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Bridges

Types
by

Michael Russell

on 21 June 2011

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Transcript of Bridges

BRIDGE
TYPES Suspension Bridge Arch Bridge Cantilever Bridge Cable Stayed Bridge Girder/Beam Bridge Truss Bridge Suspension Bridge Arch Bridge Cantilever Bridge Cable Stayed Bridge Truss Bridge Girder/Beam Bridge Of all the bridge types in use today, the suspension bridge allows for the longest spans. At first glance the suspension and cable-stayed bridges may look similar, but they are quite different. Though suspension bridges are leading long span technology today, they are in fact a very old form of bridge. Some primitive examples of suspension bridges use vines and ropes for cables. The development of metals brought the use of linked iron bars and chains. But it was the introduction of steel wire ropes that allowed spans of over 500m to become a reality. Today the Akashi Kaikyo bridge boasts the world's longest center span of any bridge at 1,991 meters.
A typical suspension bridge is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself it usually a truss or box girder though in shorter spans, plate girders are not uncommon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables.
The main cables are stretched from one anchor over the tops of the tower(s) and attached to the opposite anchor. The cables pass over a special structure known as a saddle. The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower. The development of metals brought the use of linked iron bars and chains. But it was the introduction of steel wire ropes that allowed spans of over 500m to become a reality. Today the Akashi Kaikyo bridge boasts the world's longest center span of any bridge at 1,991 meters.
A typical suspension bridge is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself it usually a truss or box girder though in shorter spans, plate girders are not uncommon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables.
The main cables are stretched from one anchor over the tops of the tower(s) and attached to the opposite anchor. The cables pass over a special structure known as a saddle. The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower. From the main cables, smaller cables known as hanger cables or hanger ropes are hung down and attached to the girder. Some suspension bridges do not use anchors, but instead attach the main cables to the ends of the girder. These self-anchoring suspension bridges rely on the weight of the end spans to balance the center span and anchor the cable.
Thus, unlike normal bridges which rest on piers and abutments, the girder or roadway is actually hanging suspended from the main cables. The majority of the weight of the bridge and any vehicles on it are suspended from the cables. In turn the cables are held up only by the tower(s), there is an incredible amount of weight that the towers must be able to support.
As explained in the cable stayed bridge section, steel cables are extremely strong yet flexible. Like a very strong piece of string, it is good for hanging or pulling something, but it is useless for trying to push something. Long span suspension bridges, though strong under normal traffic loads, are vulnerable to the forces of winds. Special measures are taken to assure that the bridge does not vibrate or sway excessively under heavy winds.
The most famous example of an aerodynamically unstable bridge is the Tacoma Narrows Bridge in Washington state, USA. This page on the Tacoma Narrows Bridge Disaster at the University of Bristol has some excellent photos and short movies showing why aerodynamic stability is important. After girder bridges, arch bridges are the second oldest bridge type and a classic structure. Unlike simple girder bridges, arches are well suited to the use of stone. Many ancient and well known examples of stone arches still stand to this day. Arches are good choices for crossing valleys and rivers since the arch doesn't require piers in the center. Arches can be one of the more beautiful bridge types. Arches use a curved structure which provides a high resistance to bending forces. Unlike girder and truss bridges, both ends of an arch are fixed in the horizontal direction (i.e. no horizontal movement is allowed in the bearing). Thus when a load is placed on the bridge (e.g. a car passes over it) horizontal forces occur in the bearings of the arch. These horizontal forces are unique to the arch and as a result arches can only be used where the ground or foundation is solid and stable. 4
DESIGN
TYPES The hinge-less arch uses no hinges and allows no rotation at the foundations. As a result a great deal of force is generated at the foundation (horizontal, vertical, and bending forces) and the hinge-less arch can only be built where the ground is very stable. However, the hinge-less arch is a very stiff structure and suffers less deflection than other arches. HINGE-LESS ARCH The two hinged arch uses hinged bearings which allow rotation. The only forces generated at the bearings are horizontal and vertical forces. This is perhaps the most commonly used variation for steel arches and is generally a very economical design. TWO HINGED ARCH The three-hinged arch adds an additional hinge at the top or crown of the arch. The three-hinged arch suffers very little if there is movement in either foundation (due to earthquakes, sinking, etc.) However, the three-hinged arch experiences much more deflection and the hinges are complex and can be difficult to fabricate. The three-hinged arch is rarely used anymore. THREE HINGED ARCH TIED ARCH The tied arch is a variation on the arch which allows construction even if the ground is not solid enough to deal with the horizontal forces. Rather than relying on the foundation to restrain the horizontal forces, the girder itself "ties" both ends of the arch together, thus the name "tied arch." A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework. A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. Compared to other bridge types, the cable-stayed is optimal for spans longer than typically seen in cantilever bridges, and shorter than those typically requiring a suspension bridge. This is the range in which cantilever spans would rapidly grow heavier if they were lengthened, and in which suspension cabling does not get more economical, were the span to be shortened. At these distances the Cable Stayed Bridge is very economical as they allow for a slender and lighter structure. A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges due to the fact they could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials. In most cases the design, fabrication, and erection of trusses is relatively simple. However, once assembled trusses take up a greater amount of space and, in more complex structures, can serve as a distraction to drivers. Beam bridges are the most simple of structural forms, being supported by an abutment at each end of the deck. No moments are transferred through the support hence their structural type is known as simply supported. The simplest beam bridge could be a slab of stone, or a plank of wood laid across a stream. Bridges designed for modern infrastructure will usually be constructed of steel or reinforced concrete, or a combination of both. The concrete used can either be reinforced, prestressed or post-tensioned. Beam bridges are not limited to a single span. Some viaducts can have multiple simply supported spans supported by piers A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. In a common variant, the suspended span, the cantilever arms do not meet in the center; instead, they support a central truss bridge which rests on the ends of the cantilever arms. The suspended span may be built off-site and lifted into place, or constructed in place using special traveling supports. A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. Thus, in a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever's supports, the bridge superstructure often takes the form of towers above the foundation piers. TWO MOST COMMON CABLE STAY ARRANGEMENTS HARP DESIGN FAN DESIGN In a harp design, the cables are made nearly parallel by attaching them to various points on the tower(s) so that the height of attachment of each cable on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, all the cables connect to or pass over the top of the tower(s). The lighter weight of the bridge, though a disadvantage in a heavy wind, is an advantage during an earthquake. However, should uneven settling of the foundations occur during an earthquake or over time, the cable-stayed bridge can suffer damage so care must be taken in planning the foundations. The modern yet simple appearance of the cable-stayed bridge makes it an attractive and distinct landmark. The unique properties of cables, and the structure as a whole, make the design of the bridge a very complex task. For longer spans where winds and temperatures must be considered, the calculations are extremely complex and would be virtually impossible without the aid of computers and computer analysis. The construction of cable stay bridges is also relatively difficult. The cable routing and attachments for the girders and towers are complex structures requiring precision fabrication. There are no distinct classifications for cable-stayed bridges. However, they can distinguished by the number of spans, number of towers, girder type, number of cables, etc. There are many variations in the number and type of towers, as well as the number and arrangement of cables. Typical towers used are single, double, portala and A-shaped towers. Cooper River Bridge
Charleston, NC Sundial Bridge
Redding, CA Copenhagen-Malmo Bridge
Denmark/Sweden In the bridge illustrated above, vertical members are in tension, lower horizontal members are in tension, shear, and bending, outer diagonal and top members are in compression, while the inner diagonals are in tension. The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord (a horizontal member of a truss) is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated, but with additional strength added to other members in compensation. The ability to distribute the forces in various ways has led to a large variety of truss bridge types.
The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. EXAMPLETRUSS DESIGNS A Pratt truss includes vertical members and diagonals that slope down towards the center, the opposite of the Howe truss.[9] It can be subdivided, creating Y- and K-shaped patterns. The Pratt Truss was invented in 1844 by Thomas and Caleb Pratt. This truss is practical for use with spans up to 250 feet and was a common configuration for railroad bridges as truss bridges moved from wood to metal. They are statically determinate bridges, which lend themselves well to long spans. The relatively rare Howe truss, patented in 1840 by Massachusetts millwright William Howe, includes vertical members and diagonals that slope up towards the center, the opposite of the Pratt truss.[9] In contrast to the Pratt Truss, the diagonal web members are in compression and the vertical web members are in tension. HOWE TRUSS PRATT TRUSS WARREN TRUSS The Warren truss was patented in 1848 by its designers James Warren and Willoughby Theobald Monzani, and consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. Loads on the diagonals alternate between compression and tension (approaching the center), with no vertical elements, while elements near the center must support both tension and compression in response to live loads. This configuration combines strength with economy of materials and can therefore be relatively light. WHIPPLE PRATT TRUSS A whipple truss is usually considered a subclass of the Pratt truss because the diagonal members are designed to work in tension. The main characteristic of a whipple truss is that the tension members are elongated, usually thin, at a shallow angle and cross two or more bays (rectangular sections defined by the vertical members). KINGPOST TRUSS One of the simplest truss styles to implement, the king post consists of two angled supports leaning into a common vertical support. QUEENPOST TRUSS The queenpost truss, sometimes queen post or queenspost, is similar to a king post truss in that the outer supports are angled towards the center of the structure. The primary difference is the horizontal extension at the center which relies on beam action to provide mechanical stability. This truss style is only suitable for relatively short spans. Truss bridges became a common type of bridge built from the 1870s through the 1930s. Examples of these bridges still remain across the United States, but their numbers are dropping rapidly, as they are demolished and replaced with new structures. As metal slowly started to replace timber, wrought iron bridges in the U.S. started being built on a large scale in the 1870s. Bowstring truss bridges were a common truss design seen during this time, with their arched top chords. Companies like the Wrought Iron Bridge Company of Canton, Ohio and the King Bridge Company of Cleveland, Ohio became well-known companies, as they marketed their designs to different cities and townships. The bowstring truss design (photo) fell out of favor due to a lack of durability, and gave way to the Pratt truss design, which was stronger. Again, the bridge companies marketed their designs, with the Wrought Iron Bridge Company in the lead. As the 1880s and 1890s progressed, steel began to replace wrought iron as the preferred material. Other truss designs were used during this time, including the camel-back. By the 1910s, many states developed standard plan truss bridges, including steel Warren pony truss bridges. As the 1920s and 1930s progressed, some states, such as Pennsylvania, continued to build steel truss bridges, including massive steel through truss bridges for long spans. Other states, such as Michigan, used standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built. 1870 1940 Calhoun St. Bridge - 1884
Trenton, NJ Washington Crossing Bridge - 1904
Washington Crossing, NJ Southern Pacific RR Bridge- 1912
Tempe, AZ Pasco Kennewick Bridge - 1922
Tri-Cities, WA Rip Van Winkle Bridge - 1935
Catskill, NY Akashi Kaikyo Bridge,
Awaji Island/Kobe, Japan
World's Longest Suspension Span - 6,532' Tacoma Narrows Bridges
Tacoma, WA Golden Gate Bridge
San Francisco, CA Jiangyin Bridge
Yangtze River, China The Great Belt Bridge
Between Zealand and the Island Sprogø, Denmark Commodore Barry Bridge,
Chester, PA to Bridgeport, NJ Connel Bridge
Connel, Scotland Chico River Bridge
Durango, Mexico Chico River Bridge
Durango, Mexico Chico River Bridge
Durango, Mexico Octavio Frias de Oliveira Bridge
Sao Paulo, Brazil Octavio Frias de Oliveira Bridge
Sao Paulo, Brazil Millau Viaduct
Millau, France
World's Tallest Bridge - 1,125'
World's Longest Cablestayed Deck - 8,070' Hoover Dam Bypass
Nevada/Arizona Hoover Dam Bypass
Nevada/Arizona Hoover Dam Bypass
Nevada/Arizona Hoover Dam Bypass
Nevada/Arizona Hoover Dam Bypass
Nevada/Arizona Hoover Dam Bypass
Nevada/Arizona Quebec Bridge
Quebec City, Quebec
World's Longest Cantilever Span - 1,800' Bridge building doesn't get any simpler than this. In order to build a beam bridge, all you need is a rigid horizontal structure (girder) and a support (abutment/bent) at each end to rest it on. These components directly support the downward weight of the bridge and any traffic travelling over it. However, in supporting weight, the bream bridge endures forces in both tension and compression. The Diagram below illustrates these forces. Many beam bridges use concrete or steel beams to handle the load. The size of the beam, and in particular the height of the beam, controls the distance that the beam can span. By increasing the height of the beam, the beam has more material to dissipate the tension. Yet even with a truss, a beam bridge is only good for a limited distance. To reach across a greater length, you have to build a bigger truss until you eventually reach the point at which the truss can't support the bridge's own weight.
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