There are seven main types of bridges: the beam, truss, arch, suspension, cantilever, cable-stay and drawbridge. Bridge types differ in length, function and the load they can bear. Each holds weight in a different way. It is the balance between the downward forces (the weight and gravity) and the upward forces (supports) that causes a bridge to stand and carry loads. Many bridges use a combination of designs.
The beam bridge is the simplest and cheapest and the most common. The first one was a tree falling over a stream. The beam type made up of a pair of girders that rest on piers and support a deck. Usually they can span up to 250 ft. It has to withstand compression (pushing down of the weight of the bridge, the load and gravity) on top and tension (stretching) on the bottom. The piers balance the downward forces by pushing up.
If the linear structural elements are arranged in a lattice, such as a triangular pattern, you get a truss bridge and each member shares only a portion of the weight on any part of the structure. Truss bridges get their strength from the rigidity of the triangle. It is commonly used for railroad bridges because it can span great distances and carry heavy loads. If the truss is shaped into an arch, even longer bridges are possible. Most American bridges are girder designs. Straight steel beams, girders, lying on abutments, support concrete slab decks. Longer girder bridges up to 700 ft have steel deck.
Arch bridges were built 6000 years ago. The Romans took arch bridges to grander designs. Nearly all their bridges used the arch design because it could support more weight than a flat bridge. The arch is most successful when its two legs can be set on either bank. The traditional shape is made from a series of blocks cut to fit together perfectly. These "voissoirs" are wedge-shaped and gradually take the curve of the arch. The forces are spread around the arch to the legs that are kept from spreading out (buttressed) by the abutments. The arch is built curving in from the ends, supported by scaffolding called falsework until the keystone is in place. The Romans often built multi arch bridges and cofferdams to cross rivers for its armies. Arch bridges have evolved with the changing of fashion and materials (from stone to concrete and steel). In 1809, Perronet transformed and perfected the stone arch by finding that pier that are 1/5 of the arch are ideal for safety and that forces thrust downward and horizontally all the way to the abutments. Arch bridges can span up to 1800 feet.
Find pictures of two arch bridges. One should be a complete semicircle. The Darby Bridge was built from 1777-1779 near Coalbrookdale, London. It was the world's first iron bridge, 140 foot single arch and is a semicircle. The other picture of an arch bridge should be a shallower arch that is only the top of a semicircle. T. Seyrig designed the Luis I bridge in Oporto, Portugal. Completed in 1885, it has an upper road tangent to the circle at the top and a lower road that forms a secant through the arch above the diameter of the circle. This means the bridge arch is less than a semicircle.
Two students will form an arch by pressing their palms together high over their heads and walking their feet back. See Ice-Breaker Activities.
The cantilever design, a beam type, is more common for long spans. It consists of two independent beams/levers called cantilevers that extend from opposite banks and are weighted by piers. The downward force in the middle is counteracted by the weights on the side. Pulling out a kitchen drawer and a diving board demonstrate the cantilever concept. The two ends are joined in the middle by a beam, girder or truss. A pier firmly anchors one end and the arm extends freely out. Most cantilever bridges are made of steel or pre-stressed concrete. In 1890 the Forth Bridge in Scotland, the first to be made entirely of steel, broke records with spans of 1,710 feet each.
Have the students make a human cantilever bridge. To prove to the public that the new Firth of Forth Bridge would be stable after the Firth of Tay disaster in Scotland, the designers had three men act out the principle of a cantilever bridge. A photograph is easily obtained on:
Long ago, suspension bridges were built in Asia and South America of vines, ropes and bamboo. How was the first cable connected to the other side, especially if there was a deep gorge to cross? Originally, it was attached to an arrow and shot over. Kites have been used. Today helicopters fly the first cable to the opposite bank. In a suspension bridge the cables are anchored in one abutment, carried over the top of one tower, draped over the other tower and carried down to the opposite anchor in another abutment. The deck is supported by suspenders attached to the cables and ride on saddles that allow the cables to slip back and forth as the bridge load changes. Steel cable suspension bridges, begun in Great Britain in the early 1800's, are used for the longest spans of 2000 to 7000 feet. They are light, strong, aesthetic but are the most expensive to construct. Two huge extremely strong cables are draped over two high, sturdy towers that may rise hundreds of feet into the air. The ends of the cable are deeply embedded in concrete and the anchorages are usually fastened to solid rock. The deck can be high enough for even the largest ships to pass under. After the Civil War, the design of suspension bridges advanced when John A. Roebling (Niagara and Brooklyn Bridge creator) developed wire cables and stiffening trusses which minimized sway or twisting. He also learned to add more stays after the Wheeling Bridge disaster. At the same time, St. Louis, Missouri, desperately needed a train bridge to cross the 1500 foot span over the dangerous Mississippi River. They needed to take economic advantage of the cross country rail route soon to be available. Eads, a man who had never before built a bridge but knew the unpredictable river, accomplished the feat. He used innovative materials (steel), foundations and superstructure construction. Most of the weight of the bridge is transferred through the cables to the anchorages. Cables of high tensile wire can support an immense load. The longest main span is in Japan and measures 6,529 feet long.
How much more than a mile is this length?
6529 - 5280 = 1249
Write this length as a decimal of a mile rounded to hundredths.
6529/5280 = 1.24 mi.
Research at least three suspension bridge tower designs and draw them on graph paper as accurately and detailed as possible. Label each with the bridge name and the dimensions of the towers. The scale should be the same for all three so their relative sizes can be easily compared. The Golden Gate, Brooklyn Bridge and the 1940 Tacoma Narrows Bridge are a good start. The Narrows Bridge art deco towers are 50 ft wide at the base and 39 feet wide across the top. Research the height and draw a diagram to scale.
There are also good pictures of the massiveness of the anchorages in three great suspension bridges.
A fascinating hands-on web site demonstrates step by step the building of this bridge.
Combination spans are often used to bridge even longer stretches of water. The San Francisco-Oakland Bay Bridge is noted for its three long spans, two of which are suspension spans and the third a cantilever.
The cable-stayed design is the most modern, coming into prominence in the 1950's, and resembling sailboats. It may look similar to the suspension bridge but the two types of bridges support the load of the roadway in different ways. In suspension bridges, the cables ride freely across the top of the towers, sending the load down to the anchorages. In cable-stay bridges the cables are attached to the towers, which bear the weight alone. Computer modeling that constructs a three dimensional virtual bridge model allows engineers to make strong bridges of light, minimal material. The longest is in Japan and spans 2920 feet. The deck is hung from diagonal cables that exert force towards the towers as well as vertically. This makes the tension in the steel cables very high and thus very stiff. The cables stabilize the towers from both sides.
Tie a 5 foot rope to each of your elbows and place the middle of the rope on your head. A partner can help you. The rope acts like a cable-stay to hold up your elbows. Next take a longer length of rope and hold it your outstretched hands, place it over your head and experience two cable-stays.
The last category is the moveable bridge. There are several types and many are located in the area. The bascule bridge has a moveable deck that can be lifted quickly, either straight up or as the typical drawbridge. The swing bridge is built on a huge wheel that moves the bridge at a right angle. Pontoon bridges are quick to build and take down. Armies in ancient times used sealed and inflated animal skins or small boats floating end to end. Today metal cylinders or hollow concrete boxes are filled with air and attached to permanent piers or supports. In 1940 a permanent pontoon bridge more than two miles long was built on a lake near Seattle, Washington.