Tham khảo tài liệu 'structure steel design's handbook 2009 part 16', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SECTION 13__ TRUSS BRIDGES John M. Kulicki . President and Chief Engineer Joseph E. Prickett . Senior Associate David H. LeRoy PE. Vice President Modjeski and Masters Inc. Harrisburg Pennsylvania 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 . 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
