TYPES OF TRUSSES AND BRIDGES.

FIG. 4.
A PRATT LOW TRUSS HIGHWAY BRIDGE: SEVEN 100-FT. SPANS.

The loads are sometimes carried on the top chord as in Fig. 5, which is a highway bridge built for the U. S. Government in the Yellowstone Park. and intermediate posts are composed of 2 channels laced;

In this truss the end-posts, top chords while the lower chords, hip verticals,

7191*

71-15 2-7 09

Girder Tubes -150-30 dia Section.

Floor Plan.

main ties and counters are composed of eye-bars. The floor beams are I beams 15 inches deep weighing 50 lb. per lineal foot (15" I @ 50 lb.), while the joists are 7" Is and 7" [s. A deck highway bridge is shown in Fig. 6.

Types of Trusses and Bridges.-The simplest type of bridge is the beam bridge, (a) Fig. 7. Beam bridges commonly consist of I beams which span the opening, and are placed near enough

(b) Beam Leg Bridge. (e) Low Pratt Truss. Half Hip.

(c) Truss Leg Bridge.

(f) Low Pratt Truss. Full Slope.

FIG. 7. TYPES OF SHORT SPAN HIGHWAY BRIDGES.

together to carry the floor of the bridge. Where foundations are relatively expensive the beams may be carried on posts as in (b) Fig. 7. A truss leg bridge is shown in (c) Fig. 7. Types (b) and (c) unless constructed with great care make inferior structures and are not to be recommended. A Warren truss is a combination of isosceles triangles as shown in (d) Fig. 7 and in (c) Fig. 8. The Pratt truss has its vertical web members in compression while its diagonal web members are in tension, as shown in (e) and (ƒ) Fig. 7 and in (b) Fig. 8. The Warren truss is commonly built with riveted joints while the Pratt truss is usually built with pin-connected joints. The Warren low truss with riveted joints as shown in (d) is generally preferred in place of the low

Pratt truss in either (e) or (ƒ) Fig. 7. The Howe truss has its vertical web members in tension, and its inclined web members in compression as shown in (a) Fig. 8. The upper and lower chords and the inclined members of a Howe truss are commonly made of timber, while the vertical tension members are iron or steel rods.

The Whipple truss, (e), Fig. 8, is a double intersection Pratt truss. This truss was designed to give short panels in long spans which have a considerable depth. The stresses in the Whipple truss are indeterminate for moving loads, and its use has been practically abandoned, the Baltimore truss, (g), Fig. 8, being used in its place. The quadrangular Warren truss, (d), Fig. 8., and Fig. 10, with riveted joints, is used as a standard truss for through highway bridges, with spans of from 80 to 170 feet, by the American Bridge Company. Like the Whipple truss its stresses are indeterminate for moving loads.

For spans of from, say, 170 to 240 feet it is quite common to use pin-connected trusses of the Pratt type having inclined chords as in (ƒ), Fig. 8, and Fig. 11.

The Baltimore truss, (g), Fig. 8, is a Pratt truss with parallel chords in which the main panels have been subdivided by an auxiliary framework. The auxiliary framework may have struts

as in (g), or ties as in (i), Fig. 8. The Baltimore truss with inclined upper chords, (i), Fig. 8, is called a Petit truss. The stresses in Baltimore and Petit trusses are statically determinate for all conditions of loading. These trusses are economical in construction and satisfactory in service, and have entirely replaced the Whipple truss for long span bridges.

FIG. 10.

WORKS.

The K-truss shown in (h) and (j), Fig. 8, is more economical than the Petit truss, and in addition has smaller secondary stresses, and is rapidly coming into general use.

The types of simple bridge trusses described above are those that are in the most common use, although quite a number of other types of trusses have been used and abandoned.

FIG. II.

PARKER OR CAMELS-BACK, PIN-CONNECTED HIGHWAY BRIDGE, BUILT BY AMERICAN BRIDGE COMPANY.