In building a bridge, as in rearing many other structures, girders of various contours are used. In bridge building the I-beam is most employed. When the roadway proceeds on the top chord, as DH, in the [queen-post] figure, page 21, we have a deck bridge; when it is built on the bottom chord, as CM, we have a through bridge.
HOWE TRUSS
PRATT TRUSS
The Burr Bridge Simplified by Howe and Pratt.
The Burr bridge of 1804, already mentioned, included an arch and was in part sustained by struts projecting from abutments. These features were omitted by William Howe in the bridge which he patented in 1840, and which was, as far as is known, the first successor to a design of Palladio in employing a simple truss for long spans. The Howe truss was built of wood, except its terminal tie-rods, which were of iron; it has been repeated thousands of times throughout the world. In 1844 Thomas W. and Caleb Pratt patented a bridge which in design was the converse of Howe’s. Its diagonals of iron were used in tension, while its vertical struts of timber were in compression; in the Howe pattern the diagonals were in compression, the verticals in tension. This plan, by shortening the struts, diminished the cross-section necessary in a truss. When wrought iron took the place of wood for bridges, the Pratt design became the most popular of all, combining as it did more desirable features than any of its rivals. To-day for long spans the Baltimore truss is much in favor. Its stresses, that is, its resistances to change of form under strain, are readily ascertained; the shortness of its panels means strength; and its diagonals have the inclination which wide and varied experience has shown most desirable. The roadway, it will be observed, is upheld by sub-verticals, that is, by verticals which reach the floor from half the height of a panel.
Diagram of Baltimore truss.
