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A suspension bridge is a bridge that consists of two multiple column pillars (or pylons), one on either end of the central span, with two or more cables slung between them. The bridge deck is suspended from vertical cables or rods attached to the main cables. The main cables continue beyond the pillars to deck–level supports, and further continue to connections with anchors in the ground. The bridge will usually have two smaller spans, running between either pair of pillars and the land, which may be supported by suspender cables or not, — in the latter case there will be very little arc in the outboard main cables.
The design 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. Since then, suspension bridges have been built all over the world.
Advantages over other bridge types
- The center span may be made very long in proportion to the amount of 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 very tall ships.
- Neither temporary central supports nor access from beneath is required for construction, allowing it to span a deep rift or busy or turbulent waterway.
- Being relatively flexible it can flex under severe wind and seismic conditions, where a more rigid bridge would have to be made much stronger and so also heavier.
Disadvantages over other bridge types
- Lacking stiffness the bridge may become unusuable in turbulent and strong wind conditions and so require temporary 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.
- Towers exert a large torque force in the ground, and thus require very 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.
Assuming a fairly 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 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.
Cable types
Cables in older bridges may be made from chain or linked bars, but modern bridge cables are made from multiple strands of wire.
Construction sequence
- Where the towers are founded on underwater piers, caissons are sunk and any soft bottom is excavated for a foundation. If bedrock cannot be reached, pilings are driven to bedrock or hard soil, or a large concrete pad may be constructed. The foundation piers are then extended to above water level.
- Where the towers are founded on dry land, deep foundations 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 will have multiple protruding open eyebolts.
- 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 1cm in diameter, 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 anchor eyebolt.
- 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 eye-bolt (or a set of eye bolts) from one anchorage to the other. The sub-cables will have a hexagonal cross section and are held together with 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 apropriate shape to conform to the 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 my be used to extend the deck one section at a time. During the construction the finished portions of the deck will appear to 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.
The largest suspension bridges in the world
(by length of centre span)
- Akashi-Kaikyo Bridge (Japan) 1,991 metres - 1998
- Great Belt Bridge (Denmark) 1,624 metres - 1998
- Humber Bridge (England) 1,410 metres - 1981 (The largest from 1981 until 1998.)
- Jangyn Bridge (China, Yangtse River) 1,385 metres - 1997
- Tsing Ma Bridge (Hong Kong) 1,377 metres - 1997 (with road and metro)
- Verrazano Narrows Bridge (USA) 1,298 metres - 1964 (The largest from 1964 until 1981.)
- Golden Gate Bridge (USA) 1,280 metres - 1937 (The largest from 1937 until 1964.)
- Höga Kusten Bridge (Sweden) - 1,210 metres - 1997
- Mackinac Bridge (USA) 1,158 metres - 1958
- Minami Bisan-Seto Bridge (Japan) 1,118 metres - 1988
- Second Bosporus Bridge (Turkey) 1,090 metres - 1988
- First Bosporus Bridge (Turkey) 1,074 metres - 1973
- George Washington Bridge (USA) 1,067 metres - 1931 (The largest from 1931 until 1937.)
- Third Kurushima-Kaikyo Bridge (Japan) 1,030 metres - 1999
- Second Kurushima-Kaikyo Bridge (Japan) 1,020 metres - 1999
- Tagus Bridge (Portugal) 1,013 metres - 1966
- Forth Road Bridge (Scotland) 1,006 metres - 1964)
The Strait of Messina Bridge, with a center span of 3,300 m, is planned to connect Italy and Sicily but construction has not yet begun. 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
See also
- 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.
- cable-stayed bridge
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