ESTIMATION OF PRESENT PRESTRESSING FORCE IN EXISTING PRESTRESSED CONCRETE BRIDGE

Abstract

Prestressed Concrete (PSC) structures especially bridges are being largely used all over the world due to their multifarious advantages in terms of structural behaviour, economy as well as aesthetical aspects. However, many PSC structures deteriorate with time due to several factors which may lead to certain loss in their initial applied prestress. This loss may affect the structure behaviour and its service life adversely and need to be checked. Such loss of prestress will also result degradation in stiffness of the structure. A precise determination of prestress losses in prestressed concrete members is a complicated problem because the rate of loss due to one factor, such as relaxation of prestressing cables, is continually being altered by changes in stress due to other factors, such as creep of concrete. It has been known that any change in the stiffness of the structure will change its natural frequency also. Hence, it may be possible to ascertain the los by measurement of natural frequency at regular intervals. In this study, an effort has been made to determine the natural frequency of a PSC structure by carrying out Vibration Study on it and determining the existing prestressing force to ascertain the loss of prestress. To achieve this objective, following methodology has been adopted: 1. The theoretical study has been carried out to establish a relationship between natural frequency and prestressing force for a simply-supported prestressed concrete bridge of composite section having parabolic profile of prestressing cables. 2. Experimental study has been carried out to determine the natural frequency of the bridge and measurement of prestressing force, using load cells which were installed during the construction of the bridge. 3. Thereafter, present prestressing force has been estimated using the developed relationship between natural frequency and prestressing force. 4. Results of estimated prestressing force were then compared with the measured prestressing force.