Bridging the Gap

Bridges make it possible to cross gaps without falling in, getting wet, or going the long way around.

The first bridges were probably made from tree trunks lying from one side of a stream to the other.

Then people discovered how to make bridges stronger and therefore longer and able to carry heavier weight.In this picture people have piled up

stones to make columns to support the bridge. Then they laid long, flat stone slabs between the columns. Some of these early bridges still survive.

A Sagging Problem

If you walk over a plank across a wide gap, it bends in the middle. The longer the plank, the more it bends.

Bridge builders realized they needed to solve the problem of how to support a bridge in the middle.

Over the centuries, bridge builders developed different types of bridges as the needs of people changed and technology improved.

Most bridges can be grouped into the following types:

•Beam

•Arch

•Truss

•Suspension

Beam Bridges

One of the earliest types of bridges was the beam bridge. They are still very common. Beam bridges are the simplest type of bridge construction which uses beams resting on supports or piers. Each end of the bridge is supported by an abutment.

The biggest limitation of beam bridges is that the maximum span (length between supports or piers) is only about 200 feet (60 meters). These bridges are best for crossing over short distances such as streams, highways, and other narrow spaces .

The beam type is the simplest type of bridge. The beam bridge could be anything as simple as a plank of wood to a complex structure. It is made of two or more supports which hold up a beam.

Beam Bridges:

Ancestor: log bridge

Descendent: truss bridge

Carries: pedestrians, autos, trucks, light & heavy rail

Span length: short

Material: timber, iron, steel, reinforced concrete

Design effort: low

Falsework required: none unless made from poured concrete

The Beam Bridge

Beam Bridge

Types of Beam Bridges

Arch Bridges

The next advancement in bridges came when the Romans developed the arch bridge. In arch bridges, compression pushes the weight away from the arch and against the side walls and the stones of the arch itself. These forces allow the arch bridge to span greater distances than the beam bridge. One disadvantage of the traditional stone arch is its heavy weight which requires large, extensive foundations.

The Romans used the arch design to build aqueducts to carry water to communities where they lived. Due to the strength and stability of the arch design, some of these aqueducts still exist today.

In the arch type of bridge, weight is carried outward along two paths, curving toward the ground.

Arch Bridge

Ancestor: masonry beam bridge

Descendent: truss arch bridge

Carries: pedestrians, vehicles, light & heavy rail, water

Span range: short, but often set end-to-end to form a long total length

Material: masonry, concrete, iron, cast iron, timber, structural steel

Design effort: low

Falsework required: Yes

The Arch Bridge

Types of Arch Bridges

Truss Bridge

The truss bridge became a common type of bridge from the 1870s through the 1930s. Many of these bridges still exist today . This type of structure has a simple design and is cheap to construct. As with the beam bridge, the span between the supports or piers is short. The advantage of the truss design is the

additional strength and stability the trusses give to a bridge. A simple truss bridge can be constructed in a short amount of time.

Basic Truss Designs

The trusses extend above the deck but are open.

The deck sits on top of the trusses.

The deck goes through the trusses which are connected at the top.

Truss Bridge Facts

Ancestor: beam bridge

Descendent: cantilever bridge, truss arch bridge

Carries: pedestrians, pipelines, vehicles, light & heavy rail

Span range: short to medium

Materials: timber, iron, steel, reinforced concrete

Design effort: medium

Falsework required: depends on length, materials

Truss bridge crossing Rio Grande River

Trussed Arch Bridge: A combination of the arch and truss bridge design

As bridge building advanced, engineers began to combine 2 or more bridge designs to meet the requirements of the terrain of the land, the type of load carried by the bridge and the distance the bridge was needing to span. One of the combinations was called the trussed arch bridge. There are several famous bridges using this design.

Sidney Harbour Bridge, Sidney, Australia – Trussed Steel Arch Bridge

Suspension Bridges

Long ago people learned to make hanging bridges with ropes. Today, the longest bridges in the world are suspension bridges which work in the same way.

Huge towers of steel or concrete are set into firm foundations. Steel cables are slung between them and firmly anchored behind the towers.

Side cables hang down to hold the roadway, called the deck. The deck is hoisted up in sections and fastened to the side cables.

The Brooklyn Bridge in New York City, the first suspension bridge built with steel cable, took 14 years to build. Construction began in 1869 and ended in 1883.

Suspension Bridge

The deck (trafficway) of a suspension bridge is hung by cables which hang from towers. The cables transfer the weight to the towers, which transfer the weight to the ground.

Suspension Bridge

Suspension Bridge Facts

Ancestor: simple suspension bridge

Descendent: cable-stayed bridge

Carries: pedestrians, vehicles, light rail

Span range: medium to long

Material: steel rope, multiple steel wire strand cables or cast chain links

Design effort: medium to difficult

Falsework required: none

The Golden Gate Bridge in San Francisco, completed in 1937, is considered one of the most famous suspension bridges in the world.

Longest cable-stayed bridge in North America is located in South Carolina

The New Bridge on the Block: The Cable-Stayed Bridge

The cable-stayed bridge may look very much like the suspension bridge, but it has some fundamental differences.

Cable-stayed bridges have towers, but cables from the towers go directly to the road deck, instead of spanning from tower to tower.

An advantage of cable-stayed bridges is that any number of towers may be used. This type of bridge form can be easily built with a single tower, as with a pair of towers. However, a suspension bridge is usually built with a pair of towers. The cable-stayed bridge is stiffer which reduces deck movement under live loads.

Bridge Vocabulary

Compression-the act of pressing or pushing

Deck-a platform extending horizontally that carries the roadway

Falsework-scaffolding or framework placed under the bridge to support it while it is being constructed

Engineer-person how uses math and science principles to design and construct structures and machines

Girder-a horizontal beam used for support

Span-portion of a bridge between two supports

Stress-the force acting upon a body.

Tension-the act of pulling or expanding

Truss-a rigid triangular framework

http://imageshack.us/

      Suspension/Cable-stayed - The deck (trafficway) of a suspension bridge is hung

by cables which hang from towers. The cables transfer the weight to the towers, which transfer the weight to the ground.Cable-stayed bridges have towers, but cables from the towers go directly to the road deck,

instead of spanning from tower to tower.                               Cantilever - In the cantilever type of bridge, two beams support another beam, which is where the deck or trafficway is. The two beams must be anchored, and this must be done well.

Iron Bridge

Until about 200 years ago, most bridges were made of wood or stone.

Then a new bridge was built in Great Britain. It was the first bridge in the world to be made of iron.

Iron was easy to use, but soon steel replaced iron since it was stronger and lasted longer. Bridge building was changed forever.