Barr Suspension Bridges 001

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Mike Barr

10/20/2010

Suspension Bridges The Fundamental Components and How They Work



AudienceBridges are an integral part of everybodys life but are often taken for granted. This paper willfocus specifically on suspension bridge, their key components, and how they work. This article is

targeted towards individuals who have a generalized knowledge of bridges, but are unfamiliar with

the innovation of a suspension bridge and their mechanics. The basics of each component and how

they work will be presented. From this, the reader will have a better understanding and a deeperappreciation of suspension bridges and bridges as a whole.

IntroductionSuspension bridges are a type of bridge that

suspends the roadway through an

intricate use of cables and towers. Theywere invented and have been used since the

early 19th century. The first built modern

style suspension bridge was the Jacobs

Creek Bridge in Westmoreland County,

Pennsylvania, in 1801. From here on, rapiddevelopment began of the modern

suspension bridge. Over the next few years

a number of more suspension bridges werebuilt su ch as the Union Bridge in

England/Scotland (1820), the Menai

Suspension Bridge in Wales (1826), and the

Clifton Suspension Bridge in England (1864).These early suspension bridges ranged from 259 feet to 1,368 feet while today the technology has

advanced to have suspension bridges capable of being almost two and half miles long. Modern

suspension bridges are the pinnacle of current bridge technology. They are capable of carrying upto twelve lanes of traffic over thousands of feet, and are visually stunning. They are a modern

enginnering infrastructure marvel.

Figure 1 - Union Bridge on the England to Scotland border

Figure 2 - The Pearl Bridge (1998) in Japan Total Length of 2.4 miles



Key ComponentsThis section will list and briefly discuss each of main components in a suspension bridge and howthey work.

Figure 3 - Labeled Components of Suspension Bridge - Image from howstuffworks.com How Suspension Bridges Work

Towers

The Towers are the backbone of thesuspension bridge and are designed to take

the entire load transferred to it from the maincables and the deck truss. Generally there are

two of them like displayed above, and the

main cables are strung through the top of

them, not attached to them. The towers aredeeply and firmly entrenched into the earth

which is where they dissapate the

compression forces acting on them. Towersare hundreds of feet tall and can be fairly thin

as they are restraining only vertical

compression (straight downward) forceswith no horizontal forces. They are built withmany open areas and holes to help prevent

swaying from dynamic wind loads.

Main Cables

The main cables of suspension bridges arestrung across the top of the bridge and are

the primary carriers of tension load. They areseveral feet in thickness and weigh thousands

of pounds per foot. They are attached to the

anchors on the ends of the bridged while

simply strung through holes at the top of thetowers. Being so thick and heavy, they are

capable extremely large loadings.

Anchors

Anchors are placed on each side of theroadway, at each end of the bridge. They are

either encased in massive blocks of concrete

or deeply entrenched into the earth. The main

cables are attached to them and spreadthroughout the anchor evenly. Tension loads

transferred from the main cables into the

anchors, dissapate into the earth or encasing

concrete.



Suspending CablesThe suspending cables are thinner, shorter

cables that primarily hold up the deck truss.

They are connected to the deck truss at one

end and the main cables at the other. Theyare designed to stretch and flex as varying

loads cross the bridge. These loads are then

transferred into the main cables.

Deck TrussThe deck truss is below the roadway and is

what holds it up. They are built with many

open holes and areas (truss like). This is to

help prevent dynamic problems such aswaving and rippling from seimic loadings like

earthquakes and high speed winds.

Structural Behavior

Figure 4 Force Diagram - Image from howstuffworks.com How Suspension Bridges Work

The structural behavior of suspension bridges can be a complex problem, but looking at its

fundamentals, is a fairly simple system. The cables, both main and suspending, are holding the deck truss which is holding the roadway and traffic. This puts these cables into tension. The suspending

cables transfer their tension into the main cables which are strung across the whole length of the

bridge. At the ends, they are attached to anchors, while only passing through the holes at the top of

the towers. This puts the anchors in tension with the earth, and the towers in compression with the

earth. Both towers and the anchors are able to evenly dissapate the forces into the earth.



Cons of Suspension Bridge Design

y Lightness Since suspension bridges are built to flex and move, they are very light.Because of this they cant take as heavy of concentrated loads as say a beam bridge.

y Seismic and Wind Loadings Again, since these types of bridges are meant to be able

to flex, seimic loadings like earthquakes and high speed winds can make a suspensionbridge sway and ripple. Modern designs include extra support at key locations but in

extreme cases, seismic loads are extremely dangerous for suspension bridges.

y Cost The materials needed for a suspension bridge are inexpensive and the structure

itself is extremely efficient. Unforunately, the labor and time that goes into building oneis very costly. From the design itself to the laborers, there is a huge amount of time and

money that goes into the construction of a suspension bridge.

y Access The process of building a suspension bridge can cause serious terrainaltercations. Traffic surrounding the area can be disturbed and the waterflow

(assuming the bridge is going over water) will be altered. If the waterway is high traffic

for boats, building a suspension bridge can be very problematic.

Conclusion

Bridges are truly an essential part of everyones life and are often overlooked and taken for granted.

Suspension bridges are no exception to this. They are often recognized for their distinctive shapeand appearance, but that makes it easier for us to ignore the importance of them. The design of

suspension bridges have made it possible to efficiently cross distances never thought possible by

bridges, while also being a visually stunning structure. Still, suspension bridges are a developing

technology and they will only become more efficient and common in the future.



Works Cited

David, Isaiah. How Does a Suspension Bridge Work? eHow . n.p. n.d. Web.

<http://www.ehow.com/how-does_4672299_suspension-bridge-work.html> 19 October 2010

Dooley, Keith. Cons of Suspension Bridges. eHow . Web.

<http://www.ehow.com/about_5108424_cons-suspension-bridges.html> 19 October 2010

Hawk, Thomas. Golden Gate Bridge at Dusk 21 August 2006. Web Image.

<http://www.flickr.com/photos/thomashawk/221827536/in/photostream> 19 October 2010

Morrissey, Michael. "How Bridges Work." HowStuffWorks.com. 01 April 2000.

<http://science.howstuffworks.com/engineering/civil/bridge.html> 19 October 2010

Wikipedia contributors. "Suspension bridge." Wikipedia, The Free Encyclopedia. 18 Oct. 2010. Web. 20

Oct. 2010.

How Suspension Bridges Work. HowStuffWorks.com. 12 October 2003

<http://static.howstuffworks.com/pdf/ups-suspension-bridge.pdf> 19 October 2010

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Barr Suspension Bridges 001