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Concrete In Australia : March 2008
PERSPECTIVE The fascinating evolution of bridge design by Rob Wheen The design of concrete bridges and their continuing evolution since my youth has always held a great fascination for me. I decided on Civil Engineering as a result of an early discovery of bridges at school and my delight with them has persisted. My fi rst contact with a real bridge was during the construction of the Mann River bridge at Jackadgery in northern NSW in 1958. I was a student and it was the fi rst time I ever saw prestressing. I was hooked. It was not the fi rst prestressed concrete bridge in Australia, but it was certainly an early one. I also had the good fortune to live, as a student, near the Gladesville Bridge during its construction and haunted the site at weekends watching progress. One of the highlights of my student days was to be present on the fi rst rib of the arch as it was jacked clear of the falsework. It was only with the perspective of time that I truly realised what had been undertaken “under my very eyes”. A world record 1000 foot (305m) concrete arch span and a highly innovative method of construction all made possible with the use of flatjacks. The then NSW Department of Main Roads’ other Rob Wheen at Roseville Bridge, which spans the upper reaches of Middle Harbour in Sydney. Wheen worked on the design of the deck of this bridge in the early 1960s under the leadership of NSW DMR engineer Albert Fried. achievements were also impressive. I think of Albert Fried’s unique Rip Bridge, a cantilever truss bridge of 183m main span at the entrance to Brisbane Water near Gosford. It was assembled from precast concrete elements and stitched together with BBR prestressing tendons to leap across the tiderace known as The Rip. The balanced cantilever bridges at Pheasant’s Nest and Moonee Moonee creek and the mighty Gateway Bridge in Brisbane are all outstanding examples of a construction method which would have been inconceivable without prestressing. I recall with pleasure hearing the visiting German Professor Fritz Leonhardt speak at the DMR about the development of his incrementally launched prestressed concrete bridges. He spoke of the idea as having been in his mind for some years but not achievable because of friction effects. The sentence I took from his lectures was his assertion that he read in the Reader’s Digest about this new material Tefl on with its 1% to 2% friction coeffi cient and knew immediately that he could successfully launch bridges. There have been some notable examples, though not a large number, of incrementally launched bridges in locations around the country. Leonhardt also spoke about cable stayed concrete bridges, and the past 20 years have seen some remarkable structures. Sydney’s 345m span Anzac Bridge is a fi ne example and one that RTA and Baulderstone Hornibrook can be justifi ably proud of. Some years ago I had the good fortune to visit and admire Virlogeux’s 856m span Normandie Bridge across the Seine in France, but one has to concede that he did use steel for the central 624m of the main span. One form of prestressed concrete construction that appealed to me when I discovered it in 1965 was Finsterwalder’s concrete stress-ribbon structure. Modest spans have been built around the world but so far as I’m aware none has been built in Australia. Perhaps we will see one here some day. I see an interesting future in the now mature prestressing fi eld as a result of the development of high-strength and high- modulus synthetic materials. Aramid fi bres (eg Kevlar) and carbon fi bres have strengths perhaps 50% greater than high tensile steel and have elastic moduli as high as 50% of that of high tensile steel. Their properties are ideal for prestressing and they don’t corrode. The challenge is to fi nd simple practical ways of anchoring tendons made from these materials. Now that we have moved to 500MPa steels, we could be moving to an era where fl exural cracking under service loads, hitherto relatively unimportant, may start to cause problems if not carefully considered. I worry too, that we have yet to see some of the consequences of the reduced ductility that we seem so willing to accept. I well recall the durability problems that we created with our move from prescriptive mixes to purely strength specifi cations for concrete. In some case these have taken decades to manifest themselves. It may not seem too signifi cant but I believe that the development of concrete pumping technology, along with the advent of superplasticisers, has revolutionised the field of concrete construction. We can now produce high performance concretes, and strengths readily achieved have at least doubled in the past 15 years (yet we still remain vulnerable to “the bloke who places the grey stuff”). Just when we think that we have the whole game under control new possibilities open up. I have no doubt that the years to come will see developments at present only glimpsed and some not yet imagined. Rob Wheen is an honorary associate professor and former head of the School of Civil Engineering at the University of Sydney. Concrete in Australia Vol 34 No 1 17