ENGINEERING. 



243 



Chicago, Burlington and Quincy, replaces the 

 ferry by which passengers and freight have 

 heretofore been transferred. The selection of 

 the location for the bridge was a difficult 

 problem, on account of the varying channel 

 of the river at this point, affected by the 

 irregular volume of the Platte, which flows 

 into it about a mile above the town. Oppo- 

 site the town the river-bed is a mile wide, 

 though the narrow channel-bed alone is oc- 

 cupied by the river at low water. The chan- 

 nel is constantly shifting from one side of the 

 river-bed to the other opposite the town, but 

 some distance below the river flows through a 

 narrow passage between a dike and a high 

 bluff. The width here is only 800 feet; but 

 below the river widens, and the variations in 

 the position of the channel above throw the 

 channel now against one bank and now 

 against the other. It was necessary, there- 

 fore, to place the bridge as near as the con-? 

 figuration of the banks would allow to the 

 narrow passage in which there is the least 

 variation in the position of the channel. The 

 shifting nature of the current also necessitated 

 the adoption of a high-bridge instead of a 

 drawbridge plan. At the point selected the 

 width of the river, except in the flood season, 

 is about 800 feet. The bridge crosses this in 

 two spans, 50 feet in clear height above high 

 water. On the east side are three deck-spans 

 of 200 feet each, crossing the sand-bar, which 

 is submerged in the spring floods; beyond 

 which is an iron viaduct, 1,440 feet long. On 

 the west side an iron viaduct, 120 feet long, 

 reaches from the extremity of the high bridge 

 to the western approach, which is a cutting, 

 80 feet deep at the deepest point, leading in a 

 curve to Plattsmouth. The bridge is 3,000 

 feet long from abutment to abutment. The 

 main bridge-structure measures 804 feet be- 

 tween the centers of the outside piers. The 

 foundations of the three piers of the main 

 structure are carried down to bed-rock, 30 to 

 50 feet bslow extreme low water. The first 

 pier on the flat also has its foundation laid on 

 the solid rook, and the second one rests on 

 piles. The work on the bridge was com- 

 menced in July, 1879. The pier on the west 

 shore was sunk by blasting through a mass of 

 bowlders 28 feet deep. The two other river- 

 piers, and the one on the sand-flat resting on 

 piles, were sunk by the plenum pneumatic pro- 

 cess. The caissons were of a new pattern, 

 designed and furnished by William Sooy 

 Smith. The entire space above the roof was 

 filled as they were sunk by Portland cement 

 concrete, the foundation being thus built 

 downward; but the walls of the caisson were 

 inclined inward, so that little of the weight 

 rested on the roof of the working chamber, 

 and the caisson was sunk by continuous press- 

 ure, instead of intermittently blowing off the 

 compressed air, as is usual. The concrete 

 foundation extends from the solid rock, which 

 is horizontally stratified limestone, nearly up 



to the low-water mark. The piles under the 

 pier on the east side of the river-bed have a 

 penetration of 28 feet, the masonry com- 

 mencing three feet below low water. All the 

 rest of the masonry has concrete foundations. 

 The stone used is the finest kind of magnesian 

 limestdne, and it is laid in Portland-cement 

 mortar. The sides of the three main piers, from 

 the foundation up to seven feet above high 

 water, have an incline from the perpendicular 

 of one in twenty-four. At the top their thick- 

 ness is eight feet, their length 33 feet. Both 

 ends are circular arcs, meeting and forming an 

 edge, the radius of the arcs being about three 

 fourths of the thickness of the pier. The 

 arch with circular sides of these dimensions is 

 expected to meet best the conditions of the 

 Missouri River, which carries down great 

 quantities of drift-wood. The extreme varia- 

 tion in the level of the river at this point is 

 17 feet. The superstructure of the iron via- 

 ducts and of the deck-spans over the sand-bar 

 is of iron; but in the channel-spans steel was 

 largely employed. The bridge was designed 

 throughout to bear a running load of 2,000 

 pounds per linear foot, and to resist a wind- 

 pressure of 30 pounds per square foot when a 

 train is crossing, equivalent to 50 pounds per 

 square foot when empty. The iron viaduct, 

 of riveted-plate girders resting on wrought- 

 iron posts, has forty-eight spans on the east 

 and four on the west side, each of 30 feet, is 

 constructed entirely of wrought-iron, except 

 the cast-iron bed-plates. The three deck- 

 spans, 30 feet deep and 16 feet apart between 

 the centers, are of the single-system Pratt 

 truss, with inclined end-posts, and eight 

 panels of 25 feet in each truss. The wrought- 

 iron trusses, secured with steel pins, rest on 

 cast-iron pedestals anchored in the masonry. 

 The superstructure of the main bridge was 

 made by the Keystone Bridge Company. The 

 two channel-spans are just 402 feet each be- 

 tween the centers of the piers. The trusses 

 are 50 feet deep, and placed 22 feet apart 

 from center to center. The plan is the double- 

 system Pratt truss with inclined end-posts. 

 Each span h/is sixteen panels of 25 feet each. 

 The top chord, the tension-members, end- 

 posts, the jaw-nuts on the bottom chord, and 

 all of the smaller parts are of steel, except the 

 rest of the nuts, which are of iron. The 

 main ties and the bars of the bottom chord 

 were rolled by the Kloman process, the motion 

 of the rollers being reversed while the steel 

 is between them. The intermediate posts are 

 formed of two channels laced at the sides, and 

 are pinned in the center to the diagonal bars, 

 as well as to the top and bottom chords, the 

 pins in the center being also connected with 

 transverse struts between opposite posts, braced 

 by diagonal rods extending to the top later- 

 al struts. The open-hearth steel used in the 

 structure, specified to contain not over 0*35 per 

 cent, of carbon, was tested by making -f-inch 

 bars bend 180 degrees around their own diam- 



