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Prestress Loss in Four Box Girder Bridges in Northern Nevada
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Four new post-tensioned, box girder bridges in Northern Nevada were instrumented during construction to collect data on the possible adverse effects of high variation in relative humidity (RH) and temperature on prestress losses. The bridges were monitored at short intervals during the first month after stressing and in two-month intervals afterward. For each bridge, the measured data consisted of concrete surface strains on two girders, creep strains on concrete cylinders at the bridge sites, shrinkage strains on cylinders at the site and at the laboratory, and deflection of the midspan relative to the ends of the structures. The actual time-dependent prestress losses due to creep and shrinkage were calculated from the measured strain data. The measured losses were compared to two current loss prediction methods: the AASHTO lump-sum estimate of prestress losses and the time-step method of determining losses by Naaman. To determine the effects of changes in RH on the losses, the climatic data of the area were incorporated into the calculated losses. The losses predicted by AASHTO for the two-span bridges were within 7 percent of the measured data (extrapolated). For South Meadows, AASHTO underestimated the losses by 7 percent, while for Mount Rose, AASHTO overestimated the losses by 7 percent. The single span bridges studied exhibited a greater variation between the measured data and the AASHTO predicted values. Lifetime creep and shrinkage losses were overestimated by AASHTO on the Zolezzi bridge by 31 percent and underestimated by AASHTO on the Old Virginia bridge by 32percent. The losses predicted by the time-step method using calculated ultimate coefficients were lower than the extrapolated measured losses on all bridges by an average of 19 percent. It is recommended to moist cure the bridge for 7 to 10 days following the stressing to minimize losses (Abstract by authors).
Report No. CCEER-99-2