PLANS FOR BRIDGE

Manawatu Standard, Volume LIII, Issue 21, 21 December 1932, Page 6

PLANS FOR BRIDGE

TO SPAN MANAWATU RIVER. FEATURES OF STRUCTURE. With the confirmation by the Palmerston North City Council on Monday evening of the Combined Bridge Committee’s decision to call tenders for the new bridge to span the Manawatu River at the end of Fitzherbert Avenue, replacing the present structure, progress is steadily being made towards the work. It is anticipated that the building operations will occupy 20 months. The plans for a reinforced concrete bridge of modern design are of elaborate nature, and the details of the proposed structure are set out in seventeen different drawings which have been finalised. STEEL AND CONCRETE.

Quantities scheduled for the projected new bridge indicate its massive nature. Over 330 tons of reinforcing steel are required. This will all be of British manufacture, and will have to be tested by the Public Works Department’s consulting engineer at Home. The piers will be carried on 240 reinforced concrete piles, and the aggregate length to be driven is 9600 feet. This work is such as to occupy considerable time. It is estimated that 3400 cubic yards, of concrete will be needed, and in addition provision is made for 27 tons of roller bearings to go under the big spans in order to take up the strain of expansion and contraction. The centre line of the bridge will be some 40ft downstream from the existing one, and the approach will be of the same construction as at the southern end. The structure will leave Fitzherbert Avenue at the same place as the present one on the Esplanade side, but will take a slightly different angle across the river. Curved wing walls on each side will mark the northern approach from the Palmerston North end, where the entrance to the bridge will be guarded by the two high pylons carrying the end of the first big arch. FOUNDATIONS FOR PIERS. The bridge will be 27 feet above normal summer water level (3 feet on the gauge) at mid-river. There are twelve groups of piles (ten piers and two abutments). The main piers will be 40 feet by 10 feet in dimension. They will rest on rafts of concrete supported by piles in order to distribute the load more evenly. The two main pillars of each pier will be connected with a concrete web wall. The depths to which the piles will have to be sunk varies from 40 to 45 feet. The river bed formation, as determined by tests, has a surface layer of loose gravel, beneath which there is compact gravel, with a thin layer of clay further down in places on the line of the bridge. Below this again there is gravel steadily increasing in compactness. No hard obstructions were encountered in the driving of the test piles.

CANTILEVER SIDES. Having a total length of 835 feet 3 inches between abutments, the projected structure will be only a foot or two longer than the present one. It will have an overall width of 40 feet 8 inches, including a footway four feet wide on the downstream side. A space 4ft. 9ins. wide has been provided on the upstream side, instead of a footway to carry a water main of 21 inches diameter for the city supply, this is to be constructed as an entirely new section to replace the two pipes of 12 inches and 9inches diameter respectively, which are carried on the existing bridge. There will be a twocar roadway 22 feet wide, laid down in a 1 1/2 inch bitumenised surface finish over the concrete decking which is to be carried directly by strong transoms 3 feet deep and 17 inches wide. There will be four concrete railings consisting of pre-cast concrete panels running along the length of the bridge, from the sides of which the footway and pipe line section will be swung in cantilever fashion. PROVIDING FOR STRESS. Four bowstring arch spans, with a distance of 112 feet between each, are an impressive feature of the design. It is provided that their construction shall be entirely of concrete. Their two sides will be tied overhead by strongly reinforced concrete ribs as cross braces, and the lowest of these will have an 18 foot clearance from the decking. The length between each of the six beam spans is 55 feet.

The design of the bridge has points of similarity with that being erected over the Clutha River in the South Island, and the newly-finished Pahiatua bridge is somewhat like it, though on a much smaller scale. The plans have been prepared to combine the essentials of economical construction, strength and durability. Phosphor bronze bearings, eliminating the possibility of rust, are provided to take expansion and contraction movements in the beams. Temporary hinges in the arch ribs will take any stress due to the gradual settlement of the bridge. As they are too heavy for plates, nests of four cast steel rollers will be on castings on the piers. THE EXISTING BRIDGE.

The building of the new bridge will sound the death-knell of the totara structure which has withstood the battering of storm and flood since Palmerston North was in its infancy, and has carried the traffic of one of the main arterial roads of this centre for over half a century. Its ultimate fate is as yet undecided, but it will have in any event to be demolished, and the massive cylinders will have to be reduced to the water line, it is stated. There is an enormous quantity of heavy timber in the structure.