Bridge K-12 Projects and Experiments
Suspension Bridge
A suspension bridge is a type of bridge where the main load-bearing elements are hung from suspension cables.
While modern suspension bridges with level decks date from the early
19th century, earlier types are reported from the 3rd century BC. Simple suspension bridges, for use by pedestrians and livestock, are still constructed, based upon the ancient Inca rope bridge.
Suspended well 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 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.
History
The suspension bridge is one of the oldest types of bridge. Early
simple, or catenary, suspension bridges consisted of three or more cables made from vines,
where people walked directly on the ropes to cross. Simple suspension
bridges with decking made from planks resting on two cables date back
at least to 285BC[1] in China, and other bridges of similar type are recorded in Tibet. Seven bridges of this type were reportedly built in China in 95BC at Chengdu by Li Bing.[1] A Chinese multi-span simple suspension bridge with bamboo cables is reported at Quan-Xian, documented from 960AD and possibly dating back to the 3rd century BC.[1]
Simple suspension bridges using iron chains are also documented in China and the Himalayas, although their earliest date is unclear. One example, the Luding Bridge, dates from 1703, spanning 100m using eleven iron chains.[1] Several are attributed to Tibetan monk Thang-stong rGyal-po, who reportedly built several in Tibet and Bhutan in the 15th century, including one at Chuka.[1]
Claims that more modern suspension bridges with a horizontal deck also
originated in Tibet or China remain largely unsubstantiated.
The first design for a bridge resembling the modern suspension bridge in the West is attributed to Faust Verantius, 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 Pennsylvania,
in 1801. This was widely publicised from 1810 onwards, beginning a
period of rapid development of the modern suspension bridge.
Early British chain bridges included the Dryburgh Abbey Bridge (1817) and 137m Union Bridge (1820), with spans rapidly increasing to 176m with the Menai Suspension Bridge (1826).
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 18m.[1] The first permanent wire cable suspension bridge was Guillaume Henri Dufour's Saint Antoine Bridge in Geneva of 1823, with two 40m spans.[1] The first with cables assembled in mid-air in the modern method was Joseph Chaley's Grand Pont Suspendu in Fribourg, in 1834.[1]
Structural behaviour
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 analyze than a cable stayed design, 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
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 seismic movements than 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 types 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[1]; James Smith's Micklewood Bridge [2]; and a proposal by Robert Stevenson for a bridge over the River Almond near Edinburgh[2].
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 is for 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. This 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 parallel-strand cables can be
largely prepared in mid-air.
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 accidentally from the Tacoma Narrows bridge
collapse). 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.
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)
- 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 single or 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.
- 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
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Main cable seat at top of tower
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Suspender cables and saddle on main cable
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The largest suspension bridges in the world
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The size of a suspension bridge typically refers to the length of the main span.
- Akashi-Kaikyo Bridge (Japan) 1991 m — 1998
- Great Belt Bridge (Denmark) 1624 m — 1998
- Runyang Bridge (China) 1490 m — 2005
- Humber Bridge (England, United Kingdom) 1410 m — 1981
- Jiangyin Suspension Bridge (China) 1385 m — 1997
- Tsing Ma Bridge (Hong Kong, China) 1377 m — 1997 (largest with both road and rail)
- Verrazano Narrows Bridge (USA) 1298 m — 1964
- Golden Gate Bridge (USA) 1280 m — 1937
- Yangluo Bridge (China) 1,280 m — 2007
- Höga Kusten Bridge (Sweden) 1210 m — 1997
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 ties extending diagonally from tall pylons, also
called towers.
Other famous suspension bridges
- 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.
- Zähringen 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.[3] The eastern portion (a cantilever bridge) is currently being replaced with a self-anchored suspension bridge which will be the longest of its type in the world.
- Tacoma Narrows Bridge (USA) 853 m - 1950 & 2007. The largest twin suspension bridge in the world.
Infamous suspension bridges
- 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 and ultimately its nickname "The Galloping
Gertie"). This led to its collapse only months after completion. The
collapse was captured on film.
Picture Gallery
Akashi-Kaikyo Bridge at night
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25 de Abril Bridge in Lisbon, Portugal
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A suspension bridge falls: Tacoma Narrows Bridge collapses
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The Ambassador Bridge- Longest suspension bridge from 1929-1931
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New York's famous Brooklyn Bridge
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San Francisco-Oakland Bay Bridge under construction
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Driving on the 2nd largest suspension bridge, Denmark's Great Belt Bridge (Storebæltsbroen).
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Busan, the Republic of Korea's Gwangan Grand Bridge, with a
suspension section of around 500 meters but with an overall length of
7,420 meters
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The famous Golden Gate Bridge in San Francisco
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References
- ^ a b c d e f g h i Peters, Tom F., "Transitions in Engineering: Guillaume Henri Dufour and the Early 19th Century Cable Suspension Bridges", Birkhauser, 1987, ISBN 3764319291
- ^ a b Drewry, Charles Stewart, "Memoir on Suspension Bridges", Longman, Rees, Orme, Brown, Green & Longman, London, 1832
- ^ McGloin, Bernard. Symphonies in Steel: Bay Bridge and the Golden Gate. Virtual Museum of the City of San Francisco. Retrieved on 2008-01-12.
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
- superficially similar to a suspension bridge, but cables from the
towers directly support the roadway, rather than the road being
suspended indirect by additional cables from the main 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,
although either the upper guardrail or lower footboard cables may be
the main structural cables.
External links
Wikimedia Commons has media related to:
Suspension Bridge Types
There are several types of suspension bridge:
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Suspension Bridge"
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