Thursday, 31st July 2008

The Hampden Bridge over the Murrumbidgee River, Wagga Wagga I wouldn't walk over this, see the water through the walkway?? Yikes!

Percy Allan in 1893 designed a standard type of timber truss bridge, called the Allan truss, which continued to be built at least until 1920. In its original form it was used for 21.3m and 27.4m spans. In 1895 he extended to the design to span 33.5m, the first bridge of this type being at Wagga Wagga, 1895. The system used timber Howe trusses, with timber top and bottom chords, timber compression diagonals and tension rods as verticals. Its chief features were because it had no counterbraces, it was possible to adjust the geometry by screwing up the verticals, counteracting the effects of shrinkage, the diagonal members were cut off square at their ends, pressing against special cast iron details and double timbers were used to allow for replacement. The shorter spans were half through. The deeper 33.5m spans were through trusses, with an upper lateral system. The span of 33.5m for a timber truss was, at the time of opening, exceeded only by the Lachlan River road bridge at Cowra (1893, 48.8m). Allan trusses of the same span were later built at Inverell, 1896 (since removed) and Morpeth, 1898. The bridge is of considerable technological significance.