Types of beam bridges include cantilever bridges, which can extend over long distances. Cantilever bridge is a balanced structure extending out diagonally on both sides of a pier. The sections meet in the middle or are supported by a center span. The cantilever is counterbalanced for added strength, which allows the bridge to cover greater distances. Beam and cantilever bridges can be formed from a truss, a rigid structure usually built from triangles.
A simple beam bridge is flat across and supported at two ends. A longer beam bridge may also be held up along its middle by piers that stand in the river. The weight of that bridge by itself, plus any load it carries, plus gravity, are the downward forces that are spread across the length of the bridge. The upward forces that hold the bridge up comes from the pier.
There are several common variations of the beam bridge mentioned above. A Clapper Bridge is simple, shallow kind of Beam Bridge that connects "stepping stones" across the stream. A floating pontoon bridge is another kind of Beam Bridge, supported by upward forces of water. Another type of Beam Bridge is the truss, which is lightweight but strong because of the open, diagonal (or triangular) elements along the sides.
Truss bridges are based on the fact that a triangle cannot be twisted or distorted. And, by putting triangles next to each other, their strength becomes even greater. The most common truss combination is a "king post" which is the vertical support that results when two triangles share the same side.
Each length of wood that goes into making a truss is called a member. The long horizontal members are called chords. The vertical members are called the posts. The slanting members are diagonals. Two kinds of stress are involved in truss construction: tension and compression. A member being pulled is under tension; a member bearing weight is under compression.2
Truss bridges can be traced back to Italy when, in the1500's architect Andrea Palladio worked out the principles of wooden trusses in Treatise de Architecture. Italian bridge builders disregarded Palladio's work because they preferred to work with masonry and stone. In 1742 Palladio's ideas made it to America and became very popular. America had plenty of wood and plenty of carpenters who could construct bridges. After 1850, bridge traffic increased, and loads became heavier and truss had to be made stronger. Wood was replaced by cast iron and eventually wrought iron. During the 1870's one in every four wrought iron bridges in America collapsed. Today truss bridges are made from steel or reinforced concrete.
There are three types of arches, the false arch, the ribbed arch, and the true arch. The false arch resembles a pair of steps meeting in the middle. If weight were placed on top, the arch would probably not support it. A true arch is made up of wedged bricks (voissoirs) fitted together between vertical supports.3 The ribbed arch is formed as a series of arch openings, sharing a common wall with next arch.
True arches are built in from the end, towards the middle, using a wood frame called "false work". The final wedge, known as the keystone, is set in place at the keystone, is set in place at the top of the arch. Then sidewalls (spandrels) are built up between the arches and filled with rubble.
Ribbed arches are actually several rows of arches next to each other. Long, flat stones are laid across the ribs for support. This technique reduces the weight of the arch and cuts construction time and materials. Arch bridges also originated in China, due to its rainy season and the affects on beam bridges. Arch shapes openings were the solution because they allowed floodwaters to flow without taking the bridge with them. An added benefit of an arch bridge was they could be made extremely strong using a minimum of materials. Romans brought arch bridges to Europe. The Romans' most innovative arch bridge wasn't actually a bridge to travel on but to carry water. This type of system is called an aqueduct.
Suspension bridges are capable of spanning great distances using a minimum of material. They can be built high enough to let large ships pass underneath. The result is a bridge with a graceful appearance and a feeling of strength. Years after construction, additional decks can be added to handle increasing traffic.
Suspension bridges sag unless the load is distributed along the length of the bridge. A stiffening truss below the bridge's deck helps distribute the load evenly and stabilize the bridge in high winds. The cables of a suspension bridge are under constant tension. Using high-tension steel cables, spans can be increased to 4,200 feet. Parts of a suspension bridge include:
• Towers: bear the weight of the bridge, often made of wood, stone and steel.
• Anchorage: hold the cable ends and are made of stone, concrete or natural rock.
• Cables: support the deck and are made from vines, split bamboo woven into ropes, or steel wire.
• Piers: support the tower, made of concrete. May require caissons (underwater work chambers).
In July of 1940, the unique Tacoma Narrows Bridge opened. At seventy-two times longer than it was wide, and without a stiffening truss, crossing the bridge was like riding a roller coaster. The flexible bridge was quickly nicknamed "Galloping Gertie" and was a successful tourist attraction for four months.4
In November of the same year, a gale wind began to twist the bridge like a ribbon. The bridge was quickly cleared. The wind eventually died down but it was too late. The concrete slabs began to drop out of the center span until the entire center feel down. A professor was there with his movie camera and caught the collapse on film. His film became a valuable tool for bridge builders. It also helped to illustrate the need to test the effects of wind on a model structure. This was a major factor for developing the wind tunnel test now practiced today and which helps to set bridge building standards.
When the George Washington Bridge was completed in 1931 it was the longest suspended span from 1931-1937. It revolutionized long-span design by eliminating the need for heavy costly and stiffening trusses, resulting in a strong bridge that was elegant and economical. Designer/engineer Othmar Herman Amman planned for the bridge's future growth in the original plans. In 1946 the original six traffic lanes were increased to eight, using area left for that purpose; fourteen lanes became available in 1962, when a lower level, also calculated in the original design was added.5
The Verrazano-Narrows Bridge connects New York's borough of Brooklyn with Staten Island. The bridge took seventy years to plan and six years to build. Following completion in 1965, it became the world's longest suspension bridge.6 At 4,260 feet, the bridge is so long that the curve of the earth had to be considered in the design. Though the towers are vertical, they are almost 2 inches further apart at the top than the base. The longest bridge to date is the Akaski Kaikyo Bridge in Japan, built in 1998 and spans 1991 meters or approximately 6,620 feet.