Pittsburgh's historic Smithfield Street Bridge, opened in 1883, has a unique lenticular (convex lens-shaped) truss structure. As traffic increased over the years and changed from trolleys and horse carriages to motorized vehicles, bridge vibrations also increased, creating the potential for dangerous structural fatigue or even failure. In 1994 the Mackin Engineering Company removed the trolley tracks and strengthened the deck to allow a third traffic lane. The firm used finite element analysis to visualize the bridge's natural frequencies and its vibrations under dynamic conditions. Courtesy of Algor, Inc., Library of Congress.