The design and construction of composite slab and girder bridges, with particular reference to the Leven Bridge at Ulverstone

The theory on which this particular type of structure is based is given in the January issue of the "Journal of the Institute of Engineers" in a paper entitled "The design and construction of composite slab and girder bridges" and reference will be made to the subject matter of this paper. The paper can be regarded as part of this thesis. Experiments on test pieces designed to investigate the possibility of reinforcing a structure consisting of a concrete deck slab supported by steel joists for shear between the top of the steel section and the concrete, were first conducted and indicated that the theoretical expectations were realised in practice A 34ft span two beam bridge with a 10ft roadway was designed and constructed for testing the type of structure on a larger scale (Commonwealth Engineer, April 1933) Later a 1/6 scale model of one span of the proposed Leven bridge was also constructed for testing purposes, but rather from the point of view of load distribution between the beams than from strength considerations. Two, three, four and five beam highway bridges of this type have now been constructed and it is of interest to note that the structural and economic advantage to be gained is likely to lead to a very extensive use in the future of this class of construction. The subject matter of the thesis, outlines the principles of the design and construction of composite beam bridges in general, and describes the Leven Bridge, which is a four beam bridge of this type, having 7 spans each 61 feet long. The deck of the Leven bridge carries a roadway 20ft between the kerbs, with a 4ft footway on each side. Reference is made, in describing the work, to the Journal paper and to the working drawings for the bridge. The Leven River rises in the vicinity of Mt. Pearce about 35 miles from the coast, and flows into Bass Strait at Ulverstone on the North West Coast of Tasmania; the Coast Highway passes through the town less than a mile from the sea. The existing bridge is .a timber structure 49 years old, and is in such a state of decay that it was considered to have reached the end of its useful life. Consideration was therefore given to the renewal of the structure which is a vital link in the communication system in this part of the State. A survey, of the locality, was therefore made and consideration given to five sites on which the bridge could be renewed; these are shown on Drawing 65L-2. Eventually the site marked 1.B. was selected by the Parliamentary Committee investigating the proposals and a decision made to renew the bridge with a permanent structure at an estimated cost of 213,700, exclusive of land resumption. Bore holes 66 feet apart were put down on the centre line of this site and indicated that a suitable foundation could not be obtained less than 35 feet below the river bed. A 10ft rise and fall of tide had also to be reckoned with. A number of designs were examined for the superstructure, including a three span high through truss bridge, several arrangements of welded pony trusses, and several arrangements of welded plate girders, Of these proposals a 7 span cantilever welded plate girder bridge was recommended as most suitable; the above designs were all prepared for a 18ft roadway and one 4ft footway. Later, however, an examination of the composite slat and girder type showed a marked saving over the latter bridge, but as the necessary funds had already been voted by Parliament, it was decided to increase the roadway width to 20feet, and add another footway, as it was estimated that the additional work could be undertaken for the difference in costs of the two types