What is a bridge?
A structure used to span a distance between two points.
Bridges are usually used to make transportation easier for vehicles, pedestrians, or trains.
Bridges are typically built over a body of water to connect two pieces of land, or overtop of a valley to get from one side to the other.
The postcard on the left shows the Golden Gate Bridge in San Francisco. This bridge connects the peninsula of San Francisco to Marin County. It is a steel suspension bridge and it is one of the most iconic bridges in the World.
Bridges are usually used to make transportation easier for vehicles, pedestrians, or trains.
Bridges are typically built over a body of water to connect two pieces of land, or overtop of a valley to get from one side to the other.
The postcard on the left shows the Golden Gate Bridge in San Francisco. This bridge connects the peninsula of San Francisco to Marin County. It is a steel suspension bridge and it is one of the most iconic bridges in the World.
Introduction to Bridges video
Watch this 25 minute video by Assignment Discovery Video called "Understanding Bridges" to learn about types of bridges, bridge maintenance, bridge construction, and what happens when bridges fail!
Why do we have bridges?
As mentioned above, bridges are usually used to span the distance between one bank of land and another.
This could be as simple as a log placed across a stream (as shown on the right), or as complicated as a massive structure such as the Golden Gate Bridge above. Although the MAIN function of bridges is to transport people and goods from one side to another, engineers also like to try and make these structures unique so that they also function as a piece of art that becomes a widely recognized symbol for that city. Many bridges around the world are beginning to get more and more complex and artistic in their design because they also (just like some of the towers) become a tourist attraction for travellers to come visit. For example: look at the 2 pedestrian walkways photographed below. Although both of these bridges have the same function as the log on the right, the engineers have built these structures to look interesting so that people will want to come see these bridges! |
The science behind bridges
The simplest way to explain bridges is to explain that bridges are under pressure when something goes over them. The roadway or path is built on top of a strong BEAM that runs from one side of a bank to the other. When a weight is put on the bridge, it makes the top of the bridge want to sink down under the weight (COMPRESSION), and it makes the bottom of the bridge stretch
out (TENSION). It makes the bridge want to bend like you could see with an eraser. This is why bridges must be built out of strong materials is so that they don’t break under the weight of cars!
out (TENSION). It makes the bridge want to bend like you could see with an eraser. This is why bridges must be built out of strong materials is so that they don’t break under the weight of cars!
The forces of compression and tension won’t be too much of a problem if the bridge is:
1) only meant to span a short distance
2) doesn’t have to hold very much weight
3) has a strong beam
Remember the picture up above of a man crossing the river on a log? That is effective because it follows those 3 rules! Even though the beam IS under pressure when someone walks over it, it supports the weight of a person by transferring the weight outwards to the banks. This is how all bridges work – the bridges transfer weight from the middle outwards to the pieces that are solid on shore which are called ABUTMENTS. Then the weight can travel downwards into the ground which is very solid and so it makes it easier for the bridge to stay up! If a bridge isn’t quite strong enough as a simple beam bridge, engineers will add columns in the middle of the bridge to help distribute some weight directly to the ground. Those pieces put in the middle are called PIERS and they go deep into the ground so that the bridge is secure.
1) only meant to span a short distance
2) doesn’t have to hold very much weight
3) has a strong beam
Remember the picture up above of a man crossing the river on a log? That is effective because it follows those 3 rules! Even though the beam IS under pressure when someone walks over it, it supports the weight of a person by transferring the weight outwards to the banks. This is how all bridges work – the bridges transfer weight from the middle outwards to the pieces that are solid on shore which are called ABUTMENTS. Then the weight can travel downwards into the ground which is very solid and so it makes it easier for the bridge to stay up! If a bridge isn’t quite strong enough as a simple beam bridge, engineers will add columns in the middle of the bridge to help distribute some weight directly to the ground. Those pieces put in the middle are called PIERS and they go deep into the ground so that the bridge is secure.
Although this helps to describe the basic science behind bridges, there are other types of bridges that work to distribute the weight in different ways as well. Sometimes (like in the picture of the Golden Gate Bridge) the piers of a bridge extend to be above the beam, and then cables are used to connect the piers to the beams. Sometimes arches are used to distribute weight more evenly above and below the bridge as well. Next we will observe some of the different types of bridges!
Types of bridges
First we will observe the 3 main types of bridges, then we will look at some variations and combinations of these bridges!
Beam bridges
The first category of bridges is the one that we have already been observing: a beam bridge. Beam bridges are the simplest and most inexpensive kind of bridge. They were also some of the earliest ones to be used. Beam bridges consist of a horizontal beam supported at each end by abutments. If a bridge needs some support in the middle then piers are put in place to help distribute the weight. The weight of the beam pushes straight down on the piers. The farther apart the piers - the weaker the beam becomes!
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arch bridges
The arch bridges are usually used to span a deep gap. They are one of the
cheaper bridges to build. The weight transfers down the curves of the arch to the abutments which keep the shape of the arch from collapsing. Arches that are pieced together (like a brick arch for example)have a KEYSTONE at the top which is the crucial piece to hold the shape of the arch. |
suspension bridges
The last of the 3 main types of bridges is a suspension bridge.
Suspension bridges are used to span the longest distances. They are one of the more expensive bridges to build. The weight of the bridge is held up by the big cables which are strongly anchored into the abutments at the shore on either end. The cables are thread through the top of the piers instead of being directly attached to the piers which allows for some movement and flexibility of the bridge. The weight on these bridges is mostly transferred through the piers and along the main cable.
Suspension bridges are used to span the longest distances. They are one of the more expensive bridges to build. The weight of the bridge is held up by the big cables which are strongly anchored into the abutments at the shore on either end. The cables are thread through the top of the piers instead of being directly attached to the piers which allows for some movement and flexibility of the bridge. The weight on these bridges is mostly transferred through the piers and along the main cable.
cable-stay bridges
This bridge is a variation of a suspension bridge. This is one of the more modern types of bridges. All of the weight is directly held up by the massive, strong piers in the middle. When cars drive over these bridges the weight pulls on the big cables, which is then transported up towards the piers, then transfers back down to the ground by travelling from the top to the bottom of the pier.
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Truss bridges
TRUSS bridges come in all shapes and sizes.
They usually have one strong beam and then they are help up with a series of triangles which are very strong. The triangles help to distribute the weight of vehicles. Many other types of bridges also have triangular trusses in their designs. |
Cantilever bridges
This bridge is built like two long diving boards that are facing
each other and then joined in the middle with a connecting piece. All of the weight in the middle of the bridge is pushed towards shore to the piers and the abutments which distribute the weight. The middle part is not as strong. These bridges are good because they don’t need the piers in the middle, so tall boats can still go under the middle of the bridge. These bridges use lots of truss pieces to help distribute the weight from getting too heavy in the middle.
each other and then joined in the middle with a connecting piece. All of the weight in the middle of the bridge is pushed towards shore to the piers and the abutments which distribute the weight. The middle part is not as strong. These bridges are good because they don’t need the piers in the middle, so tall boats can still go under the middle of the bridge. These bridges use lots of truss pieces to help distribute the weight from getting too heavy in the middle.
Covered bridges
Covered bridges were created for functionality back in the days of horses and buggies. They were useful so that the bridges would be safer in bad weather, so that horses wouldn’t get spooked going over rivers, and so that bridges wouldn’t need to be fixed as often. They would mostly be categorized as a truss bridge because their walls that have trusses in them to help distribute the weight of the roof - it is basically just a truss bridge with a roof! These bridges usually have a pointed roof so that snow can slide off of them... These bridges are usually not very long, but we have the longest one in the World here in New Brunswick, Canada. Most of these bridges aren’t long enough to need piers in the middle. |
Test your bridge knowledge!
On your own or by a class see if you can answer all of the bridge questions below correctly! Some might be tricky and some might take some debating with your classmates! Keep an eye out for bridges you have maybe seen before, or any famous bridges that you recognize! Good Luck!
(Click HERE for a larger version of the quiz on FunnelBrain)
(Click HERE for a larger version of the quiz on FunnelBrain)
GOOD NEWS!
Now you are a Bridge Expert and you are ready to move on to the
BRIDGE BUILDING FINAL PROJECT!
Now you are a Bridge Expert and you are ready to move on to the
BRIDGE BUILDING FINAL PROJECT!
Graphics Used and Edited on this page: (All Images were re-uploaded 30/10/2013)
Golden Gate bridge http://marathi.wunderground.com/wximage/viewsingleimage.html?mode=singleimage&handle=RigsRocks&number=20
“Understanding Bridges” Assignment Discovery http://www.youtube.com/watch?v=hwzXXXUAKVU Retrieved
28/10/31
Man on log http://www.agefotostock.com/en/Stock-Images/Rights-Managed/MXX-MX08A0027
Calgary Peace Bridge http://www.youthareawesome.com/the-bridge-leading-to-nowhere-the-peace-bridge/
Henderson Waves http://www.worldtoptop.com/henderson-waves-bridge/
5 Simple bridge diagrams http://kids.britannica.com/elementary/art-88577/There-are-six-basic-bridge-forms
(cropped)
Keystone picture http://www.makingthemodernworld.org.uk/learning_modules/maths/02.TU.03/?section=4
Suspension forces http://www.pbs.org/wgbh/buildingbig/bridge/susp_forces.html
Cable-stay and cantilever forces (edited) http://architecture.phillipmartin.info/architecture_bridges_cable_stayed.htm
Truss bridge http://kids.britannica.com/comptons/art-18375/A-single-span-truss-bridge-with-forces-of-tension-represented
Assorted truss bridge designs http://teachers.sduhsd.k12.ca.us/delliott/files-poe/wp%20ff%20bridges/bridge%20requirements.htm
Covered Bridge http://coveredbridgechips.com/
Quiz made on http://www.funnelbrain.com/qc-203772-these-bridges-are-often-found-in-gardens-what-kind-of-bridge-do-you-think-it-is.html
Golden Gate bridge http://marathi.wunderground.com/wximage/viewsingleimage.html?mode=singleimage&handle=RigsRocks&number=20
“Understanding Bridges” Assignment Discovery http://www.youtube.com/watch?v=hwzXXXUAKVU Retrieved
28/10/31
Man on log http://www.agefotostock.com/en/Stock-Images/Rights-Managed/MXX-MX08A0027
Calgary Peace Bridge http://www.youthareawesome.com/the-bridge-leading-to-nowhere-the-peace-bridge/
Henderson Waves http://www.worldtoptop.com/henderson-waves-bridge/
5 Simple bridge diagrams http://kids.britannica.com/elementary/art-88577/There-are-six-basic-bridge-forms
(cropped)
Keystone picture http://www.makingthemodernworld.org.uk/learning_modules/maths/02.TU.03/?section=4
Suspension forces http://www.pbs.org/wgbh/buildingbig/bridge/susp_forces.html
Cable-stay and cantilever forces (edited) http://architecture.phillipmartin.info/architecture_bridges_cable_stayed.htm
Truss bridge http://kids.britannica.com/comptons/art-18375/A-single-span-truss-bridge-with-forces-of-tension-represented
Assorted truss bridge designs http://teachers.sduhsd.k12.ca.us/delliott/files-poe/wp%20ff%20bridges/bridge%20requirements.htm
Covered Bridge http://coveredbridgechips.com/
Quiz made on http://www.funnelbrain.com/qc-203772-these-bridges-are-often-found-in-gardens-what-kind-of-bridge-do-you-think-it-is.html