FITZHERBERT BRIDGE

Manawatu Standard, Volume LV, Issue 196, 18 July 1935, Page 9

FITZHERBERT BRIDGE

NEW STEUCTUEE REPLACES OLD IMPORTANT HIGHWAY LINK On Saturday His Excellency the Governor-General, Lord Galway, is to declare open for traffic the now 1* itzherbert bridge, built of ferro-concrete, which constitutes an important link in the main highways system and now graces the southern entrance to Palmerston North with its architectural beauty. This monument to engineering skill supersedes a grand old servant of the district—the wooden bridge alongside, which for nearly sixty years has withstood the constant strain of traffic, storm, and flood. Shortly to be demolished, the old wooden bridge was built when swift moving cars and heavily laden transport lorries were not dreamed of, except in some fantastic vision of the future.' Yet this staunch strucure has met the changing and increasing traffic requirements decade after decade, also some strong earthquake shocks. It has met the needs of two generations and even now it will take explosives to shift the sturdy piers which no flood could dislodge.

THE FIRST BRIDGE. Levels for a new bridge over the Manawatu River at Palmerston North had been taken as far back as 1875 by Mr Beere and party, and the bridge was built, in 1877-78, of totara which was pit-sawn at Aok ante re and rafted to the bridge site, the price being in the vicinity of (is per 100 super feet. The construction work was carried out under a contract let to Mr Ilile, and was supervised by the late Air J. T. Stewart, whose headquarters were at Eoxton, and who was responsible for a great deal of the earliest engineering work in this district. The bridge is as old as Palmerston North itself, for it was on July 12, 1877, that Palmerston North, then with a population of 800, was constituted a borough after a town board area had been proclaimed a year earlier. When first built, the bridge consisted of 43ft. of beam spans, seven 85ft. truss spans, and about 80ft. of timber in a sloping ramp on the Fitzherbert, or south, side. This ramp terminated on what was then a low terrace marking the bank of the river, but it was very soon washed away, so another 85ft. span was added. The next addition was a second 85ft. span, and at the same time three pairs of cast-iron cylinders were sunk to replace timber piers, the remaining timber foundations being widened and strengthened. REPAIRS OR REBUILDING. Just over thirty years ago, it is interesting to note, a conference was held between the Palmerston North Borough Council and the Fitzlierbert Road Board to discuss the question of extensive repairs to the old bridge or the building of a new one. The addition of the last 85ft. span, the sinking of new cylinders and general strengthening of the structure winch followed were evidently the outcome of this discussion. The south approach was again washed out in 1904, and a 54ft. span was built. The north, or city, end of the bridge, which has a total ieHgtli of 865 ft. 7in. remains as originally constructed. Speaking generally, the bridge has kept its shape remarkably well, indicating the class of the early workmanship and material. During recent years a quantity of the timber has been replaced, the bottom chords of the main trusses reaching such a state that it became necessarv to strengthen them by bolting on heavy timber flitches, while a number of transoms had also to be renewed. Though the control of the bridge is vested in the Palmerston North City Council, the bridge has been maintained equally by the City Council and the Kairanga County Council, but with the advent of the Main Highways Board the county’s contribution was subsidised by the latter. NEED FOR REPLACEMENT. Nine years ago Messrs J. R. Hughes (city engineer) and J. E. Menzies (then engineer to the Kairanga County) took up, quite unofficially, tne question of rebuilding the bridge. Jjeyond a few preliminary sketch lilans and estimates, however, not a great deal was done, except for periodical reports to the respective local bodies that the bridge was rapidly deteriorating. Strong representations for a now bridge were made by the two engineers in 1930. They were instructed to make a joint report, and, after a very thorough examination, estimated at between £4OOO and £SOOO the cost of repairing the bridge sufficiently to prolong its life for ten years. It was considered that 22,000 super, feet of timber, excluding piles, required renewing. Tips proposition was rejected by both local bodies, which then took up much more seriously the rebuilding project, then estimated to cost £40.000. Finance was arranged in 1931, when Doth local bodies accepted the offev of tho Main Highways Board to subsidise the cost of a new bridge on a £2 for £1 basis. A bridge committee consisting of three county and three city representatives was then established. PLANS AND FINANCE. 'The preparation of the plans was undertaken by Mr Hughes, and Messrs Menzies and L. T. Brown were appointed to carry out the work under his instructions. Several designs of reinforced concrete structures were drawn and rough estimates made before the present design was adopted and quantities assessed. While the plans were being prepared, thirteen test bores up to 40ft. in depth were sunk along the line of the new bridge, and test piles driven to determine the nature of the river bed. The building of the bridge was commenced just over two years ago, after the tender by the Fletcher Construction Company, of £30,516 15s, had been accepted, the city engineer’s estimate of the cost being £32,650. Certain additional expenditure outside the scope of the actual contract was entailed, including the straightening of the present road, down to tlie intersection of the Tiritea and _ Fitzherbert East Roads. The provision on .the bridge for carrying the city’s pipe line carrying the water supply from Tiritea was assessed at a cost of £BOO. Additional works and their estimated cost were as follow:- —Road formation on county side, £748; filling ill and sealing of approaches, £sllbitumen surfacing of bridge deck, £256; purchase of land, £100; providing earthquake safeguards, £120; lighting of bridge, £350 ; supervision during construction, £850; contingencies, £4OO. This totalled £3335, and added to the bridge estimate of £32,650, made £35,985.

£33,851 15s, or somewhat less than the estimate on which the immediately preceding figures had been based. The additional amount of £3355 bore tho Highways Board’s subsidy of £2 for £l.

Supplementary expenditure of an unforeseen nature has been incurred since the estimates were first taken out. STRENGTH OF TIERS. When work on the bridge was commenced in April, 1932, pile driving was begun at the southern end and was comparatively, easy 'until the main hank of the river was reached. From that point across to the city end the driving varied in hardness, and this retarded progress until later two pile-drivers were put into operation. The bridge is 27ft. above normal summer water level (3 feet on the gauge) at mid-river. There are 12 groups of piles (ten piers and two abutments). The main piers are 40 feet by 10 ft. in dimension. They rest on rafts of concrete supported by piles in order to distribute the | load more evenly. The two main I pillars of each pier are connected with a concrete web wall. The depth to which the piles have been sunk varies from 40 to 45ft. CANTILEVER STYLE SIDES. With a total length of S3Bft. 9-Jin. between abutments, the bridge has an overall width of 401 t. Sin., including a roadway 4ft. wide on the downstream side. The centre line is some 40ft. downstream from the old bridge, and takes a slightly different angle across the river. A space 4ft. Din. wide has been provided on the upstream side, instead oi a footway to carry the water main of 21 inches diameter for the city supply. This has been constructed as an entirely new section to replace the two pipes of 12in. and Din. diameter respectively, which are carried on the existing bridge. There is a two-car roadway 22ft. wide, and this has just received a I Jin. emulsified surface finish over the concrete decking which is carried directly, by strong transoms 3ft deep and 17in. wide. There are four concrete railings consisting of pre-cast concrete panels running along the length of the bridge, from file sides of which the footway and pipe line section are swung in cantilever fashion. • Four bowstring arch spans, with a distance of 112 ft. between each, are the most>impressive feature of tlie design. Their height is 30ft. from the under side of tlie structure they support. Their two sides are tied overhead by strongly reinforced concrete ribs as cross braces, tlie lowest of which have an ]Bft. clearance from the decking. The length between each of the six beam spans is 55ft. The whole of the deck and its load is carried by the suspension rods from the arch rib. There are ten suspension rods to each arch, each containing six one-inch diameter steel rods. Tlie horizontal thrust in the arch ribs is taken by twenty ljin. steel rods. DISTRIBUTION OF LOAD. Tho design of the bridge has points of similarity with that, erected over the Clutha River in the •South Island and the Paliiatua bridge is somewhat like it, though on a .much smaller scale. The plans were inepared to combine the essentials of economical construction, strength and durability. Phosphor bronze bearings, eliminating tlie possibility of rust, are provided to take expansion and contraction movements in the beams. Temporary hinges in tlie arch ribs take any stress due to the gradual settlement of the bridge. As tliey are too heavy for plates, nests of four mild steel rollers ride on castings on the piers, and are enclosed in ail oil-proof steel box Above each set of rollers is a massive cast steel hinge and bearing plate so that the span can deflect and still give a uniform bearing on the rollers.. SOLIDITY OF STRUCTURE. . Four thousand cubic yards of concrete and 400 tons of reinforcing steel were used in the construction of the bridge, also 30 tons of steel eastings and 1J tons of phosphor bronze. Tlie total length of the concrete piles is 10,000 ft. For temporary piles 4000 lineal feet of pinus insignia were used, and staging and boxing for the concrete work required about 100,000 super, feet of timber. The loading provided for is the Public Works standard for first-class bridges. This consists of two traction engine trains, each comprising a 16-ton engine followed by two 10-ton trailers. These trains can ho eitherpassing one another or running parallel along the bridge. In addition, 50 per cent, extra for impact was allowed for in designing certain portions of tho structure. No lives were lost during construction operations, and there were only three serious casualties, with a number of minor accidents. The. bridge is under the control of the engineer of the Palmerston North City Council, Air J. R. Hughes, AI.N.Z.Soc., C-.E. Tlie resident engineer who was largely responsible for the preparing of the designs and for the supervision of the construction was Air J. 10. Alenzies, AI.N.Z.Soc., C.E. Tho general superintendent of tho work was Air H. R. Calder.

The approximate apportionment therefore became : Mein Highways Board. 2-3rds. of £36.000, £24.000. City Council, l-6th. of £36.000, £6OOO, plu« l-6tli. nf £BOO. £134. £6134. Kairanga County, l-6tli. of £36,000, £6OOO less l-6th. of £BOO, £134, £5866; total, £36,000. The additional works, estimated to cost £3355, did not enter into the contract, and were the direct responsibility of the two councils concerned. These brought the cost of the bridge to