Pakistan Journal of Science (Vol. 65 No. 1 March, 2013) 26 INFLUENCE OF GIRDER AGE AT CONTINUITY AND CONSTRUCTION SEQUENCE ON THE TIME DEPENDENT RESTRAINT MOMENTS IN CONTINUOUS PRESTRESSED CONCRETE GIRDER BRIDGES A. Hameed, M. Saleem, A.U. Qazi and J. Zhang * Department of Civil Engineering, University of Engineering and Technology, Lahore * Professor, College of Civil Engineering, Chongqing Jiaotong University, China Corresponding Author: asifhameed@uet.edu.pk ABSTRACT: One of the alternate to minimize the expansion joints in prestressed concrete bridges is to use continuous bridges in place of simply supported bridges by connecting the ends of the girders over the supports with a continuity connection. More continuity means shallow sections or longer spans which will consequently reduce the total cost of the bridge. Unlike the simply supported bridges, the design and construction of continuous bridges connected with the continuity connection require some additional consideration due to time-dependent restraint moments. Restraint moments are developed at the continuity connection due to creep and shrinkage effects which makes the design of continuous bridges different from conventional bridges. In this study, the construction stage time dependent analysis method is proposed using age adjusted effective modulus method (AEMM). In the proposed method of analysis, firstly, the change in curvature at number of sections due to creep, shrinkage and relaxation is calculated. Secondly, the long-term deflections and rotations are determined by numerical integration of curvatures for a number of sections along the length of the member. Once the long term deflections and rotations are determined, the restraint moments are calculated by the force method of structural analysis. A parametric investigation is then carried out to study girder age at continuity and different construction scenarios on the time dependent restraint moments. By delaying girder age at continuity connection and casting of the deck concrete over the span leads to lower positive restraint moments and more continuity. Key words: Continuous Prestressed Concrete Bridge, Restraint Moment, Creep and Shrinkage. INTRODUCTION It is common practice, in current concrete bridge design, to include joints in the bridges. The elimination or minimizing of joints is important as they are expensive to buy, install, maintain and repair. Some times repair costs can be as high as replacement costs. Beginning in the late 1950’s, the advantages of making a multi -span, simply supported prestressed girder bridge continuous by connecting the ends of the girders over the supports with a continuity diaphragm connection began to be investigated. Continuity of precast girders can be achieved by providing continuous reinforcement in the deck over the piers and a concrete diaphragm between the ends of the girders at interior supports (Freyermuth,1969). This type of connection has been used successfully in several states for many years. The girders act as simple span members for dead loads, before the continuity connection is cast. Once the continuity diaphragm and deck are cast, the composite girder/deck section will carry live loads and superimposed dead loads as a continuous structure (Mirmiran et al, 2001). Unlike the simply supported bridges, the design and construction of continuous bridges require some additional considerations, due to time-dependent restraint moments which make the design of jointless bridges different from other conventional bridges (Peterman et al., 1998). Consequently a good understanding of the behavior of continuous bridges under the time-dependent restraint moment is essential in order to appropriately design the continuity connection reinforcement. Restraint Moments: In continuous composite precast girders bridges, the precast girders are often cast several days before they are installed. Therefore, most of the concrete shrinkage in these members will have occurred before the cast in place (CIP) concrete is cast. As the fresh C1P concrete cures, its shrinkage will then be partially restrained by the precast members to which, it has been connected. In the case of continuous construction, the continuous ends are not allowed to rotate and a negative restraint moment is produced (figure1).