SCIENTIFIC INTRICACY

Waikato Times, Volume 119, Issue 19892, 22 May 1936, Page 5

SCIENTIFIC INTRICACY

DESIGNING A BRIDGE. STRUCTURE AT FAIRFIELD. VINDICATION OF SITE. Something of the intricacy in design, specification, estimation and supervision entailed in the erection of the new bridge across the Waikato River at Fairfield was given by the Waikato County engineer, Mr A. A. Woodward, when he addressed the Hamilton Society of Model and Experimental Engineers last evening. The difficulties faced by the designers and builders were points stressed by the speaker.

Discussing the choice of the site at Fairfield, Mr Woodward said that for many years residents in the Fairfield and Rototuna districts of the Waikato County had been endeavouring to secure better access to the town by a bridge at the northern end of the borough. In 1929, owing to the necessity of repairing and strengthening the present traffic bridge, the need of another route together with th-e desirability of keeping travelling stock out of the town, the borough engineer had prepared a report in which he stressed the necessity of meeting the adjoining local bodies in an endeavour to agree on the erection of a new bridge. Finally the Fairfield site was selected.

“It might be asked why, apart from the cost, Fairfield was considered the most suitable site,” the speaker said. “Hamilton is rapidly becoming a three bridge town and now that the Fairfield Bridge is being built at the northern end the borough can, when the conditions are favourable, build a bridge somewhere in the vicinity of Bryce Street or Rosstrevor Street., approximately half way between the old traffic bridge and the Fairfield bridge. This should meet the requirements of the town for many years. Another reason for the choice of the Fairfield site was that at the present time it would have been difficult to carry a loan to build a bridge in the borough.” Difficulty of Foundation. Mr Woodward explained that the bridge, which had been designed by Messrs Jones and Adams, of Auckland, was a concrete bowstring arch structure with one centre span of 130 ft., two spans of 128 ft. 6in., and two land spans of 35ft. each, a total length of 457 ft. The carriage way was 20ft. wide and there were two five feet footpaths cantilevered outside the arches. Some difficulty had been experienced in deciding suitable foundations to carry such a heavy structure. Test piles driven showed that there was apparently no limit to the depth a pile could be driven in the Waikato River. One pile was driven to 145 ft., and was still going easily, but the driving proved that there was a heavy strata between a depth of 20ft. and 65ft which, provided a sufficient number of piles were driven, would carry the bridge. It was decided to drive 126 wooden piles in piers C and D in the river and 60 piles in piers B and E at the edge of the river. On the completion of the piers the piles were driven down about 30ft. below the bottom of the concrete, sawn off and, in the case of piers C and D in the river would be embedded six feet in a concrete block 50ft. long, 15ft. wide and 10ft. in depth. The top of this block Avould be two feet below the bed of the river which had a summer level of 10ft. to 12ft. At the present time, however, it was about 14ft. in depth so that the bottom of the cofferdam completed was about 22ft. below the summer level of the river. From these concrete blocks the piers would be built. Timber Under the River Bed. Mr Woodward enlarged on the technical methods that were employed and referred to the difficulties of bridge building in the Waikato. It appeared, he said, that the whole of the Waikato area was at one time a large inland lake covered in parts with timber which had been buried with sand and pumice brought down from the high lands above Arapuni. Often in pile-driving—and several instances had occurred in the work on the Fairfield bridge—this timber had been struck by the piles. Nothing was left to chance in building the foundations and the load each pile was to carry was computed and the pile was driven until its set was sufficient. There were several engineering formulae used for determining the resistance of the pile. The magnitude of the bridgebuilder’s task was emphasised by Mr Woodward when he outlined the volume of materials used in the construction of the bridge at Fairfield. The lineal feet of timber piles required amounted to 14,232 ft., while 1708 ft. of reinforced concrete piles were also necessary- There were 825 tons of cement and 2965 cubic yards of reinforced concrete, while 297 tons of steel were involved. “Owing to the high hank on the Hamilton-Ngaruawahia main highway at the western entrance to the bridge and the grade of this road some difficulty has been experienced in making a suitable approach. This has been overcome by splaying out the short land span and building a wide approach road to No. 1 bridge and also up the hill,” Mr TVoodward added. “An additional 20ft. width of this road will be continued up the hill to connect with a new road being put in to give direct access to Frankton. The additional width of road will be fenced off and used as a stock route.” , , , At the close, of his address, which (touched upon many general aspects of the bridge-builder's craft, Mr Woodward -was accorded a unanimous vote of thanks.