Truss-bridge.
A bridge which depends for its stability upon the application of the principle of the truss.
Short bridges of this class may be formed by a single truss; larger structures are composed of a system of trusses or bays so connected that the spaces between the abutments and the piers may each be regarded as constituting a single compound truss.
Fig. 6708 shows several trusses for bridges of short span.
a, deck-truss for 30 feet span.
b, deck-truss for from 24 to 48 feet span.
c, deck-truss for 70 feet span.
d, deck-truss for 100 feet span.
The extensive use of wrought iron is a characteristic feature in bridges of this kind.
that material being generally employed in all parts which have to resist a tensile strain, and the various systems each aim to solve the problem of combining the maximum of strength with the minimum of lightness, by dispensing as far as possible with compressive forces, and relying as little as may be on the rigidity of materials.
Rider's, one of the earlier American forms, is composed of an upper and a lower chord, — the former of cast, and the latter of wrought, iron, — connected by upright posts of cast and diagonal braces of wrought iron, and has been successfully employed in bridges of moderate span.
In
Fink's which has been extensively used upon railways in this country, the foot of each principal strut, or king-post, is connected with the ends of the top-chord by a pair of diagonal bars which support the weight of one half the truss: each halfspan is again subdivided by a strut and two diagonal tensionbars, extended one to the nearest end of the top-chord.
and the other to the top of the center-post; each quarter-span is similarly divided into eighths, and these, again, for spans of more than 100 feet, into sixteenths.
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Principles of through-bridge trusses. |
In
Bollman's truss, the load upon each panel is transferred to the end of the truss by a pair of suspension bars The former railroad bridge at
Harper's Ferry was of this kind, having four parallel trusses for a double line, with a clear span of 124 feet, the span being divided into eight panels.
The depth of the truss was 17 feet 6 inches, and with a weight of 122 tons, at a speed of 8 miles an hour, the deflection at the center is stated to have been but 1 3/8 inches.
a (
Fig. 6709),
Fink system.
b,
Bollman system.
c,
Howe or
Jones system.
d,
Murphy-
Whipple system.
e, Linville system.
f, Post system.
g, system of triangles.
| The Louisville railway-bridge is of iron trusses, and has Piers | 28. |
| Spans | 29.
from 30 to 400 feet. |
Length of superstructure.
5,294 feet.
Hight above high water, 50 1/2 feet.
Hight above low water, 101 feet.
Wood in superstructure, 610,000 feet, board measure.
Masonry in piers, 30,000 cubic feet.
Iron in bridge, 8,723,000 pounds.
Cost, $1.555,000.
The railway bridge of the
Susquehanna, at
Havre de Grace, was built from the plans of
George A. Parker.
Its cost was about $1,500,000. The work occupied about 1,000 men for 4 years. The piers which support the superstructure are 13 in number, and are of stone carefully laid in cement within caissons of boiler-iron, which reach a point above the line of running ice. From this hight the piers are finished with cut-stone, laid in courses.
The draw-pier is circular, with a diameter of 24 feet 8 inches at the top of the caisson, while each of the other piers has a width of 8 feet, and a length of 35 feet 4 inches at top of caisson, and a width of 7 feet 3 inches at top of cut-stone.
Beside the piers which carry the superstructure, there are at the draw 2 guard-piers, one above and one below, which serve to protect the draw from injury and to aid vessels in passing.
It is substantially the Howe truss.
The bridge across the entrance to the
Niagara River, at
Black Rock, designed for the use of the
Grand Trunk, Great Western, Canada Southern, New York Central,
Erie, and
New York West
[
2639]
Shore, and Chicago Railways, has a total length of 3,550 feet, 1,300 feet of which are over trestle-work upon
Squaw Island, 450 feet over
Black Rock Harbor, and the remainder over the main branch of the river.
The river portion of the bridge has 8 piers and 2 abutments.
Owing to the depth of water, from 12 to 45 feet, and a current of from 5 1/2 to 10 miles an hour, varying with the state of the wind, considerable difficulty was experienced in the construction of the piers, which were founded upon caissons.
Four of the spans are 190, and three others 240 feet wide, in the clear, and there are two draw-openings, each 160 feet wide.
In
Black Rock Harbor are two drawopen-ings, each of 90 feet, and a fixed span of 220 feet; these are supported on 2 piers and 2 abutments.
The masonry is of sandstone, and the superstructure of wrought-iron, from the Phoenixville works, the whole amount of iron used being over 2,000 tons.
The 190-foot spans weigh 130, and the 240-foot spans 208 tons each.
After completion, one of the 190-foot spans was loaded with 210 tons of rails, equally distributed over the floor-beams, — a weight greater than that of a continuous line of locomotives covering the span, — and left in that condition for three days; the deflection amounted to but one inch, and, on the removal of the load, the truss resumed exactly its former position.
The entire cost of the bridge was about $1,500,000.
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Bridge-trusses. |
Fig. 6710, 6711, show several forms of trusses adopted by the principal bridge-building firms in the
United States.
See also iron bridge.
a, Kellogg Bridge Company,
Buffalo, N. Y.
b, American Bridge Company.
c, Phoenixville Bridge Company,
Philadelphia.
d, Watson Manufacturing Company,
Paterson, N. J.  |
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Bridge-trusses. |
e, Detroit Bridge and Iron Works,
Detroit, Mich.
f, Baltimore Bridge Company,
Baltimore, Md.
g,
Kellogg and
Maurice,
Athens, Pa.
h, Niagara Bridge Works,
Niagara, N. Y.
i,
Macdonald, New York.
j, Louisville Bridge and Iron Company,
Louisville, Ky.
k, Patapsco Bridge and Iron Works,
Baltimore, Md. 
