FIG. 58.-St. Paul Highway Bridge, Mississippi River, Minn. does for both viaduct and aqueduct. The arch supporting the roadway and spandrels is composed of cast-iron circular pipes of 48 inches internal diameter and 11 inches thick, having flanges at the ends of each of the seventeen sections, by which means they are bolted together. The clear span of the arch is 200 feet and the rise 20 feet. The tubes were at first jacketed with oak staves, to prevent the water from freezing, but this jacketing caused so great an expansion as to produce leakage, Washington, D. C. and was afterwards removed without injury to the pipes from thermal changes. The bridge is on Pennsylvania avenue between Washington and Georgetown, D. C., where it is subjected to a large amount of travel. A similar structure was subsequently erected with a span of 120 feet over College Branch on the Washington Aqueduct. FIG. 57.-Kinzua Viaduct, on a branch of the New York, Lake Erie, and Western R. R., near Bradford, Pa. SPECIAL FEATURES OF SOME AMERICAN BRIDGES. A modified form of Pratt truss as constructed by Kellogg & Maurice consists of square panels having the middle point of the lower chord supported by a tie extending to the top of the adjacent post on the side to- FIG. 59.-Highway and Aqueduct Bridge over Rock Creek, wards the pier. This enables the section of the lower chord (which is of wood) to be reduced at the extremities of the truss in proportion to the stresses. The upper chord is composed of four pieces throughout. St. Paul Highway (see fig. 58).-As early as 1854 it was proposed to bridge the Mississippi at St. Paul by a series of trusses in which the channel-span should not be less than 300 feet. The span, as constructed in 1858, was reduced to 240 feet, then a "long span;" but the peculiar feature of this bridge consists in the fact that its roadway is built upon a grade of up to the channel-span, where it is, caused by the bluffs, about 125 feet high, on the left (there the west) bank of the river. The eastern approach consists of an embankment 1600 feet in length, followed by 375 feet of trestling in bents 30 feet apart; then seven spans of 140 feet each, succeeded by the channel-span of 240 feet; and a short span of 80 feet, over the St. Paul and Omaha Railroad, to the top of the bluff. The piers are built in steps, so that each truss is 7 feet higher than its predecessor, and the roadway is supported on the top chords by bents. The short spans are of wood. The channel-span, of iron, 63 feet above high-water, was rebuilt in 1875-76. It was designed and erected by J. S. Sewell, C. E. the short spans were rebuilt in 1870. All The distinguished engineer Mr. C. Shaler Smith has constructed several bridges of such original design as to deserve mention in this article. The first we shall notice is in the Royal Gorge of the Arkansas, on the Denver and Rio Grande Railroad, in Colorado, erected during the summer of 1879 (see fig. 60). 66 It is a continuous plate girder in three spans, 275 feet long and 7 feet deep, which is suspended by rods from arch-braces" spanning the chasm at a height of 47 feet. On either side the walls of the cañon rise almost perpendicularly 1800 feet, and the entire river, contracted into a width of 50 feet, flows beneath with a fall of 6 feet in the length of the bridge, which is parallel with the stream. The bridge has a grade of 3 feet per 100. As the two middle supports are yielding, it required very delicate computations to determine the stresses, and ingenuity to provide for thermal changes. The Minnehaha bridge (fig. 61) on the Chicago, MilThe erection of the arch-braces in a position so inacces-waukee, and St. Paul Railroad is a continuous girder FIG. 60. sible is said by the engineer to have been almost as difficult as the erection of the Kentucky River bridge. on four supports, of which the two middle ones are yielding and variable. The effects of temperature and load on the piers are eliminated by hinging the middle span at the centre; the bottom chord is a stiff member, and is continuous from end to end. The principal feature of this bridge is the rocker-pier system. At the Kentucky River bridge the piers were fixed, and their tops connected with the truss. The bending strains arising from a train of 1000 tons being brought to rest on the bridge in a space of 100 feet when the middle span was expanded to its utmost from temperature, were then calculated, and the material necessary to resist these strains was added to the columns and braces of the piers (see fig. 62); whilst at Minnehaha the pier is a single post hinged at the top (a, fig. 63), and having a rocker-bearing below, of which the radius is the height of the pier. In other words it is precisely the same as though the bridge were on wheels, and all of the wheel cut away except one spoke. The entire bridge is free to expand at both ends, but up to twenty-one months from the date of its completion (July, 1880) it had moved only half an inch either way. There are ultimate stops to too great a play. This principle, which admits of a great reduction in the amount of masonry, was first introduced by this engineer in 1871 at the Rock Island bridge over the Mississippi. AMERICAN US. EUROPEAN PRACTICE. The characteristic differences between American and European methods of bridge construction may be briefly stated to be (a) the use of pins in place of rivets; (b) the assemblage of the pieces, so far FIG. 61.--Minnehaha Bridge, on Chicago, Milwaukee, and St. Paul R. R. as possible, in the shop, rather than at the site of the structure; (c) the reduction of the number of members FIG. 62. latticed girders of European engineers, thus offering less resistance to wind-pressure; (d) the consideration of the stresses due to violent wind-storms (pressure varying from 40 to 56 pounds per square foot, as well as of the area upon which it impinges, being taken at double that of the vertical projection of the truss), whilst in Europe no well-defined rules seem to exist for the determination of the wind-strains, some countries using very low (ordinary) velocities, whilst others neglect it altogether, with such disastrous results as occurred at the Tay bridge in 1879; to a minimum by the use of open trusses composed of sim- and finally (e) the ratio of depth to length of span, the ple systems, rather than the plate, tubular, or closely-practice in America being in favor of deep trusses. These differences lead directly to many important | the elastic limits of the material of which that member results, concerning which Ernst Pontzen, a distin- is composed, is evident. guished Austrian engineer, remarks: A bridge built on the American plan will always offer more safety than one built on the European plan, even though the maximum theoretical strength of both may be the same. FIG. 63. The variations from the calculated strength will be different in the latter, according to the care bestowed upon the work in the place of construction, while this will not be the case in American bridges, in which the same care is bestowed in the shops on the length of the various elements and the holes for the bolts, which are accurately drilled. With the same safety the trussed bridges of America may be made lighter, as well for the reasons already given as for the absence of the joints and packing-plates which are necessary to strengthen the weakened parts in riveted trusses and at the ends of tension-rods, as also to prevent warping. "But the main argument in favor of the trusses with bolt and pin connections is the fact that a great advantage is gained by means of the lighter, quicker, and less expensive construction." The latest American practice in the construction of iron bridges is best exemplified by that of the Pennsylvania Railroad Company. It has adopted the system of using "solid rolled I-beams for all short spans, up to such lengths as they may be available for the required live loads; then plate girders to spans of 50 to 60 feet, and in some cases even 70 to 80 feet, above which they become too wasteful and expensive in material, and are replaced by open trusses. "In small deck bridges the general practice is to use two I-beams under each rail of the track; then for longer spans up to 30 to 50 feet, where 'built' girders are employed, to place one girder under each rail; and as the span increases to adopt only three girders or trusses for two tracks, spacing them so that when both tracks are loaded each truss will carry the same weight. "In through bridges for double track, with three trusses, the centre truss sustains double the load of either outside truss." (Jos. M. Wilson, C. E.) FACTORS OF SAFETY. It has been the practice to assume some maximum load as being the greatest to which the bridge can ever be subjected, and then, to insure absolute safety, so to increase the dimensions of the members as to reduce the unit strains to from one-sixth to one-tenth of that necessary to produce rupture. This increment of strength adds also to the weight of the structure, as well as to the cost, and is at best an imperfect method of making liberal allowances for such contingencies as may arise from imperfections of material or manufacture, and from shocks produced by any external forces. Frequent experiments upon the strength of materials have enabled constructors to determine not only the ultimate breaking strains, but, what is of far greater importance, that limit of strength beyond which the material cannot be strained without producing a permanent set or change of form under the application of a given load at frequent intervals of time. This limit is known as the limit of elasticity, and it is now regarded as the vital point in preparing materials for bridges, that it shall be greater than that produced by any possible strain to which a member can be subjected. In a combination bridge, therefore, as of wood or iron, or of various grades of iron or of steel, the absurdity of the former practice of applying a constant factor to all the members of a truss becomes at once apparent, and the necessity of proportioning each member so as best to resist its individual strains, and keep them within In former specifications the ultimate strength of wrought iron was required to be from 55,000 to 65,000 pounds per unit, and the elastic limit would perhaps reach 20,000 pounds, or about one-third the breaking strength; whilst at present the elastic limit is increased to about 25,000 pounds, whilst the ultimate strength may be as low as 45,000 to 50,000. So soon as the load on a structure produces a strain beyond the elastic limit of any indispensable member, just so soon will its destruction become merely a question of time. Mr. O. Chanute, C. E., chief engineer of the New York, Lake Erie, and Western Railroad, is authority for the following distribution of safe working loads on the several members of a bridge: "Late specifications have pretty well discarded all mention of a factor of safety,' and we now limit the strains to the several parts of bridges, in accordance with their posicertain numbers of pounds per square inch upon each of tion and importance in the structure, and more particularly the frequency with which they are likely to be strained up to the calculated maximum amount. "Thus, for floor-beam hangers, which are sure to be loaded to the full calculated amount by the passage of nearly every locomotive, and which have no chance to stretch gradually, both by reason of their short lengths and because of the sudden application of the loads, we generally limit the tensile strains to 6000 or 7000 pounds to the square inch. Upon the bottom flanges of riveted plate-girders, which are also strained nearly to the full calculated amount by every train, and in which riveting is frequently imperfect, we limit the strains to 7000 or 8000 pounds per square inch, while we admit 10,000 pounds upon the bottom flanges of solid rolled beams, where no such imperfections are possible. On bottom chords, main ties, and main diagonals, which can only be strained to the calculated amounts when the bridge is loaded with the assumed maximum train, which generally consists of locomotives or of two of the heaviest engines followed by a train of the heaviest cars in the service, and which, as they advance, impose gradually their strains, we limit the latter to 10,000 pounds to the square inch; while on the lateral bracing ties, which cannot be loaded to the calculated amount unless the wind blows a hurricane and a train is standing on the bridge at the same time, we admit strains in tension of 15,000 pounds to the inch. "The above all refer to tensile strains. In compression members we follow the same general practice, and limit the strains to 5000 or 6000 pounds to the square inch on the top flanges of riveted girders, and to 10,000 pounds for solid rolled beams, while we calculate top chords, posts, and struts by formulæ which result in strains of about 8000 pounds to the inch for 1 diameter, down to 4000 pounds for 70 diameters. "With these proportions, involving, as will be seen, quite a number of different factors of safety,' we assume that but we are still learning by experience and modifying our our bridges are equally strong and safe in all their parts; views year by year. It must be admitted that our knowledge of the strength and safe loads upon compression members is as yet very deficient and imperfect. It has been mainly obtained by experiments upon small pieces; and for want of sufficiently powerful machines to test fulltheir behavior than about that of tension parts, which, sized compression members, we know very much less about being generally subdivided into a number of parallel pieces, we have been enabled to test with quite satisfactory accuracy. "There is, at last, one machine in the United Statesthat of the Government at the Watertown Arsenal—which is capable of testing full-sized compression pieces, and we may reasonably hope that important results and information will follow from its use. It is understood that the officers in charge are authorized to make tests for parties who may apply at actual cost, which is about $16 per day. As such experiments should be made upon a uniform plan, it is much to be hoped that Congress will make an appropriation for a series of experiments upon compression members of various shapes, lengths, and materials. There is reason to believe that the results of such experiments may lead to important changes in our current bridge-practice, and materially add to the safety of our bridges." The British rules for standard working loads are as | practice is to load the bridge with engines and measure the follows: "1. The working load for railway-bridges 400 feet in length and upward does not exceed ton per running foot on each line. "2. No more locomotives than will cover 100 feet in length follow each other without interruption; hence, the working load per foot diminishes as the span increases from 100 up to 400 feet. "3. Engines may be arranged on bridges less than 100 feet long so as to produce greater strains than could be due to the engine-load if it were of uniform density; hence, the equivalent working load per foot increases as the span diminishes from 100 feet downward. "4. Bridges less than 40 feet in span are subject to concentrated loads from single 9% FIG. 64.-Phoenix Wroughtiron Columns. engines, as well as to extra deflection from high-speed trains. "5. The standard locomotive is assumed to be 24 feet long, and to have six wheels with a 12-foot base-to have half its weight resting on the middle wheels and one-quarter on the leading and trailing pairs respectively. "6. Standard engines are assumed to weigh from 24 to 32 tons. This makes the standard load vary from 1 to 14 tons." No definite rule has been made by the Board of Trade for the proof load of railroad bridges, but the common deflection. For common road bridges Mr. Stoney has found by experiment that it is possible so to condense people in a crowd that each person will require but one square foot of standing room, and will produce a statical pressure of about allow 100 pounds per square foot of floor-surface. 150 pounds; but this is an extreme case; the practice is to The standard proof load in France for suspension bridges was 41 pounds per square foot, but it has recently been doubled, and the strains on members of IIHI FIG. 65.-Sections of Keystone Columns. truss bridges are limited to about 8535 pounds per square inch. In Germany the strains are about 10,000 pounds to the inch. In determining the safe load of bridges due regard must be paid to the ratio existing between the live and dead load. It is customary in both Europe and America to regard a live load as twice as injurious as a dead one, and hence the aggregate load is obtained by adding the weight of the bridge per lineal foot to twice that of foot. The maximum concentrated load for different spans, as applicable to panel points or supports at these intervals, is, for spans of the greatest weight on drivers of engine. This latter | For any greater span the load is 150 tons per lineal unit varies according to the service required and style of engine, but to insure safety all bridges should be so proportioned as to carry the heaviest engines manufactured, and to leave a liberal margin for future increase in weight of rolling stock. The general rule, however, must not be applied indiscriminately to bridges of any span, as it is evident that the relation between the length of wheel-base, concentration of load over same, and length of span will influence to a large extent the stress upon the several parts of the structure. The following exemplification of the best American practice is that of the Messrs. Wilson Bros., engineers of bridges on the Pennsylvania Railroad, giving the equivalent uniform load per lineal foot: Span. 5 feet, Load. 14:00 tons. Span. 11 feet, Load. 25.60 tons. Since the method of connecting the several parts of a bridge has enabled Americans to compete successfully with foreign builders, it may be desirable to note some of the peculiarities of this system as exemplified in the structures of a few of the oldest companies. During the transition period cast-iron joint-boxes were used quite freely, and still retain their place in combination bridges. Hollow cast-iron columns were used for compression, and long square links for the tension-members of the lower chords. These earlier forms are seen in the first bridges of Whipple and Fink (figs. 28, 38). The loops of square bar iron were subse |