Page:Final Report - The Columbia River Interstate Bridge.pdf/27

 25 ft. deep, and is separated from the main river by an island about 1,500 ft. wide. The U. S. Government has built a dyke some distance above the bridge site and the island to divert practically all of the low water ﬂow down the main channel of the river, the top of the dike being about 6 ft. above low water. The bed of this slough is principally sand. The Columbia Slough provides some drainage near the Oregon side of the valley, is about 300 ft. wide and 20 ft. deep at low water. The bottom is of very soft clay and sand; hard and compact gravel is found about 80 ft. below low water. The average elevation of the ground across the river valley is from 12 to 15 ft. above low water, so that much of it is inundated at every high water period.

The roadway is 38 ft. wide between curbs on the steel spans, 38 ft. wide between rock shoulders across Hayden Island, 30 ft. wide on the balance of the Union Avenue approach and 18 ft. wide on the Derby Street approach. Over the steel spans there is one sidewalk 5 ft. wide. Double track street car tracks for both standard and narrow gauges are placed in the middle of the roadway on the steel structure, spaced 10 ft. 815 ins. center to center. The vertical clearance on the truss spans is 21 ft. 6 ins. The roadway on the steel structures across Hayden Island and on the Union Avenue approach is paved with bitulithic pavement and on the Derby Street approach with macadam.

The bridge over the Collunbia River consists of a series of through riveted truss spans with curved top chords; three spans 275 ft. long and ten spans 265 ft. long. together with a small deck girder span at the Vancouver end, making a total length of 3,531 ft. 57/3 ins. between end shoes. Provision for navigation on the river is made by a vertical lift span. The central of three 275 ft. spans is arranged to lift between towers on the other two, so as to afford a channel 250 ft. wide at right angles to the current of the river and 150 ft. high above ordinary high water.

For the through spans the trusses are spaced ~11 ft. center to center, with the roadway between and the sidewalk beyond one truss. The floor is a reinforced concrete slab 51,4 ins. thick, and the sidewalk is also of reinforced concrete. The arrangement of the beams and of the concrete slab for the floor merits attention, as it is new and original. Between usual ﬂoor beams are longitudinal stringers spaced about 9 ft. center to center. On top of the stringers, extending entirely across the roadway, there are 8 ins. I beams spaced about 33 ins. apart. These roadway cross beams are bent to conform to the crown of the roadway, and the concrete slab rests immediately on top of them. The six