CONSTRUCTION STRESSES IN CONCRETE CABLE- STAYED BRIDGES

Abstract

The cable-stayed bridge is one of the relatively recent innovate bridge form. It consists of three principal components, namely, girders, towers (pylons) and cable stays. The girder is supported elastically at points along its length by inclined cable-stays so that it can span a much longer distance without intermediate piers. The cables are connected to the tower to transmit the load to the foundation. The cable-stayed bridge being a highly indeterminate structure, it becomes extremely important to evolve a suitable construction procedure. Since the bridge is designed for the completed structure during the construction of the bridge excess stresses may develop. Hence it becomes very important to find the stresses during each stage of construction, and monitor cable tensions and deflections to achieve not only safety at every stage but also to achieve the final profile of the bridge correctly. In the present study the cable-stayed bridge at NAINI, which is near Allahabad has been considered for examining and evolving the sequence of construction. For this bridge, the cantilever method of erection has been used. To begin with, backward analysis has been performed in which the bridge is first assumed to be completed, and then the analysis is followed by removing the loads at each stage of construction. The forces at each stage are monitored so that there should not be excessive tensions in cables. Stresses in girder are checked so that they should be within permissible limits. Deflection profiles are generated which are used to position the deck correctly during construction. It is also necessary to check the bridge for earthquake forces during construction. These have, therefore, been checked at some of the construction stages. For the earthquake analysis 3-D model is developed and analysis is done by response spectrum method.