Design of continuous prismatic prestressed concrete spine beams with variable prestressing forces

Abstract

Design of continuous prismatic prestressed concrete continuous spine beams is a complex task, primarily due to the secondary moments that occur as a result of the prestressing effects. In prestressed concrete members, it is useful to select the smallest possible section while satisfying the criteria like the use of a particular construction technique, constructability and stress limits. The methods that can be employed for selecting the smallest section dimensions by considering the global and local bending, constructability, restrictions on depth etc. are highlighted. It is advantageous to minimise the total cable force (∫Pds) used for a continuous prestressed concrete prismatic spine beam with respect to economy and constructability. It is shown in this paper that the total cable force cannot be minimised by using a constant cable force throughout the length. For that, different cable forces could be used in span and support regions. It is also shown that for any given section, there is a maximum cable force that should not be exceeded over the supports. The selection of secondary moments to ensure that the cable profile zone resulting from the minimum cable forces will lie within the section is also highlighted. When the cable force changes, there will be point moments and point forces that will act at those sections. A straight forward method is presented for dealing with these when finding the cable profile that will satisfy the stress limits and also generate the secondary moment distribution already selected at the beginning of the design.