THE HISTORY OF BRIDGES TRANSPORTATION IN AMERICA Why Do We Need Bridges? To get from one place to another over an obstacle. COVERED WAGONS In the 1850s, covered wagons heading West had to cross streams or rivers. Without adequate bridges in place, the pioneers would sometimes travel many miles out of the way to find a safe place to cross a river. CROSSING A RIVER Consider how long it would take to cross a river today if bridges were not included in the road systems. DELIVERING GOODS Consider how the movement of people, goods, and services would be affected without highways, which include bridges. The entire American economy as we know it would be stifled. EARLY BRIDGES The earliest bridges consisted mainly of logs that had fallen or were placed across rivers. While inexpensive, they were less than reliable. Moving heavy loads across them was often impossible. LOG BRIDGE 1920 FOOT BRIDGE Some bridges are made to withstand foot travel and small animals (This is a famous foot bridge in Lucerne, Switzerland. It bends across a river) BEAM BRIDGE Consists of a horizontal beam supported at each end by piers Flat across Supported at the two ends The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet. TRUSS BRIDGE Consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. ARCH BRIDGE Has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet. A simple bridge reaching across a river in an arching shape SUSPENSION BRIDGE Can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting. VOCABULARY Aqueduct - A bridge or channel for conveying water, often over long distances Cable - The staying or suspending bridge element; in modern suspension bridges, the main supporting cable is hung from towers, and formed from steel wire bound in strands. VOCABULARY Deck - the deck is the surface that all the traffic goes across on. Span- a span is a beam that is supported by two or more support structures. Famous Bridges Navajo Bridge Hoover Dam Bypass Essential Questions To Cover Why Do We Need Bridges? How Did People Travel Without Bridges? What were early bridges probably made from what materials were utilized? What are the 5 main types of bridges?