THE OPAWA BRIDGE.

Marlborough Express, Volume III, Issue 107, 14 March 1868, Page 4

THE OPAWA BRIDGE.

As this great and useful work is on the eve of completion, we shall follow our usual custom, and give a technical description of it. As our readers are mostly aware, the Opawa is an outlet from the Wairau river, which, escaping at the Breach above Ben wick, has for some time past, owing to the shifting of the shingle and enlargement of the opening, been much swollen, and is now quickly eating away the banks all along its course. The old bridge where the Picton Boad crosses it, had from this cause become unsafe and in bad repair, consequently the Government found it necessary to erect a new one. Mr. Han wright having submitted a plan of his own, was entrusted with its execution, and although he has been but a short time at work, it is all but completed. i The bridge is 170 feet long, and is built in five spans; the middle span being a truss of 108 feet long, 18 feet wide, and 17 feet high,—that is, from out to out of chord. It is supported on piles 12 x 12, which are driven eight feet below the present bed of the river. The trusses are composed of upper and lower chords, braces, counter braces, and twenty-eight vertical screw suspension rods. The upper part of the bridge consists of twelve beams and eleven sets of lateral bracing; the roadway contains seven sets of sway bracing, eight tension rods, and forty-six flooring beams, and will be covered with three-inch planks laid longitudinally. The inclination of the braces are 45 degrees ; the ends of each brace and counter brace rest against angle blocks. The upper chords are composed of three lines of chord timber, bolted together with ninety |- screw bolts to each chord. The cross section of the upper chords are 315 square inches, calculating the maximum strain at 900 lbs. per square inch, it will give about 141 tons; the weight of the bridge and its load will be about 50 tons, leaving in favor of stability 91 tons. The lower chords are composed of four lines of chord timber bolted together with 118 screw bolts, and 28 combination keys. The cross section of the lower chords is 420 square inches ; calculating the maximum strain at 700 lbs. per square inch, we have 147 tons, leaving in favour of stability 97 tons. The trusses are in eight panels, each one containing two braces and one counter brace. The eight braces next to the pier panel have a united cross section of 432 square inches ; calculating the maximum strain at 400 per square inch, we have about 151 tons, leaving in favour of stability 101 tons. The eight braces alone are sufficiently strong to bear with perfect safety two or three times the whole weight of the bridge and its load without assistance from any of the other braces in the truss. There are eight vertical suspension rods next to the pier panel to receive the weight of half the bridge, 1|- inches diameter ; the united cross section will be 14 square inches, and the strain about 3,571 pounds ; calculating the maximum strain of iron at 10,000 lbs. per square inch, we have 45 tons in favour of stability. Lastly, there are twelve. 1§ inch intermediate suspension rods, two to each panel, to resist the other half weight of the bridge. The trusses are firmly secured with eight anchor bolts to the bolsters. We understand that the bridge will bo opened again for traffic on Wednesday next, when the flooring, &c., will be finished, and the whole work, including the approaches, will be ready, to hand oyer in a complete and perfect state to the Government, by Friday next at the latest. We cannot take our leave of the work without again testifying to the skill and expedition evinced, by the contractor* Mr. Hanwright, and we congratulate the province in securing his services for such an important work, in which he has, we believe, given general satisfaction.