Browsing by Author "Ranasinghe, WMDN"

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item: Conference-Full-text
A design method for prismatic prestressed continuous box girder bridges
(1998) Ranasinghe, WMDN; Jayasinghe, MTR
Prestressed concrete is one of the most difficult building materials to design due to many reasons such as number of different solutions available for the same problem and the time dependent changes that take place. When used for continuous box girder bridges, there are added problems associated with secondary moments. In this paper, a straight forward design method has been presented that localises the iterations involved in the design process as much as possible so that the calculations involved can be minimised. The guidelines to determine the cross sectional dimensions has also been discussed. A complete design example has been presented for a three span continuous bridge. A simple method for modelling a box girder as a grillage of beams also has been presented.
item: Conference-Full-text
Design of continuous prismatic prestressed concrete spine beams with variable prestressing forces
(2000) Ranasinghe, WMDN; Jayasinghe, MTR
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.
item: Thesis-Abstract
Structural design of prestressed concrete continuous double tee beam bridges
Ranasinghe, WMDN; Jayasinghe, MTR
One of the applications of prestressed concrete is in continuous bridges. These can be either double tee or box girders. The longitudinal shape can be prismatic, semi-prismatic or non- prismatic. The design of these bridges otTer a considerable challenge to the design engineer due to the presence of secondary moments which are induced due to the prestressing forces. These introduce a considerable complexity to the design process. In this research, an attempt is made to develop a design method that would minimize the complexity associated with the design of prismatic prestressed concrete double tee bridges. In concrete bridges, generally the minimization of self weight is important. This is used as the criterion to start the design process. The governing criteria for the various components of the cross section is used to determine the smallest section that is practically possible. The methods to take account of short term and long term effects such as creep and shrinkage are also included for the cross section selected. Once the cross section is available, it is necessary to find appropriate cable forces. Design methods for both constant and variable cable forces are presented based on the line of thrust. It is shown that the use of variable cable forces could reduce the total cable forces thus leading to a saving in tendons. For the selected cable forces, it is necessary to ensure that cable profile will be available within the selected section. The line of thrust can be transformed to fit within the section by selecting a suitable set of secondary moments. Thus, the secondary moment can be selected in a straightforward manner for both constant and variable cable forces. Thus the cable profile selected should ensure that it fits within the limits of cable profile zone and also generate the selected secondary moments. This is not a trivial task. In order to simplify this task, a design method was introduced for both constant and variable cable forces. In the case of variable cable forces, there is a possibility to have point moments and forces acting at the cable force change points. A method to deal with such forces is also introduced. Therefore, it is possible to consider that this thesis presents a complete design method for the preliminary design of prismatic double tee prestressed concrete beam bridges either with constant or variable cable forces. It has also shown that it is possible to minimize the complexity of difficult design tasks by approaching the problem in modular fashion. Keywords - Prestressed Concrete, Double Tee bridges