Report No.: CCEER-89-4
Title: Direct Field Measurement of Prestress Losses in Box Girder Bridges
Authors: J. Shields and M. Saiidi
Date: December 1989
Sponsoring Agency: Nevada Department of Transportation
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557
The Golden Valley Interchange in Reno, Nevada and the Sunset Road Interchange in Henderson, Nevada were studied for prestress losses and deflections. The time-dependent prestress losses were measured directly using electrical strain gages bonded to the prestressing tendons inside the bridge web girders. The measured time-dependent prestress losses on the Golden Valley Interchange matched a calculated time-step analysis very well during the first six months of this study. Prestress losses which were found based on the measured surface concrete strains reflected considerable scatter during this period. During the second six months after the initial stressing, the measured tendon strains showed a relatively large stress loss which was attributed to the dry weather. The predicted prestress loss which was based on the concrete surface strains showed a more consistent trend during the second six months. Anchorage set effect at midspan was measured and compared favorably with predicted values.
A parametric study was conducted on some important factors influencing the calculated prestress loss. The effect of relative humidity, creep and shrinkage correction factors, and time function was studied (Abstract by authors).