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Truss Bridges

A truss is a structure that acts like a beam but with major components, or members, subjected primarily to axial stresses. The members are arranged in triangular patterns. Ideally, the end of each member at a joint is free to rotate independently of the other members at the joint.
If this does not occur, secondary stresses are induced in the members. Also if loads occur other than at panel points, or joints, bending stresses are produced in the members.
Though trusses were used by the ancient Romans, the modern truss concept seems to have been originated by Andrea Palladio, a sixteenth century Italian architect. From his time to the present, truss bridges have taken many forms.
Early trusses might be considered variations of an arch. They applied horizontal thrusts at the abutments, as well as vertical reactions, In 1820, Ithiel Town patented a truss that can be considered the forerunner of the modern truss. Under vertical loading, the Town truss exerted only vertical forces at the abutments. But unlike modern trusses, the diagonals, or web systems, were of wood lattice construction and chords were composed of two or more timber planks.
In 1830, Colonel Long of the U.S. Corps of Engineers patented a wood truss with a simpler web system. In each panel, the diagonals formed an X. The next major step came in 1840, when William Howe patented a truss in which he used wrought-iron tie rods for vertical web members, with X wood diagonals. This was followed by the patenting in 1844 of the Pratt truss with wrought-iron X diagonals and timber verticals.
The Howe and Pratt trusses were the immediate forerunners of numerous iron bridges.
In a book published in 1847, Squire Whipple pointed out the logic of using cast iron in compression and wrought iron in tension. He constructed bowstring trusses with cast-iron verticals and wrought-iron X diagonals.

These trusses were statically indeterminate. Stress analysis was difficult. Latter, simpler web systems were adopted, thus eliminating the need for tedious and exacting design procedures.
To eliminate secondary stresses due to rigid joints, early American engineers constructed pin-connected trusses. European engineers primarily used rigid joints. Properly proportioned, the rigid trusses gave satisfactory service and eliminated the possibility of frozen pins, which induce stresses not usually considered in design. Experience indicated that rigid and pinconnected trusses were nearly equal in cost, except for long spans. Hence, modern design favors rigid joints.
Many early truss designs were entirely functional, with little consideration given to appearance.
Truss members and other components seemed to lie in all possible directions and
to have a variety of sizes, thus giving the impression of complete disorder. Yet, appearance of a bridge often can be improved with very little increase in construction cost. By the 1970s, many speculated that the cable-stayed bridge would entirely supplant the truss, except on railroads. But improved design techniques, including load-factor design, and streamlined detailing have kept the truss viable. For example, some designs utilize Warren trusses without verticals. In some cases, sway frames are eliminated and truss-type portals are replaced with beam portals, resulting in an open appearance.
Because of the large number of older trusses still in the transportation system, some historical information in this section applies to those older bridges in an evaluation or rehabilitation context.

—–13.1. Specifications
—–13.2. Truss Components
—–13.3. Types of Trusses
—–13.4. Bridge Layout
—–13.5. Deck Design
—–13.6. Lateral Bracing, Portals, and Sway Frames
—–13.7. Resistance to Longitudinal Forces
—–13.8. Truss Design Procedure
—–13.9. Truss Member Details
—–13.10. Member and Joint Design Examples-LFD and SLD
—–13.11. Member Design Example-LRFD
—–13.12. Truss Joint Design Procedure
—–13.13. Skewed Bridges
—–13.14. Truss Bridges on Curves
—–13.15. Truss Supports and Other
—–13.16. Continuous Trusses