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Bridge Materials :

Selection of materials is very important in bridge construction, as in any structural project. Early wood or vine bridges would eventually rot. We still see many wooden road bridges and walk bridges. Today's wooden bridges life span is expanded by treating the wood with chemicals. These bridges will still eventually rot and are limited by their lack of strength. Wooden decking were easily destroyed by animal hooves at one time, and today traffic and snow plow blades can quickly wear the deck. Besides not being weather proof, wooden bridges can burn. Many communities or areas have experienced more than an inconvenience when local bridges have burned due to lightening, accidents or even vandalism.
Stone bridges have long since been strong and long lasting bridges. They could span longer distances than wooden beam bridges as well, but could not span extremely long distances as would be required in time. The development of railroads caused a revival of semicircular-arch construction, and cut stone was used in certain localities where it was economical. The longest stone (or masonry)-arch railroad viaduct is at Rockville, Pennsylvania, where four tracks of the Penn Central Railroad were carried over the Susquehanna River. The Rockville viaduct, opened in 1902, includes 48 arches with 21.3m(70ft) spans and is 1161m(3809ft) long. A 3657.6m (12,000ft) viaduct of 222 arches over the Lagoon of Venice connects that city with the mainland.

The demand for longer spans than what arch bridges could safely serve led to the development of suspension bridges. Early suspension bridges were destroyed by the movement caused by the traffic of people and animals.

Not only did the style of bridge change in time, so did the material of choice. In the beginning of the 1800's Iron was replacing wood, and by the late 1800's, steel starting to replace iron. Iron is much stronger than wood and, as a result, can span greater distances. It is plentiful, in fact it makes up 5% of the earth's crust. It is malleable, or can be shaped and bent quit easily. It is also weather proof (when painted) and heat resistant.

Steel is simply an iron and carbon alloy (which means a mixture of both iron and carbon that has been put through a special process) Other alloys (additives) can also be added to the recipe to make it really hard, strong, wear resistant and so on. As a result steel has become a much more reliable material and versatile than iron.

History of iron and steel bridges

Each new material or design technique builds off the lessons of the past. Modern bridges often use steel in the form of cables or girders, and/or concrete as construction materials. Steel is durable and is relatively light considering the strength and size of the components. The aestitics are also very appealing when built in steel. In other words, people find that the structure can be made to look very beautiful, or cool. Another very big plus is that steel components can be made else where and assembled on the site. This speeds up the construction of the bridge considerably.

The first steel bridge was built across the Mississippi at St. Louis, Missouri, by the American engineer James Buchanan Eads, and opened in 1874. The three arches span 153, 158.5, and 153m(502, 520, and 502ft). The bridge crossing the Niagara River between Queenston, Ontario, and Lewiston, New York, which opened in 1965, employs a 304.8m (1000ft) steel arch, the longest fixed steel-rib-arch span in the world at that time. The New River Gorge Bridge at Fayetteville, West Virginia, completed in 1977, has a span of 518.5m (1700ft).
"The Seal Island Bridge, Cape Breton ,N.S.

is a relatively inexpensive, versatile, tough, but it is a heavy material. It is widely used in modern bridge building, as well as the building of other modern day structures. It is made by mixing the proper proportions of sand, stone, cement, and water. When the water dries up, the concrete becomes hard and dry.


1) The next link helps illustrate material characteristics. Check out the different materials listed in the "Choose One" menu. For each material; move the tab up and down the bar to witness the different properties they present. ***In addition***, take note to the cost and weight of the different materials at the bottom of each page.

Concrete can be poured into forms on site to become permanent fixtures such as in the case of the abutments erected as one of the first stages of the construction of the Margaree Bridge.
Concrete can also be poured into a molds such as the puzzle shaped wall pieces being unloaded above, or as beams or other prefabricated (made ahead of time to be assembled later or else where) components to be transported and assembled on site.

Concrete is a material that has good compression properties, but does not hold up so well under tension. However, reinforced concrete. is made when steel mesh or rods (rebar) are embedded into the concrete for increased strength in tension. wpe1A.gif (265955 bytes)
It was soon after the turn of the 20th century when the development of reinforced concrete brought about great progress in concrete-arch bridge construction. The Esla Bridge over the Esla River, Spain, with a 196.6m (645ft) span, was completed in 1940. The Columbia, Pennsylvania, highway viaduct is 2089.7m (6856 ft) long, with twenty-eight 56.4m (185ft) concrete arches.
Even stronger, pre-tensioned concrete has embedded steel bars or cables which are stretched into tension before the concrete hardens. In post-tensioned concrete, the embedded steel bars or cables are stretched into tension after the concrete hardens.


2) For this bridge activity, try the bridge game :


Exercise: From the hyperlinks above answer the following:

1.) What are 3 problems that arise when you choose wood to build your bridge?

2.) What are 5 reasons that iron became the material of choice over such building materials as wood?

3.) Why did steel replace wrought and cast iron bridge materials?

4.) Steel remains a very popular bridge material for a number of reasons. Click on the following link, scroll down to Advantages of steel bridges which are listed in red. List and briefly explain 4 the many advantages of using steel for bridge construction.

5.) What are the pros and cons when it comes to concrete? What are ways to improve the properties of concrete?



Larger bridges over canons and various water ways without disrupting such things as marine traffic, fish migration and so-on can be quite a challenge. Building huge structures on land has it's problems, just think of the problems of building them in water.
For example; concrete piers must rest on bedrock. Before reaching bedrock there is water and sand to get through. Piles are often driven into the bottom to support a temporary work platform. They may also be driven to be used as anchorage for the piers. This is done by hoisting a very large heavy weight called a pile driver and dropping it on the pile. The effect is to literally hammer the pile into the ground.
wpe14.gif (184315 bytes) In this pic, the piles are connected with a girder that will support the temporary bridge/work platform.

In order to excavated and do the necessary work around the bottom of the piers, caissons must be put in place at each location.wpe34.gif (213399 bytes) A caisson is constructed of two basic parts. The upper part is basically a reinforced tube, and the lower is a pressurized work area. The lower area must be kept under pressure to keep water from leaking into the caisson. In the picture to the left you can see the top of the caisson that is holding back the water so that they can prepare the base of the abutment. They have already driven in the piles that will anchor it in place.

Once the sand has been removed and bedrock has been reached, the caisson itself, or forms built inside, is filled with concrete and becomes the bridge pier.

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These pics show the rebar sticking out of mold with in the caisson. You can also see the gang plank the workers use to get to the ladders to get down inside.

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Here is the 2 pc steel split cylindrical mold used to cast the leg portion of the pier. You can also see the 2nd, beam support mold which will be used next.

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They set the second mold on top, pour the second portion and remove the mold once the cement is set. Now the pier is one piece of reinforced concrete.

wpe2E.gif (172386 bytes) Once the pier is ready, the caisson can be removed. Once the piers and abutments are done, it is time to suspend the beams that will support the deck/roadway. The temporary work bridge that was used for access to the area is still needed for hoisting the beams into place, It will gradually be removed as each beam segment is completed.

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The Margaree bridge, replaced bridge a deteriorated wooden beam bridge. The new replacement bridge was a beam bridge as well. The beams were curved prefabricated steel components that were trucked in, lifted into place and bolted together.


The deck was made of prefabricated cement pads that were laid across the beams one after another. A second roadway layer was poured on top. Once the bridge and access ramps were complete, the old bridge had to be removed.


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Concrete and steel are the materials of choice in modern bridge building today, but how do you go about building a bridge? Small bridges, or bridge supports, can be suspended across short spans using cranes. In the case of the West Mabou bridge (shown below), a replacement bridge was suspended across the original abutments. They assembled the new bridge on one side and with the use of rollers, they kept pushing it across as they assembled it until it reached the other side. They had to be sure that more of the bridge was on the road side to prevent the bridge from toppling into the water before they could grasp the other side. They actually made it longer than necessary to keep the heavy end on the road side of the rollers. Once the other side was supported, they then disassembled the extra lengths.

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The assembly of the West Mabou (Bailey) truss type beam bridge.

A Coffer Dam is like a caisson. It doesn’t have the pressurized work area, and is completely open to the top. It can only be used in shallow areas where bedrock is near the water surface.


3)Now solve the 4 bridge problems presented in this The Bridge Challenge. Take the time to solve all four problems, as you will be responsible for this information.

Interesting Facts:

An arch made of stone doesn’t even need mortar. Ancient Romans built arch bridges (and aqueducts), which are still standing, and structurally sound, today. These bridges and aqueducts are real testaments to the natural effectiveness of an arch as a bridge structure.

The following is an excellent site to play and learn about ,not only bridges, but other structures and structure design: