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A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. While modern bridges of this type date from the early 19th century, earlier bridges without vertical suspenders (simple suspension bridges) date from the 7th Century in Central America.

Suspended-deck suspension bridge with a distinctly arched deck
This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework.

The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. In some circumstances the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables.

History

Finley's Jacob's Creek bridge
The suspended-deck suspension bridge is one of the older forms of suspension bridge. Lacking a sufficiently level and stable deck, most simple suspension bridges are not suited for modern roads and railroads. Advances in materials and design led to the development of the suspended deck bridge, a modern bridge capable of carrying vehicles and light rail. In the late 7th Century, the Mayan empire had the earliest known suspended-deck suspension bridge, the Maya Bridge at Yaxchilan. Claims that suspension bridges with a horizontal deck (presumably a suspended deck) originated in Tibet or China remain largely unsubstantiated. The first design for a bridge resembling the modern suspended deck bridge in the West is attributed to Faust Vrančić, whose 1595 book “Machinae Novae” included drawings both for a timber and rope suspension bridge, and a hybrid suspension and cable-stayed bridge using iron chains. However, the first such bridge actually built was James Finley’s iron chain bridge at Jacob’s Creek, in Westmoreland County, Pennsylvaniamarker, in 1801. This was widely publicised from 1810 onwards, beginning a period of rapid development of the modern suspended-deck suspension bridge.

Early British chain bridges included the Dryburgh Abbey Bridge (1817) and 137 m Union Bridgemarker (1820), with spans rapidly increasing to 176 m with the Menai Suspension Bridgemarker (1826). The Clifton Suspension Bridgemarker shown above (designed in 1831, completed in 1864 with a 214 m central span) is one of the longest of the parabolic arc chain type.

Development of wire cable suspension bridges dates to the temporary simple suspension bridge at Annonaymarker built by Marc Seguin and his brothers in 1822. It spanned only 18 m. The first permanent wire cable suspension bridge was Guillaume Henri Dufour’s Saint Antoine Bridge in Genevamarker of 1823, with two 40 m spans. The first with cables assembled in mid-air in the modern method was Joseph Chaley’s Grand Pont Suspendu in Fribourgmarker, in 1834.

The Otto Beit Bridge was the first modern suspension bridge outside the United States built with parallel wire cables.

Structural behavior

Structural analysis

The main force in a suspension bridge of any type are tension in the cables and compression in the pillars. Since almost all the force on the pillars is vertically downwards and they are also stabilized by the main cables, the pillars can be made quite slender, as on the Severn Bridgemarker, near Bristolmarker, Englandmarker.

In a suspended deck bridge, cables suspended via towers hold up the road deck. The weight is transferred by the cables to the towers, which in turn transfer the weight to the ground.

Assuming a negligible weight as compared to the weight of the deck and vehicles being supported, the main cables of a suspension bridge will form a parabola (very similar to a catenary, the form the unloaded cables take before the deck is added). One can see the shape from the constant increase of the gradient of the cable with linear (deck) distance, this increase in gradient at each connection with the deck providing a net upward support force. Combined with the relatively simple constraints placed upon the actual deck, this makes the suspended deck bridge much simpler to design and analyze than a cable-stayed bridge, where the deck is in compression.

Advantages over other bridge types

A suspension bridge can be made out of simple materials such as wood and common wire rope.
  • Longer main spans are achievable than with any other type of bridge
  • Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost
  • Except for installation of the initial temporary cables, little or no access from below is required during construction, for example allowing a waterway to remain open while the bridge is built above
  • May be better able to withstand earthquake movements than can heavier and more rigid bridges


Disadvantages compared with other bridge types

  • Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck vibrating under high winds
  • The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur
  • Some access below may be required during construction, to lift the initial cables or to lift deck units. This access can often be avoided in cable-stayed bridge construction


Variations

Underspanned suspension bridge

Micklewood Bridge as illustrated by Charles Drewry, 1832
In an underspanned suspension bridge, the main cables hang entirely below the bridge deck, but are still anchored into the ground in a similar way to the conventional type. Very few bridges of this nature have been built, as the deck is inherently less stable than when suspended below the cables. Examples include the Pont des Bergues of 1834 designed by Guillaume Henri Dufour; James Smith’s Micklewood Bridge; and a proposal by Robert Stevenson for a bridge over the River Almond near Edinburghmarker.

Suspension cable types

The main suspension cable in older bridges was often made from chain or linked bars, but modern bridge cables are made from multiple strands of wire. This contributes greater redundancy; a few flawed strands in the hundreds used pose very little threat, whereas a single bad link or eyebar can cause failure of the entire bridge. (The failure of a single eyebar was found to be the cause of the collapse of the Silver Bridgemarker over the Ohio river). Another reason is that as spans increased, engineers were unable to lift larger chains into position, whereas wire strand cables can be largely prepared in mid-air from a temporary walkway.

Deck structure types

Most suspended deck bridges have open truss structures to support the roadbed, particularly owing to the unfavorable effects of using plate girders, discovered from the Tacoma Narrows Bridge marker bridge collapse. Recent developments in bridge aerodynamics have allowed the re-introduction of plate structures. In the picture of the Yichang Bridgemarker, note the very sharp entry edge and sloping undergirders in the suspension bridge shown. This enables this type of construction to be used without the danger of vortex shedding and consequent aeroelastic effects, such as those that destroyed the original Tacoma Narrows bridge.

Use other than road and rail

The principles of suspension used on the large scale may also appear in contexts less dramatic than road or rail bridges. Light cable suspension may prove less expensive and seem more elegant for a footbridge than strong girder supports. Where such a bridge spans a gap between two buildings, there is no need to construct special towers, as the buildings can anchor the cables. Cable suspension may also be augmented by the inherent stiffness of a structure that has much in common with a tubular bridge.

Construction sequence (wire strand cable type)

Suspender cables and suspender cable band on the Golden Gate Bridge in San Francisco.
Main cable diameter is 36 inches, and suspender cable diameter is 3 1/2 inches.


Typical suspension bridges are constructed using a sequence generally described as follows. Depending on length and size, construction may take anywhere between a year and a half (construction on the original Tacoma Narrows Bridge took only 19 months) to as many as a decade (the Akashi-Kaikyō Bridge's construction began in May 1986 and was opened in May, 1998 - a total of twelve years).
  1. Where the towers are founded on underwater piers, caissons are sunk and any soft bottom is excavated for a foundation. If the bedrock is too deep to be exposed by excavation or the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil, or a large concrete pad to distribute the weight over less resistant soil may be constructed, first preparing the surface with a bed of compacted gravel. (Such a pad footing can also accommodate the movements of an active earthquake fault, and this has been implemented on the foundations of the cable-stayed Rio-Antirio bridgemarker. The piers are then extended above water level, where they're capped with pedestal bases for the towers.
  2. Where the towers are founded on dry land, deep foundation excavation or pilings are used.
  3. From the tower foundation, towers of single or multiple columns are erected using high-strength reinforced concrete, stonework, or steel. Concrete is used most frequently in modern suspension bridge construction due to the high cost of steel.
  4. Large devices called saddles, which will carry the main suspension cables, are positioned atop the towers. Typically of cast steel, they can also be manufactured using riveted forms, and are equipped with rollers to allow the main cables to shift under construction and normal loads.
  5. Anchorages are constructed, usually in tandem with the towers, to resist the tension of the cables and form as the main anchor system for the entire structure. These are usually anchored in good quality rock, but may consist of massive reinforced concrete deadweights within an excavation. The anchorage structure will have multiple protruding open eyebolts enclosed within a secure space.
  6. Temporary suspended walkways, called catwalks, are then erected using a set of guide wires hoisted into place via winches positioned atop the towers. These catwalks follow the curve set by bridge designers for the main cables, in a path mathematically described as a catenary arc. Typical catwalks are usually between eight and ten feet wide, and are constructed using wire grate and wood slats.
  7. Gantries are placed upon the catwalks, which will support the main cable spinning reels. Then, cables attached to winches are installed, and in turn, the main cable spinning devices are installed.
  8. High strength wire (typically 4 or 6 gauge galvanized steel wire), is pulled in a loop by pulleys on the traveler, with one end affixed at an anchorage. When the traveler reaches the opposite anchorage the loop is placed over an open anchor eyebar. Along the catwalk, workers also pull the cable wires to their desired tension. This continues until a bundle, called a "cable strand" is completed, and temporarily bundled using stainless steel wire. This process is repeated until the final cable strand is completed. Workers then remove the individual wraps on the cable strands (during the spinning process, the shape of the main cable closely resembles a hexagon), and then the entire cable is then compressed by a traveling hydraulic press into a closely packed cylinder and tightly wrapped with additional wire to form the final circular cross section. The wire used in suspension bridge construction is a galvanized steel wire that is pre-coated with anti-corrosion inhibitors.
  9. At specific points along the main cable (each being the exact distance horizontally in relation to the next) devices called "cable bands" are installed to carry steel wire ropes called Suspender cables. Each suspender cable is engineered and cut to precise lengths, and are looped over the cable bands. In some bridges, where the towers are close to or on the shore, the suspender cables may be applied only to the central span. Early suspender cables were fitted with zinc jewels and a set of steel washers, which formed the support for the deck. Modern suspender cables carry a shackle-type fitting.
  10. Special lifting hosts attached to the suspenders or from the main cables are used to lift prefabricated sections of bridge deck to the proper level, provided that the local conditions allow the sections to be carried below the bridge by barge or other means. Otherwise, a traveling cantilever derrick may be used to extend the deck one section at a time starting from the towers and working outward. If the addition of the deck structure extends from the towers the finished portions of the deck will pitch upward rather sharply, as there is no downward force in the center of the span. Upon completion of the deck the added load will pull the main cables into an arc mathematically described as a parabola, while the arc of the deck will be as the designer intended — usually a gentle upward arc for added clearance if over a shipping channel, or flat in other cases such as a span over a canyon. Arched suspension spans also give the structure more rigidity and strength.
  11. With completion of the primary structure various details such as lighting, handrails, finish painting and paving are installed or completed.


The longest suspended-deck suspension bridge spans in the world



Suspension bridge are typically ranked by the length of their main span.
  1. Akashi-Kaikyo Bridgemarker (Japanmarker), 1991 m — 1998
  2. Xihoumen Bridgemarker (Chinamarker), 1650 m — 2007
  3. Great Belt Bridgemarker (Denmarkmarker), 1624 m — 1998
  4. Runyang Bridgemarker (China), 1490 m — 2005
  5. Humber Bridge (Englandmarker, United Kingdommarker), 1410 m — 1981. (The longest span from 1981 until 1998.)
  6. Jiangyin Suspension Bridge (China), 1385 m — 1997
  7. Tsing Ma Bridgemarker (Hong Kongmarker, China), 1377 m — 1997 (longest span with both road and metro)
  8. Verrazano-Narrows Bridgemarker (USAmarker), 1298 m — 1964. (The longest span from 1964 until 1981.)
  9. Golden Gate Bridgemarker (USA), 1280 m — 1937. (The longest span from 1937 until 1964.)
  10. Yangluo Bridge (China), 1280 m — 2007


Other suspended-deck suspension bridges

See also: History of longest vehicle suspension bridge spans.


Infamous suspended-deck suspension bridges



Gallery

File:Most na Suvom.jpg|Suspension bridge without river in Zrenjaninmarker, SerbiamarkerFile:Akashi-kaikyo_bridge_night_shot_small.jpg‎|Akashi-Kaikyo Bridgemarker at nightFile:Ponte_25_Abril_Lisboa.JPG‎|25 de Abril Bridgemarker in Lisbonmarker, PortugalmarkerFile:AmbassadorBridgesunsetting1.jpg|The Ambassador Bridgemarker — Longest suspension bridge from 1929–1931.File:Brooklyn Bridge Postdlf.jpg|New York’s Brooklyn BridgemarkerFile:SF-Oakland-Bay-Bridge-Construction.jpg| San Francisco – Oakland Bay Bridgemarker under constructionFile:Great Belt Bridge JvdC.jpg|Driving on the 2nd largest suspension bridge, Denmark’s Great Belt Bridge marker.File:Golden Gate Bridge Aerial.jpg|The Golden Gate Bridgemarker in San FranciscoFile:Ortakoey Istanbul Bosporusbruecke Mrz2005.jpg|Ortaköy Mosque and the Bosphorus Bridgemarker in IstanbulFile:Brooklyn Bridge by David Shankbone.jpg|Brooklyn Bridgemarker with Manhattan Bridgemarker in backgroundFile:SFOakBrWestPartVEast.jpg|Western portion of the San Francisco – Oakland Bay Bridgemarker — two bridges with a common central anchorageFile:GoldenGateBridge.jpg|Golden Gate Bridgemarker, California, USAFile:Mackinac-Bridge-Snowstorm-February-20-2006.jpg|Mackinac Bridgemarker in a snowstorm, during high winds, the bridge has to be closed.File:Mackinac Bridge Mod 1.jpg|The Mackinac Bridgemarker at night.File:Ozolnieki bridge by Igors Jefimovs.jpg|Suspension Bridge in Ozolnieki, Latviamarker.File:Budapest Chain Bridge.jpg|Széchenyi Chain Bridgemarker in Budapestmarker, HungarymarkerFile:HennepinAveBridge.jpg|Hennepin Avenue Bridgemarker in Minneapolis, MinnesotamarkerFile:2009-09-02 06-08-46philabenfranklinsunrise4w.JPG|Ben Franklin Bridge at sunrise, longest suspension bridge from 1926-1929.File:Humber_Bridge.png|Humber Bridge near Kingston-upon-Hullmarker had the longest span from 1981 until 1998.

See also



References

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