BRIDGES. 



tain size is avoided in the longer spans, as 

 rolled bars of a much greater width than nine 

 inches cannot be depended upon for snch uni- 

 form strength and tenacity as the smaller bars. 



The difference consists in the arrangement 

 of the tie bars. In the channel span of 177 

 feet, the ties cross three of the panels formed 

 by the vertical posts ; in the 140 feet and 88- 

 feet spans they cross t\vo panels, while in the 

 76 feet span they cross but one panel. 



When the ties cross three panels diagonally, 

 as in the channel span, the truss partakes some- 

 what of the character of a lattice ; and the 

 principle is capable of being extended still far- 

 ther for longer spans by making the ties cross 

 four or more panels according to the length of 

 the girder. 



The work of erecting the bridge was com- 

 menced the last week in June, 1865, and pro- 

 gressed without interruption until the whole 

 of the iron work was finished, on the 1st of 

 February, 1866. 



Before commencing the iron work of the 

 several trusses, a series of blocks were laid 

 across longitudinal timbers placed under the 

 position to be occupied by each girder, for the 

 purpose of supporting it during construction. 

 These blocks were of the proper height to give 

 the required camber to the girders, and were 

 placed under each post. Upon these were first 

 placed the plates of the lower chord, which 

 were then riveted together in their proper 

 places. Next, the posts were placed in posi- 

 tion and riveted to the plates of the lower 

 chord. The top chord was then put on, first 

 the side plates and angle irons, then the hori- 

 zontal plates and covers. After the plates were 

 all riveted, the camber blocks upon which the 

 girders were built were removed by striking 

 the wedges upon which they rested, leaving the 

 girders supported by the ends. 



During the construction of the bridge, as 

 soon as any part was finished and the track 

 placed upon, it, heavy trains, weighing about 

 one ton to the foot, were run over it to test its 

 safety. These loads were not so heavy as it 

 was designed ultimately to subject the bridge 

 to as a test, on account of the rest of the bridge, 

 where the iron work was not completed, not 

 being in a condition to bear the extra strain. 



The channel span, however, was subjected to 

 a severe test by loading it with railroad bars, 

 in addition to a heavy train of four cars loaded 

 with iron, with the engine and tender ; in all, 

 about 220 tons. This would be about 1 J tons 

 to the foot. "With this load the deflection of 

 the girders was |" on one side, and T y on 

 the other. "When the load was removed there 

 was a permanent deflection of only T ' ff " on one 

 side and none on the other. 



The cost of the iron, delivered in New York, 

 was $24:1.55 per ton, in United States curren- 

 cy, $] 17.18 of which was premium paid upon 

 gold. The total cost of the iron work of the 

 bridge erected and completed was $277.41 per 

 ton, or 12-^ cents per pound. 



The Susquehanna Bridge. This bridge, de- 

 signed and executed under the direction of 

 George A. Parker, Esq., is situated nearly one 

 mile above the mouth of the Susquehanna River, 

 and four miles below the head of navigation and 

 tide-water, and has been built by the Philadel- 

 phia, Wilmington, and Baltimore Railroad Com- 

 pany, at an expense of nearly $2,000,000. The 

 engineering difficulties involved in building it 

 were, principally, the unusual depth of water, 

 the unstable nature of the bottom at certain 

 points, and the more than common violence of 

 the ice freshets peculiar to its locality. It is 

 composed of thirteen spans, seven of 250 feet 

 9 inches each in the clear, east of the draw, 

 and five of nearly the same dimensions, west 

 of the draw. 



The draw span is 175 feet long in the clear. 

 The whole length of the superstructure of the 

 bridge, including the draw, from abutment to 

 abutment, is 3,273 feet 9 inches. Its height is 

 25 feet, and its width 22 feet 6 inches. 



The piers are all of solid granite masonry, 

 sheathed from the bottom to the height of ex- 

 treme high water (eleven feet above ordinary 

 high water) with plate iron. The masonry 

 above water is cut to joints of one-eighth of an 

 inch, and where exposed to lateral pressure is 

 clamped in the courses vertically and horizon- 

 tally. At the top of the sheathing the piers 

 are eight feet wide, and their sides batter to 

 the bottom at the rate of five-eighths of an inch 

 to the vertical foot. They terminate at each 

 end in triangular starlings seven feet long on 

 the top, which have a double sheathing of 

 wrought iron. They do not project like the 

 ordinary ploughshare-shaped ice-breakers of 

 American bridges, but have a concave outline 

 at their salient edge ; not being exposed to the 

 momentum of the ice-fields moving down long 

 planes, this modification of the ordinary form 

 seemed necessary; as these piers have only to 

 meet, when subjected to their greatest strain, a 

 steady crushing pressure, resistance to which 

 cannot be much aided by any mechanical con- 

 trivance, but which must be met in the main 

 by simple inertia and irrefragibility. An uncom- 

 mon degree of inertia (proportioned to bulk) is 

 given to these structures by their iron sheath- 

 ing, and also by the extraordinary density of 

 the stone of which they are composed ; the 

 latter being Port Deposit granite, weighing 

 more than one hundred and sixty-five pounds 

 to the cubic foot. They are 35 feet 4 inches 

 long, and 7 feet 4 inches wide at the bridge 

 seat. The draw pier is circular, 24 feet 8 inches 

 in diameter at the top of the iron sheathing. 



The abutments are of solid masonry of the 

 same character as that of the piers, but not iron 

 cased below the water line. Above high-water 

 line they are hollow, and contain offices and ap- 

 pliances necessary for the uses of the bridge 

 and the railroad. Their upper story is of iron, 

 corresponding in a:rchitectural character with 

 the covering of the superstructure, which they 

 flank. 



