Chapter IX: Iron Bridges

“A little knowledge is a dangerous thing.”

213. Within the past ten years iron has been brought extensively into use for railroad bridging; and when employed by those who understand its chemical and mechanical nature is unequalled for strength, durability, and elegance of appearance; but when, as is too often the case in America, it is intrusted to men who neither know nor care for any thing but the price they get for it, nothing can be more unsafe. No material requires so complete a knowledge of its properties, to be safely used, as cast-iron.

NATURE AND STRENGTH OF IRON.

214. The table below shows the properties of the several descriptions of iron used in engineering.

┌───────────┬──────────┬───────────┬───────────┬──────────────────────┐ │ Wrought │Cast-Iron.│Iron Wire. │ Boiler │ Designation of the │ │ Iron. │ │ │ Plate. │ quality. │ ├───────────┼──────────┼───────────┼───────────┼──────────────────────┤ │ 480│ 450│ ——│ 480│Weight per cubic foot │ │ │ │ │ │ in lbs. │ │ │ │ │ │Resistance to │ │ 15000│ 4500│ 25000│ 12740│ extension in lbs. │ │ │ │ │ │ per sq. inch. │ │ │ │ │ │Resistance to │ │ 11000│ 25000│ ——│ 7500│ compression in lbs. │ │ │ │ │ │ per sq. in. │ │ │ │ │ │Expansion per degree │ │ .0000066│ .00000608│ .00000685│ .0000066│ Fahrenheit in │ │ │ │ │ │ lengths. │ │.0000000424│.000000106│.0000000446│.0000000524│Extension per lb. per │ │ │ │ │ │ square inch. │ │ .000000149│.000000083│ ——│ .000000189│Compression per lb. │ │ │ │ │ │ per square inch. │ │ │ │ │ │Ratio of extensive to │ │ 90 to 66│ 20 to 111│ ——│ 127 to 75│ compressive │ │ │ │ │ │ strength. │ │ │ │ │ │Resistance to │ │ 12500│ 17500│ ——│ ——│ detrusion, or │ │ │ │ │ │ shearing. │ │ 55│ 31│ ——│ ——│Relative transverse │ │ │ │ │ │ strength. │ └───────────┴──────────┴───────────┴───────────┴──────────────────────┘

_Column four refers to boiler plate when built into tubes._

After wrought iron has become a little compressed, its power to resist a crushing force is very much increased.

215. The tenacity of wrought iron is increased by heating. Experiments upon thirty varieties gave the following mean result, the temperature ranging from 500° to 700° Fahrenheit.

Strength when

Cold. Hot. Cooled.
60,000 64,000 70,000

216. Stirling’s process of toughening cast-iron, by the addition of malleable scrap, increases the strength in the following ratio:—

The mean tensile strength of cast-iron being 18,000 lbs.
And the compressive strength being 105,000 lbs.
When Stirling-toughened the tensile strength is 23,000 lbs.
And the compressive strength 130,000 lbs.

The strength of cast-iron increases rapidly up to the twelfth or fifteenth recasting, when it is nearly doubled; after the fifteenth melting the strength decreases.

217. Wrought iron exposed for some time to vibration, as in the case of railroad axles, or iron which has been wrought with light hammers, loses its toughness and becomes “short,” (crystalline). The fibre may be restored in such cases by reheating and cooling slowly.

218. GENERAL RATIOS OF THE STRENGTH OF IRON.

Tension. Compression. Cross Strain.
Cast, 300 1,666 31.68
Wrought, 1,000 733 55.40

OF THE STRENGTH OF BOILER PLATES.

219. The strength of rolled boiler plates is no greater in the _direction of the fibres than crosswise_, but is more regular; whence the length of the fibre must be placed as nearly as possible with the direction of the force.

A mean of twelve experiments, by Mr. Fairbairn, gives the tensile strength of wrought iron plates as 50,960 lbs. per square inch; and the compressive strength of plates, when built into tubes, as 30,464 lbs., or for safe use in practice, for extension, 12,740 lbs., and for compression, 7,500 lbs. In the remarks upon girder bridges the matter of riveting will be considered.

CLASSIFICATION OF IRON BRIDGES.

220. Iron bridges may be classified as follows:—

Those entirely of _cast-iron_, or Arch and Girder bridge.
Those of _wrought iron_ alone, or Tubular and Girder.
Those of _iron wire_, or Suspension bridges.
Those of _cast and wrought iron_, or Trussed bridges.

The order in which these bridges may be placed as regards cost of construction, and extent of application, is as follows:—

Number. Span. Description of bridge.
1 10 to 50 feet Cast-iron girder.
2 50 to 200 feet Cast and wrought combinations.
3 200 to 2000 feet Suspension.
4 200 to 500 feet Cast arch.
5 25 to 100 feet Boiler plate girder.
6 100 to 500 feet Tubular.

Numbers 2, 3, and 5, are the forms which are in use upon American roads. No. 1, is very liable to failure, requires much more knowledge and care in building, and is far more expensive than a wooden truss, or trussed girder. No. 4, is very expensive, and causes a greater obstruction to the water-way than any other. The enormous expense of No. 6, should, and will prevent its adoption in the United States. Let us look at the principles of construction of numbers 2, 3, and 5.

COMBINATIONS OF CAST AND WROUGHT IRON.

221. Under this head come all of the iron trussed frames used in this country.

As before observed, skill in bridge construction consists in using always that material which with the least expense is the best able to resist the particular strain to which it may be exposed. Thus wrought iron must always be used to resist tension, and cast-iron compression. Posts, braces, and upper chords should always be cast, while ties and lower chords should be made of wrought iron.

The strength of a railroad bridge must be such as to resist all extra shocks and strains, such as are produced by derailment of engines, and breakage of axles; also incidental strains arising from change of form by expansion and contraction of the metal, and from high winds and gales.

Every part of a bridge not resisting some force is worse than useless, as it adds to the weight. Lightness not only increases the economy directly, but indirectly by removing a part of the permanent load.

222. Foremost in class number two stands Wendel Bollman’s Iron Suspension and trussed bridge. For simplicity of construction and directness of action, this bridge is unsurpassed. The weight at each post is transferred at once to the abutment or pier. The upper chord is of cast iron, hollow, octagonal without, and circular within. The posts consist of an ¶ᕼ¶ casting, the central web cast open and the flanges whole. The top is adjusted to the chord, and the bottom to the tension or suspending rods. These latter are of wrought iron, rectangular in section, joined when the length requires it by an eye bolt. Each set after leaving the foot of the post, passes through the chair at A B, fig. 101, and is secured by a nut. The junction of the tension rod A C, and the counter rod B C, is attached indirectly to the foot of the post by a pendulum or link; which serves to equalize the effect of expansion upon the rods. Vibration and reaction are prevented by the panel diagonal ties D H, and C E. The floor is supported by flanges at the foot of each post. The lateral bracing consists of a system of hollow cast-iron posts, and of wrought diagonal tie rods. A lower chord is plainly unnecessary, its place being taken by the rods C B, F B, F A, G A.

A bridge of this description upon the Baltimore and Ohio Railroad of the following dimensions,

Clear span, 124 feet
Length of top chord, 128 feet
Length of panel, 15 feet
Height of truss, 17 feet
Width, 16 feet
Lbs. of cast-iron, 65,137
Lbs. of wrought iron, 33,527
Whole weight, 98,664
Weight per lineal foot, 796

was subjected to the following tests.

Three locomotives with tenders attached, and weighing in all one hundred and twenty-two tons, (nearly one ton per foot,) were run over the bridge at eight miles per hour, when the deflection at centre was one and three eighths inches, and at the first post nine sixteenths of an inch. The following tests were applied to a bridge of seventy-six feet span upon the Washington branch of the same road:

An engine and tender weighing forty tons, caused a deflection of five eighths of an inch. A fast passenger train deflected the bridge nine sixteenths of an inch.

Two engines and tenders, back to back, at rest, and weighing in all 77½ tons, caused a deflection of 11/16 inch, The same at ten miles per hour, 13/16 inch, Engines head to head at four miles per hour, 13/16 inch, Engines head to head at eight miles per hour, 13/16 inch, Engines head to head at twenty miles per hour, 14/16 inch.

The extreme expansion of the one hundred and twenty-eight feet chord from heat, was five sixteenths of an inch at each end, or five eighths of an inch in all, or 1/2457th of the length; and that without the slightest derangement of masonry. The rod C B, being five times as long as C A, expands five times as much, but at the same time the lengths D A, D B, being so nearly proportional to C A, and C B, expand also in the ratio of one to five; and thus no bad result is experienced.

The estimate of strains upon this bridge is extremely simple; the whole consisting of as many separate systems as there are posts. Each set of rods sustain a rectangle equal to one panel, i. e., the two adjacent half panels. Thus A C, and C B, support the rectangle _m m_, _m m_, the rods A F, F B, the rectangle _n n_, _n n_. Allowance must of course be made for the inclination of the rods. The dimensions of the central pair will of course be the same; but those of the other sets will vary. The diagonals D H, and H L, prevent reaction; and must be able to resist the action produced by the variable load upon one panel (as noticed in