Part of the success of the Berlin Iron Bridge Company can be attributed to its patented lenticular-truss design, so-called because both the top and bottom chords are curved, creating a profile that resembles a convex lens. Like any truss, the lenticular functions to transfer loads on the deck of the bridge out to the ends, which rest on solid abutments.
The lenticular truss was also called the parabolic truss because the curves of the top and bottom chords approximated the shape of a parabola. Because of the curved chords, the truss was deepest in the middle, where the greatest strength was needed, and therefore it was economical in its use of iron. This advantage, however, was partially outweighed by the fact that the lenticular truss's multiple angles were more complicated to manufacture. Regardless of the economics, the design's unique profile provided something distinctive that Berlin agents could point out when trying to convince local highway officials of their product's superiority. And the long life of the company's bridges--approaching 120 years in the case of many survivors--shows that the lenticular truss was a sound design from an engineering point of view.
Like most other bridges of the period, the Berlin Iron Bridge Company's lenticular trusses were manufactured from wrought iron, not steel. Until the 1890s, structural shapes such as bars, plate, angles and I-beams were not readily available in steel. Once the American steel industry reached a certain point, however, steel became the material of choice for bridges, since it is approximately twice as strong as wrought iron. Some of the later Berlin bridges were built using steel, but most of their production used wrought iron. All of the company's lenticular trusses made use of pinned connections to join the major parts of the bridge. Many engineers of the period favored pinned connections over riveted connections because they thought the pinned joints transmitted forces more consistently.
Although the basic mechanics of the lenticular truss was the same, there were many variations in detail depending upon the length of the truss and the purpose it was intended to serve. Pony trusses--in which there was no horizontal bracing above the roadway--served for shorter spans, up to about 80 feet. Longer bridges were built as through trusses, braced both horizontally and diagonally above the roadway. Click the image below to see the difference between the two types.
Other variations included tapered lattice-girder uprights vs. straight uprights and tapered floor beams vs straight floor beams. In each case, the tapered shape saved some material but was more time-consuming to fabricate. Some pony trusses (generally the longer or wider ones) had the bottom chord pin-connected to the end posts, while others had their bottom chord secured with large nuts threaded onto the ends of the eyebars. In terms of nonstructural elements, the bridges varied widely as well. The railings could be fancy iron lattices decorated with cast-iron rossettes, a series of simple iron rods, or even plain wooden fencing, depending upon how much the town wanted to spend and how important the appearance of the bridge was. Some Berlin bridges had decorative finials atop the end ports, and among the through trusses, some had elaborate portal ornamentation and builders plates listing the date of the bridge and the names of the selectmen at the time it was purchased.
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