A
suspension bridge is a type of
bridge in which the deck (the load-bearing portion) is hung below suspension
cables on vertical suspenders. Outside Tibet and Bhutan, where the first examples of this type of bridge were built in the 15th century, this type of bridge dates from the early 19th century.
[1][dubious – discuss] [2] Bridges without vertical suspenders have a long history in many mountainous parts of the world.
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.
[edit]History
The earliest suspension bridges were ropes slung across a chasm, with a deck possibly at the same level or hung below the ropes so that the rope has a
catenary shape.
Drawing of the Chakzam bridge south of Lhasa, constructed in 1430, with cables suspended between towers, and vertical suspender cables carrying the weight of the straight deck below.
The first bridges resembling the modern suspension bridge were built in Tibet and Bhutan by the Tibetan architect and engineer
Thangtong Gyalpo, with the most famous example dating to 1430.
[citation needed]The first design for a bridge resembling the modern suspension bridge in the West is attributed to
Fausto Veranzio, 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 (see gallery below). However, the first such bridge actually built was
James Finley’s iron chain bridge at Jacob’s Creek, in
Westmoreland County, Pennsylvania, in 1801.
[3]This was widely publicised from 1810 onwards, beginning a period of rapid development of the modern suspension bridge.
[edit]Wire-cable
Development of wire-cable suspension bridges dates to the temporary simple suspension bridge at
Annonay built by
Marc Seguin and his brothers in 1822. It spanned only 18 m.
[4] The first permanent wire cable suspension bridge was
Guillaume Henri Dufour’s Saint Antoine Bridge in
Geneva of 1823, with two 40 m spans.
[4] The first with cables assembled in mid-air in the modern method was
Joseph Chaley’s Grand Pont Suspendu in
Fribourg, in 1834.
[4]The
Otto Beit Bridge (1938–39) was the first modern suspension bridge outside the United States built with parallel wire cables.
[5][edit]Structural behavior
[edit]Structural analysis
The main
forces 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 Bridge, on the Wales-England border.
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 suspension bridge much simpler to design and analyze than a
cable-stayed bridge, where the deck is in compression.
[edit]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 to withstand earthquake movements than heavier and more rigid bridges
[edit]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
[edit]Variations
[edit]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;
[4] James Smith’s Micklewood Bridge;
[6] and a proposal by
Robert Stevenson for a bridge over the River Almond near
Edinburgh.
[6]Roebling's Delaware Aqueduct (begun 1847) consists of three sections supported by cables. The timber structure essentially hides the cables; and from a quick view, it is not immediately apparent that it is even a suspension bridge.
[edit]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 Bridge 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.
[edit]Deck structure types
Most suspension 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 (1940) bridge collapse. Recent developments in bridge aerodynamics have allowed the re-introduction of plate structures. In the picture of the
Yichang Bridge, 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.
[edit]Forces acting on suspension bridges
Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. Dead load refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is made. Live load refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature, precipitation, and winds. Dynamic load refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration when building a bridge.
[edit]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.
[edit]Construction sequence (wire strand cable type)
One of the main cable saddles atop the Golden Gate Bridge.
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).
- 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 fault, and this has been implemented on the foundations of the cable-stayed Rio-Antirio bridge. The piers are then extended above water level, where they are capped with pedestal bases for the towers.
- Where the towers are founded on dry land, deep foundation excavation or pilings are used.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 has been coated with corrosion inhibitors.
- 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.
- Special lifting hoists 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.
- With completion of the primary structure various details such as lighting, handrails, finish painting and paving are installed or completed.
[edit]The longest suspension bridge spans in the world
Suspension bridges are typically ranked by the length of their main span. These are the ten bridges with the longest spans, followed by the length of the span and the year the bridge opened for traffic:
- Akashi Kaikyō Bridge (Japan), 1991 m — 1998
- Xihoumen Bridge (China), 1650 m — 2009
- Great Belt Bridge (Denmark), 1624 m — 1998
- Runyang Bridge (China), 1490 m — 2005
- Humber Bridge (England, United Kingdom), 1410 m — 1981. (The longest span from 1981 until 1998.)
- Jiangyin Suspension Bridge (China), 1385 m — 1997
- Tsing Ma Bridge (Hong Kong, China), 1377 m — 1997 (longest span with both road and metro)
- Verrazano-Narrows Bridge (USA), 1298 m — 1964. (The longest span from 1964 until 1981.)
- Golden Gate Bridge (USA), 1280 m — 1937. (The longest span from 1937 until 1964.)
- Yangpu Bridge (China), 1280 m — 2007
[edit]Other suspended-deck suspension bridges
- Brooklyn Bridge (USA, 1883), The first steel-wire suspension bridge.
- Bear Mountain Bridge (USA, 1924), The longest suspension span (497 m)from 1924 to 1926. The first suspension bridge to have a concrete deck. The construction methods pioneered in building it would make possible several much larger projects to follow.
- John A. Roebling Suspension Bridge (USA, 1866), Then the longest wire suspension bridge in the world at 1,057 feet (322 m) main span.
- Mackinac Bridge (USA, 1957), The longest suspension bridge between anchorages in the Western hemisphere.
- Roebling's Delaware Aqueduct (USA, 1847) The oldest wire suspension bridge still in service in United States.
- Si Du River Bridge (China, 2009), The highest bridge in the world (at least 472 meters).
- San Francisco – Oakland Bay Bridge (USA, 1936), Until recently, this was the longest steel high-level bridge in the world (704 m).[7] The eastern portion (a cantilever bridge) is being replaced with aself-anchored suspension bridge which will be the longest of its type in the world.
- Union Bridge (England/Scotland, 1820), The longest span (137 m) from 1820 to 1826. The oldest in the world still in use today.
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