the Acting Commissioner of Public Works declared the bridge open. A ribbon - placed with some difficulty by Mr Wilson - was then cut by Mrs Edgar, wife of the acting commissioner before the company enjoyed a luncheon served by the ladies. Naturally this was accompanied by the expected round of toasting and mutual back-slapping which seem to be traditional on such occasions.
Thereafter, with the exception of some discussion of the bridge with respect to the necessary adjustments to shire boundaries, the media appears to make no further mention of its existence. After the trials and tribulations over the 40 year lifespan of the previous bridge, this speaks volumes for the quality and the endurance of this revolutionary bridge which over 103 years later still survives as a tribute to the engineering skill and vision of CCP Wilson, shire engineer.

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