Time-dependent analysis of continuous precast prestressed concrete beams
A literature survey on the subject of construction, analysis and design of precast prestressed concrete beams made continuous has been conducted from which various methods of achieving continuity between the precast units have been identified. A need to re-evaluate the analysis and design of such structures has been established with special emphasis on the time-dependent aspects. An extensive evaluation of the time-dependent properties of concrete and the time- dependent behaviour of reinforced and prestressed concrete structures has been conducted. The evaluation used widely accepted methods of prediction for the material properties and four different analytical procedures for the behaviour of the structures. In a preliminary evaluation of the material properties, the methods of prediction based on the CEB78, CEB90 and ACI recommendations have been compared. This was followed by a more extensive parametric study of the factors affecting the material properties of plain concrete using the CEB90 recommendations. In a preliminary evaluation of the analytical procedures for structures subject to varying stress, the time- dependent behaviour of reinforced concrete sections using the Effective Modulus Method (EMM), Rate of Creep Method (RCM), Age-adjusted Effective Modulus Method (AEM) and Superposition Method (SSM) have been compared. This was followed by a more extensive evaluation of the factors affecting time-dependent behaviour of simply supported prestressed concrete beams using the AEM method. An evaluation of the parameters affecting the time-dependent behaviour of continuous precast prestressed concrete beams has been conducted using the SSM method which was based on a finite element analysis of prestressed concrete structures using the commercially available package, ADAPT. Both the AEM and ADAPT analyses have been compared against each other, and verified satisfactorily against a large number of experimental results. A special procedure to include cracked section properties has also been included into ADAPT analysis and it was found to improve time-dependent predictions of cracked reinforced concrete beams. Using the available features in ADAPT, the evaluation of continuous precast prestressed concrete has been extended to members connected using other methods of achieving continuity. Based on this study, several recommendations have been made on the treatment of the methods of prediction for the material properties and for the analysis of structures under varying state of stress, and on the analysis and design of continuous precast prestressed beams using unstressed reinforcement to provide continuity.