Suspension Bridges

A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders.

1.Components

Anchorage (blocks or tunnel type), towers, main cables, hangers, stiffened (box) girder and deck, substructure and foundation.

2.Form

 An arch upside down or the shape of a catenary.

3.Anchorage

3.1 The anchorage, constructed as a concrete block or by tunneling the ground, forms a giant weight to anchor the bridge and transmit the tension generated through the cables firmly to ground.

General Type Anchorage

3.2 The anchorage type is determined by the terrain where the anchorage points are constructed.

3.3 A general type anchorage that supports unidirectional forces transmitting through cables. (One block for one bridge).

3.4 An anchorage of a type that supports bi-directional forces transmitting through cables. (One block for two bridges)

    __located between two bridges and serving as two anchorage points.

    __cable strands crossing each other in the air before entering the anchorage.

Bi-directional forces transmitting type 

__The Kurushima Kaikyo Bridge use four anchorages for which three types of forms are employed.

3.5 .1 A tunnel type anchorage which allows minimum alterations to be made to the existing landform.

3.5.2 Cable anchor frames securing the cables to the tunnels.

4.Tower

Tower of a Suspension Bridge

4.1  The main tower functions to transmit forces through the cables and into the main tower foundation.The towers can be prefabricated in the plants or cast in situ.The blocks for the main tower were fabricated at the shop in blocks 6 m in length. The blocks are erected in the field using a climbing type of tower crane in building-block fashion to form the main tower.Towers of Kurushima Kaikyo Bridge: The blocks for the main tower were fabricated at the shop in blocks 6 m in length. The blocks are erected in the field using a climbing type of tower crane in building-block fashion to form the main tower.

5.Cables 

5.1 Cables usu. of high tensile steel wires support bridge girders and other loads, including vehicle loads, and transmit these dead and live loads into the anchorage points.

5.2 Wires, strands, and ropes

5.2.1Galvanized bridge wire for parallel wire bridge cables. Recommended diameter .196 inch.

5.2.2 Galvanized bridge strand--consists of several bridge wires, of various diameters
twisted together.

5.2.3Galvanized bridge rope--consists of six strands twisted around a strand core.

5.3 Types of Cables

5.3.1 Parallel wire cables: This type of cable is made up of a large number of individual wires parallel to one another. Neither the cables nor the wires are twisted in any manner. The wire is shipped to the site of the bridge on reels and the individual wires are installed or' "spun" on the bridge and later compacted together to form a round cross­section. Cables of this type are used on monumental structures, such as the Golden Gate Bridge and the George Washington Bridge.

5.3.2 Parallel Strand Cables, Closed Construction--These consist of several prefabricated Galvanized Bridge Strands, all laid parallel and in contact with one another. Wood or aluminum fillers are used to bring the cable to a circular cross-section, after which the whole cable is wrapped with wire for protection. The cable may contain 7, 19 37, 61, 91 or 127 strands.

5.3.3 Parallel Strand Cables, Open Construction--This type of cable consists of several prefabricated galvanized bridge Strands which are all laid parallel to one another and not in contact. The strands are usually arranged in the form of a rectangle and the cable may contain 2, 4, 6, 9, 12, 16, 20, 24 or 30 strands.

5.3.4 Parallel Rope Cables, Open Construction--These are the same as Parallel Strand Cables except that Galvanized Bridge Rope is used in place of Bridge Strand.

5.3.5 Single Rope or Single Strand Cables--These are used for small structures.

5.4 A single-stranded cable with a hexagonal in cross section is formed by tying together 127 high-tension galvanized steel wires each about 5 mm in diameter. (as used for Kurushima Kaikyo Bridge)

5.5 Erecting cables

5.6 Squeezing the cables

__After all the strands are laid, they are squeezed to form one single cable with a circular cross section.

__The strands are first tapped manually using a wooden maul to form a cable roughly circular and then squeezed using a hydraulic squeezing machine to form a circular cross section.

Multi Stranded Cable

5.7 Cable strength

5.7.1 Strength of cables
One single steel wire (about 5 mm in diameter) is strong enough to hoist three passenger cars (1.2 tons each), and one stranded cable (consisting of 127 steel strands) is strong enough to hoist six space shuttles (74 tons each).

5.7.2 Length of cables

The main cables for the Kurushima Kaikyo Bridges weigh 16,000 tons, and the strands are long enough to run round the earth two and one half times.

6.Stiffening Girder

6.1 The stiffening girder functions as a driveway for vehicles. The girder was designed with a cross section in the shape of a slim box to reduce vibrations in strong winds to a minimum.

6.2 Erecting box girder with barge.

Stiffening Box Girder

Stiffening girder sections, each 36m in length, are prefabricated in the plant and loaded on a self-propelled barge for transport to a site directly below each erection points. There they are lifted into position by a lifting beam and secured to hanger ropes.

7.Features of Suspension Bridges

7.1. Aesthetic, light, and strong

7.2 Span range: 2,000 to 7,000 feet -- far longer than any other kind of bridge

7.3 Most expensive to build

7.4 Complicated in force bearing and distribution.