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CHAPTER 3

ECONOMY

3.1 GENERAL

Economy is an important parameter to determine which type of bride structure is

to be adopted for a given site condition. Relative costs of bridges depend on the

number and length of spans and number of piers that affect the method

construction. Studies of comparative costs of cable stayed bridges and other 

types of bridges are few; consequently, a designer should perform a detailed

investigation of economics of the total structure including cost involving erection

until sufficient data is available to make general decision quickly. Here some

works on economic aspects of cable stayed bridges have been discussed.

3.2 REVIEW ON ECONOMICS OF CABLE STAYED BRIDGES

In his survey of the bridges in Germany (1966), Thul (Ref. 2) has compared the

center span length to the total length of the bridge for three-span continuous

girder bridges, cable- stayed bridges, and suspension bridges. His investigation

shows that the cable stayed bridge fills the void left by the continuous girder and

suspension bridges in the range of 210 m (700 ft) to 300 m (1000 ft), with a

corresponding center span to total length range of 50 to 60%. In his comparative

study, Thul has shown that the cable-stayed concept can be economical for 

bridges with intermediate spans. However, with greater experience in design and

construction, the application of longer main spans of cable-stayed bridges has

increased.

In the economical study on Canadian bridges (1969), Er. P.R. Tailor (Ref. 2) has

concluded that for Canada highways cable stayed bridges with center span

ranging from 210 m (700 ft) to 240 m (800 ft) are 5 to 10% more economical than

other types of comparable bridges.

When Thul wrote: ‘It is considered highly unlikely or unrealistic to build bridges

with very long spans using cable-stayed construction. Such span lengths will be

reserved for suspension bridges because there are considerable difficulties in

construction of cable-stayed bridges,” he apparently did not foresee the effects of 

 

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improved technology and modern techniques of erection and construction, as

perceived by Leonhardt. Leonhardt, 1970, (Ref. 2) concluded that cable-stayed

bridges are particularly suited for spans in excess of 600 m (2000 ft) and may

even be constructed with spans of more than 1500 m (5000 ft).

In their study (1998), M. Nagai, X. Xie, H. Yamaguchi and Y. Fujino (Ref. 9) have

presented a steel volume of a 1400-meter cable-stayed bridge model and

discussed economy of the cable-stayed bridges with a span exceeding 1000

meters. The employed girder for the cable-stayed bridge has the span/depth ratio

of 400 and the span/width ratio of 56, respectively. These values are higher than

those of conventional cable-stayed bridges. The safety against static and

dynamic instabilities of this model is examined by elasto-plastic finite

displacement analysis under in-plane load, finite displacement analysis which

takes displacement-dependent wind load into account and flutter analysis based

on modal coordinate. From this study, it is shown that the cable-stayed system

with a span up to 1600 meters is expected to be an economical alternative.