Effects of Restraint Moments in Integral Abutment Bridges

The integral abutment bridge concept has received considerable interest among bridge engineers owing to their enormous benefits due to elimination of expensive joints and reduced installation and maintenance costs. The superstructure of integral abutment bridges is made continuous through a composite cast-in-place concrete deck slab over prestressed concrete or steel girders and continuity diaphragms. The girders are often rigidly connected with the abutments. Restraint moments develop in the superstructure due to the continuity and time-dependent creep, shrinkage and thermal effects. This makes the design of integral abutment bridges different from other conventional bridges. There is a need to develop an analytical procedure to determine the secondary effects induced due to creep, shrinkage and temperature. The objective of this paper is to present the state-of-the-art on restraint moments in continuous precast prestressed girder bridges and its relevance to integral abutment bridges. Approximate and more rigorous methods are available to determine the effects of creep and shrinkage. The Portland Cement Association (PCA) method reports the influence of creep of the precast girders, and of the differential shrinkage between the precast girders and the situ-cast deck slab on continuity behavior. The Construction Technology Laboratories Inc. (CTL) approach was based on a series of computer simulations to study the effects of variation in time-dependent material behavior and variation in bridge design parameters on the resultant service moments in the bridge girders. A modified restraint-moment calculation method (P-method) was developed for full-span prestressed concrete form panels that accounts for the length and stiffness of the diaphragm, the different initiation time for creep and the restraint of cast-in-place concrete shrinkage. A flexibility based analytical tool was presented to predict time-dependent restraint moments and the effectiveness of the continuity connection under service live loads. Non-linear time-dependent analysis was reported to examine the design considerations for integral abutment bridges.