In “Highways II” we learned the difficulties experienced by General Knox’s troops from canyons, ravines, and bodies of water. Man constructed bridges to overcome these obstacles.
Prehistoric man used fallen trees and stepping stones to cross bodies of water. As spans increased and trees broke, they added mid-span supports of upright trees or piles of stones the Romans named ‘piers.’ Multiple piers at spaced intervals gradually widened spans.
Stones placed horizontally called “clapper” bridges were used in northwestern Europe.
Jungle vines suspended across jungle crevasses eventually led to suspension bridges still in use today. In 513 BC Darius I, Emperor of Persia used 720 vessels to form a pontoon bridge to cross the 3540-foot Bosphorus dividing Asia Minor and Greece.
Roman engineers discovered that stacking wedge-shaped stones created an arch that would transmit stress horizontally into its foundation creating the mold for bridges in Europe for the next 2000 years. [The Hoover dam does horizontally what an arch bridge does vertically.]
They got double duty from multiple-arch aqueducts with water inside road topside.
Romans invented an early form of concrete, but they found it strong under compression and weak in tensile strength. They overcome these deficiencies in steel spans by embedding in them steel reinforcing rods [“rebar”].
In the last two centuries iron and steel have become the material of choice. Iron beams having “I” ,“L,” “U,” and “square” or “box” cross-sections proved the best. Vertical support beams configured into “N” and “X” and “Z” designs lengthened horizontal spans.
All of these developments are illustrated in the pictures that follow.
 David Miller, “Bridges,” Chartwell Books, Edison, NJ 2006.