Report No.: CCEER-94-5
Title: Prestress Losses in a Post-Tensioned R/C Box Girder Bridge in Southern Nevada
Authors: M. "Saiid" Saiidi, Eric Hutchens, and Dean Gardella
Date: October 1994
Sponsoring Agency: Nevada Department of Transportation
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557
A post-tensioned, simply supported box-girder bridge, in southern Nevada was instrumented in June 1992 during construction. The study was triggered by concern over the low relative humidity in southern Nevada and its potential adverse effects on prestress force in prestressed bridges. The variation of prestress forces and deflection was then monitored over a 24-month period. The measured data consisted of tendon strain on four cables, concrete surface strain on two girders, and the deflection of the midspan relative to the ends of the superstructure. The change in strain values were converted to stresses and the prestress loss components of creep and shrinkage were recorded. These prestress loss components were then compared to four current loss prediction models; the 1992 AASHTO specifications, the ACI Committee 209R-92 report, the PCI Committee report of 1975, and Naaman's time-step method.
The measured data showed that the actual losses were less than those predicated by AASHTO and that, unlike a bridge which had been previously studied in northern Nevada , the low relative humidity did not cause excessive losses for the Greenway bridge. Assuming that the data for this bridge represent the behavior of other bridges in southern Nevada, it appears that southern Nevada bridges should perform adequately despite the low ambient relative humidity.
The study also showed that the AASHTO method to be more conservative than the ACI method and Naaman's method, which were slightly conservative. The PCI method predicted considerably less losses than those measured (Abstract by authors).