« AnteriorContinuar »
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 it 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 Fig. 57.-Kinzua Viaduct, on a branch of the New York, prevent the water from freezing, but this jacketing Lake Erie, and Western R. R., near Bradford, Pa.
caused so great an expansion as to produce leakage, 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 towards the pier. This enables the section of the lower Fig. 59.-Highway and Aqueduct Bridge over Rock Creek, chord (which is of wood) to be reduced at the extremi
Washington, D. C. ties of the truss in proportion to the stresses. The and was afterwards removed without injury to the pipes upper chord is composed of four pieces throughout from thermal changes. The bridge is on Pennsylvania
St. Paul Highway (see fig. 58). -As early as 1854 avenue between Washington and Georgetown, D. C., it was proposed to bridge the Mississippi at St. Paul by where it is subjected to a large amount of travel
. a series of trusses in which the channel-span should not A similar structure was subsequently erected with a be less than 300 feet. The span, as constructed in 1858, span of 120 feet over College Branch on the Washingwas reduced to 240 feet, then a "long span;' but the ton Aqueduct. peculiar feature of this bridge consists in the fact that The distinguished engineer Mr. C. Shaler Smith has its roadway is built upon a grade of to up to the chan-constructed several bridges of such original design as nel-span, where it is is, caused by the bluffs, about 125 to deserve mention in this article. The first we shall feet high, on the left (there the west) bank of the river. notice is in the Royal Gorge of the Arkansas, on the The eastern approach consists of an embankment 1600 Denver and Rio Grande Railroad, in Colorado, erected feet in length, followed by 375 feet of trestling in bents during the summer of 1879 (see fig. 60). 30 feet apart; then seven spans of 140 feet each, suc- It is a continuous plate girder in three spans, 275 feet ceeded by the channel-span of 240 feet; and a short long and 7 feet deep, which is suspended by rods from span of 80 feet, over the St. Paul and Omaha Railroad, arch-braces” spanning the chasm at a height of 47 to the top of the bluff. The piers are built in steps, so feet. On either side the walls of the cañon rise almost that each truss is 7 feet higher than its predecessor, and perpendicularly 1800 feet, and the entire river, conthe roadway is supported on the top chords by bents. tracted into a width of 50 feet, flows beneath with a The short spans are of wood. The channel-span, of fall of 6 feet in the length of the bridge, which is paraliron, 63 feet above high-water, was rebuilt in 1875–76. lel with the stream. The bridge has a grade of 3 feet It was designed and erected by J. S. Sewell, C. E. All per 100. As the two middle supports are yielding, it the short spans were rebuilt in 1870.
i required very delicate computations to determine the
stresses, and ingenuity to provide for thermal changes. The Minnehaha bridge (fig. 61) on the Chicago, Mil
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 vs. EUROPEAN PRACTICE.
The characteristic differences between AmerFIG. 60.
ican and European methods of bridge construcsible is said by the engineer to have been almost as dif- tion may be briefly stated to be (a) the use of pins in ficult as the erection of the Kentucky River bridge. 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 latticed girders of European engineers, thus offering less structure; (c) the reduction of the number of members 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 FIG. 62.
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- 1 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 In former specifications the ultimate strength of on the American plan will always offer more safety wrought iron was required to be from 55,000 to 65,000 than one built on the European plan, even though the pounds per unit, and the elastic limit would perhaps maximum theoretical strength of both may be the same. 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 FIG. 63.
structure produces a strain The variations from the calculated strength will be beyond the elastic limit of any indispensable member, different in the latter, according to the care bestowed just so soon will its destruction become merely a quesupon the work in the place of construction, while this tion of time. Mr. 0. Chanute, C. E., chief engineer will not be the case in American bridges, in which the of the New York, Lake Erie, and Western Railroad, is same care is bestowed in the shops on the length of the authority for the following distribution of safe working various elements and the holes for the bolts, which are loads on the several members of a bridge: accurately drilled. With the same safety the trussed bridges of America may be made lighter, as well for the
“Late specifications have pretty well discarded all menreasons already given as for the absence of the joints tion of a factor of safety, and we now limit the strains to and packing-plates which are necessary to strengthen the several parts of bridges, in accordance with their posithe weakened parts in riveted trusses and at the ends tion and importance in the structure, and more particularly of tension-rods, as also to prevent warping. “But the frequency with which they are likely to be strained up the main argument in favor of the trusses with bolt to the calculated maximum amount. and pin connections is the fact that a great advantage "Thus, for floor-beam hangers, which are sure to be loaded is gained by means of the lighter, quicker, and less to the full calculated amount by the passage of nearly every
locomotive, and which have no chance to stretch gradually, expensive construction."
both by reason of their short lengths and because of the The latest American practice in the construction of sudden application of the loads, we generally limit the iron bridges is best exemplified by that of the Pennsyl- tensile strains to 6000 or 7000 pounds to the square inch. vania Railroad Company.
Upon the bottom flanges of riveted plate-girders, which are
also strained nearly to the full calculated amount by every It has adopted the system of using “solid rolled I-beams train, and in which riveting is frequently imperfect, we for all short spans, up to such lengths as they may be limit the strains to 7000 or 8000 pounds per square inch, available for the required live loads; then plate girders to while we admit 10,000 pounds upon the bottom flanges of spans of 50 to 60 feet, and in some cases even 70 to 80 feet, solid rolled beams, where no such imperfections are possible. above which they become too wasteful and expensive in On bottom chords, main ties, and main diagonals, which can only material, and are replaced by open trusses.
be strained to the calculated amounts when the bridge is “In small deck bridges the general practice is to use two loaded with the assumed maximum train, which generally I-beams under each rail of the track; then for longer consists of locomotives or of two of the heaviest engines spans up to 30 to 50 feet, where 'built' girders are em- followed by a train of the heaviest cars in the service, and ployed, to place one girder under each rail; and as the which, as they advance, impose gradually their strains, we span increases to adopt only three girders or trusses for limit the latter to 10,000 pounds to the square inch; while two tracks, spacing them so that when both tracks are on the lateral bracing ties, which cannot be loaded to the calloaded each truss will carry the same weight.
culated amount unless the wind blows a hurricane and a "In through bridges for double track, with three trusses, train is standing on the bridge at the same time, wc admit the centre truss sustains double the load of either outside strains in tension of 15,000 pounds to the inch. truss." (Jos. M. Wilson, C. E.)
“The above all refer to tensile strains. In compression
members we follow the same general practice, and limit FACTORS OF SAFETY.
the strains to 5000 or 6000 pounds to the square inch on It has been the practice to assume some maximum solid rolled beams, while we calculate top chords, posts, and
the top flanges of riveted girders, and to 10,000 pounds for load as being the greatest to which the bridge can ever struts by formulæ which result in strains of about 8000 be subjected, and then, to insure absolute safety, so to pounds to the inch for 1 diameter, down to 4000 pounds for increase the dimensions of the members as to reduce 70 diameters. the unit strains to from one-sixth to one-tenth of that
“With these proportions, involving, as will be seen, quite necessary to produce rupture. This increment of a number of different 'factors of safety,' we assume that
our bridges are equally strong and safe in all their parts; strength adds also to the weight of the structure, as well but we are still learning by experience and modifying our as to the cost, and is at best an imperfect method of views year by year. It must be admitted that our knowmaking liberal allowances for such contingencies as may ledge of the strength and safe loads upon compression arise from imperfections of material or manufacture, members is as yet very deficient and imperfect. It has and from shocks produced by any external forces. been mainly obtained by experiments upon small pieces ; Frequent experiments upon the strength of materials and for want of sufficiently powerful machines to test fullhave enabled constructors to determine not only the their behavior than about that of tension parts, which, ultimate breaking strains, but, what is of far greater being generally subdivided into a number of parallel importance, that limit of strength beyond which the pieces, we have been enabled to test with quite satisfacmaterial cannot be strained without producing a per- tory accuracy. manent set or change of form under the application of "There is, at last, one machine in the United States a given load at frequent intervals of time. This limit that of the Government at the Watertown Arsenal—which is known as the limit of elasticity, and it is now regarded is capable of testing full-sized compression pieces, and we as the vital point in preparing materials for bridges, tion will follow from its use. It is understood that the that it shall be greater than that produced by any officers in charge are authorized to make tests for parties possible strain to which a member can be subjected. who may apply at actual cost, which is about $16 per day. În a combination bridge, therefore, as of wood or iron, As such experiments should be made upon a uniform plan, or of various grades of iron or of steel, the absurdity of it is much to be hoped that Congress will make an approthe former practice of applying a constant factor to all priation for a series of experiments upon compression mem. the members of a truss becomes at once apparent, and reason to believe that the results of such experiments may the necessity of proportioning each member so as best I lead to important
changes in our current bridge-practice, to resist its individual strains, and keep them within 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:
deflection. For common road bridges Mr. Stoney has found “1. The working load for railway-bridges 400 feet in crowd that each person will require but one square foot of
by experiment that it is possible so to condense people in a length and upward does not exceed 1 ton per running foot standing room, and will produce a statical pressure of about on each line.
“2. No more locomotives than will cover 100 feet in 150 pounds; but this is an extreme case; the practice is to length follow each other without interruption; hence, the allow 100 pounds per square foot of floor-surface. working load per foot dimin
The standard proof load in France for suspension ishes as the span increases
bridges was 41 pounds per square foot, but it has refrom 100 up to 400 feet.
cently been doubled, and the strains on members of “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 Fig. 64.–Phænix Wroughtin span are subject to con
Fig. 65.-Sections of Keystone Columns. centrated loads from single engines, as well as to extra deflection from high-speed truss bridges are limited to about 8535 pounds per trains.
square inch. In Germany the strains are about 10,000 “5. The standard locomotive is assumed to be 24 feet pounds to the inch. long, and to have six wheels with a 12-foot base—to have In determining the safe load of bridges due regard half its weight resting on the middle wheels and one-quar- must be paid to the ratio existing between the live and
“6. Standard engines are assumed to weigh from 24 to 32 dead load. It is customary in both Europe and Amertons. This makes the standard load vary from 1 to 1f tons." ica to regard a live load as twice as injurious as a dead
No definite rule has been made by the Board of Trade one, and hence the aggregate load is obtained by adding for the proof load of railroad bridges, but the common the weight of the bridge per lineal foot to twice that of
6 7 8 9
FIG. 66.-Connections for Through Bridges, Keystone Bridge Co., Pittsburg. the greatest weight on drivers of engine. This latter For any greater span the load is 1.50 tons per lineal unit varies according to the service required and style foot. The maximum concentrated load for different of engine, but to insure safety all bridges should be so spans, as applicable to panel points or supports at these proportioned as to carry the heaviest engines manufac- intervals, is, for spans oftured, and to leave a liberal margin for future increase
Load. in weight of rolling stock.
5 feet, 14:00 tons.
11 feet, 25.60 tons. The general rule, however, must not be applied in
27.00 discriminately to bridges of any span, as it is evident
28:30 that the relation between the length of wheel-base,
29:50 concentration of load over same, and length of span
10 will influence to a large extent the stress upon the
31:20 several parts of the structure.
DETAILS. The following exemplification of the best American practice is that of the Messrs. Wilson Bros., engineers
Since the method of connecting the several parts of of bridges on the Pennsylvania Railroad, giving the a bridge has enabled Americans to compete successfully equivalent uniform load per lineal foot:
with foreign builders, it may be desirable to note some
of the peculiarities of this system as exemplified in the Span. Load.
Load. structures of a few of the oldest companies. 5 feet, 4.80 tons. 18, 19 feet, 2:55 tons. During the transition period cast-iron joint-boxes 4.00
were used quite freely, and still retain their place in 3:40
combination bridges. Hollow cast-iron columns were 3:00
used for compression, and long square links for the ten2.90
2:00 10 2.80
sion-members of the lower chords. These earlier forms 11-13 2.70
are seen in the first bridges of Whipple and Fink (figs. 14-17 2.60
28, 38). The loops of square bar iron were subse
28 32 40 48
quently modified by bending the ends of the rod around duce the length of pin, they were made flat, with forged the pin and welding them down; and still later, to re- 1 eyes upon their enlarged ends. For cast iron, as used
FIG. 67.-Keystone Deck Bridge-Details. in compression-members, various forms of rolled bars consist in the forms of their cross-sections and arrangehave been substituted, known as channels }, "I," "T,"ment of their connections. The Phoenix Co. use a closeangle L, cruciform +, deck 1, and other shapes.
Fig. 70.—Details of Through Bridge, Shoe of End Strut. FIG. 73.-Panel-post, Chord, and Wind-brace.
The distinctive features of the columns manufactured flanges (fig. 64), whilst the Keystone column is comby the Keystone, Phønix, Kellogg, and other companies posed of angular sections united by bolts running dia