Effects of Axial Force on Frequency of Prestressed Concrete Bridges

Report No. CCEER-92-4The purpose of the study presented in this report was determine if vibration frequencies of prestressed concrete member can be used to establish prestress losses. A post-tensioned concrete bridge (called the Golden Valley bridge) which had been instrumented for another study was the primary subject of the study. The bridge was a simply-supported, multi-cell box girder. Because the actual stresses in the bridge were known, it was possible to determine if the changes in the measured frequencies of the bridge would correlate with the prestress losses. The dynamic characteristics of the bridge were measured on days 105, 202, and 455 from the prestressing completion date. The bridge was excited by impact from a heavy truck or by ambient truck traffic. Several data sets were collected in each test, and the frequencies were determined from the free-vibration acceleration data using a fast Fourier transformation method. The theoretical prediction for homogenous members was that, as the prestress force decreased, the frequency would increase because a reduction in the axial compressive load should stiffen the element. The measured frequencies for the bridge showed an opposite trend. To verify the field observation, a 12-ft. long prestress member was built and tested in the laboratory under different axial loads. The laboratory data showed the same trend as that observed in the field. The difference between the theoretical and measured results was attributed to the fact that, in concrete members with moderate axial loads, prestress forces tend to close shrinkage and other microcracks, and hence stiffen the element. Based on the measured data for the laboratory specimen, an empirical equation was developed that accounts for the effect of axial force on the rigidity of the element. This expression was attempted for the Golden Valley bridge and led to a reasonable estimate of the fundamental frequencies at different prestress forces