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CHAPTER 2BRIDGE COMPONENTS

2.1 GENERAL

The cable stayed bridges are composed of three basic components:

Figure 2.1: A typical Cable stayed bridge

The cables extending from one or more towers of the cable-stayed bridge

support the superstructure at many points along the span. The cable system is

ideal for spanning natural barriers of wide rivers, deep valleys, or ravines, and

for vehicular and pedestrian bridges crossing wide interstate highways

because there are no piers that will form obstructions. For the most part,

cable-stayed bridges have been built across navigable rivers where navigation

requirements have dictated the dimensions of the spans and clearance above

the main water levels.

Cable Stayed BridgeComponents

Cable Stays Pylons/Towers Deck/Superstructure

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The most successful span arrangements are of three basic types: they may be

categorized as:

oTwo spans: symmetrical or asymmetrical,

oThree spans: symmetrical or asymmetrical,

oMultiple spans.

The versatile cable-stayed bridge concept lends itself to a large variety of

geometrical configurations. The arrangement of the cables, type of

superstructure, and style of the towers can be easily adjusted to suit, the

numerous requirements of site conditions and aesthetics for highway and

pedestrian bridges.

2.2 TRANSVERSE CABLE ARRANGEMENT

According to the arrangement of cables in transverse direction, cable stayed bridges

can be categorized as:

1. Single Plane System,

2. Double Plane System.

In transverse direction, cables may be symmetrically or asymmetrically placed, and

may lie in oblique or vertical plane.

Figure 2.2: Transverse cable arrangement: (a) Single plane – vertical, (b) Single plane –vertical/lateral, (c) Double lane – vertical, (d) double plane – oblique.

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2.2.1 Single Plane System

The single plane cable arrangement is generally used with roadway deck with cables

passing through the median strip and anchored below the roadway. This arrangement

is not only economical but aesthetically pleasing also.

A possible disadvantage of the single plane cable system is the fact that relatively

high concentrated cable force is transferred to the main girder, thereby requiring a

larger connection and girder to support the cable force. Additional reinforcement and

stiffening of the deck web plates, and bottom flange will normally be required in order

to distribute the concentrated load uniformly throughout the cross-section of the

superstructure members.

In a single-plane cable arrangement, the cables support vertical or gravity loads

only. The torsional forces that develop because of the asymmetrical vehicular

loading and/or wind forces must be resisted by a torsionally stiff box girder in

order to transmit the unbalanced forces to the piers.

Although the single-plane cable system has been used symmetrically with

respect to the longitudinal centerline, on vehicular bridges, it has been

constructed off-center or asymmetrically for pedestrian bridges. In the

asymmetrical applications, the plane of the cables is at the edge of the walkway.

Because the walkway loadings are small, the unbalanced system produces only

small torsional forces that are easily resisted by the walkway structure.

2.2.2 Double Plane System

The two principal double planar cable systems are: one system consisting of a

vertical plane located at each edge of the superstructure and another system in

which the cable planes are oblique, sloping toward each other the edges of the

roadway and intersecting at the towers along the longitudinal centerline of the

deck. The tower in the oblique double plane arrangement is generally of the A-

frame type in order to receive the sloping cables that intersect along the

centerline of the roadway.

Using the two-plane cable system, the anchorages may be located either on the

outside of the deck structure or within the limits of the deck roadway. With the

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cable anchorages on the outside of the deck, an advantage is gained, because

no portion of the deck roadway is required for the connection fittings. A

disadvantage is the fact that additional reinforcement may be required to transmit

the eccentric cable loadings of shear and moment into the main girders of the

superstructure.

For those applications in which the cable anchorage lies within the limits of the

bridge deck, the overall width of the deck must be increased for the full length

of the bridge in order to provide room for the anchorage fittings. This additional

width of roadway deck usually results in an increased cost for the

superstructure.

2.3 LONGITUDINAL CABLE ARRANGEMENT

There are four basic cable configurations for cable stayed bridges as:

Ø The radiating/fan type system is an arrangement wherein the cables

intersect at a common point at the top of the tower.

Ø The harp type, as the name implies, resembles harp strings – the cables

are parallel and equidistant from each other. The required number of cables

are spaced uniformly along the tower height and, as a result, also

connected to the roadway superstructure with equal spacing.

Ø The semi-harp type is a combination of the radiating and harp types. The

cables are connected at equal spacing to the top of pylon and also along

the superstructure. Because of the small spacings concentrated near the

top of the pylon, the cables are not parallel.

LongitudinalCable

Arrangement

Radiating/Fan Harp StarSemi-harp

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Ø In the star arrangement, the cables intersect the pylon at different heights

and then converge on each side of the pylon to intersect the roadway at a

common point. This pattern is only used on Norderelbe Bridge, Humburg

for its unique aesthetics.

Figure 2.3: Longitudinal Cable arrangement

Using the four basic longitudinal cable configurations, a great variety of

combinations are possible, such as: the Great Belt Bridge has a hybrid system

combining fan and star arrangement.

The selection of cable layout and number of cables is dependant on the length

of span, type of loading, number of roadway lanes, height of towers, economy,

and the designer’s individual sense of proportion and aesthetics.

2.3.1 Influence of Number of Cable Stays:

Some bridges have relatively few cable-stays while others may have many

stays intersecting the deck such that the cables provide a continuous elastic

supporting system.

When only a few cables support the deck structure, cable forces would be

large, which requires massive and complicated anchorage systems connecting

to the pylon and superstructure. The connections become source of heavy

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concentrated loads requiring additional reinforcement of webs, flanges, and

stiffeners to transfer the loads to the bridge girders and distribute them

uniformly throughout the deck system. A large number of cables simplifies the

cable anchorages to the bridge girders and distributes the forces more

uniformly throughout the deck structure without major reinforcements to the

girders and floor beams. Therefore a large number of cables can provide

continuous support, thus permitting the use of a shallow depth girder.

2.3.2 Influence of Cable layout:

Cable layout significantly influences the axial compression and moments in

deck as well as pylon.

In fan type of arrangement, the cables converge at the top of the pylon with the

cables having the maximum angle of inclination to the bridge girders. As the

cables in fan layout are in an optimum position to support the gravity dead and

live loads and simultaneously produce a smaller compressive force in deck

than that in case of other layouts.

The harp system with cables connections distributed throughout the height of

pylon results in an efficient pylon design compared to the fan system, which

has all the cables attached to the top of pylon. For fan arrangement, the

concentrated load at the top of the pylon produces large shears and moments

along the entire height of the pylon, thus increasing the cost. Also it adds

difficulties in anchoring the cables to the pylon.

The semi-harp arrangement represents a compromise between the extremes

of the harp and fan systems and is specially useful when it becomes difficult to

accommodate all the cables at the top of the pylon.

2.4 STAY CABLE TYPES

The stay cables of the cable stayed bridges fall into the following categories:

1) Parallel-bar cables

2) Parallel-wire cables

3) Stranded cables

4) Locked-coil cables

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(a) (b) (c) (d)

The choice of one of these types depends on the mechanical properties required

(modulus of elasticity, ultimate tensile strength, durability etc.) as well as on

structural and economical criteria.

2.5 PYLON

The pylons are of many shapes and varieties to accommodate different cable

arrangements, bridge site conditions, design requirements, aesthetics and

economy, as follows:

o Single cantilever to support a single plane arrangement,

o Two cantilever towers to support the double plane cable system

o Portal frame

o A-frame

o Modified A-frame

o Diamond shaped

o Modified diamond

2.5.1 Choice of Bearing at the Base of Pylon

The decision to use a fixed or hinged base for the tower either to the pier or the

superstructure must be based on knowledge of the magnitude and relationship

of the vertical and horizontal forces acting on the tower. A fixed base induces

large bending moments at the base of the tower, whereas a hinged base does

not and may be preferred. However, the increased rigidity of the total structure

resulting from the fixed base of the towers may offset the disadvantage of the

Figure 2.4: Stay cable types: (a) Parallel-bar cables, (b) Parallel-wire cables, (c)

Stranded cables, (d) Locked-coil cables.

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large bending is that a fixed base may be more practical to erect and may be

less costly than inserting a heavy pinned bearing, which requires the tower to

be externally supported until the cables are connected. The design engineer

and contractor should discuss these considerations early in the design stage of

the project in order to arrive at the most economical solution.

2.5.2 Height of Pylon

The height of the tower is determined from several considerations, such as the

relation of tower height to span length, the type of cable arrangement, and the

general aesthetic proportions of all the spans and towers visualized as an

entity.

2.5.3 Influence of the Pylon Inertia

With the increase of pylon inertia, Maximum bending moment in pylon

decreases whereas the effect is opposite in deck. So, this effect is favorable

although the designer should not go for very stiff pylon – instead it is better to

make fuller use of the stays.

2.6 DECK

The deck, used synonymously with superstructure, for cable-stayed bridges

takes as many forms as there are structural systems. Basically, however, two

types of deck girders have been used most frequently: the stiffening truss and

the solid web types. Past experience with the two systems indicates that the

stiffening truss type is seldom used in current designs. The stiffening trusses

require more fabrication, are relatively more difficult to maintain, arc more

susceptible to corrosion.

An increase in tensional rigidity is achieved by using box type cross sections,

[Fig. 2.5c and d]. They may range from the single cell or multicell box with

rectangular sides to a similar trapezoidal type with sloping sides. In each of

these types the roadway width extends beyond the edges of the single boxed

girders.

When the roadways require a large number of traffic lanes, the transverse

width requires several box-girder systems to support the deck structure, [Fig.

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2.5e and f]. Twin boxes, either of the rectangular or trapezoidal shapes, have

been used to advantage when large deck widths are required.

2.6.1 Influence of Deck Inertia

The maximum moments in the deck increase considerably with the increase of

deck inertia. So, high inertia of deck system is not favorable as it attracts

considerable bending moments without appreciably reducing the forces in

pylons and cables. However, designer should take optimum deck inertia taking

into account the aerodynamic effect.

Figure 2.5: Girder types: (a) twin I-girder, (b) multiple I-girder, (c) rectangular box girder, (d) trapezoidalbox girder, (e) twin rectangular box girder, (f) twin trapezoidal box girder.