Lenticular Truss Bridges

Lenticular truss bridge is named after its unique structure that combines upper parabolic chord and lower opposite curved one together shaping like a lense, also called "fish belly truss" from another poetic imagination the shape raises or "parabolic truss" or "elliptical truss" from very scientific observation. The last two namings were common in the 19th century.
This type of truss bridges was first common in Europe and then introduced into the US in the middle of 19th century. In the US, William O.Douglas had patented in 1878 (in this patent the structure is called "elliptical truss"), and allowed his Berlin Iron Bridge Company, Berlin, CT alone to build lenticular truss bridges. The company is said to have built 400 or more truss bridges (the figure may include other types of trusses together the company have ever built and there is no exact record about how many lenticular truss bridges were built) until it was merged to the American Bridge Company in 1900, which was purchased by US Steel a year after the merger, in 1901. The monopolist steel maker did not continue to build lenticular truss bridges, because they found that the lenticular structure did not improve the structural strength of the bridge over any one structure, despite the lenticular combination of upper and lower chords rose both the cost and complexity of design.
So, lenticular truss bridges are significant not only for its unique shape and few examples but also for the historic value that they were only built during the last two dacedes of 19th century, like dinasors had enjoyed a historic age long long ago and then suddenly vanished at all.
Today, about 50 lenticular truss bridges are still remaining all over the country. Most of them, however, are located in the New England and middle state region for the geographic reason that the patented builder was once located in Connecticut.