The problems incident to the replacing and strengthening of old bridges frequently tax the resources of the engineer and demonstrate his ability to overcome difficulties. Only a few examples may be cited to indicate the character of this work. In 1900 the Niagara cantilever bridge had its capacity increased about 75 per cent by the insertion of a middle truss with out interfering with traffic. In 1897 the entire floor of the Cincinnati and Covington suspension bridge was raised four feet while the traffic was using it. It may be of interest to state that the two new cables, 101⁄2 inches in diameter, which were added to increase the capacity of the bridge, have just about three times the strength of the two old ones, 12 inches in diameter, and which were made a little over 30 years before. In the same year the old tubular bridge across the St. Lawrence River was replaced by simple truss spans without the use of false works under the bridge and without interfering with traffic. On 25 May 1902 the Pennsylvania R.R. bridge over the Raritan River and canal at New Brunswick, N. J., was moved sidewise a distance of 141⁄2 feet. Five simple spans 150 feet long and a drawbridge of the same length, weighing in all 2,057 tons, were moved to the new position and aligned in 2 minutes and 50 seconds. The actual times that the two tracks were out of service were respectively 15 and 28 minutes. On 17 October 1897, on the same railroad near Girard Avenue, Philadelphia, an old span was moved away, and a new one, 235 feet 7 inches long, put in exactly the same place in 2 minutes and 28 seconds. No train was delayed in either case. HENRY S. JACOBY,

College of Civil Engineering, Cornell. Bridge Construction, Modern Methods of: An instructive exposition of these methods requires a brief consideration of the principles of design and the controlling factors therein.

All framed structures may be divided into two classes those designed to sustain only a permanent or "dead load," which acts with unvarying forces, and those which sustain not only the dead load consisting of their own weight, but also the action of a "live load" applied by the movement of railway trains, ordinary vehicles and horses, men, etc., over them.

Roof trusses, cranes, cantilevers, etc., are of the first class, while bridges belong to the second.

All bridge structures may be conveniently divided into three classes (1) "beam bridges," (2) "suspension bridges," (3) "arch bridges." The first exert only vertical pressures upon the supporting piers and abutments; the second exert a horizontal pull on the towers and anchorages; while the third exert a horizontal push in addition to the vertical pressures.

"Beam bridges" are of a great variety of forms, including those commonly known as simple bridges, drawbridges, continuous bridges, and cantilever bridges. Over 90 per cent of the modern bridges are simple bridges, of which there are two classes truss bridges and girder bridges. In the former the floor is supported by two or more frame structures, called trusses: in the latter, the floor is supported by solid built-up beams. Girder bridges are generally sed for short spans, seldom exceeding 100 feet; but the truss bridges are used for larger spans, and also for spans as short as 50 feet.

A simple framed structure or truss is one composed of straight "members" or parts joined together by pins or rivets, so as to form a rigid framework. The most rigid form is that of a triangle, as it is the only figure the shape of which cannot be altered without changing the length of its sides. It is, therefore, the "truss element," and all framed structures, no matter how complicated in construction, may be treated as a combination of triangles when no superfluous members are present.

The forces such structures are designed to resist are those of tension, compression, and shearing. These forces when external are called "strains," while the corresponding internal forces developed in the several members of the structure to resist the strains, are called stresses. Owing to the frequent confused and indiscriminate use of the terms strain and stress by some authors, a great deal of popular misunderstanding exists as to their exact designations. According to the best authorities, however, a strain is the distortion of a body under the action of one or more external forces, and it is the immediate cause of the stress developed in the body to resist that distortion. Under certain conditions, however, a stress may by reaction and transmission act as an external force, as in the case of the stresses in the masonry supports of a bridge which react upon the superstructure and are in effect external forces and have to be treated as such. In solid bodies, within certain limits, the intensity of the stress is equal to the amount of the strain, and as enunciated by Hookes' Law, equal increments of one develop equal increments of the other; but, it is evident that stresses cannot thus continue to increase indefinitely in proportion to the strains, and a point is necessarily reached when a greater increment of distortion is required to develop a given increment of stress. This point is known as the "elastic limit." Below this point, a distorted body returns to its original form and size when the straining force ceases to act; but, beyond the limit, the strain increases more rapidly than the stress, or than the straining force which is always the equal of the stress, and the body does not return to its original dimensions, since a portion of the distortion remains as a "permanent set."

In the designing of a truss, the members are guarded against "taking a set," by keeping the working stresses well within the elastic limit of the material of construction.

A bridge truss is designed to act as a beam and it is, therefore, usually subjected to longitudinal strains of tension or compression only, and develops in its members corresponding tensile or compressive stresses. Fig. I shows a Panel Upper Chord

Hanger

Counter Tie

Compression Members.

FIG. 1.

Tie Rod End Post

Tension Members

- Simple Truss.

simple bridge truss with its several members designated as struts, ties or tie-rods, etc., according to the character of the stresses developed in them by the combined action of the dead and live loads.

In the "struts the stresses are compressive; in the "ties" tensile. The upper and lower "chords" are placed in compression or tension according to the direction of action of the external forces applied to the structure. In a truss supported at the ends and bearing a downward acting load, the upper chord is always in compression, and the lower chord always in tension. "Counterbraces" are designed to resist both

tensile and compressive strains alternately, as they may be applied by the changing positions of the load. No truss-member can act simultaneously in full tension and compression, but may do so partially, since the stress developed in any member by the strains of two or more external forces is equal to and of the same sign as the algebraic sum of all those forces. Thus a tie may resist a compressive strain without becoming a counterbrace, or a strut may resist a tensile strain without becoming a tie, so long as the contrary strains are smaller than those for which the member is designed, and which continue to act at the same time.

Fig. 2 shows the action of a truss. As already stated, a truss acts as a beam, and a load applied at (h) will be carried to the abutments at (a) and (v) along the several members as indicated by the arrows, leaving the post (hi), the ties (ij) (kl) (mn), and the hanger (cb) and (tw) idle, and they may be removed without weakening the truss under that particular loading, since it would still remain a combination of triangles properly joined. Fig. 3 shows the conditions with the load applied at (j), with the ties (ij) and (jm) under stress, and the ties (hk) and (kl) idle. Fig. 4 shows

structure, provided all the members are properly designed to carry their respective loads.

The economy and the efficiency of the truss lies in the panel or quadrilateral system. Fig. 5 shows its development from the triangular

Да

FIG. 5.

king-post truss by the addition of the side panels. It appears that this system was first introduced by Palladio about 1570, but was little used until the close of the 18th century, when it was re-discovered by Burr, and came into extensive use in the United States and is the progenitor of nearly all the forms of bridge trusses now in use in this country, its most valuable feature-a constant angle for the inclined members and its panel system being and to many other later forms of trusses. transmitted to the "Long," "Pratt," "Howe,"

The "Bollman," "Warren," and "Fink" trusses, embodying the pure triangular types

two equal loads applied at the middle points (j) and (1). Since the part of the load at (j) going to (v) is just balanced by the part of the load at (1) going to (a), there is no stress in (jk) (lm) (jm) and (kl), nor in the counter ties (hk) and (mn).

From an inspection of these conditions it is obvious that a truss is not weakened on account of a lack of symmetry in the arrangement of its individual members, or as a whole

FIG. 9. Post.

FIG. 10. Baltimore.

KAM

FIG. 11.- - Kellogg.

structure. These considerations led to the development of the quadrilateral type in which each member is required to resist a strain of only one particular character. Of these, the more important other than those already men

II,

tioned, are the "Post," "Baltimore," "Kellogg," and "Whipple-Murphy," shown by Figs. 9, 10, 12, respectively. The "Whipple-Murphy" was the first to approach to the modern iron truss-bridges. The first bridge of this type, a span of 146 feet, was built by Whipple in 1852, near Troy, N. Y., on the Rensselaer and Saratoga Railway.

joints, thus making them continuous from one end to the other. The web members are riveted to the chords directly, or by means of special plates, which are riveted to both.

Other forms of bridge trusses than those already described, which have many claims to economy and excellence of design, are the "parabolic bowstring," the "double bowstring" or "lenticular," the "Pegram," and the "Petit" trusses, shown by Figs. 13, 14, 15, and 16.

Pratt

Howe

Murphy-Whipple

FIG. 12.

In 1861, Linville introduced wide forged eyebars and wrought iron posts in the web system, while two years later, Murphy substituted wrought iron for all of the compression members, and established in this country the distinctive practice of eye-bars and pin connections, which is still applied to long span steel truss-bridges. The credit of originating the correct theory of truss action, however, belongs to Whipple.

In Europe, the prevailing method of construction is the riveted system, which in this country is limited to plate girders and lattice

FIG. 16. Petit.

A plate girder bridge consists of two or more girders connected by systems of lateral and transverse bracings. In its simplest form a plate girder is composed of a vertical web plate to the top and bottom of which are riveted pairs of horizontal angle irons, forming the flanges, and to the ends, vertical angles which transmit the load to the support. The structural forms of girders are modified in many ways to adapt them for different purposes. Increase in the ratio of the depth of the web to its thickness necessitates the addition of "stiffeners." These are vertical angles riveted on to the web in pairs on opposite sides at intervals along the span. As the span increases, two or more web plates are used, spliced end to splicing. See Fig. 17. end. In long spans the flanges also require

Cover Plate

Flange

-Web Plate Joint

Floor

FIG. 13. Parabolic-Bowstring.

Web Plate

Web Plate

-Splice Plate

Floor

FIG. 14. Double-Bowstring or Lenticular.

FIG. 15. Pegram.

trusses of less than 200 feet span. In this system the chords are formed of angles or channel and plates riveted together with splice

FIG. 17.- Plate-Girder Web-Splice.

There are three general classes of truss and girder bridges used for railroad and highway purposes: (1) "through-bridges," in which the roadway or floor is carried directly by and attached to the bottom chord joints or web plates, and the lateral bracing joins the upper chord joints, and encloses a space for the passage of the load; (2) "deck bridges," in which the roadway is carried on the tops of the girders; and (3) "pony trusses," for short spans, carrying the roadway at the bottom joints but too low to allow upper lateral bracing. so that the trusses are held in place by bracing incorporated with the floor systems. In short spans the girders are arranged to slide upon their supports, base plates being riveted to their