=66. Introductory.=—Although the bridge structures of to-day serve the
same general purposes as those served by the most ancient structures,
they are very different engineering products. It is not long, in
comparison with the historic and prehistoric periods during which
bridges have been built, since the science of mechanics has been
sufficiently developed to make bridge design a rational procedure; and
it is scarcely more than a century since the principles of mechanics
were first applied to the design of bridge structures in such a way as
to determine even approximately the amount of stress produced in any
member by the imposed load. Naturally the first efforts made toward a
truly rational bridge design were in fact simple and crude and only
loosely approximate in their results. Probably the first analytic
treatment of bridges was given to the design of arches in masonry and
then in cast-iron. As the action of forces in structures became better
known through the development of mechanical science, the applications
of the latter became less crude and approximate and the approach to the
refined accuracy of the present day was begun.
=67. First Cast-iron Arch.=—These older structures, nearly all of them
arches or more or less related to the arch, first appeared in cast-iron
in the latter part of the eighteenth century, when nothing like an
accurate analysis of forces developed by the application of a given
load was known. The first cast-iron arch was erected over the Severn
in England near Coalbrookdale in the year 1779. This bridge had a
span of 100 feet, and the under surface of the arch or soffit at the
crown was 45 feet above the points at the abutment from which the arch
sprang, or, as civil engineers put it, the arch had a span of 100 feet
and a rise or versine of 45 feet. Other cast-iron arches were built in
England soon after.
=68. Early Timber Bridges in America.=—Timber bridges have been built
since the earliest historic periods and even earlier, but the widest
and boldest applications of timber to bridge structures have been made
in this country, beginning near the end of the eighteenth century and
running to the middle of the nineteenth century, when timber began to
be displaced by iron. Timber bridges and those of combined iron and
timber are built to some extent even at the present day, but the most
extended work of this class is to be found in the period just named.
In 1660 what was called the “Great Bridge” was built across the
Charles River near Boston, and was a structure on piles. Other similar
structures followed, but the first long-span timber bridge, where
genuine bridge trussing or framing was used, appears to have been
completed in 1792, when Colonel William P. Riddle constructed the
Amoskeag Bridge across the Merrimac River at Manchester, N. H., in
six spans of a little over 92 feet from centre to centre of piers.
From that time timber bridges, mostly on the combined arch and truss
principle, were built, many of them examples of remarkably excellent
engineering structures for their day. Among these the most prominent
were the Bellows Falls Bridge, in two spans of 184 feet each from
centre to centre of piers, over the Connecticut River, built in 1785-92
by Colonel Enoch Hale; the Essex-Merrimac Bridge over the Merrimac
River, three miles above Newburyport, Mass., built by Timothy Palmer
in 1792, consisting actually of two bridges with Deer Island between
them, the principal feature of each being a kind of arched truss of 160
feet span on one side of the island and 113 feet span on the other;
the Piscataqua Bridge, seven miles above Portsmouth, N. H., in which
a “stupendous arch of 244 feet cord is allowed to be a masterly piece
of architecture, planned and built by the ingenious Timothy Palmer of
Newburyport, Mass.,” in 1794; the so-called “Permanent Bridge” over
the Schuylkill River at Philadelphia, built in 1804-06 in two arches
of 150 feet and one of 195 feet, all in the clear, after the design
of Timothy Palmer; the Waterford Bridge over the Hudson River, built
in 1804 by Theodore Burr, in four combined arch and truss spans, one
of 154 feet, one of 161 feet, one of 176 feet, and the fourth of 180
feet, all in the clear; the Trenton Bridge, built in 1804-06 over
the Delaware River at Trenton, N. J., by Theodore Burr, in five arch
spans of the bowstring type, ranging from 161 feet to 203 feet in the
clear; a remarkable kind of wooden suspension bridge built by Theodore
Burr in 1808 across the Mohawk River at Schenectady, N. Y., in spans
ranging in length from 157 feet to 190 feet; the Susquehanna Bridge at
Harrisburg, Pa., built by Theodore Burr in 1812-16 in twelve spans of
about 210 feet each; the so-called Colossus Bridge, built in 1812 by
Lewis Wernwag over the Schuylkill River at Fairmount, Pa., with a clear
span of 340 feet 3¾ inches; the New Hope Bridge, built in 1814 over the
Delaware River, in six 175 feet combined arch and truss spans, and a
considerable number of others built by the same engineer.
Some of these wooden bridges, like those at Easton, Pa., and at
Waterford, N. Y., remained in use for over ninety years with only
ordinary repairs and with nearly all of the timber in good condition.
In such cases the arches and trusses have been housed and covered
with boards, so as to make what has been commonly called a covered
bridge. The curious timber suspension bridge built by Theodore Burr
at Schenectady was used twenty years as originally built, but its
excessive deflection under loads made it necessary to build up a pier
under the middle of each span so as to support the bridge structure
at those points. These bridges were all constructed to carry highway
traffic, but timber bridges to carry railroad traffic were subsequently
built on similar plans, except that Burr’s plan of wooden suspension
bridge at Schenectady was never repeated.
[Illustration: MOHAWK BRIDGE AT SCHENECTADY N.Y.
Built by Theodore Burr.
FIG. 1.]
[Illustration: PATENT BRIDGE “COLOSSUS”
Across the River Schuylkill at Philadelphia.
Single Arch 340 feet 3¾ inches.
Built by Lewis Wernwag.
FIG. 2.]
=69. Town Lattice Bridge.=—A later type of timber bridge which was
most extensively used in this country was invented by Ithiel Town in
January, 1820, which was known as the Town lattice bridge. This timber
bridge was among those used for railroad structures. As shown by the
plan it was composed of a close timber lattice, heavy plank being used
as the lattice members, and they were all joined by wooden pins at
their intersections. This type of timber structure was comparatively
common not longer ago than twenty-five years, and probably some
structures of its kind are still in use. The close latticework with its
many pinned intersections made a very safe and strong frame-work, and
it enjoyed deserved popularity. It was the forerunner in timber of the
modern all-riveted iron and steel lattice truss. It is of sufficient
significance to state, in connection with the Town lattice, that its
inventor claimed that his trusses could be made of wrought or cast-iron
as well as timber. In many cases timber arches were combined with them.
[Illustration: FIG. 3.]
=70. Howe Truss.=—The next distinct advance made in the development
of bridge construction in the United States was made by brevet
Lieutenant-Colonel Long of the Corps of Engineers, U.S.A., in 1830-39,
and by William Howe, who patented the bridge known as the Howe
truss, although the structure more lately known under that name is a
modification of Howe’s original truss. Long’s truss was entirely of
timber, including the keys, pins, or treenails required, and it was
frequently built in combination with the wooden arch. The truss was
considerably used, but it was not sufficiently popular to remain in
use.
[Illustration: Howe Truss-Bridge.]
[Illustration: FIG. 4.]
The Howe truss was not an all-wooden bridge. The top and bottom
horizontal members, known as “chords,” the inclined braces between
them and the vertical end braces, all connecting the two chords, were
of timber, and they were bolted at all intersections; but the vertical
braces were of round iron with screw ends. These rods extended through
both chords and received nuts at both ends pressing on cast-iron
washers through which the rods extended. These wrought-iron round rods
were in groups at each panel-point, numbering as many as existing
stresses required. The ends of the timber braces abutted against
cast-iron joint-boxes. The railroad floor was carried on heavy timber
ties running entirely across the bridge and resting upon the lower
chord members. It was a structure simple in character, easily framed,
and of materials readily secured. It was also easily erected and could
quickly be constructed for any reasonable length of span. It possessed
so many merits that it became widely adopted and is used in modified
form at the present day, particularly on lines where the first cost
of construction must be kept as low as possible. The large amount of
timber in it and the simple character of its wrought-iron or steel
members greatly reduces its first cost.
=71. Pratt Truss.=—In 1844 the two Pratts, Thomas W. and Caleb,
patented the truss, largely of timber, which has since been perpetuated
in form by probably the largest number of iron and steel spans ever
constructed on a single type. The original Pratt trusses had timber
upper and lower chords, but the vertical braces were also made of
timber instead of iron, while the inclined braces were of round
wrought-iron with screw ends, the reverse of the web arrangement in the
Howe type. This truss had the great advantage of making the longest
braces (of iron) resist tension only, while the shorter vertical braces
resist compression. As a partially timber bridge it could not compete
with the Howe truss, because it contained materially more iron and
consequently was more costly. This structure practically closed the
period of development of timber bridges.
=72. Squire Whipple’s Work.=—What amounted to a new epoch in the
development of bridge construction in this country practically began
in 1840 when Squire Whipple built his first bowstring truss with
wrought-iron tension and cast-iron compression members. While the
Pratts and Howe had begun to employ to some extent the analysis of
stresses in the design of their bridge members, the era of exact bridge
analysis began with Squire Whipple. He subjected his bridge designs to
the exacting requirements of a rational analysis, and to him belongs
the honor of placing the design of bridges upon the firm foundation of
a systematic mathematical analysis.
=73. Character of Work of Early Builders.=—The names of Palmer, Burr,
and Wernwag were connected with an era of admirable engineering works,
but, with bridge analysis practically unknown, and with the simplest
and crudest materials at their disposal, their resources were largely
constituted of an intuitive engineering judgment of high quality and
remarkable force in the execution of their designs never excelled in
American engineering. They occasionally made failures, it is true,
but it is not recorded that they ever made the same error twice, and
the works which they constructed form a series of precedents which
have made themselves felt in the entire development of American
bridge-building.