Of the nearly hundred lenticular truss bridges that are estimated to have been built between about 1878 and 1895 in the Commonwealth of Massachusetts, only nine are known to exist. Only three of these bridges are currently open and in use for vehicular traffic. Of the remaining six, three are closed to vehicular traffic, two have been restored and relocated as pedestrian bridges, and one has been dismantled and is currently in open storage, awaiting reuse. The authors visited each of the bridges in Massachusetts, measured the dimensions of the critical members, and documented their condition.

During the latter part of the 19^th Century the Berlin Iron Bridge Co. of East Berlin, Connecticut manufactured and erected almost 800 lenticular truss bridges in the United States (Darnell 1979). While most of these bridges were built in New England, a few were constructed in Ohio and Texas. These bridges are sometimes referred to as "pumpkin-seed bridges", "cats-eyes bridges", "elliptical truss bridges", or "parabolic truss bridges" because of their unique lense shape. Lenticular bridges were only used for vehicular traffic and were generally considered too light to be used for railroad and trolley loads.

Darnell (1979) described a number of these bridges and gives a detailed account of the Berlin Iron Bridge Co. The Berlin Iron Bridge Co. also built conventional steel truss bridges and even built a few pedestrian suspension bridges, the most notable in Keesville, N.Y. and Milford, N.H. In addition to bridges, the Berlin Iron Bridge Co. also built roof trusses, water towers, and steel frames for buildings, as the ads below show.

The accompanying figure shows the location of the remaining lenticular truss bridges in Massachusetts.

The design of the individual elements was exquisitely simple, yet functionally good, and lent itself to economic fabrication at the plant. The use of riveted plates, angles, and channel sections to build the top chords and vertical posts is characteristic of all the bridges. The construction brings to mind a child’s Erector set and one can easily imagine the individual top chord elements, vertical posts, lower chords, bracing bars and other smaller components being transported to the site by horse carts. Assembly at the site would have been relatively easy and fast, even by today’s standards. There are relatively few variations in components among the surviving bridges, and the components suggest a simple modular design concept. After all, the market for iron bridges at the end of the 19^th century was highly competitive and therefore any means to reduce fabrication costs would have been exercised.

In comparison with typical bridge sections of today, the components for a lenticular pony truss were relatively light and could have been handled by workers at the site. For example, if we use the Fort River Bridge in South Amherst as an example, a 14.75 ft. long upper chord would weigh about 640 lbs; an 8 ft. long vertical post would weight about 130 lbs; and a 14.75 ft. long lower eye chord would weight about 105 lbs. Each of these members is constructed of individual components riveted together. The weight of the upper chord is such that it would likely have been handled by a tripod or boom and jib. The other elements could easily be placed by two workers.

A listing of other existing lenticular truss bridges known to the authors is given in Table 2.

Chamberlin, W.P., 2000. Hadley, New York’s Parabolic Truss: "Catalog Bridges" and the Engineering of Structural Art. Proceedings of the 7^th Historic Bridges Conference, pp. 94-105.

Darnell, V., 79. Lenticular Bridges from East Berlin, Connecticut. Journal of the Society for Zindustrial Archeology. Vol. 5, No. 1, pp. 19-32.

Drew, B.A., 1979. Lenticular Bridges in the Berkshires. Journal of the Society for Industrial Archeology, Vol. 5, No. 1, pp. 69-70.

Drew, B.A., 1990. Spanning Berkshire Waterways. Attic Revivals Press, Great Barrington, Ma., 36 pp.

Fleming, R., 1928. Early Parabolic Truss Bridges Gradually Disappearing. Engineering News Record, Vol. 100, No. 10, pp. 748-749.