Cable Stayed Bridge

Introduction

The oldest attempt a cable stayed bridge dates back to 1784, When a German carpenter designed a structure entirely in timber. However the first actual bridge was of hybrid design part suspension and part stayed. The bridge at storm bound, Sweden (in 1955)is considered as the first modern steel cable stayed bridge. The main reasons for the success of this type of bridge are speedy construction and economical feasibility, In addition to their structural and economical advantages. Cable stayed bridges owe their popularity to their elegant transparent appearance.

General Design

The three fundamental load bearing elements in this structure are cables, deck and pylons.
 
2.1: Layout of cable stays.

This is one of the fundamental items in the design of cable stayed bridges. It influences not only the structural performance of the bridge but also the method of erection and the economics.

In transverse direction, one central plane, tow lateral planes, three or more planes can be used with the aim of reducing the cross sectional forces when the deck is wide.

Longitudinal layout of C.S.Bs can be harp pattern-The cables are parallel and cross each other at a constant angle, Fan pattern-This brings all the stays together to the top of pylons, semi harp pattern an intermediate solution between the extremes of harp and fan patterns or Asymmetric layout. However from the point of view of economy of stays, an angle of 45° is the optimum.

The maximum spacing of the stays depend on several parameters, in particular on width of shape of deck. As a general rule spacing between 15m and 25m are adopted. 
2.2: Deck

   2.2.1: steel decks (2.5-3.5 kg/m²)

It is possible to limit it is self weight to a value which is about one-fifth of that of a concrete deck. But use of a steel cross section is today, two or four times as expensive as its equivalent in concrete.

   2.2.1: Concrete deck s (10-15 kg/m²)

The idea of a multiple stay system, initially for steel structures, rapidly led to the construction of decks in concrete-poured in situ or prefabricated. The high self weight of concrete decks is not a major factor in case of small and medium spans.

   2.2.1: Composite deck s (6.5-8.5 kg/m²)

The use of mixed concrete and steel design in C.S.Bs can show considerable advantages. The fact that the dead weight of a composite deck is somewhat greater than of a pure steel deck is generally not a critical disadvantage, except in bridges with very large spans.

2.3: Pylons

Because of their very function as load bearing elements, centralizing the loads, the pylons have a governing influence on the overall architectural effect of a C.S.Bs .In present market conditions steel pylons are more expensive than those in concrete. A reinforced concrete pylon or even a presterssed one may be seen to be the answer, because of the appreciable savings in subsequent maintenance cost. The number of bays erected, the proposal placing of the stays and the local conditions are all relevant parameters in this element of design.

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