Time Dependent Deflections of Pretensioned Beams

Eight long-span (127-ft) pretensioned AASHTO Type IV bridge beams made with high strength concrete and low-relaxation strand were instrumented in the field. Longtime deformations and internal beams temperatures were measured. Measurements were taken periodically from the time the beams were cast, through construction, and continued for one year after they were placed in service. Deformation measurements included concrete surface strains, prestressing strand strain, and quarter point and midspan camber or deflection. Internal concrete temperature were measured with thermocouples which were located in critical locations of the beam. The measured time dependent camber or deflection was compared to the results of several previously reported analytical techniques. These analytical results did not accurately predict the measured response.

A modification of the PCI multiplier technique for calculating longtime camber and deflection was developed and programmed for use on a microcomputer. These new multipliers were used to accurately predict the time dependent response of the instrumented beams. This technique was then used to calculate the senitivity of time dependent caber or deflection to some of the more important variables such as concrete strength, creep, and construction time schedule. Based on the results of this sensitivity study, one could expect camber or deflection of long span beams, similar to those studied, to vary from 2 to 6 in. at erection and from -0.75 to 2 in. at the end of the service life.