The Smithfield Street bridge (1883) is considered by most to be Pittsburgh's most historically significant bridge for several reasons: 1) it replaced two bridge structures by well-known bridge engineers, Lewis Wernwag and John A. Roebling (creator of the Brooklyn Bridge);  2) it was the first American use of the lenticular (lens-shaped) truss design; and 3) it was one of the first major bridges in the US built primarily with steel, and is probably the oldest extant major steel truss remaining. The graceful Smithfield Street Bridge is also the oldest remaining river bridge in Allegheny County and has been designated a National Historic Civil Engineering Landmark.