Restraint Moments Due to Thermal Gradients in Continuous Prestressed Concrete Girder Bridges

This research is motivated by the need to develop new bridges, using new materials and construction details to achieve the goal of building longer span bridges that are safe and durable. The study of precast/prestressed concrete bridges made continuous started early in the 1960’s. Continuous bridges have many advantages compared to simple span bridges such as reduction in structural depth, riding comfort and reserve load capacity under overload conditions. They also eliminate joints that require a lot of maintenance and cause deterioration in some cases. The design and construction of continuous bridges requires the consideration of many factors that are not considered in the design of simply supported bridges. For example positive restraining moment that can cause cracking near the bottom of the girders/continuity diaphragm at the supports develops in continuous bridges are caused by time dependent factors such as creep, shrinkage and thermal gradient. This research presents an analytical approach for calculating restraint moments in Prestressed Concrete continuous bridge girders. The RESTRAINT program, which is capable of predicting restraint moments due to creep and shrinkage, was modified to add a feature for calculating the restraining moment due to thermal gradient. A detailed parametric study was carried out using the modified version of mRESTRAINT to investigate the effect of various parameters on restraint moments in continuous girders. The results from the parametric study were then used to establish optimum girder age at time of establishing continuity such that an allowable restraining moment is not exceeded. A reliability study was also conducted on two span continuous bridge with equal span lengths to estimate the probability of cracking at girder’s ends due to positive restraint moments obtained from study. The reliability showed that prestressed girders have a high (68% - 93%) probability of cracking if made continuous as a result of the combined effect of thermal gradient, creep and shrinkage