***Bridges_notes***

**__Bridges:__** They are structures that connect one part of a place to another for people and vehicles to get across an obstacle or long distances easily and in a shorter time. The design will vary depending of its use and its materials. Usually bridges are made of wood, concrete, steel, bricks, stone, etc.

**__Types of bridges:__** __The beam bridge: __It is the earliest and simplest king of bridge; it is made of a simples beam supported at the end of both sides but unsupported in the middle. Most simple beam bridges are made of wood laid on stones supports. Lately these bridges have been made from steel or concrete, depending on how long it has to be and the weight it is going to support.



__ The arch bridge: __ These bridges are formed by a semicircle shape. They where originally built of stone or brick. This kind of bridge dissipates the weight from the center of the deck to the abutments.



__ The suspension bridge: __ A suspension bridge is one where cables are strung across the obstacle and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. For suspension bridges steel cables are commonly used.

**__Materials used in bridges __**

Stone
> >  >
 * Natural stone is one of the oldest bridge construction materials. It is strong and able to resist erosion caused by wind and water. Materials such as granite and limestone are naturally attractive and will last for centuries with little or no maintenance. At the same time, stone is heavy and has high installation costs. It is often used to build bridge piers and footers, which allow the upper portions of the bridge to be built from more affordable and lighter materials.

<span style="background-color: #ffffff; color: #444444; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; text-align: left; vertical-align: baseline;">Cement
> >
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-family: inherit; text-align: left; vertical-align: baseline; vertical-align: baseline;"><span style="background-color: #ffffff; color: #444444; display: block; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; vertical-align: baseline;">Concrete is popular for all types of bridge construction because of its affordability and strength. Concrete requires little maintenance, although it tends to hold up poorly against saltwater and erosion. Though it can be easily shaped and formed, concrete is often thought of as unattractive because of its dull, gray finish. When used on longer spans, concrete can be reinforced with steel bars or subject to a treatment known as "pre-stressing" to help increase its strength.

<span style="background-color: #ffffff; color: #444444; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; text-align: left; vertical-align: baseline;">Steel
> >
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-family: inherit; text-align: left; vertical-align: baseline; vertical-align: baseline;"><span style="background-color: #ffffff; color: #444444; display: block; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; vertical-align: baseline;">Steel is the one of the strongest bridge materials available, and can be used to span distances that are not possible with other products. It is 10 to 100 times stronger than concrete and weighs less. Steel bridges are susceptible to rust and corrosion, however, and tend to require a lot of maintenance. Many steel bridges are painted to improve their appearance. Aluminum is sometimes used in place of steel because of its anti-corrosive properties.

<span style="background-color: #ffffff; color: #444444; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; text-align: left; vertical-align: baseline;">Wood
>
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-family: inherit; text-align: left; vertical-align: baseline; vertical-align: baseline;"><span style="background-color: #ffffff; color: #444444; display: block; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%; vertical-align: baseline;">Wood is not as reliable as other bridge construction materials, and should only be used on relatively simple structures. It is one of the more affordable bridge-building materials, and is easy to work with using basic tools and equipment. Wood bridges are primarily chosen for their natural beauty, and are used for pedestrian access or light vehicle traffic. Because wood can swell and rot when exposed to moisture, wood bridges will last longer when protected from rain by a chemical treatment.