Ferro-Concrete Bridges in Taranaki.

Progress, Volume III, Issue 4, 1 February 1908, Page 120

Ferro-Concrete Bridges in Taranaki.

About four years ago the Taranaki CountyCouncil were given power by the ratepayers to raise a loan for the purpose of re-construct-ing the bridges of the county. With one exception these bridges were of wood, and were in a bad state of decay. The Council began b}*- renewing with Australian timber, on concrete abutments. These being found expensive, it was decided to rebuild in steel, and two bridges, one of 90 ft. and the other of 60 ft. span, were built at what was considered a low cost. On account of the excessive rainfall in Taranaki these two bridges quickly beganto scale, and had to be scraped and painted within a year. This induced the Council to have all bridges built in concrete, and instructions were given to their engineer to prepare plans for several bridges in ferroconcrete. A bridge has been built over the Oakura river, 62 ft. span, of ribbed plate design having four reinforced ribs, or beams, with parapets and decking, reinforced with round steel bars and expanded metal, at a cost for the superstructure of £396, or £6 10s per foot length, and 8s 6d per square foot of decking ; also over the Waiongona, 45 ft. arch, Piakau, 30 ft. arch, Manganui, and others. The Manganui bridge consists of three spans, one 60 ft., one 30 ft., and one 20 ft., on piers 32 feet above the river bed. This bridge is of rolled steel girders understrutted and cased in concrete. It has a fair amount of steel in it — over 16 tons — and is so designed that the girders are capable of carrying all the load as well as the concrete — in fact live and dead load — in themselves. The largest bridge constructed by the Council in f erro-concrete is that over the Waiwakaiho river, near New Plymouth, on the site of a massive puriri timber bridge, built 50 years ago, that had become unsafe for heavy traffic. This new bridge is of four spans, two of 60 ft., and two of 30 ft., on concrete abutments and three piers. The width of the bridge is 30 ft., consisting of a 21 ft. roadway and two cantilever sidewalks of 4 ft. each. The total length of the bridge is 190 ft. The design is a combination of the Winch and Hennibique construction, the flat topped arches being broken into ribs by light wooden frames that remain embedded in the concrete. The rise in the arches is only 4 ft. 6 in., the radius of the long span arches being 96 ft., and the short spans 28 ft. The thickness of concrete^at the crown of the arches is one foot six inches, and at the piers 6ft.

The reinforcement consists of 12 parallel rows of 40 ft>. steel rails running horizontally along the whole length of the bridge, about 6in. from the floor level, and the same number of 40 lb. rails following the curve of the arches at about 6in. from their lower surface. The horizontal members are placed base downward, and those following the curves base upward, and the two are riveted together at the arch crowns. These reinforcing frames are spaced two feet apart, and are continuous from end to end of the bridge, and are connected at the abutments by steel rails. A network of rods connects the two members and laces the whole reinforcement together. The vertical rods are simply turned round pther rods that run horizontally through the webs of the rails, and are very roughly put together, but the effect to the rigidity of the reinforcement is very marked. The frames are bolted to the piers and abutments with heavy iron bolts. Expanded metal was used for the bracketed footwalks. The concrete consists of Portland cement and, principally, beach gravel in different proportions, the ring of the arch and grouting being 4 to 1, and all other parts of the bridge 6to 1. Broken shingle taker from the river bed was used in piers and abutments, and in the heavier ' parts of the bridge. The concrete was made very wet, and was well spaded

and rammed around the steel. The roadway is divided from the footways by par? pets six inches thick of concrete reinforced with rods, and having eight posts or newels on each side, open spaces being left at the piers. The f ootwalks have iron railing, consisting of angle iron stanchions and gas piping rails, the top rail being 2| in. and three others of | in. This is the only exposed iron in the bridge. A nosing runs the whole length of the outer edge of footwalks, throwing the rain water back into the drains and preventing any flow over the sides. The bridge has a very nice appearance now that it is finished, and timber centreing away, and should meet all requirements of traffic for many years. The cost of the bridge complete, with asphalt flooring, is £3500, without including the value of old piers that are used in the new construction, valued at £600, but including the cost of removing the old bridge and building a temporary bridge to carry the large amount of traffic passing over the road. These cost about £550. The cost of superstructure is estimated at £15 per foot, or 10s per square foot of floor surface. Comparing this with steel bridges the cosl is much less for ferro-concrete- The Waiongona steel bridge of 90 ft. span, 14 ft. wide, having timber decking, cost 11s 6d per square foot, and the Waipuku steel bridge, 60 ft. span, and 14 ft. wide, with timber decking, cost 13s per square foot. It should be noted that the Taranaki county is peculiarly adapted for concrete construction, having large supplies of clean stone suitable for the work, which can be procured at a fairly low cost. But considering the durability and absence of maintenance, the concrete, at a much higher cost than steel or wood, would be the cheaper. The contract for the Waiwakaiho bridge was carried out by Mr. 1,. G. P. Spencer, to plans and specifications prepared by the engineer to the council, Mr. John Skinner.

Of all applications of machinery for industrial purpuses the manufacture of horse slioes by the use of a tram of rolls and quick-working machines occupies a position that is most important in these days of strenuous life. The extensive use of horses by the' citizens of various nations, has created such a demand' for the shoes, that it has resulted in the building Ox mills that give employment to thousands of workmen and add to the wealth and comforts of the people.