Unlike timber and stone bridges, metal bridges were an industrial product. Fabricators such as Connecticut's Berlin Iron Bridge Company bought iron in basic forms--I-beams, angles, plates--and cut and riveted it together to make bridge components. The companies employed traveling agents to sell their products to local highway officials.
Once the industry was established, iron trusses dropped in price, and towns throughout America bought them as replacements for their timber bridges. Pony trusses (low trusses without overhead bracing) served for small bridges. Longer crossings required through trusses or, if the river ran in a deep gorge, a deck truss, in which the roadway ran along the top of the bridge.
Nineteenth-century engineers created a rich diversity of truss types. Although all trusses work by transmitting the roadway load to the abutments, they differed from one another in the way that their members were arranged. Some had their diagonals resisting stretching (tensile) forces, while others had them bearing compressive forces. The various patterns were named after the engineer who devised them (Pratt, Warren, and Whipple trusses) or after the shape formed by the upper and lower chords. The lenticular (lens-shaped) truss was the specialty of Berlin Iron Bridge Company. Bridge fabricators also distinguished their products by using proprietary components, such as patented columns or connectors.
As the turn of the century approached, truss design became more standardized. A few basic patterns, with variations, sufficed where dozens had competed before. Bridges no longer relied on patented components. Riveted connections superceded the pinned joints that had characterized American trusses throughout the 19th century, and steel replaced wrought iron for structural work.
The coming of the automobile meant that highway trusses had to be designed for increasingly heavy loads. Except for the use of ever larger and stronger members, however, only minor refinements occurred after 1900. Engineers continued to specify trusses for highway bridges until about 1940. After that, the availability of large concrete and steel beams (and the ability to transport them to any site) made trusses obsolete. Beam bridges served not only for minor town roads but also for major undertakings such as the 3,200'-long, 23-span Charter Oak Bridge, completed in Hartford in 1942.
The major innovations of the 20th century were long-span steel bridges intended to provide high-level passage across wide bodies of water. Engineers designed suspension bridges, cantilevered trusses, and steel arches of ever-greater span. Connecticut, however, has only two examples of the period's high-technology steel engineering, the 600'-long arches of the Arrigoni Bridge in Middletown and the Gold Star Bridge in New London, a 540'-span deck truss built in 1943.