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Suspension bridge

A suspension bridge is a bridge that consists of two multiple column pillars, 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, and are anchored in the ground. The bridge usually also has 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), and has become widely used since.

The slender lines of the Severn Bridge, near Bristol, England

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.

Clifton Suspension Bridge over the River Avon in Bristol, England. The bridge was designed by the great Victorian engineer Isambard Kingdom Brunel. Although not visible here, the Clifton Gorge below is about 240 feet deep (73 metres)

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 Swarovski 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.

A successful synthesis from the finest and noble materials: the collectors cases, curio cabinets, display cabinets and display showcases can combine practical and art-full in a challenging finish.

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) 1,990 meters or 6,529 feet - 1998

Great Belt Bridge (Denmark) 1,624 meters or 5,328 feet - 1998

Humber Bridge (England) 1,410 meters or 4,624 feet - 1981

Jangyn Bridge (China, Yangtse River) 1,385 meters - 1997

Tsing Ma Bridge (Hong Kong) 1,377 meters - 1997

Verrazano Narrows Bridge (USA) 1,298 meters or 4,260 feet - 1964

Golden Gate Bridge (USA) 1,280 meters or 4,200 feet - 1937

Höga Kusten Bridge (Sweden) - 1,210 meters - 1997

Mackinac Bridge (USA) 1,158 meters or 3,800 feet - 1958

Minami Bisan-Seto Bridge (Japan) 1,118 meters or 3,668 feet - 1988

Second Bosporus Bridge (Turkey) 1,090 meters or 3,576 feet - 1988

First Bosporus Bridge (Turkey) 1,074 meters or 3,524 feet - 1973

George Washington Bridge (USA) 1,067 meters or 3,500 feet - 1931

Third Kurushima-Kaikyo Bridge (Japan) 1,030 meters - 1999

Second Kurushima-Kaikyo Bridge (Japan) 1,020 meters - 1999

Tagus Bridge (Portugal) 1,013 meters - 1966

Forth Road Bridge (Scotland) 1,006 metres - 1964)

A bridge with a center span of 3,290 m or 10,800 feet is proposed to connect Italy and Sicily across the Strait of Messina but construction has not yet begun. Hypothetical bridges have also been proposed for the Strait of Gibraltar and the Sunda Strait with longest spans of several kilometers. 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

Menai Suspension Bridge (north Wales, 1826)

Conwy Suspension Bridge (north Wales, 1826)

Wheeling Suspension Bridge (http://wheeling.weirton.lib.wv.us/landmark/bridges/susp/bridgdex.htm) (USA, 1849)

Clifton Suspension Bridge (England, 1864)

Brooklyn Bridge (USA, 1883)

Manhattan Bridge (USA, 1909)

Williamsburg Bridge (USA, 1903)

Royal Gorge Bridge (USA)

San Francisco-Oakland Bay Bridge (Western portion) (California, USA, 1936) This is two complete bridges end-to-end with a central anchorage.

Lions Gate Bridge (British Columbia, Canada, 1938)

Tacoma Narrows Bridge, (USA, 1940). The Tacoma Narrows are prone to sustained and moderately strong winds, with which the bridge had a tendency to resonate, leading to its collapse only months after completion. Replaced using the same towers but a different deck structure.

Regency Bridge, Texas, USA. A small rural one lane bridge seen weekly on the television show Texas Country Reporter.

Ambassador Bridge (Michigan-Ontario, USA-Canada, 1929)

External links

Structurae: Suspension Bridges (http://www.structurae.net/en/structures/stype/s1001.cfm)

Swarovski