Time-dependent analysis of partially prestressed composite members

The object of the research is to develop a computer based procedure for calculation of stresses and deformations of partially prestressed continuous beams and frames. The analysis accounts for the nonlinearity produced by cracking, creep and shrinkage of concrete, as well as relaxation of prestressed steel. Composite members made of precast prestressed girders and cast-in-place topping and joints are within the scope of the proposed method. Second stage post-tensioning can be introduced to the members to make them a continuous beam or frame. The procedure accounts for cracking under the weight of cast-in-place topping, any superimposed loads and the time-dependent stress redistribution. Thus, creep, shrinkage and relaxation are taken into account for cracked prestressed concrete members. Attempt is made to limit the input data to information commonly available to designers. Both shored and unshored construction are allowed for. Linear creep law is used, and the time-dependent effects, such as prestress losses and deflection growth, are automatically generated. The step-by-step method is employed to evaluate the variations of stresses and deformations with time. The time period is divided into discrete time intervals depending upon time at which various loads are applied. The increments of stresses and deformations during each interval are calculated with consideration of the time-dependent effects occurring in preceding intervals. Updating values are determined by summing up their increments up to the interval considered. A rational procedure is presented for analysis of the cracked composite section. It accounts for possible subsequent cracking as well as initial cracking. Therefore, the procedure is continued to check for additional cracking due to successive loading applications and time-development of stresses. A computer program is developed for analysis of partially prestressed composite members. It is especially useful for analysis of continuous beams or frames with multi-stage construction and prestressing, including use of temporary falsework among its diverse applications. The output data give instantaneous as well as gradually developing stresses in concrete and steel at every sections considered. The program also provides variation of displacement with time along the centroidal axis of the structure