Bridges

[JimboJ]

As requested for your viewing pleasure.

History of Bridges

Although bridges often go unnoticed, their function is essential to modern America. In the 1850s, covered wagons heading West had to ford streams or rivers. Without adequate bridges in place, the pioneers would sometimes travel many miles out of the way to find a safe place to cross a river. Other times they waited for days or weeks for flooded rivers to recede until it was safe to cross. Crossing rivers was a dangerous aspect of early travel. Narrow wheels sank into the muddy river bottoms, and the swift, unpre- dictable currents took a toll in lives and property.

Consider how long it would take to cross a river today if bridges were not included in the road systems. Consider how the movement of people, goods, and services would be affected without highway systems, which include bridges. The entire American economy as we know it would be stifled.

Figure I
An early log bridge.

https://perth.uwlax.edu/globalengineer/draft/project/Page_11.jpg

Early Bridges
The earliest bridges consisted mainly of logs that had fallen or were placed across rivers. While inexpensive, they were less than reliable. Moving heavy loads across them was often impossible.

https://perth.uwlax.edu/globalengineer/draft/project/Page_12.jpg

Figure 2
A primitive rope bridge.


Primitive bridges were nothing more than a cable of rope stretched across a river. People could cross, but the rope was not practical for moving heavy loads. These rope bridges were the fore- runners of modern suspension bridges.

https://perth.uwlax.edu/globalengineer/draft/project/Page_13.jpg

Figure 3

An 18M ft. long rope bridge. Sophisticated rope bridges, like the one located in China and shown in Figure 3, are still used in many parts of the world. This particular bridge spans 1,800 feet. It allows light loads to be moved across the river.

While suitable for foot travel and pack,- animals, motorized traffic would have an impos- sible time crossing it. Imagine the effects of high winds on a rope suspension bridge like the one.

https://perth.uwlax.edu/globalengineer/draft/project/Page21.jpg

The stone arch bridge design is among the strongest and most durable. Stone is a naturally strong and enduring material for a bridge. In stone arch bridges, the stones are in compression (pressing against each other). The costs for labor and for transporting stones for these types of bridges are great. The bridge shown in Figure 4 was built during the time of the Roman Empire nearly 2,000 years ago.

https://perth.uwlax.edu/globalengineer/draft/project/Page22.jpg

Figure 5

A steel arch bridge.

Steel arch bridges are also capable of covering great spans and bearing heavy loads. The bridge in Figure 5 is 1,652 feet long. Note the cable suspended from the arch to the roadbed. This bridge is a combination of the arch, truss, and suspension bridges. In this case, the steel arch is under compression and the cables holding up the roadbed are in tension.

https://perth.uwlax.edu/globalengineer/draft/project/Page23.jpg

Figure 6

A suspension bridge.

The suspension bridge design is beautiful and very functional. It is capable of covering longer spans than any other type of bridge and is extremely efficient in terms of materials. The George Washington Bridge in New York City, shown in Figure 6, spans 3,600 feet. Suspension bridges use steel cable under tension to hold up theroadbed.

Cables supporting suspension bridges are made of individual strands capable of holding thousands of pounds. The George Washington Bridge uses four such cables. Each cable is three feet in diameter, one mile long, and includes over 26,000 strands. Each cable can support nearly 80,000 tons. The individual strands amount to a total of 107,000 miles ... enough to circle the earth four times. Each tower supporting the cables consists of 20,000 tons of steel and more than one million rivets!

https://perth.uwlax.edu/globalengineer/draft/project/page32.jpg

Figure 7

A cantilever bridge.

Cantilever bridges are a type of truss bridge. They are built toward the center of the river from each bank. The bridge is then joined with a section in the middle. The Quebec Bridge shown in Figure 7, which crosses the St. Lawrence River in Canada, is one of the world's longest cantilever bridges, spanning 1,800 feet. It was completed in 1917 and is still in use today.

One of the most challenging steps in building a cantilever bridge is setting the center section in place. Figure 8 shows the center section being set in place on the Carquinex Strait Bridge in California. The 700-ton, 150-foot long span was floated out under the bridge, then lifted into place with counterweights. Counter-weights are weights applied to offset the weight of the object being lifted.

https://perth.uwlax.edu/globalengineer/draft/project/page33.jpg

Figure 8
Raising the center section of a cantilever bridge.

https://perth.uwlax.edu/globalengineer/draft/project/page34.jpg

Figure 9
A wood truss bridge.

Where an abundance of wood exists, a simple truss-designed bridge serves well. Wood bridges are easy to build, yet capable of carrying heavy loads. The truss design is much more rigid and stronger than simple designs where boards are laid across a river. Figure 9 shows a simple version of a truss design bridge.

https://perth.uwlax.edu/globalengineer/draft/project/page35.jpg

Figure 10
A steel truss bridge.

Steel truss bridges like the one shown in Figure 10 are not as capable of great spans as other bridge designs. However, they can be quickly assembled, or even prefabricated, and can support great loads. This type of bridge is used extensively for railroad bridges. Trusses distribute the load to a large number of steel beams.


James
____________________
[00:42] <NSR-Andy> which one's the missionary position
[00:42] <NSR-Andy> i've heard it's good
[9:42pm] <Shaun> you came first mike | <Shaun> yes gav, because I really want to fuck you

[TheBoyChris]

Well written. Informative. Consise.

One question.

...

Why?

[Sparks!]

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Current Toys: 06 Yamaha WR250F | Nissan 350Z GT | Tech 4 Homes

[dainesefreak]

Roads?

[Adam_P]

Were you looking to up your karma by any chance Jim?
____________________
Colin McRae MBE 1968 - 2007 RIP
Orwell Rolls in his Grave
God is imaginary

[JimboJ]

As a fountain of knowledge it's only fair I share it with some people in the hope we can educate the world.

After posting the informative "History of Bricks" I was asked to do one on Bridges and cranes. Since I have a brilliant understanding of bridges I chose to do one on Bridges. Karma has nothing to do with it

Yes it was stolen from a website

James
____________________
[00:42] <NSR-Andy> which one's the missionary position
[00:42] <NSR-Andy> i've heard it's good
[9:42pm] <Shaun> you came first mike | <Shaun> yes gav, because I really want to fuck you

[JimboJ]

Just something that cropped up earlier with Mcjam, If you don't find something interesting does that make it boring? to you maybe but others may think it's interesting?

James
____________________
[00:42] <NSR-Andy> which one's the missionary position
[00:42] <NSR-Andy> i've heard it's good
[9:42pm] <Shaun> you came first mike | <Shaun> yes gav, because I really want to fuck you

[Stevie Northants]

I started reading that expecting a punch line nearer the end,it didn't happen.
I am sure a lot of people were quite informed and maybe some of the younger members of this forum can use some or all of it to help/cheat on an assignment.
Personally I can honestly say it wasn't for me,but I'm not knocking it,just don't normally read stuff like that on the bike forums i frequent.
____________________
Stevie
'98 Gixer 600,red and black(the fastest colour )
Why put off until tomorrow what you can get someone else to do for you today?