Cable stay bridge design

Cable stayed bridges

D. J. Farquhar Mott Macdonald

Following a brief history of cable stayed bridges this chapter describes the various

materials and forms of construction that have been adopted for the major structural

components of these bridges, focussing in turn on the cable system, the pylon and

the deck. By the use of examples the most appropriate use of these materials and

component forms is discussed. A step-by-step approach is given for the preliminary

design of the cable stayed bridge from outline proportions of the structure to the

static and dynamic analysis including requirements for erection calculations and wind

loading on stays. The dynamic behaviour for the cable stayed bridge includes the

phenomenon of stay oscillation, which is reviewed in detail including discussion of the

various types of dynamic cable response together with the available preventative

measures.

Introduction

The use of inclined stays as a tension support to a bridge

deck was a well-known concept in the nineteenth century

and there are many examples, particularly using the

inclined stay as added stiffness to the primary draped

cables of the suspension bridge. Unfortunately, at this

time, the concept was not well understood. As it was not

possible to tension the stays they would become slack

under various load conditions. The structures often had

inadequate resistance to wind-induced oscillations. There

were several notable collapses of such bridges, for example

the bridge over the Tweed River at Dryburgh (Drewry,

1832), built in 1817, collapsed in 1818 during a gale only six

months after construction was completed. As a result the use

of the stay concept was abandoned in England.

Nevertheless, these ideas were adapted and improved by

the American bridge engineer Roebling who used cable

stays in conjunction with the draped suspension cable for

the design of his bridges. The best known of Roebling’s

bridges is the Brooklyn Bridge, completed in 1883.

The modern concept of the cable-stayed bridge was first

proposed in postwar Germany, in the early 1950s, for the

reconstruction of a number of bridges over the River

Rhine. These bridges proved more economic, for moderate

spans, than either the suspension or arch bridge forms. It

proved very difficult and expensive in the prevailing soil

conditions of an alluvial floodplain to provide the gravity

anchorages required for the cables of suspension bridges.

Similarly for the arch structure, whether designed with

the arch thrust carried at foundation level or carried as a

tied arch, substantial foundations were required to carry

these large heavy spans. By comparison the cable-stayed

alternatives had light decks and the tensile cable forces

were part of a closed force system which balanced these

forces with the compression within the deck and pylon.

Thus expensive external gravity anchorages were not

required. The construction of the modern multi-stay

cable-stayed bridge can be seen as an extension, for larger

spans, of the prestressed concrete, balanced cantilever

form of construction. The tension cables in the cable-stayed

bridge are located outside the deck section, and the girder is

no longer required to be of variable depth. However, the

principle of the balanced cantilever modular erection

sequence, where each deck unit is a constant length and

erected with the supporting stays in each erection cycle, is

retained.

The first modern cable-stayed bridge was the Stroms￾mund Bridge (Wenk, 1954) in Sweden constructed by the

firm Demag, with the assistance of the German engineer

Dischinger, in 1955. At the same time Leonhardt designed

the Theodor Heuss Bridge (Beyer and Tussing, 1955)

across the Rhine at Dusseldorf but this bridge, also

known as the North Bridge, was not constructed until

1958. The first modern cable-stayed bridge constructed in

the United Kingdom was the George Street Bridge over

the Usk River (Brown, 1966) at Newport, South Wales

which was constructed in 1964. These structures were

designed with twin vertical stay planes. The first structure

with twin inclined planes connected from the edge of the

deck to an A-frame pylon was the Severins Bridge (Fischer,

1960) crossing the River Rhine at Cologne, Germany. This

bridge was also the first bridge designed as an asymmetrical

two-span structure.

The economic advantages described above are valid to

this day and have established the cable-stayed bridge in

its unique position as the preferred bridge concept for

major crossings within a wide range of spans. The long￾est-span cable-stayed bridge so far completed is the

Tatara Bridge in Japan with a main span of 890 m. At the

time of writing (2008) several other bridges are planned,

or are in construction, with main spans in excess of

1000 m, notably the Sutong Bridge (1088 m) and the

Stonecutters Bridge (1018 m), which are both in China.

ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers www.icemanuals.com 357

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doi: 10.1680/mobe.34525.0357

CONTENTS

Introduction 357

Stay cable arrangement 358

Stay oscillations 364

Pylons 367

Deck 372

Preliminary design 376

References 380

Further reading 381

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Stay cable arrangement

Two basic arrangements have been developed for the layout

of the stay cables:

1 the fan stay system (including the modified fan stay

system)

2 the harp stay system.

These alternative stay cable arrangements are illustrated in

Figure 1.

Fan cable system

The fan system was adopted for several of the early designs

of the modern cable-stay bridge, including the Stromsmund

Bridge (Wenk, 1954). The method of supporting the stays

on top of the pylon was taken from suspension bridge tech￾nology where the cable is laid within a pylon top deviator

saddle. The floor of the saddle is machined to a radius so

that each cable stay anchored in the main span can pass

over the pylon and be anchored directly within the back or

anchor span. This arrangement is structurally efficient with

all the stays located at their maximum eccentricity from

the deck and a minimum moment is applied to the pylon.

The fan arrangement initially proved suitable for the

moderate spans of the early cable-stay designs, with a small

number of stay cables or bundled cables supporting the

deck. There were, however, obvious difficulties, with the

corrosion protection of cables at the pylon head, their sus￾ceptibility to fretting fatigue arising from bending and hori￾zontal shear stresses within the cable bundle and with the

replacement of any individual stay in the event of damage.

In addition, when this arrangement was adopted for larger

spans the size of the limited number of cables increased,

eventually becoming uneconomically large and difficult to

accommodate within the fan configuration. The anchorages

were also heavy and more complicated and the deck needed

to be further strengthened at the termination point.

Therefore when a greater number of stays were required

the modified fan layout was introduced whereby the stays

are individually anchored near the top of the pylon. This

is now the more commonly adopted system. In order to

give sufficient room for anchoring, the cable anchor

points are spaced vertically at 1.5–2.5 m. Providing the

anchor zone is maintained close to the pylon top there is

little loss of structural efficiency as the behaviour of the

cable system will be dominated by the outermost cable

which is still attached to the top of the pylon and anchored

at the supported end of the back span. The advantages of

this arrangement are as follows.

n The large number of stays distribute the forces with greater

uniformity through the deck section, providing a continuous

elastic support. Hence the deck section can be both lighter

and simpler in its construction.

n As each stay supports a discrete deck module, each module can

be erected by the progressive cantilever method without resort

to any additional temporary supports. Thus increased speed

and efficiency of the deck erection is possible.

n The concentrated forces at each anchor point are much reduced.

n With the modified fan layout it is also possible to completely

encapsulate each stay, thus giving a double protective system

throughout its length and, should damage occur, replacement

of the stay can be undertaken as a routine maintenance task.

n The large number of stays of varying length and natural

frequency increases the potential damping of the structure.

Freyssinet International has recently reintroduced the

concept of a deviator saddle at pylons in conjunction with

a modified strand system, for use in smaller-span cable￾stayed bridges and extradosed bridges. The modified

strand, known as Cohestrand1, is protected by a poly￾ethylene sheath but is filled internally with polymer resin

instead of petroleum wax. The resin compound is hydro￾phobic, resistant to water vapour and oxygen and is capable

of transferring both compression and shear forces from the

polyethylene sheath to the steel wires of the strand. The

strand can thus be continuous through the deviator

saddle without the need to remove the polyethylene

sheath. This enables more slender pylons to be constructed

without having to provide a cross-over stay arrangement.

The disadvantages of earlier saddle designs have been

addressed in that the corrosion protection of each strand

is continuous through the saddle, individual strands are

not in contact and thus not subject to fretting corrosion

and the system is replaceable strand by strand. The deviator

saddle is made of a bundle of tubes placed within a larger

steel saddle tube. All voids between the tube bundles are

filled with a high-strength fibre concrete in the factory. If

(a)

(b)

(c)

Figure 1 Alternative stay cable arrangements: (a) fan stay system:

(b) modified fan stay system; and (c) harp stay system

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