PORTABLE BRIDGE DESCRIBED

Press, Volume XCVIII, Issue 28822, 17 February 1959, Page 14

PORTABLE BRIDGE DESCRIBED

Use Of Aluminium f Alloy A standardised portable aluminium highway bridge, described in a paper by. an ’ Auckland consulting engineer, Mr T. Wi Arnold, was discussed yesterday by members of the New Zealand Institution of Engineers. New Zealand was a country of very many bridges, a large proportion of which were built in the days when timber was cheap. Of good quality, and plentiful, said the author. Most of the bridges were approaching the end of their effective life. During 1954-55, tremendous impetus was given, under the Nattional Roads Board, to a road and bridging programme by the Ministry of Works and the counties. It became obvious \to a number of engineers that, in order to cope with the replacement of thousands of small bridges, some form of prefabrication was advisable and some very ingenious methods were devised, mainly utilising mild rteel with precast concrete decks. To take full advantage of available crane and transport facilities, his firm aimed at a superstructure fully, transportable in one piece. From the weight angle, mild steel did not provide a very practicable solution. It -began to investigate aluminium alloy and was surprised to find that in a reasonable number of cases, overall costs, even with such an expensive material, did not appear to be any greater than for other materials.

Final Placing Bridge trusses for the trial bridge on the State highway about 18 miles from Wellington, taking a traffic density of 3000 to'sooo vehicles a day, were fabricated in England. The deck aection was supplied from England. The structure, which had a 30ft span and a 24ft carriageway, was finally fabricated four miles from the site and the structure weighing 76201 b was lifted, transported and placed in one hour and a half.'

After nine months service, and 1.500,000 vehicles, the bituminous deck had settled down to a hard homogenous surface and the cracks between the planks had completely disappeared. Although a little vibration of individual members occurred under certain' loads, it was no more marked than for trussed steel structures, said the paper, and the whole superstructure aid not display detrimental resonance under actual traffic conditions. After 17 months of service and observation, the bridge showed no apparent signs of deterioration of any kind.