 A simple suspension footbridge in theZillertal Alps | |
| Descendant | |
|---|---|
| Carries | Pedestrians,livestock |
| Span range | short to medium |
| Material | Rope (fiber),chain,steelwire rope; appropriate decking material |
| Movable | No |
| Design effort | low |
| Falsework required | No |
Asimple suspension bridge (alsorope bridge,swing bridge (inNew Zealand),suspended bridge,hanging bridge andcatenary bridge) is a primitive type ofbridge in which the deck of the bridge lies on two parallel load-bearing cables that are anchored at either end. They have no towers or piers. The cables follow a shallow downwardcatenary arc which moves in response to dynamic loads on the bridge deck.
The arc of the deck and its large movement under load make such bridges unsuitable for vehicular traffic. Simple suspension bridges are restricted in their use to foot traffic. For safety, they are built with stout handrail cables, supported on short piers at each end, and running parallel to the load-bearing cables. Sometime these may be the primary load-bearing element, with the deck suspended below. Simple suspension bridges are considered the most efficient andsustainable design in rural regions, especially for river crossings that lie in non-floodplain topography such as gorges.
In some contexts the term "simple suspension bridge" refers not to this type of bridge but rather to asuspended-deck bridge that is "simple" in that its deck is not stiffened.[1][2] Although simple suspension bridges and "simple" suspended deck bridges are similar in many respects, they differ in their physics. On a simple suspension bridge, the main cables (or chains) follow ahyperbolic curve, thecatenary. This is because the main cables are free hanging. In contrast, on a suspended deck bridge (whether "simple" or not) the main cables follow aparabolic curve. This is because the main cables are tied at uniform intervals to the bridge deck below (seesuspension bridge curve).
The differences between these two curves were a question of importance in the 17th century, worked on byIsaac Newton.[3] The solution was found in 1691, byGottfried Leibniz,Christiaan Huygens, andJohann Bernoulli who derived theequation in response to a challenge byJakob Bernoulli.[4] Their solutions were published in theActa Eruditorum for June 1691.[5][6]
Astressed ribbon bridge also has one or more catenary curves and a deck laid on the main cables. Unlike a simple suspension bridge however, a stressed ribbon bridge has a stiff deck, usually due to the addition of compression elements (concrete slabs) laid over the main cables. This stiffness allows the bridge to be much heavier, wider, and more stable.
The simple suspension bridge is the oldest known type ofsuspension bridge and, ignoring the possibility ofpre-Columbian trans-oceanic contact, there were at least twoindependent inventions of the simple suspension bridge, in the widerHimalaya region andSouth America.[7]
The earliest reference to suspension bridges appear inHan dynasty records on the travels of Chinese diplomatic missions to the countries on the western and southern fringe of theHimalaya, namely theHindukush range inAfghanistan, and the lands ofGandhara andGilgit.[8] These were simple suspension bridges of three or morecables made fromvines, where people walked directly on the ropes to cross. Later, they also used decking made from planks resting on two cables.[8]
InSouth America,Inca rope bridges predate the arrival of the Spanish in theAndes in the 16th century. The oldest known suspension bridge, reported from ruins,[clarification needed] dates from the 7th century inCentral America (seeMaya Bridge at Yaxchilan).
Simple suspension bridges usingironchains are also documented inTibet andChina. Onebridge on the upper Yangtze dates back to the 7th century. Several are attributed to Tibetan monkThang Tong Gyalpo, who reportedly built several in Tibet andBhutan in the 15th century, includingChushul Chakzam and one atChuka.[7] Another example, theLuding Bridge, dates from 1703, spanning 100 m using 11 iron chains.[7]
Development of wire cablesuspension bridges dates to the temporary simple suspension bridge atAnnonay built byMarc Seguin and his brothers in 1822. It spanned only 18 m.[7] However, simple suspension bridge designs were made largely obsolete by the 19th century invention andpatent of thesuspended deck bridge byJames Finley.[9] A late 18th centuryEnglish painting of a bridge inSrinagar[citation needed], then part of theGarhwal Kingdom, anticipates the invention of the suspended deck bridge. This unusual bridge, built on a floodplain, had suspended deck ramps used to access a simple suspension bridge supported from towers.
This type of bridge is known as a rope bridge due to its historical construction fromrope.Inca rope bridges still are formed from native materials, chiefly rope, in some areas of South America. These rope bridges must be renewed periodically owing to the limited lifetime of the materials, and rope components are made by families as contributions to a community endeavor.
Simple suspension bridges, for use bypedestrians and livestock, are still constructed, based on the ancientInca rope bridge but using wire rope and sometimessteel oraluminium grid decking, rather than wood.
In modern bridges, materials used instead of (fiber) rope includewire rope,chain, and special-purpose articulated steel beams.
In the northeast Indian state ofMeghalaya,Khasi andJaintia[10] tribal people have createdliving root bridges, which are a form oftree shaping. Here, simple suspension bridges are made by training the roots of theFicus elastica species of banyan tree across watercourses.[11] There are examples with a span of over 170 feet (52 m).[12] They are naturally self-renewing and self-strengthening as the component roots grow thicker and some are thought to be more than 500 years old.[13][14][15]
In theIya Valley of Japan, bridges have been constructed usingwisteria vines. To build such a bridge, these vines were planted on opposite sides of a river and woven together when they grew long enough to span the gap. The addition of planks produced a serviceable bridge.[16][17]
The very lightest bridges of this type consist of a single footrope and nothing more. These aretightropes andslacklines, and require skill to use. More commonly, the footrope is accompanied by one or two handrail ropes, connected at intervals by vertical side ropes. This style is used by mountaineers and is employed extensively in New Zealand on lesser backcountry walking tracks where examples are referred to as 'three wire bridges'. A slightly heavier variation has two ropes supporting a deck, and two handrail ropes. Handrails are necessary because these bridges are prone to oscillate side to side and end to end. Rarely, the footrope (or footrope plus handrails) is combined with an overhead rope similar to azip-line orcableway.
In some cases, such as theCapilano Suspension Bridge, the primary supports form the handrails with the deck suspended below them. This makes for more motion side-to-side in the deck than when the primary supports are at deck level, but less motion in the handrails.
Disadvantages connected with simple suspension bridges are very great. The location of the deck is limited, massive anchorages and piers generally are required, and loading produces transient deformation of the deck.[18] Solutions to these problems led to a wide variety of methods of stiffening the deck,[18][19] resulting in several other types of suspension bridge. These include astressed ribbon bridge, which is closely related to a simple suspension bridge but has a stiffened deck suitable for vehicle traffic.
A very light bridge, constructed with cables under high tension, may approach asuspended deck bridge in the nearly horizontal grade of its deck.
The bridge may be stiffened by the addition of cables that do not bear the primary structural or live loads and so may be relatively light. These also add stability in wind. An example is the 220-meter-long (720 ft) bridge across the riverDrac atLac de Monteynard-Avignonet: this bridge has stabilizing cables below and to the side of the deck.
To reduce twisting motion in response to users a bridge may employ vertical drop cables from each side at the center of the bridge, anchored to the ground below.
The lightest of these bridges, without decking, are suitable for use only by pedestrians. Light bridges with decking, and sufficient tension that crossing the bridge does not approach climbing, may be used also bypack horses (and other animals), equestrians, and bicycle riders. To walk a lighter bridge of this type at a reasonable pace requires a particular gliding step, as the more normal walking step will induce traveling waves that can cause the traveler to pitch (uncomfortably) up and down or side-to-side. The exception is a stabilized bridge, which may be quite stable.
Simple suspension bridges have applications inoutdoor recreation. They are a popular choice for tree-top trails[20] and, where the terrain is suitable, for stream crossings.[21] They may be designed without stabilizing so that the free movement of the bridge provides a more interesting experience for the user.[21]
InFrench, a rudimentary simple suspension bridge is known by one of three names, depending on its form:pont himalayen ("Himalayan bridge": a single footrope and handrails on both sides, usually without a deck);pont de singe ("monkey bridge: a footrope with overhead rope); andtyrolienne ("Tyrolean": a zip-line).[22] Zip-lines can be traversed by hanging below, or walked (by individuals with exceptional balance). A more developed version of thepont himalayen, provided with a deck between a pair of main cables, is known as apasserellehimalayenne (French, "Himalayan footbridge").[23] Examples of this type include two bridges atLac de Monteynard-Avignonet in theFrench Alps; these bridges are exceptionally long, for bridges of this type.
A simple rope bridge used to cross a river in India is pictured by W. Purser with a poetical illustration byLetitia Elizabeth Landon, as
Crossing the River Tonse by a Jhoola. in Fisher's Drawing Room Scrap Book, 1839.[24]
Notable simple suspension bridges include:
| Name | Span length | Year built |
|---|---|---|
| Capilano Suspension Bridge | 140 metres (460 ft) | 1889 |
| Arroyo Cangrejillo Pipeline Bridge | 337 metres (1,106 ft) | 1998[25] |
| Lac de Monteynard-Avignonet Drac bridge | 220 metres (720 ft) | 2007 |
| Carrick-a-Rede Rope Bridge | 20 metres (66 ft) | rebuilt 2008 |
| Ponte tibetanoCesana-Claviere | 478 metres (1,568 ft) | 2006[26] |
| Ponte nel Cielo | 234 metres (768 ft) | 2018[27] |
| Charles Kuonen Suspension Bridge | 494 metres (1,621 ft) | 2017[28] |
| Gandaki Golden Footbridge | 567 metres (1,860 ft) | 2020[29] |
| Arouca 516 | 516 metres (1,693 ft) | 2021[30] |
| Ponte tibetano diCastelsaraceno | 586 metres (1,923 ft) | 2021[31] |
| Sky Bridge 721 | 721 metres (2,365 ft) | 2022 |
| Bridge of National Unity | 723 metres (2,372 ft) | 2024 |
| Ponte tibetano diSellano | 517.5 metres (1,698 ft)[32] | 2024[33] |
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