Materials

Old Materials

Stone was one of the first materials used in building bridges.  It is typically mined or cut out of the earth and then shaped by the builders.  It has good compressive strength and looks natural rather than manufactured.

Wood is cut from large trees.  In addition to looking natural it is easy to cut and shape wood.  It has good tensile and compressive strength.  Unfortunately it usually ends up rotting due to moisture and/or burning down as a result of a fire.

Bricks are formed from clay or cement that has been "fired" or baked in very hot ovens for many hours.  It has good compressive strength but can shatter if hit.  It does have a classic manufactured look, and needs to be "glued" together using mortar, a paste that is put between the bricks.

Iron was the first metal used in bridge construction.  It is relatively soft and has low compressive and tensile strength when compared to other metal bridges.  It definitely looks like a manufactured structure.

Modern Materials

Steel is iron that has been mixed with other minerals.  It can withstand high tension and compression forces and its strength can be predicted.  When it does fail it gives its users plenty of advance notice by deforming a great deal.  It is much stronger than concrete and it is often used inside concrete to provide it with extra strength; it is then know as reinforced concrete, which is described below.

Concrete is a combination of gravel, sand, water and cement which becomes very hard as a result of a complicated chemical reaction.  It is very strong in compression but weak in tension, and will fail without warning.  It can be shaped into whatever form is needed if a mold is created for it.  Because it is much cheaper than steel to produce the two are often combined together to create reinforced concrete (which is described below) to get the benefits of both.

Reinforced Concrete is concrete that has been poured around a network of steel (called reinforcement) which is placed in key locations to provide tensile strength, while the concrete provides compressive strength.  This combination protects the steel from rusting because they bond together very well, sealing out moisture.  They also expand and contract at the same rate when heated or cooled.  Because of steel's ability to deform before breaking, reinforced concrete is much safer than regular concrete; it will give off signs that it is going to break before the concrete snaps.  Since steel is much stronger than concrete, more concrete is needed than steel.

Future Materials

Plastics are currently very expensive to produce, and are sensitive to temperature changes.  Since it is such a new material, we have no idea how well it will last over many years.  It can have very high strength and is immune to corrosion, although acid rain may damage its surface.

See also aesthetics, cost, function, and safety.