The design consists essentially of a system of girders of rectangular cross-section supported by towers. Each tower is made with two vertical bents, each having two columns 6 ft. 9 in. apart on centers at top, and battered 1 to 6. They are of square cross-section, and reinforced with from four to nine steel bars, the steel being placed for transverse strains only, and the concrete to take the compression. The longitudinal struts and transverse braces are all horizontal.
The spans vary in length from 23 ft. 6 in. to 67 ft. 5 in. c. to c. of bents, the longer spans being used at railway crossings and at streets which had to be crossed with a clear span. Expansion in the structure is taken care of by expansion joints placed at intervals of about 200 ft., consisting of a grooved steel plate on top of the bent, on which a planed steel plate on the bottom of the girder slides, while a sliding toggle near top of girder prevents any tendency to turn the girder.
The concrete, of a 1:2:4 mixture, was figured for a compressive stress of 500 lbs. per square inch and shear of 50 lbs., while the steel used was given a tensile stress of 16,000 lbs.; compression, 60,000 lbs., and shear of 10,000 lbs. per square inch of cross-section. The modulus of elasticity of concrete to steel was taken as 1 :12, and the percentage of steel to be less than 1.45 per cent.
The footings for the columns were first put in, having four rods projecting 4 ft., around which the column was built, and also a pocket left for the bottom of the column. The foundations were either hard clay or com pact gravel, and were calculated for a pressure not to exceed 6,000 lbs. per square foot. As the structure was designed so that it could be readily double-tracked when necessary, the footings were made twice as large as they would have been if designed simply for a single track ; and a 4-ft. stump of a column was left, on which the future double-track column will rest. When traffic justi fies double tracking the viaduct, it will be easy to con struct an additional girder with proper supports, on one side of the viaduct, shift the track so that it will be sup ported by the new girder and one of the original girders, and carry the traffic on this track during the construction of the additional girder and new track on the other side. Cored holes are provided to take care of fastening new work to the old when double tracking is begun. The foot ings were carried down a uniform depth of 4 ft., unless extra depth was required to get suitable foundations.
After the erection of the forms, the columns (up to the bottom of girders) and contiguous struts were poured at one continuous operation, so as to make them mono lithic. Next, the girders and floor were put in in the same manner. At first it was attempted to pour columns from the top; but, owing to the difficulty of properly ramming and working the concrete through the rein forcing bars, this method was abandoned. The column forms were then built in a U-shape, and the fourth side built up in sections as the concrete was poured.
Dumping buckets holding 3 cu. ft., and built for this express purpose, were used in dumping into column forms. These buckets were lifted by means of a traveling single-boom derrick seated on top of the viaduct, and a hoisting engine. The struts were poured from wheel barrows from the top of the structure, through a 10-in. hinged pipe of galvanized iron. Concrete for the girders and floor was hoisted on wheelbarrows by double-cage towers, to the top of the viaduct, and wheeled to position on runways on the forms.
The forms were made of 2-in. lumber, dressed on one side, and as much of the lumber as possible was cleaned and used again after forms were pulled down. The forms on the sides of the girders were removed at the end of a week, but those on the columns and the sup porting falsework of the girders were left in place 30 days or longer if the lumber was not needed.
The smaller girder forms were supported by false work, consisting of 4x4-in. pieces placed 3 ft. on centers along the viaduct and resting on planking placed on the ground, which had been leveled to firm bearing. Under the heavier girder forms, at street crossings, the false work was made of 6x6-in. timbers and the struts were supported by 4x4-in. pieces, placed at distances to suit the weight carried. Owing to the great height of the structure throughout a considerable part of its length, a prodigious amount of lumber was used in the forms and shoring. Approximately 456,000 ft. B. M. was used in the entire job, or about 16,500 ft. B. M. to each 100 cu. yds. of concrete. Of course, at street and railway cross ings, special provision had to be made to carry the great weight of the girders. It is notable that in spite of the great weight of these girders, being considerably in ex cess of 50 tons each, the forms were erected and concrete put in place without delaying a train or street-ear.