298 



ENGINEERING. 



the mainland are now separated from the 

 water by flats on which are wharves, railway 

 tracks, etc. The new bridge is for a highway. 



The structure is combined masonry, steel, 

 and wrought-iron, carrying foot and road- ways. 

 The approaches are each 660 feet long, and the 

 remaining 1,060 feet the bridge proper con- 

 sists of two steel arches and a central stone 

 pier. The carriage-way is 50 feet wide, with 

 a 15-foot side-walk on either hand. The car- 

 riage-way is laid with granite blocks, and is 

 151 feet above the river. The intrados of the 

 arch is 133 feet above the river. 



One of the most interesting engineering feat- 

 ures of the structure is the bearing of the arch 

 ribs, as illustrated in Fig. 2. At the end of 

 each rib the top and bottom chords converge, 

 and a second bearing or bed is formed, which 

 receives the projecting surface of the pin, a free 

 space being left between the skewback bearing 

 and terminal of the rib. Thus a sort of hinge- 

 joint is formed that secures a true thrust undis- 

 turbed by varying load or by changes of temper- 

 ature. As the rib can oscillate freely in such a 



FIG. 2. PIVOT-BEARING AND SKEWBACK, HARLEM 

 BRIDGE. 



bearing, no destructive strain is possible. Each 

 rib thus ends, constructionally speaking, in a 

 sort of point. As a concession to the public 

 the general lines of the rib are carried out as 

 shown in outline, but these outlined parts do 

 none of the work. It is rather a pity that these 

 superfluous plates were added. They detract 

 from the character of the structure, and the 

 public should be educated up to such devices. 

 "With an extreme range of temperature, a rise 

 and fall of the crown of the arch through a space 

 of three inches may occur, and many times this 

 amount is provided for by the pivotal bearing. 

 Each arch consists of six ribs thus con- 

 structed and supported. They are spaced lat- 

 erally 14 feet from center to center. Their rise 

 is 90 feet. They are connected by bracing that 



has two distinct functions, namely, wind brac- 

 ing, in the line of upper and lower flanges or 

 chords of the ribs; and sway bracing, which 

 extends from rib to rib at each junction of the 

 voussoirs or panels. From the upper surfaces 

 of the arch rise vertical columns, upon which 

 rest the cross floor-beams. These columns are 

 15 feet from center to center, and they de- 

 termine the varying length of the rib panels, 

 already alluded to, as each column starts from 

 the termination of a joint between the vous- 

 soirs. The two main arches, one spanning the 

 river, the other the railroads, stree's, and low 

 ground on the east bank, are identical in con- 

 struction. They contain about 7,500 tons of 

 iron and steel. 



The skewbacks, pins, and bearings are of 

 forged steel. The arch-ribs are of steel. Both 

 open-hearth and Bessemer steel are used, but 

 the tests call for an ultimate tensile strength 

 of 62,000 to 70,000 pounds to the square inch, 

 an elastic limit of not less than 32,000 pounds, 

 with a minimum elongation of 18 per cent. 

 The bracing, vertical posts, and floor-beams 

 are of wrought-iron. Most of the riveting is 

 done by machine, air riveters being used for 

 work in situ. Before being riveted together, 

 all abutting surfaces were painted. Eivets of 

 seven-eighths-inch diameter are used through- 

 out. William R. Button is the chief-engineer, 

 with Theodore Cooper as assistant. 



Pontoon Bridge at Nebraska City. Pontoon 

 bridges are generally used for temporary pur- 

 poses, but there are some notable exceptions. 

 At Nebraska City the Missouri river has two 

 arms, and the main branch has a very swift 

 current, often bearing large quantities of drift- 

 wood. The lesser arm is crossed by a perma- 

 nent crib work, 1,050 feet long. The pontoon 

 section is 1,074 feet long. A central span of 

 528 feet is closed by two swinging sections, 

 which form a V-shaped junction, with the an- 

 gle pointing down stream. When it is desired 

 to open the draw, the fasts at the apex are 

 cast off, and the two halves at once swing apart, 

 the current doing all the work. The operation 

 of closing is also aided by the current, and the 

 whole, it is said, can be effected by one man. 

 The floats are constructed so that the ordinary 

 '' flood trash " of the river is carried under 

 them by the force of the current. The con- 

 structing engineer was Colonel S. N. Stewart, 

 of Philadelphia, and the success of the bridge 

 has been such that others are already proposed 

 for the great Western rivers. The cost of the 

 structure was $18,000, and it was built in a 

 surprisingly short time. 



Just above the pontoon is a second bridge, 

 built for the use of the railroads. This also is 

 a recently completed structure. It was built 

 by the Union Bridge Works, and is of steel 

 throughout. The caissons were sunk in De- 

 cember, 1887, and January and February, 1883. 

 The first piece of metal was put in position 

 February 13, and on June 8 the last piece was 

 in place. Tlie-througb spans are 400 feet, the 



