The Canadian Northern Railway bridge over the Pembina River, Alta.
The route selected for the extension of the C.N.R. westward from Edmonton, Alta., towards the Yellowhead Pass, involves the construction of a couple of rather heavy bridges over the Pembina and McLeod Rivers, as these steams have eroded their channels far below the general level of the country. At the Pembina River the banks rise steeply from the water's edge without the usual river flats intervening between the channel and the terraced benches so often seen in deep river valleys; the absence of river flats at the Pembina River thus reducing the length necessary to be bridged.
The total length of steel works is 780 ft. 8 ins., the approach at the east end being 28 bents of the C.N.R. standard framed timber trestle 68 ft. in height. The river span is 216 ft. long, supported on steel towers on concrete pedestals, the span of these towers being 60 ft. The east and west flanking spans are respectively 162 and 135 ft. long, and the western approach is of viaduct construction.
The three truss spans are of the Pratt type, with riveted connections, the panel spacing being uniformly 27 ft., the trusses are spaced 17 1/2 ft., c. to c., and the depth of trusses is 36 ft. from centre to centre of chords. The floor beams rest on the top of the upper chord with bracket connections to chord at ends of floor beams.
Canadian Northern Railway bridge over Pembina River.
The towers are designed to take care of all lateral and longitudinal stresses, and sufficient anchorage is provided to overcome any possible uplift. The live load for which the structure is designed is Class 1 of the Dominion Government specifications.
We are indebted to W. L. Mackenzie, Bridge Engineer, for the foregoing information.