Manual method to find cable forces in extradosed bridges

Abstract

The first extradosed bridge was completed in 1994 and within the following three decades, this new bridge type has gained popularity due to its indistinguishable appearance to a cable stayed bridge. Balanced cantilever construction method was adopted in many of the extradosed bridges. Limited number of reference availability to study the extradosed bridges has limited the understanding of design concepts and practicing the designs among the engineers. Objective of this research is to develop a manual calculation method to determine the stay cable force for a prestressed concrete extradosed bridges constructed using balanced cantilever method. In addition, a detailed explanation about the design concepts, construction methods and comparison of extradosed bridge with other bridge types is also intended in this study. Determination of the stay cable force has been a challenging task for extradosed bridges due to its high statical indeterminacy. At present, modern bridge design software equipped with the required iteration functions are used mainly for this but no literature is available to manually approximate the stay cable force for an extradosed bridge constructed using balanced cantilever method. This research aims at bridging this gap and assisting the readers to understand all the basic concepts with regard to an extradosed bridge built using balanced cantilever method. Detailed literature review was conducted to understand the design concept and the initial sizing of the members. Based on the study, it was identified that the self-weight and the internal prestressing forces are the two main factors which affect the required final stay cable force. This was further verified using a computer analysis model developed using CSiBridge software. In developing a manual calculation method, it was assumed that the bridge behaves as a propped cantilever vertically supported at the stay cable anchorage locations. With the internal prestressing applied as a bending moment, self-weight applied as a uniformly distributed load (UDL) and the form traveller load applied as a point load at the cantilever end, stay cable force required was determined considering that the total displacement at cable anchorage location requires to be zero. Several assumptions were made in this simplified method and the stay cable force determined from the simplified method was found to be reasonably accurate compared with the actual force.