Suspension bridge

This article is concerned with a particular type of suspension bridge, the suspended-deck type. For an index to the several types see suspension bridge types

Suspension bridge (suspended deck type)

An early bridge of this type, the Clifton Suspension Bridge
Ancestor	Simple suspension bridge
Related	None, but see also cable stayed bridge and compression arch suspended-deck bridge
Descendant	Self-anchored suspension bridge
Carries	Pedestrians, automobiles, trucks, light rail
Span range	Medium to long
Material	Steel rope, multiple steel wire strand cables or forged or cast chain links
Movable	No
Design effort	medium
Falsework required	No

A suspension bridge is a type of bridge that has been made since ancient times as early as 100 AD. Simple suspension bridges, for use by pedestrians and livestock, are still constructed, based upon the ancient Inca rope bridge. Suspended from two high locations over a river or canyon, simple suspension bridges follow a shallow downward arc and are not suited for modern roads and railroads. Advances in materials and design led to the development of the suspended-deck suspension bridge, a modern bridge capable of carrying vehicles and light rail. Instead of the deck following the downward arc of the main load-bearing cables (or chains), these cables are suspended between towers, and vertical suspender cables carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc slightly upward for additional clearance.

The design of the modern suspended-deck suspension bridge was developed in the early 19th century. Early examples include the Menai and Conwy Suspension Bridges (both opened in 1826) in north Wales, and the first Hammersmith Bridge (1827) in west London. This type of bridge is the only practical type suitable for very long spans or when it would be hazardous to maritime traffic to add central supports. One example frequently cited for its aesthetic appeal is the Golden Gate Bridge at the entrance to San Francisco Bay.

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 (An exception is the Royal Albert Bridge (1859) where the anchors are replaced by an arch between the columns.) The roadway is supported by vertical suspender cables or rods. 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. Without special design this type is generally not suited for heavy rail applications as the bridge will flex under the concentrated load of a locomotive.

Advantages over other bridge types

A suspension bridge can be made out of simple materials such as wood and common wire rope.

• The center span may be made long in proportion to the materials required, allowing the bridge to economically span a very wide canyon or waterway.
• It can be built high over water to allow the passage of tall ships.
• Neither temporary central supports nor access from beneath is required for construction, allowing it to span a rift or waterway.
• Being flexible it can flex under wind and seismic conditions, where a more rigid bridge would have to be made stronger and heavier.
• They cost less since they are not so bulky

Disadvantages compared with other bridge types

• Lacking stiffness the bridge may become unusable in strong wind conditions and so require closure to traffic.
• Being flexible in response to concentrated loads the structure is generally not used for heavy rail crossings, which concentrate the maximum "live" loading at the location of the locomotives.
• Under severe wind loading, the towers exert a large torque force in the ground, and thus require expensive foundation work when building on soft ground.

Structural analysis

The main forces in a suspension bridge are tension in the main 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, they can be made quite slender, as they have been in, for example, the Severn Bridge, near Bristol, England

The slender lines of the Severn Bridge

Assuming a negligible cable weight compared to the deck and vehicles being supported, a suspension bridge's main cables will form a parabola (very similar to a catenary, the form the unloaded cables take before the deck is added). This can be seen from the cable's constant gradient increase 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 analyse than a cable stayed design, where the deck is in compression.

Suspension types

The suspension in older bridges may be made from chain or linked bars, but modern bridge cables are made from multiple strands of wire. This is for greater redundancy; a few flawed strands in the hundreds used pose very little threat, whereas a single bad link or eyebar can eliminate the safety margin or bring down the structure. This was found to be the cause of the collapse of the Silver Bridge over the Ohio river.

Deck structure types

A plate deck suspension bridge over the Yangtze River in China

Most suspension bridges have open truss structures to support the roadbed (particularly owing to the unfavorable effects of using plate girders, discovered accidentally). Recent developments in bridge aerodynamics have allowed the re-introduction of plate structures. In the illustration to the right, 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 Tacoma Narrows Bridge.

Other applications

Cable-suspended footbridge at DFW Terminal D

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)

New Little Belt suspension bridge, 1970 Denmark

• 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 bridge. The foundation piers are then extended to above water level.
• Where the towers are founded on dry land, deep foundation excavation or pilings are used.
• From the tower foundation, towers of multiple columns are erected using concrete, stonework, or steel structures. At some elevation there must be a passage for the deck, with the columns extending high above this level.
• Smooth open cable paths called saddles are anchored atop the towers. These allow for slight movements of the cable as the loads change during construction. The top of these saddles may be closed with an additional part after completion of the bridge.
• Anchorages are constructed to resist the tension of the cables. 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.
• A temporary suspended walkway supported by wire rope follows the curve of the cables to be constructed, mathematically described as a catenary arc.
• Another set of wire ropes are suspended above the walkway and are used to support a traveler that has wheels riding atop these cables. There will be one set of wire ropes and a traveler for each cable to be "spun"
• Pulling cables attached to winches are capable of pulling the traveler from one anchorage to the other, traveling in arcs to the tops of the two towers.
• High strength wire, typically less than 10 mm in diameter, is pulled in a loop by pulleys on the traveler, with one end affixed at an anchorage. Workers stationed along the walkway attach the passing cable to a bundle with a temporary binding. When the traveler reaches the opposite anchorage the loop is placed over an open anchor eyebar.
• The traveler is returned to the start point to pick up another loop or it is used to carry a new loop from this side.
• As loops are placed, corrosion proofing may be applied.
• In this way a complete sub-cable is created linking the eyebar (or a set of eyebars) from one anchorage to the other. The sub-cables will have a hexagonal cross section and are held together with the temporary bindings.
• Multiple adjacent sub-cables are placed adjacent to each other. While these are on a hexagonal grid, the general form for the larger cable is circular.
• 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.
• Saddles to carry the suspender cables are clamped to the main cables, each with an appropriate shape to conform to the ultimate slope of the main cables. Each saddle is an equal horizontal distance from the next, with spacing appropriate to the design of the deck.
• Suspender cables engineered and cut to precise lengths and carrying swedged ends are looped over the saddles. In some bridges, where the towers are close to or on the shore, the suspender cables may be applied only to the central span.

Lions' Gate Bridge under construction

• 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 may be used to extend the deck one section at a time. 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.
• With completion of the primary structure various details such as lighting, handrails, finish painting and paving are added.

Details gallery

• 
  Demonstration section of main cable showing component wires
• 
  Main cable seat at top of tower
• 
  Suspender cables and saddle on main cable

The largest suspension bridges in the world

The Akashi-Kaikyo Bridge between Akashi and Awaji island, Japan

Main article: List of largest suspension bridges

When a suspension bridge is called "the largest", it typically means that the length of the main span is the longest. The record is currently held by the Akashi-Kaikyo Bridge in Japan. Ranking by the largest span does not mean that a bridge is longer from end to end or even more massive, but it is a reasonably good indicator of the overall engineering achievement. The following is a list of the 15 largest suspension bridges ranked by length of center span.

1. Akashi-Kaikyo Bridge (Japan) 1991 m - 1998
2. Great Belt Bridge (Denmark) 1624 m - 1998
3. Runyang Bridge (China) 1490 m - 2005
4. Humber Bridge (England) 1410 m - 1981 (The largest from 1981 until 1998.)
5. Jiangyin Suspension Bridge (China) 1385 m - 1997
6. Tsing Ma Bridge (Hong Kong, China) 1377 m - 1997 (with road and metro)
7. Verrazano Narrows Bridge (USA) 1298 m - 1964 (The largest from 1964 until 1981.)
8. Golden Gate Bridge (USA) 1280 m - 1937 (The largest from 1937 until 1964.)
9. Höga Kusten Bridge (Sweden) - 1210 m - 1997
10. Mackinac Bridge (USA) 1158 m - 1958
11. Minami Bisan-Seto Bridge (Great Seto Bridge) (Japan) 1118 m - 1988
12. Fatih Sultan Mehmet Bridge (Turkey) 1090 m - 1988
13. Bosphorus Bridge (Turkey) 1074 m - 1973
14. George Washington Bridge (USA) 1067 m - 1931 (The largest from 1931 until 1937.)

The Humber Bridge, UK, formerly the longest suspension bridge in the world

It is also possible to rank suspension bridges by the total length of suspension. Note that some of these bridges have more than two towers, but these are actually multiple bridges. Having more than two towers without a central anchorage could be unstable in some conditions. A modern exception to this is the Chacao Channel bridge, which is not being constructed. The design for this innovative bridge has two main spans, made possible by the use of a rigid central tower composed of two side-by-side A frames. The stiffness of these frames (as opposed to the flexibility of the usual spar tower) prevents transmission of significant dynamic forces between the mainspans, ensuring dynamic stability in various wind conditions.

1. Akashi-Kaikyo Bridge (Japan) 3909 m
2. Kurushima-Kaikyo Bridge (Japan) 3260 m (suspended sections are not all contiguous)
3. Great Seto Bridge (Japan) 3186 m (two bridges with common anchorage)
4. San Francisco-Oakland Bay Bridge (USA) 2822 m (two bridges with common central anchorage)
5. Great Belt Bridge (Denmark) 2719 m
6. Mackinac Bridge (USA) 2625 m

Tsing Ma Bridge, Hong Kong

The Strait of Messina Bridge, with a center span of 3300 m, was planned to connect Italy and Sicily but was cancelled shortly before construction was set to begin. Bridges have also been suggested for the Strait of Gibraltar and the Sunda Strait with longest spans of several kilometres. The suspension cables for these longest bridges are suspended from the ends of cable-stayed struts extending diagonally from tall pylons.

Other famous suspension bridges

Golden Gate Bridge, California, USA

Western portion of the San Francisco-Oakland Bay Bridge — two bridges with a common central anchorage

• Union Bridge (England/Scotland) 137 m - 1820. The largest suspension bridge from 1820 to 1826. The oldest in the world still in use today.
• Menai Suspension Bridge (north Wales) 176 m - 1826, The largest suspension bridge from 1826 until 1834.
• Zaehringen Bridge (Switzerland) 271 m - 1834. The largest suspension bridge from 1834 until 1849. The bridge was removed in the 1920s.
• Wheeling Suspension Bridge (USA) 308 m - 1849. The largest suspension bridge from 1849 until 1851 and from 1864 to 1866
• Lewiston-Queenston Bridge (USA and Canada) 316 m - 1851. The largest suspension bridge from 1851 until it was destroyed by wind in 1864.
• John A. Roebling Suspension Bridge (USA) 322 m - 1866. The largest suspension bridge from 1866 - 1869
• Niagara Clifton Bridge 384 m - 1869. The largest suspension bridge from 1869 to 1883. Replaced in 1899.
• Brooklyn Bridge (USA) 486 m - 1883. The largest suspension bridge from 1883 until 1903.
• Williamsburg Bridge (USA) 488 m - 1903. The largest suspension bridge from 1903 until 1924.
• Bear Mountain Bridge (USA) 497 m - 1924. The largest suspension bridge 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.
• Benjamin Franklin Bridge (USA) 533 m - 1926. The largest suspension bridge from 1926 until 1929.
• Ambassador Bridge (Michigan-Ontario, USA-Canada) 564 m - 1929. The largest suspension bridge from 1929 to 1931.
• Royal Gorge Bridge (USA) 1929 The highest (384 m) suspension bridge in the world.
• San Francisco-Oakland Bay Bridge (California, USA) 704 m -1936 The western portion is two complete two tower bridges end-to-end with a central anchorage, required to avoid dynamic interactions between three main spans between the four towers. Until recently, this was the longest steel high-level bridge in the world. [1] The eastern portion, currently being replaced, will be a self-anchored suspension bridge, the longest of its type in the world.
• Delaware Memorial Bridge (USA) 655 m - 1951 & 1968. Since the second bridge opened in 1968, it has been the world's largest twin suspension bridge.

Infamous suspension bridges

File:Ponte Hercilio Luz - Dezembro 1996 - by Sérgio Schmiegelow.jpg

Hercilio Luz Bridge, Florianópolis, Brazil

• The Bridge of San Luis Rey (Fictional)
• Silver Bridge, a 1928 eyebar chain bridge that collapsed in 1967, killing forty-eight people.
• Tacoma Narrows Bridge, (USA) 853 m - 1940 The Tacoma Narrows are prone to sustained and moderately strong winds, with which the bridge had a tendency to resonate (owing to its unique plate-girder deck structure). This led to its collapse only months after completion.

See also

• Category:Suspension bridges- for all the articles about specific suspension bridges.
• List of largest suspension bridges - ordered by the length of the main span.
• Cable-stayed bridge - again, superficially similar to a suspension bridge, but cables from the towers directly support the roadway, rather than the road being suspended from cables connecting two towers.
• Inca rope bridge - which have many features in common with a suspension bridge and predates them by at least three hundred years. However in a rope bridge the deck itself is suspended from the anchored piers and the guardrails are non-structural.
• Self-supporting suspension bridge - combining elements of a suspension bridge and a cable-stayed bridge.
• Simple suspension bridge - a modern implementation of the rope bridge using steel cables.

External links

• New Brunswick Canada Suspension FootBridges
• Structurae: Suspension Bridges
• American Society of Civil Engineers History and Heritage of Civil Engineering - Bridges
• Bridgemeister: Mostly Suspension Bridges