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assisted by his brother John. This also was destroyed fecution, being constructed of many long scarfed timbers, in the campaign of 1799. These bridges were remark- involving much labor and accurate fitting. able for their originality of design and difficulty of ex- An example of much simpler design for long-span
FIG. 8.—Wernwag's Wooden Arch over the Schuylkill River at Fairmount, Philadelphia, Pa. (“The Colossus". wooden arch trusses may be cited in the bridge erected was destroyed by fire in 1839, after twenty-seven years by Lewis Wernwag in 1812 across the Schuylkill River of service. at Fairmount, Philadelphia. It was 340 feet 38 inches
In 1804, Mr. Wernwag had built an arch bridge across the in single span, and had a rise of 19 feet 11 inches. The Delaware River at Trenton, in five spans of unequal
lengths. form of this structure is shown in fig. 8. This bridge The centre span was 200 feet long, the two adjacent 180, and
ented in America is that of Theodore Burr, which was granted Feb. 14, 1806, and improved April 3, 1817. As originally designed, the posts and top chord were single, while the lower chord and arch were in pairs. Subsequently, the posts were doubled, and the chords composed of three pieces both at top and bottomIron ties were added between the main braces of each panel to serve as counterbraces. A fair sample of the original structure is shown in fig. 12, representing the Columbia Bridge, across the Schuylkill River near Peter's Island, Philadelphia. It cost about $133,000, and was built in 1834 in three months from the completion of the piers. To compensate to some extent for the increase of strains in the web members, it will be seen that the panel lengths diminish from the centre towards the ends of the bridge. There is, however, a waste of material required by the particular form of post. Recently, cast-iron check
Fig. 9.-Wernwag's Bridge at Trenton, N. J., over the
Delaware River. the shore spans 160 each. Each truss consisted of a single arch composed of eight planks 4 x 12. The roadway was suspended by chains of 15-inch square iron, the links of which were about 4 feet long and 5 inches wide, passing through the arches and between the chords and counterbraces, held in place by a key on top of the arch. The floor-beams were suspended below the roadway under the chords, and held up by the suspension chains. The bridge was destitute of lateral braces, the five trusses being connected by the floor-and-roof systems and stiffened by spur struts. (See plan, fig. 9.) This bridge has recently been replaced by a stiffened triangular iron truss bridge (fig. 10).
Numerous other bridges were erected by Mr. Wernwag; at least one of which, known as the Economy bridge, deserves notice for its simplicity. It is shown in fig. 11, representing the half of the longest spans across the Neshaminy Creek, Pa. It was simply a pair of cantilevers supporting Fig. 10.- Railroad and Highway Bridge at Trenton, S. J.. a triangular truss. There were three such ribs, connected
on Pennsylvania R. R. by cross-ties and bolts. The total span from centre to centre was 803 feet. This principle will be recognized as an braces have been substituted for the wooden struts anticipation of the latest form for very long spans. shown at a, a. In some of the bridges of this type
Among the earliest designs of wooden bridges pat- the arch is not fastened to the posts, but supports the lower chord by tie-rods passing through saddles over The form known as the Town or lattice truss was the arch and stirrup-pieces under the chord. This originally patented Jan. 20, 1820, by Ithiel Town of type has long been a favorite for common and railroad New York. It consisted mainly of timbers from 2 to 3 bridges.
inches thick and from 9 to 12 inches wide, depending
FIG. 11. - Details of Wernwag's “Economy” Bridge. on the span, placed diagonally and crossing each other | 1835, for doubling the side-trusses by “the addition of at right angles, so as to form a lattice-work, united by another similar set or series of bracing, of similar kind two or more treenails at each intersection. There were and dimensions, to be placed in a similar manner, either suitable horizontal string-pieces at the top and bottom directly opposite to the former or in any other manner, so of the trellis-work to sustain the floor and roof.
as to bring the second tier not opposite to the former, but
so that all the intersections of the braces of the latter series This design was improved by a patent issued April 3, shall fall between those of the former braces on the hori.
FIG. 12.—Details of Columbia Bridge, over Schuylkill River. zontal string-pieces. One, two, or more of these trusses, | arrangement of the members. To relieve the tension with a floor and other necessary parts, as usual, will con- on the lower chord, double wedges are driven behind stitute a bridge thus improved” (fig. 13).
the ends of those members. The spans of these bridges frequently exceeded 150 The modifications introduced by Bvt. Lt.-Col. S. H. Long, feet, in which case there were two upper and two lower U. S. Engineers, March 6, 1830, consisted of (1st) Two modes chords, sometimes of timber 6 inches thick. For com- of splicing the string-pieces--one by wooden, the other by mon road bridges of spans not exceeding 175 feet
two heretofore adopted. (24) A system of bracing and framing
iron splicing-pieces, of a different construction from any trusses were sufficient if built of 3 x 12 white pine with such that the stresses or thrusts communicated by the open squares 23 feet on a side in the clear; but many braces to the trusses are resisted by shoulders or steps, as of them warped badly, and when applied to railroad nearly as may be at right angles with the grain or fibres of purposes failed entirely. The great objection to this the timber. (34) A system of counterbracing by which the form of bridge is the unequal distribution of material frames are rendered stiff and the bridge kept in uniform with reference to the stresses. Its use in wooden struc- action. (4th) The introduction of metal bearing-plates, of tures is nearly obsolete in this country, but it is not un- and toe of each main brace and the steps in the post common in iron.
against which they thrust. (5th) A mode of keying or Several forms of improved lattice bridges have been wedging by which the centre of the bridge may be elesuggested to overcome the objections to the Town truss. vated and sustained in case of subsidence from shrinking One of these is shown in fig. 14, in which the braces, or compression of the timber. The ratio of the panel instead of being single, are arranged in pairs, one on height to its length is about 11 to 1. (See fig. 15.) On each side of the truss, and the tie, which is vertical, is Jan. 23, 1836, Col.
Long patented an application of the latmade to pass between them; the end-braces also all rest of the upper or lower string-pieces of his bridge as diagonal
tice-work of Mr. Town to the horizontal lateral stiffening on the bridge-seat, giving greater support to the struc- bracing. ture, and the truss is effectively counterbraced by this These patents were succeeded by others for various improvements in details, but it was found practically edly popular and extensively used in both wood and impossible to make these bridges, with wooden connec- iron, is clearly stated in the patent granted to George tions, sufficiently stiff for railway purposes.
W. Long, U. S. A., Fort Jackson, La., March 10, 1870. The origin of the bow-string girder, a form deserv- The object of his invention was " to secure great strength
Plan of Side-Trusses.
FIG. 13.—“Town” Covered Wooden Bridge for Railroads. in a bridge of framework of such shape as to support the framed beam of this shape will contain no superfluous greatest strength when in a horizontal position, assuming weight of material; each timber will receive a portion
of stress to support any weight placed on the bridge; and further, that this stress shall be in the longitudinal or strongest direction of them, either in thrust or tension. The form may be either an ellipse, a segment of a circle, or a triangle.'
The bridge (fig. 16) consists—"(1st) of a string. piece made of one timber, or if more they are joined with straps of iron so well bolted through the timber as to make a joint as strong as the other parts
of the string, which extends the whole length of Fig. 14.-Haupt's Improved Lattice Truss.
the span. (20) A set of posts are made to rest on that form which will make it equally strong throughout. passing under it and well bolted to the bottom, etc. (3d
the string, and made fast to it by straps of irou The semi-ellipse possesses this property, as the |A set of timbers joining the tops of the posts, or (4th
FIG. 15.-Long's “Jackson" Bridge. The braces are a set of timbers placed diagonally between | The strings may be of iron, and also ties in opposite diag. the posts, all inclining inward. ... The material for con- onals to the braces, of iron. If a chain is used for the struction may be wood, iron, or a combination of the two. strings, an inverted position of the bridge may be assumed
(making it a suspension bridge). The saving in expense of per lineal foot (see fig. 17). In this bridge the panels piers and abutments not required to resist a thrust (as in were only 7 feet long, the braces all of the same size, Burr's and Long's bridges) must be counted a material ad
FIG. 16.-Long's Bow-string Girder. vantage, especially on soft soil." Its application to roofs is also mentioned. The span is limited to 200 feet. A model 10 feet long and 11 high, made of 14-inch cypress timbers, supported 14 men whose united weight was 2140 pounds.
On July 10, 1840, William Howe of Warren, Mass., obtained a patent for a bridge in which all the members were of wood connected by keys and wedges and containing an arch. This was succeeded almost immediately by a patent by the same inventor, bearing date Aug. 3, 1840, in which iron rods are substituted for the wooden posts. The braces have a “run” of two panels. It was not until Aug. 28, 1846, that the cast-iron angleblocks and sockets extending through the chord-pieces
FIG. 17.—Howe Truss at Springfield, Mass. were patented.
and the angle-blocks of wood. Though light and badly The first bridge built by Howe was a highway span proportioned, it stood until 1853, when it was replaced of 75 feet in 1840, and during the same year he erected by a "Howe" of more modern design, and this in turn the seven spans, of 180 feet each, crossing the Connecti- was removed in 1874 to give place to a wrought-iron cut. River at Springfield, Mass., at a cost of about $2.2 structure.
FIG. 18.—“Howe" Truss Bridge over the Susquehanna River, Pennsylvania R. R. The later design in combination with an_arch is The difference between the Howe and Pratt being in clearly shown in the Rockville bridge on the Pennsyl- the substitution of wood for iron ties, and the reverse, vania Railroad (figs. 18, 19).
the strains in the members were changed from tension to compression, and vice versâ, requiring slight modifications in the joints; but experience soon gave the preference to the Howe form, because of its requiring less iron, then the more expensive material, and the simplicity of details. It was found difficult to keep the
two diagonal ties of the Pratt system in adjustment. FIG. 19.—Howe Truss over the Susquehanna River on
This latter form, however, is well adapted to structures Pennsylvania R. R. (1848-49).
in iron, where it is frequently applied. For wooden A form of truss almost identical with that of Howe bridges the Howe truss, although not the most economwas invented by Thomas W. and Caleb Pratt
, and ical, is unrivalled in simplicity of construction, rapidity patented April 4, 1844.
of erection, and general utility, and it is still extensively
used wherever timber can be obtained. They claimed the combination of two diagonal tensionbraces and straining-blocks in each panel of the truss
The Susquehanna River bridge at Rockville, on the frame of a bridge, by means of which the camber may be Pennsylvania
Railroad, which was built in 1848-49 by 80 regulated as to increase or diminish it either in whole H. Haupt, C. E., is 3680 feet long, supported on or in sectional parts of the bridge."
twenty-two piers and two abutments, founded upon cribs. The piers are 6 feet wide on top and 10 feet at The arches are in three segments, the dimensions being the springing of the arches. The spans are 160 feet at centre 11 inches + 7 inches + 21 inches deep, by 9 inches from centre to centre of piers. The following are the wide; at skew-back 11 inches each in depth, and same width. principal dimensions of the superstructure, which is a
Height of truss from out to out of chords, 18 feet.
The cost of the masonry was $96,355,84, and of the Howe deck truss stiffened with a substantial wooden superstructure $73,600, or $20 per lineal foot, making the arch, as shown in figs. 18, 19:
entire cost about $170,000. Span in the clear......
149 ft. 3 in.
This bridge, which was single-track, carried the Versed sine of lower arch.
enormous business of the Pennsylvania Railroad until No. of panels.......
16 Length of panel...........
1877, when it was replaced by one of iron (fig. 20). Width in clear between arches....
On Feb. 20, 1849, J. Dutton Steele of Pottstown, chords
5 “ Pa., took out a patent for a modification of the Howe Angle of pier and chord........... 681°
truss, in which he merely substituted a wooden vertical
96 13 « 15"
FIG. 20.-Stiffened Triangular Truss over the Susquehanna River at Rockville, Pennsylvania R. R. tie in place of the iron rod, without in any way chang- name of its inventor, Foreman. It is similar to a ing the action of the truss.
Howe, but has additional arch-braces at the end-panels.
Mr. B. H. Latrobe, chief engineer of the Baltimore and Ohio Railroad, used for some time on that road form of truss bearing some resemblance to the celebrated bridge at Schaffhausen, consisting of archbraces and straining-beams, with vertical ties and diagonal braces and counterbraces, making a well-proportioned and scientifically arranged structure of great strength, as shown in fig. 21. The bridge is not so
economical as some of those already cited. Span 145ft.
Another form of wooden bridge possessing peculiar properties is that patented by Gen. D. C. McCallum (July 15, 1851), shown in fig. 22. It is a modification of the older form of Burr, in which the upper chon! is made curved, so as to increase the depth of truss at the centre, and in which there are introduced archbraces starting from the masonry, passing through the
lower chord, and abutting against the heads of the FIG. 21.-Wooden Railroad Bridges on Balt. and Ohio R. R. ventor that these braces would sustain the bridge even
posts of the end-panels. It was the boast of the inThe wooden truss most frequently used on the Phila- if the lower chord were cut in two; and an instance acdelphia and Reading Railroad is that known by the tually occurred in which the lower chord fell under the
weight of a train into the river, leaving the upper por- complicated that it was customary to make a full-sized tion of the bridge standing. This bridge was exceed drawing on a smooth floor from which to obtain patterns ingly stiff, and for a time very popular, but it was so for framing. It is now obsolete.