Bridge Design over time: Bridge Basics

• The downward forces acting on a beam bridge are: gravity, the weight of the bridge itself, and the load that it carries. Because the bridge is flat across, these downward forces are spread evenly across the length of the bridge. The downward forces must be carried by the upward forces that come from the piers. Bridge Basics (2)

• Beam bridges that span longer distances must be strengthened by adding middle piers to help carry the weight. Early Truss/Beam Bridges

• The first beam bridges were no doubt bridges that were created by nature, such as a tree that has fallen across a stream Clapper Bridges

• The “clapper bridge” design is one of the earliest bridge constructions the builders built up the stepping-stones found across a shallow stream and connected the stones with stone slabs or logs to form a low bridge.

Cantilever Bridges

• The “cantilever Bridge” design was used by early Asian builders to span fairly wide rivers. The example here was built from wood timbers. Unlike other beam bridges, the cantilever design is supported on two levers that are weighted by piers. The downward force at the center of the bridge is counteracted by the weights.

Truss Bridges • Covered bridges are among the oldest bridges in America, and they disguise some of the earliest wooden “truss bridges”. • The truss designs used to strengthen beam bridges used the triangle as their main element. As we look at different truss designs later, you will see how the triangle has been modified and strengthened as engineers have applied science and math principles to improve designs. • Another factor that contributed to longer, stronger bridge designs through history has been the availability of better materials, such as cast iron, wrought iron, steel and concrete. • Many of today’s bridges are built of steel trusses and pre-stressed concrete.

Arch Bridge Basics • The downward forces that act on arch bridges are gravity, the weight of the bridge, and the load that it carries. These forces are spread outward by the curve of the arch, and are concentrated on the end supports. Arch Bridge Basics (2)

• Arch bridges that span longer distances are strengthened with additional supports and use the additional arches to distribute the weight of the supports. “Roman Arch” bridge

• The ancient romans developed the “Arch Bridge”. The Romans found that the arch form could support much more weight than a flat surface. The Romans also discovered that natural cement helped to strengthen their bridges and make them last much longer.

“Modern Arch” Bridge

• Some arch bridges are designed with the arch reaching above the deck of the bridge. Today, many arch bridges are built of steel with concrete supports.

Suspension Bridges

• The downward forces acting on a suspension bridge are gravity, the weight of the bridge itself, and the load that it carries. The curve of the suspension ropes or cables concentrate these downward forces inward where there is no support. “Simple Suspension Bridge”

• The simple suspension bridge droops down between the two end supports. Because of this, it’s it not capable of supporting heavy loads. Foot bridges like the one shown here are often created this way.

“Modern Suspension Bridge”

• Today, however, modern suspension bridges span great distances and support enormous weights. These bridges are built with steel cables, concrete anchorages, steel towers, and steel trusses.

Truss Examples There are thousands of truss designs found in bridges throughout our history. Often these designs are named after the person who designed them. Many are still in use today or some combination/variance. Truss Examples(cont.) Truss Examples(cont.) Truss Examples(cont.) Bridge Report

• 1 page(Double spaced/12 font) report/description on a famous structure, can be a building, bridge, monument, etc… • Include: Why it was built, where it is located, and what year it was built.

• Study for test over this presentation and over the video!!!