BRIDGE DESIGNS, Railway. Bridges have been in use from prehistoric times. Even the scientific forms, now looked upon as most modern, were developed in a crude way by the ancients, timber cantilever structures being built by the Chinese in the time of Confucius.
Bridge design, of necessity, continued in an elementary stage as long as timber and masonry were the only materials in use, and long spans were impracticable with such materials, though the "'Towne lattice spans, built entirely of plank connected with hard-wood pins, were as nearly perfect as was possible without the use of iron, and some of these spans are still doing effective duty after 90 years of continued use.
The earliest use of iron in bridges began with the use of cast-iron posts and cast-iron arches, but this material was not found entirely satisfactory for long-span bridges. The first real development began with the use of wrought iron, which was first used in this country in vertical tension members in the truss about 1840, and in inclined tension members in the truss about 1845. The use of wrought-iron tension members with wood and cast-iron compression members continued until about 1860, when Howard Carroll, an Irish engineer, assistant to Chief Engineer Gray of the New York Central, introduced the riveted lattice bridge, built entirely of wrought iron, which was developed by his successors on that road and used until very recent years. Wrought-iron plate girders were also first built about 1860.
J. H. Linville, between 1850 and 1860, in troduced on the Pennsylvania Railroad a type of truss with flat diagonals and hexagonal rolled-iron posts,.and about this same time Boll man and Fink introduced on the Baltimore & Ohio their multiple suspension types of trusses with flat diagonals and vertical iron posts. John W. Murphy adopted Whipple's ideas and developed a truss on Whipple's lines with cast iron posts and square bar tension members with loop eyes.
A long step in the evolution of properly designed compression members was made by David Reeves about 1864, when he began the use of the rolled iron, round segmental "Phce column, and about that date he also de veloped the hydraulic upset-end on tension members, which were made by the Phoenix Iron Company, first on round rods and afterward developed • into eyes on flat bars, and these two features were combined in the (Phoenix" bridge, which was far in advance of any bridge construction at that date and quickly came into very general use. While it still preserved the
use of cast iron for connections, it permanently did away with any further use of cast iron for compression members, and its simplicity and ease of erection made it by far the best type developed up to that time. Many of these bridges are still in use, and those that have been replaced have generally been removed on account of the great increase of rolling loads to be carried, and not on account of inherent defects in the design.
With the use of aeye bars'> the pin-con nected type of truss came into practically uni versal use in this country, since it had great advantages over riveted bridges in many re spects; but owing to its ease of erection, it was in many cases carried to ridiculous ex tremes, and spans as small as 25 and 30 feet were constructed with individual members so light and insignificant that the structure had no rigidity. In recent years there has been a very general disuse of this type of construction for spans under 150 feet, and the best engi neering practice now calls for riveted structures for small spans, though American engineers still use the pin-connected type for long spans almost universally.
The general principles of railroad bridge construction have not undergone much change in the last 20 years, as practically all spans are built on the general lines of the (Prate' truss, with vertical compression members and inclined tension members, or on the lines of the "War ren" truss, with all main web members inclined; but the constantly increasing weight of the loco motives in use has tended to the use of heavier and more rigid structures with long panels and as few members as possible, more substantial details, solid floors, stiff lateral and vibration bracing and other features of present engineer ing practice, which reduce vibration and add to the rigidity of the structure.